Electrical Drive Systems 324Synchronous Motors

Dr. P.J Randewijk

Stellenbosch UniversityDep. of Electrical & Electronic Engineering

Stephan J. Chapman

Chapter 5 (5th Edition)

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Outline

1 Chapman, Chapter 55.1 – Basic Principles of Motor Operation5.2 – Steady-State Synchronous Motor Operation5.3 – Starting Synchronous Motors5.4 – Synchronous Generators and Motors5.5 – Synchronous Motor Ratings

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5.1 Basic Principles of Motor Operation

Although the equivalent circuit is essentially the same,the stator current now flows (per definition) into themachine, i.e. the machine now absorbs electrical energyThe KVL equation for the machine is thus

Vφ = EA + jXSIA + RAIA (5–1)

Magnetic Field PerspectiveThe biggest difference between motor operation andgenerator operation is that:

for generator mode of operation, the rotor field, BR , pullsthe stator field, BS, and hence EA is leading. . .but for motor operation, the stator field, BS, now pullsthe rotor field, BR , and hence Vφ is leading. . .

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5.1 Basic Principles of Motor Operation (cont.)

Generator Operation – Fig. 5–3

Motor Operation – Fig. 5–04

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5.2 Steady-State Operation

Torque-Speed Characteristic “Curve”Because it is a synchronous machine, the speed isconstant regardless of the load – i.e. 0% speedregulation

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5.2 Steady-State Operation (cont.)

The torque in terms of known phasor quantities(remembering that P = ωτ

τind =3VφEA sin δωmXS

(4–22)

The maximum torque the motor can deliver is at δ = 90◦

and is called the pullout torqueIf the mechanical load’s torque exceeds the torque themotor can deliver,the stator field will start to “lap” the rotor field withdisastrous consequencesthis is called pole slipping

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5.2 Steady-State Operation (cont.)

From Fitzgerald Fig. 5–1, the torque, T (or τ inChapman) as a function of δ, can graphically bedepicted as

For stability reasons, generators and motors areoperated at |δ| < 90◦ – see operating points g and mrespectively. . .

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5.2 Steady-State Operation (cont.)

The effect of Load Changes on a SynchronousMotorSimilar to that of generator mode of operation, just withEA lagging – see Fig. 5–6 (b)

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5.2 Steady-State Operation (cont.)

The Effect of Field Current Changes on aSynchronous MotorSimilar to that of generator mode of operation, just withEA lagging – see Fig. 5–8 (b)

+ Ignore V-curves – Fig. 5–9

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5.2 Steady-State Operation (cont.)

The Synchronous Motor and Power-FactorCorrectioningBy over magnetising a synchronous motor, the powerfactor for the rest of the plant can be corrected – seeExample 5–3

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5.2 Steady-State Operation (cont.)

The Synchronous Capacitor or SynchronousCondenserPower utilities (e.g. Eskom in South Africa) have largesynchronous motor connected to their power grid atstrategic places (e.g. Muldervlei, near Stellenbosch)that are operated at no-load (mechanical) with the solepurpose to do reactive power (i.e. VAR) compensationon the power system – see Fig. 5–15

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5.3 Starting Synchronous Motors

The following methods can be used to start – i.e.synchronise – a synchronous motor with the AC supply:

using a variable voltage, variable frequency AC supply –i.e. basically a power electronic “drive”using a non-synchronous motor, e.g. an induction motor,as a pilot motor to start the synchronous motorusing a synchronous motor with damper – oramortisseur windings – i.e. basically “squirrel cage”windings (will be discussed when we do inductionmachines)

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5.4 Synchronous Generators and Motors

The following is a nice summary of the four modes ofoperation for synchronous machines – see Fig. 5–20

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5.5 Synchronous Motor Ratings

Similar to generator ratings. . .With the only difference, that for a generator, the powerfactor rating was given as lagging – for a motor it isleading. . .In both cases this has to do with how much the machinecan be over excited, i.e. the maximum value of the fieldcurrent. . .

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