CHAPTER 6MOTOR DRIVES

6.1 INTRODUCTION TO POWER ELECTRONIC DRIVES Definition of power electronic:

To convert, to process and control the flow of electric power by supplying voltages and currents in a form that is optimally suited for user loads.

Power electronic circuit convert electric power from one form to another form using electronic devices.

Power electronic circuits functions by using semiconductor devices as switches. Applications of power electronic:

high power conversion equipment such as dc transmission Everyday application such as cordless screwdriver or power supplies for

notebook computer and others. The particular switching devices used in power electronic circuit depend on the

existing state of semiconductor device technology.

Figure 6.1 Basic block diagram of power electronic system

Power electronic systems are virtually in every electronic device. For example, around us:

DC/DC converters are used in most mobile devices (mobile phone, pda and etc) to maintain the voltage at a fixed value whatever the charge level of the battery is. These converters are also used for electronic isolation and power factor correction.

AC/DC converters (rectifiers) are used every time an electronic device is connected to the mains (computer, television and etc)

AC/AC converters are used to change either the voltage level or the frequency (international power adapters, light dimmer). In power distribution networks AC/AC converters may be used to exchange power between utility frequency 50 Hz and 60 Hz power grids.

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DC/AC converters (inverters) are used primarily in UPS or emergency light. During normal electricity condition, the electricity will charge the DC battery. During blackout time, the DC battery will be used to produce AC electricity at its output to power up the appliances.

POWER SWITCHES Power switches: work-horses of PE systems. Operates in two states:

Fully on- i.e Switch closed- Conducting state

Fully off - i.e Switch opened- Blocking state

Power switch never operates in linear mode. Can be categorized into three groups:

Uncontrolled: Diode Semi-controlled: Thyristor (SCR). Fully controlled: Power transistors: e.g. BJT, MOSFET, IGBT, GTO,

IGCT

(a) (b) (c)

Figure 6.2 Photos of power switches(a) Power diode(b) IGBT(c) IGCT

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Figure 6.3 Switches comparison

6.1.1 POWER DIODE Is the simplest electronic switch. Uncontrollable On and off conditions are determined by voltages and current in the circuit. When diode is forward biased, it conducts current with a small forward voltage (Vf)

across it (0.2-3V) When reversed (or blocking state), a negligibly small leakage current (uA to mA)

flows until the reverse breakdown occurs. Diode should not be operated at reverse voltage greater than Vr

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(a) (b)Figure 6.4 Power diode

(a) Symbol(b) v-i characteristic

TYPES OF POWER DIODE There are three types of power diode:

Line frequency (general purpose)- On state voltage: very low (below 1V)- Large reverse recovery time,trr (about 25us) (very slow response)- Very high current ratings (up to 5kA)- Very high voltage ratings (5kV) - Used in line-frequency (50/60Hz) applications such as rectifiers

Fast recovery- Very low trr (<1us).- Power levels at several hundred volts and several hundred amps - Normally used in high frequency circuits

Schottky- Very low forward voltage drop (typical 0.3V) - Limited blocking voltage (50-100V)- Used in low voltage, high current application such as switched mode power

supplies6.1.2 GTO – Gate Turn Off Thyristor Behave like normal thyristor, but can be turned off using gate signal However turning off is difficult. Need very large reverse gate current (normally 1/5 of

anode current) Gate drive design is very difficult due to very large reverse gate current at turn off. Ratings: Highest power ratings switch

Voltage: Vak < 5kV Current: Ia < 5kA Frequency: f < 5KHz

(a) (b)

Figure 6.5 GTO(a) Symbol and (b) v-i characteristic

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6.1.3 TRIAC Semiconductor device that electrically equivalent to two SCRs, connected anti

parallel, although internal structure are not exactly the same as that two SCRs Behave like normal thyristor, but can be turned off using gate signal However turning off is difficult. Need very large reverse gate current (normally 1/5 of

anode current)

Figure 6.6 SCRs connected as TRIAC

Figure 6.7 TRIAC

(a) Symbol(b) v-i characteristic

6.1.4 IGBT – Insulated Gate Bipolar Transistor Combination of BJT and MOSFET characteristics.

Gate behaviour similar to MOSFET - easy to turn on and off. Low losses like BJT due to low on-state Collector- Emitter voltage (2-3V).

Ratings: Voltage: VCE<3.3kV, Current,: IC<1.2kA currently available. Latest: HVIGBT

4.5kV/1.2kA. Switching frequency up to 100KHz. Typical applications: 20-50KHz.

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(a) (b)

Figure 6.8 IGBT(a) Symbol(b) v-i characteristic

6.1.5 DIAC The construction of a diac is similar to an open base NPN transistor. The diac is similar to having two parallel Shockley diodes turned in opposite

directions The bidirectional transistor-like structure exhibits a high-impedance blocking state up

to a voltage breakover point (VBO) above which the device enters a negative-resistance region.

These basic diac characteristics produce a bidirectional pulsing oscillator in a resistor-capacitor AC circuit.

Since the diac is a bidirectional device, it makes a good economical trigger for firing triacs in phase control circuits such as light dimmers and motor speed controls.

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(a) (b)Figure 6.9 DIAC

(a) Symbol(b) v-i characteristic of bilateral trigger DIAC

6.1.6 PUT – PROGRAMMABLE UNIJUNCTION TRANSISTOR The PUT is actually a type of thyristor It can replace the UJT in some applications. It is more similar to an SCR (four-layer device) except that its anode-to-gate voltage

can be used to both turn on and turn off the device. Notice that the gate is connected to the n region adjacent to the anode. The gate is always biased positive with respect to the cathode.

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When VA - VG > 0.7 V, the PUT turns on. The characteristic plot of VAK versus IA is similar to the VE versus IE plot of the UJT.

(a) (b)Figure 610 PUT

(a) Basic construction(b) Symbol and biasing

6.1.7 SCR If the forward break over voltage (Vbo) is exceeded, the SCR “self-triggers” into the

conducting state. The presence of gate current will reduce Vbo. “Normal” conditions for thyristors to turn on:

The device is in forward blocking state (i.e Vak is positive) A positive gate current (Ig) is applied at the gate

Once conducting, the anode current is latched. Vak collapses to normal forward volt-drop, typically 1.5-3V.

In reverse -biased mode, the SCR behaves like a diode. Thyristor cannot be turned off by applying negative gate current.

It can only be turned off if Ia goes negative (reverse) This happens when negative portion of the of sine-wave occurs (natural

commutation) Another method of turning off is known as “forced commutation”

The anode current is “diverted” to another circuitry.

Figure 6.11 SCR (Thyristor)

(a) Symbol

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(b) v-i characteristic

Figure 612 SCR (Thyristor) conduction

6.1.8 UJT – UNIJUNCTION TRANSISTOR

UJT has only one p-n junction. It has an emitter and two bases, B1 and B2. r’B1 and r’B2 are internal dynamic resistances. The interbase resistance, r’BB = r’B1 + r’B2. r’B1 varies inversely with emitter current, IE r’B1 can range from several thousand ohms to tens of ohms depending on IE. UJT can be used as trigger device for SCRs and triacs. Other applications include

nonsinusoidal oscillators, sawtooth generators, phase control, and timing circuits.

(a) (b)

(c)Figure 6.13 UJT

(a) Symbol(b) Equivalent circuit(c) v-i characteristic

6.2 DC MOTOR DRIVES

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DC drives are widely used in application requiring: Adjustable speed Good speed regulation Frequent starting, braking and reversing

Some applications are: Rolling mills Paper mills Mine winders Hoist Machine tools Traction Printing presses Textile mills Excavators Crane

Until today, the variable speed applications are dominated by ac drives because of: Lower cost Reliability Simple control

6.2.1 SPEED CONTROL

Speed can be controlled by any of the following methods: Armature voltage control Field flux control Armature resistance control

Armature Voltage Control

Is preferred because of high efficiency, good transient response and good speed regulation.

But it can provide speed control only below base (rated) speed because the armature voltage cannot be allowed to exceed rated value.

In armature voltage control at full field, Ia, consequently, the maximum torque that the machine can deliver has a constant value.

Figure 6.14 Armature voltage control Vr (rated) > V1 > V2

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Field Flux Control

Is employed for speed control above base speed In normally designed motor, the maximum speed can be six times rated speed In field control at rated armature voltage, Pm Ia (because E V = constant),

therefore the maximum power developed by the motor has a constant value.

Figure 6.15 Field flux control r (rated) > 1 > 2

Armature Resistance Control

In armature resistance control, speed is varied by wasting power in external resistor that are connected in series with armature

It is an inefficient method of speed control It is used only in an intermittent load application where the duration of low speed

operations forms only a small proportion of total running time, for example in traction.

Figure 6.16 Armature resistance control (Re : external resistance)

6.2.2 METHODS OF ARMATURE VOLTAGE CONTROL

Variable armature voltage for speed control, starting, reversing and braking of dc motors can be obtained by: Ward Leonard schemes

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Transformer with taps Static ward Leonard scheme Chopper controlWhen the supply is ac, Ward Leonard schemes, transformer with taps and static Ward Leonard scheme can be used.

When the supply is dc, chopper control is used.

WARD LEONARD DRIVE

Known after the name of his inventor H Ward Leonard (1891).It consist of a separately excited generator feeding the dc motor to be controlledThe generator is driven at a constant speed by an ac motor connected to 50 Hz ac mains.The driving motor may be induction or synchronous motor.When the source of power is not electrical, generator is driven by a non electrical prime mover such as diesel or gas turbine.Motor terminal voltage is controlled by adjusting the field current of the generator.Advantages: It inherent regenerative braking which allows efficient four quadrant operation Can be employed for power factor improvement by using a synchronous motorDisadvantages: Its high initial cost Require more frequent maintenance Produce more noise

Figure 6.17 Block diagram of Ward Leonard drive

TRANSFORMER AND UNCONTROLLED RECTIFIER CONTROL

Variable voltage for the dc motor can also be obtained by either using an autotransformer or a transformer with tapping (either on primary or on secondary) followed by an uncontrolled rectifier.

A reactor is connected in the armature circuit to improve armature current waveform. Autotransformer can be used for low power rating. For high applications, a transformer with tapping is employed and tap changing is

done with the help of an on load tap changer to avoid severe voltage transient produced due to interruption of current in open circuit transition.

The scheme is employed in 25kV single phase 50 Hz ac traction. The important feature of this scheme is:

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Output voltage can be changed only in steps Rectifier output voltage waveform does not change as the output is reduced.

Figure 6.18 Armature voltage control using a transformer with taps and an uncontrolled rectifier.

Figure 6.19 On load tap changer

STATIC WARD LEONARD DRIVE

Also known as controlled rectifier fed dc drive. Are used to get variable dc voltage from an ac source of fixed voltage. For low power applications (up to around 10kW) single phase rectifier drives are

employed. For high power applications, three phase rectifier drives are used.

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Figure 6.20 Single phase and three phase controlled rectifier circuits.6.3 AC MOTOR DRIVES (INDUCTION)

Induction motor has been used in the past mainly in applications requiring a constant speed because conventional methods of their speed control have either expensive or highly inefficient. Variable speed applications have been dominated by DC drive.

Availability of thyristor and power transistor have allowed the development of variable speed induction motor drive.

SPEED CONTROL FOR INDUCTION MOTOR

Speed of induction motor can be controlled by any of the following methods: Pole changing Stator voltage control Supply frequency control Rotor resistance control

Pole changing is applicable for squirrel cage motor Stator voltage control, supply frequency control and Eddy-current coupling are

applicable for both, squirrel cage motor and wound rotor motor.

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Rotor resistance control and slip power recovery are applicable for wound rotor motor.

POLE CHANGING

For a given frequency, synchronous speed is inversely proportional to the number of poles.

Synchronous speed and therefore motor speed can be changed by changing the number of poles.

Provision for changing the number of poles has to be incorporated at the manufacturing stage and such machines are called, pole changing motor or multi speed motor.

Figure 6.21 Variable torque control

STATOR VOLTAGE CONTROL

By reducing stator voltage, speed of a high slip induction motor can be reduced by an amount which is sufficient for the speed control of some fan and pump drives.

While torque is proportional to voltage squared, current is proportional to voltage.

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Thus, as voltage is reduced to reduced speed, for the same current, motor develops lower torque.

This method is suitable for applications where torque demand reduces with speed. Variable voltage for small size motors, particularly for single phase, is sometimes

obtained using autotransformer. However, more common method is the use of ac voltage controllers.

Figure 6.22 Stator voltage control

VARIABLE FREQUENCY CONTROL FROM VOLTAGE SOURCES

Synchronous speed, therefore, the motor speed can be controlled by varying supply frequency.

Voltage induced in stator is proportional to the product of supply frequency and air gap flux.

If stator drop is neglected, terminal voltage can be considered proportional to the product of frequency and flux.

Figure 6.23 Variable frequency controls. ROTOR RESISTANCE CONTROL

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Figure 6.24 Rotor resistance control

While maximum torque is independent of rotor resistance, speed at which the maximum torque is produced changes with rotor resistance.

For the same torque, speed falls with an increase in rotor resistance. Cost of rotor resistance control is lower than the variable frequency control. A major disadvantage is low efficiency due to additional losses in resistor connected

in the rotor circuit.Tutorial 6

1. Application of power electronic devices become are used from high to low power conversion equipment such as dc transmission, cordless screwdriver or power supplies for notebook computer and others. State the characteristic and draw the symbol of the following components.

(i) Gate Turn Off Thyristor, GTO(ii) Triode for Alternating Current, TRIAC(iii) Insulated Gate Bipolar Transistor, IGBT(iv) Silicon-controlled rectifier, SCR(v) Diode for Alternating Current, DIAC

2. Draw the basic block diagram for power electronic system.

3. State the main three (3) groups of power switches by giving the examples for each group.

4. List down speed control method for DC motor.

5. Explain briefly the operation of armature voltage control in speed control in DC drive.

6. Describe the operation of field flux control and armature resistance control for speed control in DC drive.

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7. Draw the speed torque curve for separately excited DC motor under armature voltage control, field flux control and armature resistance control.

8. Repeat question 4 above for series DC motor.

9. Draw the block diagram of Wad Leonard drive.

10. State the control scheme that using the construction of circuit given below.

11. Give the advantages and the disadvantages of Ward Leonard drive.

12. State and explain briefly three methods speed control of DC motor.

13. Explain clearly each method of speed control for induction motor.

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