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This section covers all types of motors, from the elementary circuitry needed to control a variable reluctance motor, to the H-bridge circuitry needed to control a bipolar permanent magnet motor. Each class of drive circuit is illustrated with practical examples, but these examples are not intended as an exhaustive catalog of the commercially available control circuits, nor is the information given here intended to substitute for the information found on the manufacturer’s component data sheets for the parts mentioned.

 

This section only covers the most elementary control circuitry for each class of motor. All of these circuits assume that the motor power supply provides a drive voltage no greater than the motor’s rated voltage, and this significantly limits motor performance. The next section, on current limited drive circuitry, covers practical high-performance drive circuits.

 

 

Variable Reluctance Motors

Typical controllers for variable reluctance stepping motors are variations on the outline shown in Figure 3.1:

 

Figure 3.1

In Figure 3.1, boxes are used to represent switches; a control unit, not shown, is responsible for providing the control signals to open and close the switches at the appropriate times in order to spin the motors. In many cases, the control unit will be a computer or programmable interface controller, with software directly generating the outputs needed to control the switches, but in other cases, additional control circuitry is introduced, sometimes gratuitously!

 

Motor windings, solenoids and similar devices are all inductive loads. As such, the current through the motor winding cannot be turned on or off instantaneously without involving infinite voltages! When the switch controlling a motor winding is closed, allowing current to flow, the result of this is a slow rise in current. When the switch controlling a motor winding is opened, the result of this is a voltage spike that can seriously damage the switch unless care is taken to deal with it appropriately.

 

There are two basic ways of dealing with this voltage spike. One is to bridge the motor winding with a diode, and the other is to bridge the motor winding with a capacitor. Figure 3.2 illustrates both approaches:

 

Figure 3.2

 

The diode shown in Figure 3.2 must be able to conduct the full current through the motor winding, but it will only conduct briefly each time the switch is turned off, as the current through the winding decays. If relatively slow diodes such as the common 1N400X family are used together with a fast switch, it may be necessary to add a small capacitor in parallel with the diode.

 

The capacitor shown in Figure 3.2 poses more complex design problems! When the switch is closed, the capacitor will discharge through the switch to ground, and the switch must be able to handle this brief spike of discharge current. A resistor in series with the capacitor or in series with the power supply will limit this current. When the switch is opened, the stored energy in the motor winding will charge the capacitor up to a voltage significantly above the supply voltage, and the switch must be able to tolerate this voltage. To solve for the size of the capacitor, we equate the two formulas for the stored energy in a resonant circuit:

 

P = C V2 / 2

P = L I2 / 2

Where:

P — stored energy, in watt seconds or coulomb volts

C — capacity, in farads

V — voltage across capacitor

L — inductance of motor winding, in henrys

I — current through motor winding

Solving for the minimum size of capacitor required to prevent overvoltage on the switch is fairly easy:

C > L I2 / (Vb – Vs)2

Where:

Vb — the breakdown voltage of the switch

Vs — the supply voltage

Variable reluctance motors have variable inductance that depends on the shaft angle. Therefore, worst-case design must be used to select the capacitor. Furthermore, motor inductances are frequently poorly documented, if at all.

The capacitor and motor winding, in combination, form a resonant circuit. If the control system drives the motor at frequencies near the resonant frequency of this circuit, the motor current through the motor windings, and therefore, the torque exerted by the motor, will be quite different from the steady state torque at the nominal operating voltage! The resonant frequency is:

 

f = 1 / ( 2 (L C)0.5 )

Again, the electrical resonant frequency for a variable reluctance motor will depend on shaft angle! When a variable reluctance motors is operated with the exciting pulses near resonance, the oscillating current in the motor winding will lead to a magnetic field that goes to zero at twice the resonant frequency, and this can severely reduce the available torque!

 

Unipolar Permanent Magnet and Hybrid Motors

 

Typical controllers for unipolar stepping motors are variations on the outline shown in Figure 3.3:

 

Figure 3.3

In Figure 3.3, as in Figure 3.1, boxes are used to represent switches; a control unit, not shown, is responsible for providing the control signals to open and close the switches at the appropriate times in order to spin the motors. The control unit is commonly a computer or programmable interface controller, with software directly generating the outputs needed to control the switches.

As with drive circuitry for variable reluctance motors, we must deal with the inductive kick produced when each of these switches is turned off. Again, we may shunt the inductive kick using diodes, but now, 4 diodes are required, as shown in Figure 3.4:

 

 

 

 

 

 

 

 

 

Figure 3.4

The extra diodes are required because the motor winding is not two independent inductors, it is a single center-tapped inductor with the center tap at a fixed voltage. This acts as an autotransformer! When one end of the motor winding is pulled down, the other end will fly up, and visa versa. When a switch opens, the inductive kickback will drive that end of the motor winding to the positive supply, where it is clamped by the diode. The opposite end will fly downward, and if it was not floating at the supply voltage at the time, it will fall below ground, reversing the voltage across the switch at that end. Some switches are immune to such reversals, but others can be seriously damaged.

A capacitor may also be used to limit the kickback voltage, as shown in Figure 3.5:

 

Figure 3.5

The rules for sizing the capacitor shown in Figure 3.5 are the same as the rules for sizing the capacitor shown in Figure 3.2, but the effect of resonance is quite different! With a permanent magnet motor, if the capacitor is driven at or near the resonant frequency, the torque will increase to as much as twice the low-speed torque! The resulting torque versus speed curve may be quite complex, as illustrated in Figure 3.6:

Figure 3.6

Figure 3.6 shows a peak in the available torque at the electrical resonant frequency, and a valley at the mechanical resonant frequency. If the electrical resonant frequency is placed appropriately above what would have been the cutoff speed for the motor using a diode-based driver, the effect can be a considerable increase in the effective cutoff speed.

 

The mechanical resonant frequency depends on the torque, so if the mechanical resonant frequency is anywhere near the electrical resonance, it will be shifted by the electrical resonance! Furthermore, the width of the mechanical resonance depends on the local slope of the torque versus speed curve; if the torque drops with speed, the mechanical resonance will
be sharper, while if the torque climbs with speed, it will be broader or even split into multiple resonant frequencies.

 

Practical Unipolar and Variable Reluctance Drivers

 

In the above circuits, the details of the necessary switches have been deliberately ignored. Any switching technology, from toggle switches to power MOSFETS will work! Figure 3.7 contains some suggestions for implementing each switch, with a motor winding and protection diode included for orientation purposes:

 

Figure 3.7

 

Each of the switches shown in Figure 3.7 is compatible with a TTL input. The 5 volt supply used for the logic, including the 7407 open-collector driver used in the figure, should be well regulated. The motor power, typically between 5 and 24 volts, needs only minimal regulation. It is worth noting that these power switching circuits are appropriate for driving solenoids, DC motors and other inductive loads as well as for driving stepping motors.

 

The SK3180 transistor shown in Figure 3.7 is a power darlington with a current gain over 1000; thus, the 10 milliamps flowing through the 470 ohm bias resistor is more than enough to allow the transistor to switch a few amps current through the motor winding. The 7407 buffer used to drive the darlington may be replaced with any high-voltage open collector chip that can sink at least 10 milliamps. In the event that the transistor fails, the high-voltage open collector driver serves to protects the rest of the logic circuitry from the motor power supply.

 

The IRC IRL540 shown in Figure 3.7 is a power field effect transistor. This can handle currents of up to about 20 amps, and it breaks down nondestructively at 100 volts; as a result, this chip can absorb inductive spikes without protection diodes if it is attached to a large enough heat sink. This transistor has a very fast switching time, so the protection diodes must be comparably fast or bypassed by small capacitors. This is particularly essential with the diodes used to protect the transistor against reverse bias! In the event that the transistor fails, the zener diode and 100 ohm resistor protect the TTL circuitry. The 100 ohm resistor also acts to somewhat slow the switching times on the transistor.

 

For applications where each motor winding draws under 500 milliamps, the ULN200x family of darlington arrays from Allegro Microsystems, also available as the DS200x from National Semiconductor and as the Motorola MC1413 darlington array will drive multiple motor windings or other inductive loads directly from logic inputs. Figure 3.8 shows the pinout of the widely available ULN2003 chip, an array of 7 darlington transistors with TTL compatible inputs:

 

Figure 3.8

 

The base resistor on each darlington transistor is matched to standard bipolar TTL outputs. Each NPN darlington is wired with its emitter connected to pin 8, intended as a ground pin, Each transistor in this package is protected by two diodes, one shorting the emitter to the collector, protecting against reverse voltages across the transistor, and one connecting the collector to pin 9; if pin 9 is wired to the positive motor supply, this diode will protect the transistor against inductive spikes.

The ULN2803 chip is essentially the same as the ULN2003 chip described above, except that it is in an 18-pin package, and contains 8 darlingtons, allowing one chip to be used to drive a pair of common unipolar permanent-magnet or variable-reluctance motors.

 

For motors drawing under 600 milliamps per winding, the UDN2547B quad power driver made by Allegro Microsystems will handle all 4 windings of common unipolar stepping motors. For motors drawing under 300 milliamps per winding, Texas Instruments SN7541, 7542 and 7543 dual power drivers are a good choice; both of these alternatives include some logic with the power drivers.

 

Bipolar Motors and H-Bridges

 

Things are more complex for bipolar permanent magnet stepping motors because these have no center taps on their windings. Therefore, to reverse the direction of the field produced by a motor winding, we need to reverse the current through the winding. We could use a double-pole double throw switch to do this electromechanically; the electronic equivalent of such a switch is called an H-bridge and is outlined in

 

Figure 3.9

As with the unipolar drive circuits discussed previously, the switches used in the H-bridge must be protected from the voltage spikes caused by turning the power off in a motor winding. This is usually done with diodes, as shown in Figure 3.9.

It is worth noting that H-bridges are applicable not only to the control of bipolar stepping motors, but also to the control of DC motors, push-pull solenoids (those with permanent magnet plungers) and many other applications.

 

With 4 switches, the basic H-bridge offers 16 possible operating modes, 7 of which short out the power supply! The following operating modes are of interest:

 

Forward mode, switches A and D closed.

 

Reverse mode, switches B and C closed.

These are the usual operating modes, allowing current to flow from the supply, through the motor winding and onward to ground. Figure 3.10 illustrates forward mode:

Figure 3.10

Fast decay mode or coasting mode, all switches open.

Any current flowing through the motor winding will be working against the full supply voltage, plus two diode drops, so current will decay quickly. This mode provides little or no dynamic braking effect on the motor rotor, so the rotor will coast freely if all motor windings are powered in this mode. Figure 3.11 illustrates the current flow immediately after switching from forward running mode to fast decay mode.

 

 

 

 

Figure 3.11

Slow decay modes or dynamic braking modes.

 

In these modes, current may recirculate through the motor winding with minimum resistance. As a result, if current is flowing in a motor winding when one of these modes is entered, the current will decay slowly, and if the motor rotor is turning, it will induce a current that will act as a brake on the rotor. Figure 3.12 illustrates one of the many useful slow-decay modes, with switch D closed; if the motor winding has recently been in forward running mode, the state of switch B may be either open or closed:

Figure 3.12

Most H-bridges are designed so that the logic necessary to prevent a short circuit is included at a very low level in the design. Figure 3.13 illustrates what is probably the best arrangement:

 

Figure 3.13

Here, the following operating modes are available:

XY  ABCD Mode 

   

00  0000 fast decay 

01  1001 forward 

10  0110 reverse 

11  0101 slow decay 

 

The advantage of this arrangement is that all of the useful operating modes are preserved, and they are encoded with a minimum number of bits; the latter is important when using a microcontroller or computer system to drive the H-bridge because many such systems have only limited numbers of bits available for parallel output. Sadly, few of the integrated H-bridge chips on the market have such a simple control scheme.

 

Practical Bipolar Drive Circuits

 

There are a number of integrated H-bridge drivers on the market, but it is still useful to look at discrete component implementations for an understanding of how an H-bridge works. Antonio Raposo (ajr@cybill.inesc.pt) suggested the H-bridge circuit shown in Figure 3.14;

 

Figure 3.14

The X and Y inputs to this circuit can be driven by open collector TTL outputs as in the darlington-based uni
polar drive circuit in Figure 3.7. The motor winding will be energised if exactly one of the X and Y inputs is high and exactly one of them is low. If both are low, both pull-down transistors will be off. If both are high, both pull-up transistors will be off. As a result, this simple circuit puts the motor in dynamic braking mode in both the 11 and 00 states, and does not offer a coasting mode.

The circuit in Figure 3.14 consists of two identical halves, each of which may be properly described as a push-pull driver. The term half H-bridge is sometimes applied to these circuits! It is also worth noting that a half H-bridge has a circuit quite similar to the output drive circuit used in TTL logic. In fact, TTL tri-state line drivers such as the 74LS125A and the 74LS244 can be used as half H-bridges for small loads, as illustrated in Figure 3.15:

 

 

 

 

 

 

Figure 3.15

This circuit is effective for driving motors with up to about 50 ohms per winding at voltages up to about 4.5 volts using a 5 volt supply. Each tri-state buffer in the LS244 can sink about twice the current it can source, and the internal resistance of the buffers is sufficient, when sourcing current, to evenly divide the current between the drivers that are run in parallel. This motor drive allows for all of the useful states achieved by the driver in Figure 3.13, but these states are not encoded as efficiently:

XYE  Mode 

  

–1  fast decay 

000  slower decay 

010  forward 

100  reverse 

110  slow decay 

 

The second dynamic braking mode, XYE=110, provides a slightly weaker braking effect than the first because of the fact that the LS244 drivers can sink more current than they can source.

The Microchip (formerly Telcom Semiconductor) TC4467 Quad CMOS driver is another example of a general purpose driver that can be used as 4 independent half H-bridges. Unlike earlier drivers, the data sheet for this driver even suggests using it for motor control applications, with supply voltages up to 18 volts and up to 250 milliamps per motor winding.

 

One of the problems with commercially available stepping motor control chips is that many of them have relatively short market lifetimes. For example, the Seagate IPxMxx series of dual H-bridge chips (IP1M10 through IP3M12) were very well thought out, but unfortunately, it appears that Seagate only made these when they used stepping motors for head positioning in Seagate disk drives. The Toshiba TA7279 dual H-bridge driver would be another another excellent choice for motors under 1 amp, but again, it appears to have been made for internal use only.

 

The SGS-Thompson (and others) L293 dual H-bridge is a close competitor for the above chips, but unlike them, it does not include protection diodes. The L293D chip, introduced later, is pin compatible and includes these diodes. If the earlier L293 is used, each motor winding must be set across a bridge rectifier (1N4001 equivalent). The use of external diodes allows a series resistor to be put in the current recirculation path to speed the decay of the current in a motor winding when it is turned off; this may be desirable in some applications. The L293 family offers excellent choices for driving small bipolar steppers drawing up to one amp per motor winding at up to 36 volts. Figure 3.16 shows the pinout common to the L293B and L293D chips:

 

Figure 3.16

This chip may be viewed as 4 independent half H-bridges, enabled in pairs, or as two full H-bridges. This is a power DIP package, with pins 4, 5, 12 and 13 designed to conduct heat to the PC board or to an external heat sink.

 

The SGS-Thompson (and others) L298 dual H-bridge is quite similar to the above, but is able to handle up to 2-amps per channel and is packaged as a power component; as with the LS244, it is safe to wire the two H-bridges in the L298 package into one 4-amp H-bridge (the data sheet for this chip provides specific advice on how to do this). One warning is appropriate concerning the L298; this chip very fast switches, fast enough that commonplace protection diodes (1N400X equivalent) don’t work. Instead, use a diode such as the BYV27. The National Semiconductor LMD18200 H-bridge is another good example; this handles up to 3 amps and has integral protection diodes.

 

While integrated H-bridges are not available for very high currents or very high voltages, there are well designed components on the market to simplify the construction of H-bridges from discrete switches. For example, International Rectifier sells a line of half H-bridge drivers; two of these chips plus 4 MOSFET switching transistors suffice to build an H-bridge. The IR2101, IR2102 and IR2103 are basic half H-bridge drivers. Each of these chips has 2 logic inputs to directly control the two switching transistors on one leg of an H-bridge. The IR2104 and IR2111 have similar output-side logic for controlling the switches of an H-bridge, but they also include input-side logic that, in some applications, may reduce the need for external logic. In particular, the 2104 includes an enable input, so that 4 2104 chips plus 8 switching transistors can replace an L293 with no need for additional logic.

 

The data sheet for the Microchip (formerly Telcom Semiconductor) TC4467 family of quad CMOS drivers includes information on how to use drivers in this family to drive the power MOSFETs of H-bridges running at up to 15 volts.

 

A number of manufacturers make complex H-bridge chips that include current limiting circuitry; these are the subject of the next section. It is also worth noting that there are a number of 3-phase bridge drivers on the market, appropriate for driving Y or delta configured 3-phase permanent magnet steppers. Few such motors are available, and these chips were not developed with steppers in mind. Nonetheless, the Toshiba TA7288P, the GL7438, the TA8400 and TA8405 are clean designs, and 2 such chips, with one of the 6 half-bridges ignored, will cleanly control a 5-winding 10 step per revolution motor.

 

 

Abstract -We investigated a small isolated hybrid power system that used two types of power generation; wind turbine and diesel generation. The interaction of diesel generation, the wind turbine, and the local load is complicated because both the load and the wind turbine fluctuate during the day. These fluctuations create imbalances in power distribution (energy sources are not equal to energy sinks) that can affect the frequency and the voltage in the power system. The addition of energy storage will help balance the distribution of power in the power network. For this paper, we studied the interaction among hybrid power system components and the relative size of the components. We also show how the contribution of wind energy affects the entire power system and distribution and the role of energy storage under the transient conditions caused by load changes and wind turbine startups.

Index Terms - wind turbine, diesel generator, hybrid power system, renewable energy, energy storage.

 

I. INTRODUCTION

Windmills were used to pump water and mill grain, along with many other uses [1, 2, 3, 4].

Today, wind turbines are used for similar purposes (i.e., water or oil pumping, battery charging, and utility generation). One important aspect of wind turbine applications, especially in an industrial environment, is that wind turbines generate electricity without creating pollution. Wind turbines are also well suited for generating electricity in isolated places with no connections to the utility grid [2,3,4]. However, in isolated applications, especially very small applications, the power system components (sources and loads) are limited, and the system networks are weak in many cases. Thus, any changes in the power input or output of one component may affect the rest of the system more dramatically than in a larger system where the smoothing effect of many components benefits the overall system. In this paper, we analyze a hybrid power system consisting of a wind turbine, a diesel generator, a local load, and energy storage. We also present the impact of energy storage on the power system performance. The results and conclusions of this analysis apply to similar hybrid power systems.

 

 

 

II. SYSTEM CONFIGURATION

The system has two types of generation: the diesel generator and the wind turbine generator (Figure 1). The

energy storage can act as a load or as a generator depending on the need. The diesel generator provides smooth output power, whereas the output power of a wind turbine depends on the wind velocity. As the wind velocity varies, so does is the power generation. For example, if the wind speed changes very smoothly, the output power of the wind turbine will also change very smoothly. On the other hand, wind turbulence causes the output power to fluctuate. Figure 1 is a single line diagram that represents the analyzed power system. The wind turbine has an induction generator with a capacity ranging from 40 kW to 225 kW. At low wind speeds, the generator operates at 900 rpm with a rated capacity of 40 kW. At high wind speeds, the generator speed is 1,200 rpm with a rated capacity of 225 kW. We used 150 kW of energy storage as a buffer to operate as a load or a source depending on the need. This paper discusses only fixed-speed wind turbine generation and does not cover variable-speed wind turbine generation [5]. The diesel engine, which has a rated capacity of 400 kW, is operated in parallel with the wind turbine to supply the load. The local loads are mostly residential and light loads. Other loads include water pumps, compressors, and heavy equipment. An 80-kW water pump represents the transient condition of a heavy load.

 

Fig 1. One line diagram of power system

 

 

III. COMPONENTS OF POWER SYSTEM

The system we discuss in this paper consists of four major subsystems: a diesel generator, a wind turbine generator, heavy (industrial) loads, and energy storage. In the power system network, the balance of active power and reactive power must be maintained. The diesel-genset, then, must be able to keep the power balanced when the wind turbine or local load varies. This task is easy to accomplish provided the diesel genset is sufficiently sized. Although they are important, we will not cover the details of the dynamic model for electric machines used in the simulation. Many good textbooks are available on this subject.

A. Diesel Generator

In terms of an electrical system, a diesel generator can be represented as a prime mover and a generator. Ideally, the prime mover is capable of supplying any power demand up to rated power at constant frequency, and the synchronous generator connected to it must be able to keep the voltage constant at any load condition. Figure 2 is a block diagram of the diesel generator. The diesel engine keeps the frequency constant by maintaining the rotor speed constant via its governor. The synchronous generator must control its output voltage by controlling the excitation current. Thus, as a unit, the diesel generating system must be able to control its frequency and its output voltage. The inertia of the diesel genset, the sensitivity of the governor, and the power capability of the diesel engine all affect the diesel generator’s ability to respond to frequency changes. The ability of the synchronous generator to control its voltage is affected by the field winding time constant, the availability of the direct current (DC) power to supply the field winding, and the response of the voltage control regulation mechanism.

 

Figure 2. Diesel generator control block diagram

 

 

B. Wind Turbine

The main components of a wind turbine are the rotor of the turbine, which is the prime mover, and an induction generator. In general, the rotor is connected to the generator via a gearbox that matches the rotational speed. The simplest system uses a fixed-speed turbine. A fixed-speed turbine must rely on the blade-stall condition to limit the output power when the winds are at high speed. Note that, although the rotor speed of an induction generator varies with wind speed, the speed range is within a 1% to 2% slip. On the other hand, the wind speed variation may range from 5 m/s to 25 m/s; thus, in terms of the wind turbine, the induction generator operates at a relatively “fixed speed” compared to the range of wind speed variation.

C. Induction Machines

Most electric machines used in industry as prime movers are induction motors. Two applications of induction machines in the power system network fall within the scope of this study: one as the generator on a wind turbine and the other as a motor driving large pumps and compressors. By its nature, an induction machine is an inductive load. This machine absorbs reactive power either as a motor or generator. The reactive power absorbed by the induction machine comes from the line to which it is connected. In a hybrid power system, the reactive power comes from the synchronous generator of the diesel genset. In a wind turbine generator, a fixed capacitor is usually installed to supply some of the reactive power that the induction generator needs. Figure 3 shows the equivalent circuit of an induction machine connected to a power system. The power system is represented by infinite bus Es and the line impedance is represented by reactance Xs.

 

Figure 3. Equivalent circuit of an induction machine connected to power system

D. Various Loads

In the power system considered, there are two major loads. The first is a large water pump representing a typical ind
ustrial load. The second is a collection of loads for which the size and power factor can be programmed throughout the day to represent a typical village load. The voltage at the terminal of the load varies as a result of a voltage drop across the line impedance. The voltage drop across the line impedance varies depending on the size of the current and the power factor of the load. The terminal voltage for a wind turbine generator (VS), as the output current of induction machine, varies from start-up to generating mode. During start-up, voltage drops significantly at the terminal voltage of the induction machine. The voltage drop across line impedance is caused by the current surge during start-up. In addition, the phase angle of the stator current is very large and lagging. The combination of a poor power factor and a lagging, large current surge creates a voltage dip at the terminal of the induction machine during start-up. Thus, a start-up of short duration is preferable to a prolonged one

E. Energy Storage

The energy storage can be of different types (i.e. flywheel, battery, hydrogen/fuel-cell, hydropower etc.). In this paper, we assumed energy storage with a power converter interface to the power network. The power converter is connected to the energy storage at one end. With variability of wind resource, energy storage is an excellent contributor to the power system. The energy storage behaves like a large buffer to accommodate the unequal instantaneous energy in the power system. Ideally, at any instant of time, there should be a zero net exchange between the energy sources and the energy sinks (both real and reactive power). If this balance is not achieved, the voltage and frequency of the system changes to maintain equilibrium. At any instant, the energy storage behaves either as an energy source or energy sink depending of the mode of operation.

Figure 4. Energy Storage control block diagram

 

It is assumed that the energy storage has a power converter interfacing the power network. Although it is possible for the power converter to function as a reactive power compensator, the cost of a power converter is very expensive compared to other means of reactive power compensation currently available in the market. Keep in mind that the size of the power semiconductor in the power converter is limited by its current limit and its voltage limit. Thus, minimizing the current passing through the power switches will minimize the current rating of the power converter and will lower the cost. For this paper, we only used the power converter to process real power in and out of the energy storage. Figure 4 shows a block diagram of energy storage control algorithm. It uses frequency deviation to indicate a real power imbalance in the system. The frequency deviation is also used as the feedback to control the energy storage output. If the load power demand is higher than the power supply available, the frequency of the diesel generator will slowly drop. Other energy stored in the system includes the kinetic energy in the turbine blades, the diesel generator inertia, and energy in the inductors and capacitors, etc.

 

F. Balance of Energy in the System

In the isolated system we studied, the balance of real and reactive power must always be maintained. The balance of real power is maintained by the governor of the diesel generator. The balance of reactive power is maintained by the exciter of the diesel’s synchronous generator. When the load demands more power than the diesel and the wind turbine can produce, and the diesel engine has reached its highest limit, as the loads continue to increase, the governor of the diesel cannot push more power, and the rotor speed of the diesel will start to drop. The frequency of the generator will then drop until balance is reached or the system collapses. The voltage in the system is also an indicator of the balance in the system. When the reactive power demand from the loads is higher than what can be provided by the diesel generator, the capacitor, and other means of compensation, the system voltage will drop. Although the size of output and input of the energy storage is adjustable, it is limited by its ratings. For this paper, we assumed that the energy storage is capable of storing and providing long-term energy to the power network to maintain system balance. In reality, only a limited amount of energy can be stored. We will not discuss energy analysis in detail in this paper. In practice, the energy will be stored when the wind turbine produces enough power and the diesel is operating under light load. The actual loads are divided into critical and non-critical loads. Critical loads are supplied at all times and non-critical loads are served only if there is enough source and it will be shed off the system when the voltage or frequency drops below the allowable limit. With the existence of sufficiently sized energy storage, it is possible to serve all the loads (critical and non-critical) all the time.

IV. DYNAMIC ANALYSIS

The case studies look at different aspects of major power system components in the power network. The first case study investigates the diesel power component. In the power network, a diesel generator must maintain system balance by responding properly to power changes.

A. Case Study I: Diesel–Wind Turbine Interaction

A diesel generator consists of a diesel engine and a synchronous generator. The diesel engine is responsible for controlling the frequency and keeping it constant through its governor. The synchronous generator is responsible for controlling the voltage via its field winding and voltage controller. Undersized diesel engine: The ability of a diesel engine to change speed is its accelerating or decelerating power. The diesel accelerates when the input power is higher than the electrical output power of the generator (including losses).

The diesel decelerates when the input power is lower than the electrical output power of the generator (including losses). An oversized diesel engine does not have problems accelerating or decelerating, but an undersized diesel engine may create problems, during, for example, the start-up of a wind turbine or large compressor. Figure 5 illustrates a condition where the diesel is undersized with respect to the load. The genset frequency and the terminal voltage of the wind turbine generator are shown on the top graph, and the real power of the diesel, wind turbine, water pump, and local load are shown on the bottom graph. At start-up, the wind turbine uses the smaller, 40-kW generator to motor up and bring the induction machine up to speed. Because the wind speed is low, the wind turbine operates at low output power, and the local load is set to 200 kW. The diesel engine has a rated power of 400 kW. At t = 2 s, the wind turbine is turned on. As we can see, the voltage dip and the frequency dip are not very large, because the wind turbine is started using a smaller generator

 

Figure 5. Voltage, frequency, and power to illustrate

an undersized diesel genset

At t = 10 s, the 80-kW water pump is started up. The startup time for the water pump is longer than that of the wind turbine because the wind turbine is started when the rotor speed is close to the synchronous speed and the wind turbine also gets some help from the wind. The voltage drop is not very significant, but the frequency of the diesel drops about 3%. The diesel output power increases to cover the real power needed, whereas the contribution from the wind turbine is insignificant because the wind is low. For a short time, the induction generator enters the motoring range between t = 10.8 s and t = 11.3 s. After the condition is restored, at t = 14 s, the additional local load (300 kW noncritical) is turned on, bringing the total load to 580 kW. Because the diesel can carry only up to 400 kW and the wind’s contribution is very small at abo
ut 40 kW, the voltage and frequency start decreasing, and the voltage and frequency sensors detect the change. If the frequency drops below 95% and the voltage drops below 90% for an elapsed time of 0.5 s, the controller will drop the additional load (300 kW) and keep the critical load (200 kW) to regain the voltage and frequency. After the load is shed at t = 14.5 s, the frequency and voltage eventually return to normal. When the frequency drops, the wind turbine’s power contribution suddenly jumps because of a sudden increase of generating slip. Eventually, the genset frequency increases again for a short period and the induction generator enters into the motoring condition (between t = 14.5 s and t = 15 s). This condition worsens if the mechanical time constant of the wind turbine rotor (including the blade) is higher than the diesel genset time constant. In other words, the changing of the genset rotor speed is much faster than the changing of the wind turbine rotor speed. The response to the load change is shown by how fast the governor corrects the frequency and how fast the generator’s field excitation control reacts to the voltage changes. Undersized diesel engine with energy storage: As shown in the previous subsection, an undersized diesel engine cannot supply all energy needed, and it must shed some of the non-critical load to retain power-system stability. To remedy this situation, a 150-kW energy storage is installed to bring the combined output of the diesel genset and energy storage up to 550 kW. Figure 6 shows the improved power system after the energy storage is added. The same simulation is performed except it is now equipped with an energy storage. There is a significant improvement in the frequency regulation after the storage is installed to stabilize the system. The non-critical load (300 kW) survives even during low wind conditions. The frequency dips during the wind turbine start-up and the water pump start-up, and when the 300 kW load non-critical load is switched, it is reduced dramatically. Obviously, the capability of the energy storage to deliver a large amount of power instantaneously plays a major role in restoring the frequency of the power system. An additional benefit is noticed in the system voltage behavior of the wind turbine. Because the change in the frequency deviation presented to the wind turbine induction generator is small and smooth, the behavior of the stator current at the induction generator is also smooth. Thus it reduces the Ldi/dt and overall voltage drop across the line.

Oversized wind turbine:

When the wind power output exceeds the power required by the load, the synchronous generator of the diesel genset becomes a synchronous motor that tends to accelerate the rotor speed of the diesel engine. The excess energy from the wind power, then, tries to drive the diesel engine. Because the diesel engine has only a small braking capability resulting from engine compression, the frequency control can be lost when the extra power generated by the wind turbine is sufficiently high.

 

Figure 6. Voltage, frequency, and power to illustrate

an undersized diesel genset with storage

In Figure 7, the diesel generator has a rated power of 400 kW, the local load is initially set to 280 kW and at t = 4 s, and the local load is set to 100 kW. When the diesel is started, there is only a local load of 280 kW. The wind turbine is then started at t = 2 s with a 225-kW induction machine. Although the diesel genset is rated at only 400 kW and the wind turbine is started with a 225-kW induction machine, the effect of wind turbine start-up on the power system is very mild, mostly because the induction machine current is limited by a soft start. A soft start is a device that limits starting current during start-up. It consists of a pair of back-to-back thyristors installed in series with each phase of the motor winding. Because the firing angle of the thyristor can be controlled, the size of the starting current can be adjusted by controlling the firing angle of the thyristors. As we can see (Figure 5), the same wind turbine (225 kW) draws a starting power of 300 kW, but after the soft start is installed (Figure 7), the power surge during start-up drops to about 100 kW. After the wind turbine enters generating mode (at about t = 2.5 s), the local load (280 kW) is shared between the diesel genset (55 kW) and the wind turbine (225 kW). The voltage and frequency are maintained constant, and the diesel genset

 

 

 

Figure 7. Voltage, rotor speed, and power of an

oversize wind turbine

generates only a small percentage of its rated load (about 13%). This makes a significant contribution to fuel savings from the wind energy. At t = 4 s, the local load is reduced from 280 kW to 100 kW; the wind speed stays the same. As a result, the wind turbine tries to supply 225 kW, but the only load available is 100 kW. As a result, the synchronous generator of the diesel genset turns into a motor (negative power), the governor loses its speed control, and frequency runaway is triggered. This is an example of the wind turbine being oversized compared to the local load. In such a case, a dump load (water heater, water pump, battery charger, etc.) is usually deployed to keep the diesel genset generating, which prevents it from motoring. Minimum power generation of the diesel genset is usually pre-set (for example, 15%–40% of the rated load). If the generated power of the diesel genset is less than the preset value, the dump load should be deployed. The dump load must be sized so that the diesel genset will always generate power above its minimum set point. The dump loads are normally non-critical loads used to store excess electrical energy in another form, such as heat (water or space heater), electric charge (battery charging), or potential energy (water pump). Oversized wind turbine with energy storage: As shown in the previous subsection, an oversized wind turbine can drive the system into an unstable condition because of the inability of the diesel engine to keep the frequency constant. An energy storage installed in the power system network is not only useful to remedy the undersized diesel engine but also for cases where there is an excess power produced by the wind turbine. Without energy storage, the wind turbine can drive the synchronous machine into motoring region and the frequency output will be out of control. With a power converter to interface between the energy storage and the power network, the energy storage is capable of quickly absorbing excess power generated by the wind turbine and hold the generator rotor speed from a runaway condition. As shown in Figure 8, the frequency runaway can be prevented by using energy storage to capture the excess power in the power network.

Figure 8. Voltage, rotor speed, and power of an

oversize wind turbine with energy storage

B. Case Study II: Charging the Storage Under Normal Condition

The energy storage will be charged only when there is an energy surplus from the wind and the required network load is very light. Because the governor of the diesel generator will always maintain the frequency constant, the output power of the diesel generator is an indicator of the power within the system available to charge the energy storage. One benefit of charging the energy storage during this condition is that the efficiency of the diesel engine is at its peak when it is operated near its rated power. Thus, when a surplus of power is detected within the system, the energy storage will be charged and some energy will be stored within the system. The amount of energy and the size of charging power depend on the size of the surplus power. The charging process will be stopped when the energy storage reaches its limit. Maximum charging current is also limited by the energy storage and by the power converter interface. Figures 9 shows the charging process. Initially there is enough wind speed to start the wind t
urbine. The diesel generator is supplying a constant load of 280 kW (power factor = 0.995 lagging) all the time. As the wind turbine generates full power (225kW), the diesel governor redistributes the load and there is a load sharing between the wind turbine and the diesel generator. As the transient settles out, it is shown that the diesel generator is contributing a very small amount of power to the load, thus the charging mechanism is started. The energy storage is charged slowly until it reaches its limit.

 

 

Figure 9. Real power flow in the power system

 

In Figure 9, the charging of energy storage during normal condition is limited to 75 kW, which is about 50% of the rated power of the capacitor. This limit ensures that the power converter still has enough headroom to deliver or absorb power during an emergency. For example, if there is some loss of the loads in the power systems, the energy storage must absorb the loads loss to avoid a sudden change in frequency. Similarly, to compensate for a sudden load increase to the power systems (e.g. the water pump is started), the energy storage must release energy to the power system to keep constant frequency at the diesel generator. As shown in Figure 9, the real power used by the energy storage to stabilize the frequency takes precedence over the charging power used to charge the storage. This can be seen especially when the water pump is started at about t = 15 seconds.

V. CONCLUSION

After presenting an overview of the components of the power system under investigation, we described the operating characteristics of the components as they relate to voltage and frequency variations in the power network. The analysis shows the dynamic interaction among the wind turbine, diesel engine, large loads, and energy storage. It also demonstrates the dynamics of real power balance and how the system is stabilized with the controlled energy storage. The voltage regulation is very minimal and the frequency regulation is controlled very closely. The voltage regulation is controlled mostly by the balance of reactive power in the system and the time constant of the excitation system of the generator. The frequency regulation depends on the energy storage control, the size of the energy storage, the total inertia in the system (temporary energy storage).Many technical solutions can be implemented to remedy the shortcomings covered in this paper. However, as in any power generation system, the economic implications of the solutions must be carefully considered.

REFERENCES

[1] E. Muljadi, L. Flowers, J. Green, and M. Bergey. 1996. “Electrical Design of Wind-Electric Water Pumping.” ASME Journal of Solar Energy Engineering 118:246–252.

[2] J.T.G. Pierik and M. De Bonte. 1985. Quasi Steady State Simulation of Autonomous Wind Diesel Systems (Status Report). Report No. ECN-85-091. Petten, The Netherlands: The Netherlands Energy Research Foundation.

[3] A.J. Tsitsovits and L.L. Freris. 1983. Dynamics of an Isolated Power System Supplied from Diesel and Wind. Proc.IEEE 130, Part A, No. 9:587–595.

[4] J.T. Bialasiewicz, E. Muljadi, G. Nix, and S. Drouilhet. 1998. “RPM-SIM Simulator: A Comparison of Simulated versus Recorded Data. “Proceedings of WINDPOWER ’98.” Bakersfield, California, 423–432.

[5] E. Muljadi and C.P. Butterfield. 2001. “Pitch-Controlled Variable-Speed Wind Turbine Generation,” Transactions of the IEEE-Industry Applications Society.

 

 

INTRODUCTION:

 

Robot was coined by Czech playwright Karl Capek in his play R.U.R (Rossum’s Universal Robots), which opened in Prague in 1921. Robot is the Czech word for forced labor.

The term robotics was introduced by writer Isaac Asimov. In his science fiction book I, Robot, published in 1950, he presented three laws of robotics:

1. A robot may not injure a human being, or, through inaction, allow a human being to come to harm

2. A robot must obey the orders given it by human beings except where such orders would conflict with the First Law.

3. A robot must protect its own existence as long as such protection does not conflict with the First or Second Law

 

 

 

INDUSTRIAL ROBOTS:

 

Robots usually have multiple components, such as motors, sensors, microcontrollers and embedded computers. DC motors transform direct current into mechanical energy and are often used to drive the robots. Sensors collect data from environment and provide information to robots. Most often used sensors are vision, infrared, sonar and laser rangers. Many robots use embedded computers for high-level computation and microcontrollers for low-level controls.

 

 

 

 

 

 

EPSON INDUSTRIAL ROBOT

 

 

 

 

 

 

 The microcontroller directly controls motors, sensors, and polls the sensor readings. It hides the hardware details from the embedded computer, and provides an application programming interface (API) for the embedded computer. The embedded computer handles high-level computation, including motion planning, image processing, and scheduling. The separation of the microcontroller and embedded computer makes the designs more flexible. However, other components like sensing, control, communication and computation also consume significant amounts of power. It is important to consider all components to achieve better energy efficiency. This study has two major contributions. Firstly, we study power consumption of a robot

 

USES OF INDUSTRIAL ROBOTS:

            One of the most common uses for industrial robots is welding. Robot welded car bodies for example enhances safety, a robot never miss a welding spot and performs equally all through the day.

In assembling of parts many of these robots can be found in the automotive and electronics industries Packaging/palletizing, is still a minor application area for industrial robots, this application area is expected to grow as robots become easier to handle.

The food industry is an area where robots are expected to play a major role in the future. The process involves harvesting each arriving plant, cutting its steam into segments near each node, and then replanting the segments so that they can grow into new plants etc.

 

 MICROCONTROLLER AND EMBEDDED    COMPUTER

 

The microcontroller periodically sends commands to motors and sensors, polls sensors’ readings, and communicates with the embedded computer. The microcontroller’s tasks are usually fixed so the power consumption of the microcontroller can be modeled by a constant. The embedded computer is more complex than the microcontroller. Many studies have been devoted into simulation-based methods to estimate its power consumption [6] [5] [8]. The power consumption of the embedded computer may vary significantly across different programs.





 

 

 

Micro controller



 



 

 

 

Motor



Sensor

Embedded computer



 

 

 

PREVIOUS WORK:

 

[1] Both timing and energy constraints are considered; the robots carry limited energy and need to finish the tasks before deadlines

 

 

ENERGY-CONSERVATION TECHNIQUES:

 

This section explains three promising techniques for power reduction of mobile robots.

 

A. Dynamic Power Management

Dynamic power management (DPM) dynamically adjusts power states of components adaptive to the task’s need. The purpose is to reduce the power consumption without compromising system performance. Many electronic components have multiple power states; their power consumption is different at different power states. For example, processors can run on different frequencies. To save power, the processors can enter lower frequencies when the workloads are light. Another example is to shut off the power supply to the disk in an embedded computer to save the static power when there is no disk access.

A simple DPM method shuts down a component when it is idle. It is essentially a prediction problem. If we predict there is no access on this component for a reasonably long period of time, the component can be shut down to save static power. Turning on and off the component takes time and energy. If the idle period is too short, the components may actually consume more energy for turning on and off. One of the widely used prediction methods is timeout: if the component has been idle for a time period longer than the timeout, the component will be shut down. The rationale behind timeout is that the component is likely to keep idle in the near future since it has been idle for a while. Another widely used DPM technique is dynamic voltage scaling (DVS) by reducing both supply voltage and clock frequency to reduce the power consumption of processors. CMOS circuit is its dynamic power, which can be expressed by c Vdd, f, where c is the effective switched capacitance, vdd is the supply voltage and f is the clock frequency.

B. Real-Time Scheduling

Real-time systems handle tasks with deadlines. Real-time scheduling (RTS) schedules multiple tasks and meet the deadlines. If the tasks can be scheduled without missing the deadlines, we say they are schedulable. Mobile robots are real-time systems. When a robot detects an obstacle, it has to timely slow down and decides the next motion. For multiple robots coordinating to accomplish a task, timely information communicating is critical. Two often used scheduling algorithms are rate monotonic (RM) and earliest deadline first (EDF). Many other algorithms are based on these two. RM is a fixed-priority algorithm, assigning a higher priority to a task with a shorter period. EDF executes the task with the earliest deadline among all ready tasks. It has been proved that EDF is optimal with respect to minimizing the maximum lateness. Besides scheduling tasks to meet their deadlines, RTS can also schedule the tasks such that DPM can save more energy. For example, when the idle periods of a component are too short due to frequent accesses, power cannot be saved by shutting down the component. However, if we can reschedule the tasks and make the component have more long idle periods, the component may be shut down to save power.

C. Examples

In this section, we show some potential applications of DPM and RTS into energy-efficient robot designs using several examples.

 

1) Shutdown of Unused Components: Electric components consume static power in idle states. Shut
ting down the power supply when a component is idle can save the static power. When the robot stops, the sensors may be turned off. If half of the time the sensors can be shut down, the average sensing power can be reduced.

 

2) Sensing Frequency Scaling: It is intuitive that the sensing frequency should be different when robots move at different speeds. The sensing frequency needs to be higher when the speed is higher. Instead of keeping the sensing frequency that satisfies the highest speed’s need, we can reduce the sensing frequency when the robot moves slowly. If the robot moves slowly and the sensing frequency can be reduced.

 

3) Dynamic Voltage Scaling: DVS is very effective in reducing processors’ power. The processor inside the Hitachi-8s microcontroller can work at two different frequencies: 20MHz and 10MHz. The current operating system inside the microcontroller doesn’t support the frequency scaling. Therefore, we can not measure the power savings. However, if we can dynamically change the working frequency according to the workload, we can reduce the control power. This technique also applies to the embedded computer.

 

4) Trade-off between Motion and Communication:

A Team of robots may move and cooperatively execute a task. Robots need to send sensing data through wireless communication. Consider one robot needs to transfer data to another robot, but the robot is far away. If the robots can move closer, the communication power can be saved. The cost here is the motion power for moving closer. If the volume of the data is large enough, more communication power can be saved than the motion power cost.

 

5) Energy-Efficient Real-Time Scheduling for Robots:

A mobile robot is a real-time system. The robot can have many periodic tasks, such as motor and sensor control, sensing data reading, motion planning, and data processing. The robot may also have some aperiodic tasks, such as obstacle avoidance and communication. RTS can work with DPM to more effectively reduce the power consumption. For example, if a scheduler can cluster tasks closer in time and create longer idle periods, shutdown techniques can be more effective. RTS also can work with DVS to reduce processor energy consumption, as we discussed in the related work. For mobile robots, the tasks’ deadlines are different at different traveling speeds. At a higher speed, the periodic tasks have shorter periods. Therefore, we should consider both motion planning and RTS together.

 

FORTIFICATION:

 

Many fails to consider the third quadrant called fortification. This paper put some idea about that quadrant. Some shrinks the use of robots by Fleet Size Problem [3]: A fundamental question for multi-robot applications is to decide the number of robots needed (i.e., the “fleet-size problem”) to accomplish tasks. We provide a probabilistic method to decide the fleet size necessary to serve requests with random arrival times and locations. We consider five factors on which the fleet size depends: available energy, power consumption, service field, request rate, and timing constraints.

 

Though we know that many of the industrial robots uses stepper motors, servo motors, relays etc., on AC or DC. In AC supply it is must to keep the power factor under control it should not be low. If it so, heavy power loss will be occurred and the company will be liable to meet the surcharge of their electricity department.. The following are the some of the fortification technique

 

1. To improve the power factor capacitor has to be used. Though robot uses electrical motors, transformers etc., under starting state it needs high capacitance to maintain pf; on the other hand under running condition it needs minimum capacitance value. In other words it can be explained as the capacitance value changes according to the load of the robots. These will be an extra burden to cutoff the capacitor under load. To overcome this dynamic control capacitor may be used.

2. The one of the factor that decides the life of the robot is wear and tear of the electrical equipments. This falls in sparking of the relays contacts and motor brushes due to the in rush current. These can be eliminated by close circuit transient.

 

3. Due to energy conservation law, energy will be dissipated through heat. This can be reduced by silver windings. Silver has the lower resistance of current then copper and aluminum.  Due to lower resistance of electric current it reduces major electrical losses

 

4. Over load detector for example, robot with hardware platform of a Pioneer 2-DX robot [2] augmented with custom hardware for watering. To deliver water to the plant, the robot has been fitted with a water line, dispensing spout, and pump. To deliver power to wireless sensors an inductive charging coil has been positioned near the watering spout. Similarly, another paddle shaped inductive charge coil has been added to the robot to allow it to recharge itself at its “maintenance bay”. In order to support calibration, the robot includes a sensor node that was human-calibrated lastly, the robot has a maintenance bay it uses to automatically charge its own batteries and refill its water reservoir. The reliability of this approach has been demonstrated during the deployment of the robots Rhino and Minerva as autonomous museum tour guide robots [4, 7]. The high-level task ordering and dispatching software was custom-built for the Plant Care project.

 

There may be the chance to water to direct contact with the power pack. This leads to short circuit, and draws more current that the rated (PU) per unit.

 

FUTURE ENHANCEMENTS:

 

For future work, I plan to extend the current study in two directions. First, we will measure power consumption of more components, such as laser rangers, cameras, servo motors, stepper motors, and relays. Second, I plan to implement the proposed energy conservation techniques into the Pioneer robots, and conduct experiments in real applications

 

CONCLUSION:

 

In this study, I presented some of the power consumption technique of different components of an industrial robot. In this paper, I introduce one technique called fortification technique than two exiting techniques DPM and RTS for energy-efficient designs of robots. These techniques together with motion planning provide greater opportunities for reducing the power consumption and prolonging the operation time of mobile robots.

 

REFERENCE:

 [1] Yongguo Mei, Student Member, IEEE, Yung-Hsiang Lu, Member, IEEE, Y. Charlie Hu, Member, IEEE, and C. S. George Lee, Member, IEEE

 

[2].ActivMedia Robotics, http://www.activrobots.com, visited Feb. 2002.

 

[3] Y. Mei, Y.-H. Lu, C. S. G. Lee, and Y. C. Hu. Determining the Fleet Size of Mobile Robots with Energy Cons traints. In IEEE /RSJ International Conference on Intelligent Robots and Systems, pages 1420–1425, 2004.

 

[4]. Burgard, W., A. Cremers, D. Fox, D. Haehnel, G. Lakemeyer, D. Schulz, W. Steiner and S. Thrun. 1999. Experiences with an interactive museum tour-guide robot. Artificial Intelligence.

 

[5] J. R. Lorch and A. J. Smith. Apple Macintosh’s Energy Consumption. IEEE Micro, 18(6):54–63, November 1998

 

[6] D. Brooks, V. Tiwari, and M. Martonosi. Wattch: A Framework for Architectural-level Power Analysis and Optimizations. In International Symposium on Computer Architecture, pages 83– 94, 2000.

 

[7]. S. Thrun, M. Bennewitz, W. Burgard, A. Cremers, F. Dellaert, D. Fox, D. Haehnel, C. Rosenberg, N. Roy, J. Schulte and D. Schulz. 1999. MINERVA: A second generation mobile tour-guide robot. In P
roceedings of the IEEE International Conference on Robotics and Automation (ICRA).

 

[8] T. Simunic, L. Benini, and G. D. Micheli. Cycle-accurate Simulation of Energy Consumption in Embedded Systems. In Design Automation Conference, pages 867–872, 1999