ID-4th modIndustrial Drives 4th mod. CALICUT UNIVERSITY

8/7/2019 ID-4th modIndustrial Drives 4th mod. CALICUT UNIVERSITY

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Synchronous Motor Drives

Commonly used synch. Motors are

y W ound field

y Permanent Magnet

y Synchronous reluctance

y Hysteresis

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Synchronous Motor Drives contd..

y W ound field synch. Motory It has a dc field wdg.W hich is supplied from a dc source through

slip rings.y The rotor can have cylindrical or salient pole construction.y Cylindrical rotor motors have higher mechanical strength and

are employed in high power and high speed applications

y The wound field and permanent magnet synch. Motors have ahigher full load efficiency and power factor than inductionmotors

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Synchronous Motor Drives contd..y

Cylindrical RotorW

ound Field Motor

Xs : Synchronous

reactance.E : Excitation emf

: power or torque angleV : Applied voltage

y Power input to the motor Pin = 3 V Is cosy W here is the phase angle of Is with respect to V.

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Synchronous Motor Drives contd..y Since stator loss has been neglected, the power developed is Pm =

3 V Is cos

y For steady state torque to be produced, rotor field must move at thesame speed as stator field. Since rotor field has same speed as that of rotor, the rotor also runs at synchronous speed.Therefore

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Braking Motoring

-Tmax Tmax0

m

ms

Speed torque chars with fixed frequency

For a given field excitation E is constant,hence Pm &T are proportional to sin .

Pull out

Torque


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Synchronous Motor Drives contd..y Salient PoleW ound Field Motor

5

V

d axis

q axisIsIsd

Isq EIsd Xsd

Isq Xsq

Phasor diagram for salient pole motor


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Braking Motoring

-Tmax Tmax0

m

ms

Speed torque chars ith fixed frequency

The first component Synchronous torque is

proportional to sin andthe second component Reluctance Torque isproportional to sin2 .

Synchronous Motor Drives contd..


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Synchronous Motor Variable Speed Drives

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V=E ideally hence

for cylindrical type synchronous motor

W hen v/f is constant , torqueTmax is constantBelow base speed - V/f is

constantAbove base speed V is constant

and f can be varied, hence thetorque decreases.

Braking otoring

-Tmax Tmax0

m

ms

Speed torque chars ith variable frequency control

T


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Modes of Variable frequency control

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There are two modes of operation1. True Synchronous mode and2. Self controlled mode or self synchronous mode.

True Synchronous mode

Frequency fro its initial value to desired value is changed gradually sothat the difference bet een synch. speed and rotor speed is al ays s all.

his allo s rotor speed to track the changes in synch. speed.

When the desired synch. Speed is reached , the rotor runs at synch. speedafter hunting oscillations. Da per inding is used for pull intosynchronis


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Modes of Variable frequency control

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Self controlled mode or self synchronous mode.Frequency is changed so that synch. Speed is same as rotor speed.

This ensures that rotor runs at ms for all op.points.D amper winding is not required.

Sensors can be mounted on the stator to track rotor position. Thesesensors are called as rotor position sensors. The frequency of signalgenerated by these sensors are proportional to rotor speed.


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Variable Frequency control of multiple synchronous motors(True synchronous mode closed loop)

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Variable frequency control contd..

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True synchronous mode. Closed loopf*is applied to VSI through a delay circuit so that rotor speed is

able to track the changes in frequency.

Flux control block produce V* corresponding to f to maintainv/f as constant (below base speed) and V as constant (above basespeed).


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Self Controlled or Self synchronous mode


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Self controlled or self synchronous mode

In large power application wound field synchronous motorare used.

D rive employs source side converter and load sideconverterSource side converter is6 pulse line commutated thyristor

converterFor 0


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Self controlled or self synchronous mode

On the other way for 9 0


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Self controlled or self synchronous modeD ue to large LdI works as inv. It behaves like CSI during BRAKING operation andII works as inv. It behaves like CSI during MOTORING operation,

consequently motor phase current has six step wave form

120

t0

Id

iA

Id2/ 6 5 / 6

120

The fundamental component of motor phase current is


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Closed loop speed con tr ol of self con tr olled SM d riv e


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Inner current loop and outer speed loop controlTerminal voltage sensor generates reference pulses as the same

frequency as the machine induced voltages.Phase delay circuit shifts reference pulses to get constant

commutation lead angle ( lc = 180 l)

Speed m can be sensed either from voltage sensor or bytachometer.Increase in speed command produces positive speed error lc set

for motoring , machine accelerates fast, and the drive settles atthe desired speed when the dc link current balances motor and

load torques.Decrease in speed command produces negative speed error lc

set for regenerative braking , motor decelerates fast, When speederror sign changes, lc sets for motoring, and the drive settles for desired speed.

Closed loop speed con tr ol of self con tr olledSM d riv e


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Microprocessor based Closed Loop Control

A dvantages :Improvement in functions, reliability, reduction in size of control

equipment, rapid decrease in manufacturing cost.Once the algorithms are developed, the software can be used for

different plants with only a few modifications. This is not possiblededicated hardware control.It provides suitable protections against commutation failure,

cross over conduction etc..

There will be interfacing circuits between uP and the system,Interfacing circuit may increase the cost, but due to overalladvantages, uP based control is better than analog control.

A DC & D A C make digital system more economical


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Microprocessor based Closed Loop Control

L imitations:Interfacing circuit may increase the costMicroprocessor control is in serial manner, which make the

system slower than dedicated analog controlQuantization error A ssembly language programming is time consuming and

expensive


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Microprocessor based Closed Loop Control of D C D rive

BlockD iagram of a separately excited dc motor drive using microprocessor


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Microprocessor based Closed Loop Control of D C D rive contd..

The converter can be FCR orD ual Converter


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Speed

Position

Microprocessor control of a CSI fed Synchronous Motor


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Microprocessor based induction motor drive


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ID-4th modIndustrial Drives 4th mod. CALICUT UNIVERSITY