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Chapter 6Synchronous Motors

Edit by Chi-Shan Yu

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Synchronous Machine

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Chapter 6 Synchronous Motors

• Basic Principles of Motor Operation

• Steady-State Synchronous Motor Operation

• Starting Synchronous Motors

• Synchronous Generators and Synchronous Motors

• Synchronous Motor Rating

• Summary

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Basic Principles of Motor Operation

• Two dependent magnetic fields exist in the machine

• The dc field current IF produces a rotor field BR

• The three-phase ac current produce a stator field BS

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The equivalent circuit

• The equivalent circuit of a synchronous motor is the same as the equivalent circuit of a synchronous generator. (Besides, the current flow direction is reverse)

• Since the current flow direction is reverse, the relation between the terminal and internal voltage is

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The phasor diagram of a synchronous generator – with

lagging power factor load

• The induced torque is opposing the applied torque from prime mover

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The phasor diagram of a synchronous motor

• The induced torque drives the motor rotation.

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Steady-state synchronous motor operation – Torque speed curve

• The steady-state speed is constant from no-load to full-load.

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The maximum torque or pull-out torque

• While the voltages are constant, the induced torque of the synchronous motor only depends on the power angle .

• Thus, the pull-out torque is maximum induced torque.– That is, = 90 degrees.–

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The effect of load changes on a synchronous motor (what is the meaning

of pull-out torque)

• At steady-state the torque that the synchronous motor supports is equal to the load torque.

• When the load torque increases ?– The motor will slow down (rotor mechanical speed)– The stator rotation flux still remain its constant speed (stator

flux speed)– Thus, the power angle increases– Finally, the synchronous motor’s will generate a large torque

that is equal to the load torque.

• However, if the maximum induced torque is still lower than the load torque ?

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The phasor diagram of increasing load torque

• When the load increases, all the armature current IA, power angle , and the power factor angle are increase (from leading to lagging).

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Example 6-1

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The effect of field current changes on a synchronous motor

• An increase in field current increase the magnitude of EA but does not affect the real power supplied by the motor.– Why ?

– The power supply by the motor is (Pm = ×load)

• The terminal voltage V supplies to the motor is constant

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The phasor diagram – increase the field current

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What is the effect of increasing field current

• What is the meaning of the power factor of the motor changes from lagging to leading– can support Q to the electrical system– The motor is now acting like a capacitor-resistor combination

load, and the magnitude of the capacitor can be changed by the field current

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Synchronous motor V curve

• A plot of IA versus IF for a synchronous motor is the V curve.

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Underexcited and overexcited

• The internal voltage can be smaller or larger than the terminal voltage

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Example 6-2

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Homework - 2

• Use the MATLAB to complete the Figure 6.12

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EX 6-3 Power factor correction

• Why the power factor is so important in power system ?

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EX 6-3 Power factor correction

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The reason for power factor correction

4. Meanwhile, the harmonic content is also important reason

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The synchronous capacitor or synchronous condenser

• A synchronous motor can be operated overexcited to supply reactive power Q for a power system.

• No real power have been drawn from the load and the shaft of the motor is removed

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The V curve of the synchronous capacitor

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Starting the synchronous motors

• How to start the synchronous motor ?• What is the problem of the synchronous motor in

starting?

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The problem in starting

• The induced torque between rotor flux and stator flux

• The directions of the induced torque depends on the angle relation between the BS and BR

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The problem in starting

• At starting, the motor speed is increased from zero speed

• The speed of the rotor is slower than the speed of the stator flux

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The problem and how to solve

The motor will vibrate but not rotate !!

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Reducing the stator electrical frequency

• At starting, the motor operates at a low enough speed.

• The speed of the stator magnetic field can then be increased gradually up to 60Hz

• Notably– The stator voltage must also be reduced to prevent the over-

current in stator winding. (EA = K)

• The power electronics can build the inverter to achieve the variable frequency and voltage drive

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Motor starting by using damper windings

• The damper or amortisseur windings are special bars laid into notches carved in the face of a synchronous motor’s rotor, and then shorted out on each end by a large shorting ring.

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The simplified diagram of the damper winding

• The rotor shows an damper winding with the shorting bars on the ends of the two rotor pole faces connected by wires

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The reason of adding the damper windings

• At starting, the bars of damper winding are shorted, and the field winding is disconnected from the field voltage

• The motor will act as an induction motor

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Starting the synchronous motor by damper winding

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The effect of damper windings on motor stability

• The damper windings speeds up slow machines and slows down fast machines

• Thus, the stability can be increased by adding the damper windings

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The differences between the synchronous generators and the

synchronous motors

The differences between them are the direction of the induced torque and the angle relation between the internal and terminal voltage

Phasor diagra

m

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Synchronous motor rating

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Recap

• The synchronous speed• The field current and the power factor of the

synchronous motor• Synchronous condenser• The starting problem of the synchronous motor

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Appendix CSalient-Pole Theory of Synchronous

Machines

Edit by Chi-Shan Yu

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Why we discuss the salient-pole effect ?

• Salient pole rotor and cylindrical rotor

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Until now the result is

• The following result is only valid for cylindrical rotor

• The salient pole rotor has another reluctance torque

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The effect of armature reaction in a salient-pole synchronous generator

• The rotor magnetic field and the induced voltage on the stator conductor

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The effect of armature reaction in a salient-pole synchronous generator

• If a lagging load is connected to the terminals of this generator

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The effect of armature reaction in a salient-pole synchronous generator

• The stator magnetic field is no more 90 degrees behind the armature current

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The stator voltage

• Each component of the stator magnetic field produces a voltage in the stator winding by armature reaction. The total voltage in the stator is thus

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Now we may include the armature self- resistance and reactance

• The armature self-reactance is independent of the rotor angle

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How to plot the phasor diagram ?

• The armature current IA is broken into Id and Iq by angle +

• Usually, the torque angle is unknown, and the power factor angle is known.

• Without the knowledge of , how to plot the phasor diagram ?

• Once the angle is known, the armature current IA can be broken into Id and Iq.

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How to plot the phasor diagram ?

• The EA’’ has the same angle as the EA

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Example C -1

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Torque and Power equation of salient-pole machine

• The power produced by d and q axis current

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Power equation

• The power is

• The d-axis current is

• The q-axis current is

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Power equation

• The power is

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Torque equation