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INTRODUCTION TO

ELECTRIC DRIVESUnit 5Control of AC Drives

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Introduction D.C. drive has certain drawbacks. Most of them are due to

dc motors.

Need regular maintenance

They are tailor made hence not readily available for replacements.

Bulky in size.

Due to commutator sparking, they are simply not suitable inhazardous areas like chemical and petrochemical plants or in

mines.

For ratings above 500 KW, manufacture of d.c. motor poses

problems.

With these serious limitations, d.c. drive systems have

become unsuitable for energy saving applications.


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Intro (Contd)A.C. drive systems use the a.c. motor as the driven

element either induction or synchronous type.

Induction motors, particularly squirrel cage type

induction motors, have a number of advantages when

compared with d.c. motors.

Some of them are ruggedness; lower maintenance, better

reliability, lower cost, weight, volume, and inertia; higher

efficiency and the ability to operate in explosive

environments.


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Basic Principle of OperationApproximately, 60% of the worlds consumption of

electrical energy passes through the winding of squirrel-cage induction motors in the range of 1 to 125 HP.

A three phase induction motor consists of a balanceddistributed three phase winding on the stator. These

windings are displaced by 120o

in space with respect toeach other.

The squirrel cage rotor consists of a stack of insulatedlaminations. It has electrically conducting bars insertedthrough it in axial direction which are electrically shorted

at each end of the rotor by end rings, thus producing acage like structure.

The squirrel cage motor are simple, rugged and havelow cost.


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Torque Speed Characteristics of a Polyphase

Induction Motor


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Squirrel Cage Rotor Design


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Design Class A: normal starting torque, normal starting

current, low slip

This design usually has a low resistance single cage rotor. It emphasizes good running performance.

The class A motor is the basic standard design in sizes below

about 7.5 and above about 200 HP.

Design Class B: normal starting torque, low startingcurrent, low slip

This design has approximately the same starting torque as the

class A design but with 75 % of the starting current.

This design is common in 7.5 to 200 HP range of sizes.

It is used for substantially constant speed drives where starting torque requirements are not severe, such as in driving fans,

blowers, pumps and machine tools.


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Design Class C: high starting torque, low starting current

This design uses a double cage rotor with higher rotor resistance

than the class B design.

Result is higher starting torque with low starting current but

somewhat lower running efficiency and higher slip then classes A

and B designs

Typical applications are in driving compressors and conveyers.

Design Class D: high starting torque, high slip This design usually has a simple cage, high resistance rotor.

It produces very high starting torque at low starting current.

It principle uses are for driving intermittent loads involving high

accelerating duty and for driving high impact loads such as punch

presses and shears.


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Speed Control of Induction Motors The basic speed control methods are:

Stator voltage control

Variable frequency control

Rotor Resistance control

Slip power recovery scheme


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Rotor Resistance control

The methods of induction motor control describes earlier,

control the motor from the stator. Hence, they are

applicable to both squirrel cage and wound rotor

motors.

The wound rotor motor has number of disadvantages

compared to squirrel cage motor such as high cost,

weight, volume and inertia, and frequent maintenance due

to the presence of brushes and slip sings. However, the

control of a wound rotor from the rotor allows cheaperdrives to be obtained for a few specific applications.


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In a wound rotor motor, the improved starting

performance is obtained by connecting an external

resistance in series with the rotor winding.


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Chopper controlled wound rotor induction motor


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Slip Power Recovery Scheme

In the rotor resistance control, the slip power is dissipated

in the resistance and this effectively reduces the efficiency

of the motor. However, instead of dissipating it in the

resistance, the slip power can be conveniently returned to

the mains or used in useful manner making the drivemore efficient.

This is achieved by means of slip power recovery

schemes known as Kramer and Scherbius Drives.


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Scherbius Drives


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Static Kramer Drives


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Synchronous motor drives

A synchronous machine is the one in which alternating

current flows in the armature winding and d.c. excitation is

supplied to the field winding. The armature winding is on

the stator and is usually a three phase winding. The field

winding is on the rotor. The speed of synchronousmachine under steady state conditions is proportional to

the frequency of the current in the armature. The

magnetic field created by the armature currents rotates at

the same speed as that created by the field current on therotor (which is rotating at synchronous speed) and a

steady torque results.


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A synchronous motor is a constant speed machine which

always rotates with zero slip at the synchronous speed.

The synchronous motors can be of the following types: Round or cylindrical rotor motors

Salient or projecting rotor pole motors

Reluctance motors

Permanent magnet motors Switched reluctance motors

Brushless d.c. and a.c. motors


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Round (Cylindrical) Rotor Motor

The round rotor or cylindrical rotor synchronous machinehas a uniform air gap between a slotted stator and rotor.

The stator is composed of iron laminations stacked

together. The rotor is a solid forging with rotor slots milled

into its surface.A single field winding is placed in the rotor slots and a

conventional three phase distributed armature winding

is placed in the stator slots.

For synchronous motor operation, the three phasearmature winding is supplied with balanced three phase

currents, and d.c. excitation is supplied to the rotor field

winding.


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The balanced three phase armature currents

establishes a flux wave in the air gap which rotates at

synchronous speed. If the rotor also rotates atsynchronous speed, the magnetic fields of stator and rotor

are stationary relative to each other and a steady

electromagnetic torque is developed because of the

tendency of the two magnetic fields to align their axes.

The rotor field winding can be excited with direct current

supplied through slip rings and brushes from a static

phase controlled rectifier exciter or from a d.c. generator

exciter.

Round rotor synchronous machines are used for steamand gas turbine driven generators in utility and industrial

generating stations. Ratings can exceed 1500 MW for a

single unit.


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Salient Pole Motors In a salient pole machine, the stator has a conventional

three phase distributed armature winding placed in

slots. The field winding consists of a number ofconcentrated field coils placed around projecting poles onthe rotor.

To produce alternate north and south poles, the field

winding is excited with direct current. The salient pole construction results in a non uniform air

gap. The air gap is length is minimum in the polar, ordirect axis and is maximum in the inter polar, quadratureaxis.

The stator winding mmf will establish a larger air gap fluxwhen the mmf is centered on the direct axis than whencentered on the quadrature axis.

Salient pole machines have been built in unit sizes of upto 500 MW for hydrogenerator utility applications.


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Synchronous Reluctance Motors The synchronous reluctance motors has an unexcited

ferromagnetic rotor with polar projections. The reluctance torque is developed by the tendency of e

ferromagnetic material to align itself with a magnetic field.

If a synchronously rotating stator field is established by

means of a conventional polyphase winding excited abalanced polyphase a.c. supply, then the rotor runs in

exact synchronism with this field as the salient poles seek

to maintain the minimum reluctance position w.r.t. the

stator flux. Reluctance motors have been widely used in adjustable

speed multimotor drives requiring exact speed

coordination between individual motors.


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Permanent Magnet Motors

In a permanent magnet motor operating on a fixed

frequency a.c. supply, the constant rotor flux produced by

the permanent magnets generates a constant value of

excitation emf, Ef. The actual value of excitation emf

depends on the magnet material, its physical dimensions,the rotor design and the air gap length.

The elimination of field coil, d.c. supply and slip rings

reduce the motor loss and the complexity. These motors

are known as brushless motors and finds applications inrobots and machine tools.


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Switched Reluctance Motors The SRM has a doubly salient construction with projecting

poles on both stator and rotor. The machine rotorconstruction is same as that of a synchronous reluctance

motor, i.e. it does not have permanent magnet or any

winding, but the stator poles have a concentrated winding.


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The SRM has a number of inherent advantages:

The machine has simple construction, making it a potentially

cheaper alternative.

There is independent and uncoupled operation of motor phases so

that the machine will continue to operate if a phase falls.

There is no possibility of converter shoot through fault because

the winding is always in series with the devices.

Its robust rotor construction makes it more reliable and suitable for

high-speed operation.

Efficient motor cooling because all windings are on the stator.

Low rotor inertia and high torque inertia ratio.

The principle demerits of the drive are that the machine

pulsating torque is high, giving high acoustic noise, and

the machine is somewhat bulky.



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Self controlled Synchronous motor drives

(Brushless D.C. and A.C. Motor Drives)

In self control, as the rotor speed changes the armaturesupply frequency is also changed proportionately so thearmature field always moves at the same speed as therotor. This ensures that the armature and rotor fieldsalways rotates in synchronism for all operating points.

The term brushless d.c. motor is used to identify thecombination of a.c. machine, solid-state inverter, and rotorposition sensor that results in a drive system having alinear torque-speed characteristic.

The rotor position sensor controls the firing signals for the

solid-state inverter. In response to these firing signals, theinverter directs current through the stator phase windingin a controlled sequence.

In brushless d.c. motor, the torque function is trapezoidal.



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Brushless D.C. Motor

Due to the similarity in operation with a d.c. motor, theinverter fed self-controlled synchronous motor drive of fig.

is known as commutatorless d.c. motor or bushless d.c.

motor.

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The three phase half wave

brushless d.c. motor

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This circuit configuration is suitable for small brushless

d.c. motors in a wide variety of applications at power

levels from a few watts to 100 W. Typical applications forthese small brushless d.c. motors include turntable drives

in record players, spindle drives in hard-disk drives for

computers, etc.

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The three phase full wave

brushless d.c. motor

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Brushless A.C. Motor

y y

These motors are used for high power applications

(up to megawatt range), such as compressor,blowers, fans, conveyers, cement plants, etc.

The self-control is also used for starting large

synchronous motor in gas turbine and pump storage

power plants

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