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Electrical Machines Theory Period 4 SAIT-Alberta > Synchronous Motors > Flashcards

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What is a synchronous motor called when used solely for power factor correction?

A synchronous condenser.


Why would a synchronous motor be used to power factor correct instead of a capacitor bank?

It is much easier to adjust the amount of correction.


What is a disadvantage of a permanent pole motor?

This design cannot alter the power factor of the machine.


What are three advantages of a synchronous motor over a SCIM?

Operate at a leading power factor to correct a building;

Constant speed from no load to full load;

Higher efficiency.


What are four disadvantages of a synchronous motor when compared to a SCIM?

DC excitation required;

May fall out of synchronism if overloaded;

Assistance to get to speed is required;

Does not work well with frequent starting.


Why does a stator and rotor of a synchronous motor need to be constructed with the same number of poles?

So that the motor can magnetically lock and operate at synchronous speed.


How does a synchronous motor magnetically lock?

A dc excitation source is applied to the rotor poles, which creates permanent magnetic poles, these poles lock onto the rotating magnetic field of the three phase supply.


What are three methods to start a synchronous motor?

Use a shaft mounted induction motor with 2 fewer poles;

Use the load, i.e. DC dynamo;

Use amortisseur windings and a field discharge resistor.


What is torque angle?

The angle between the center of magnetically locked poles, the angle exists due to rotational drag and mechanical load.


When does pull out torque typically occur on cylindrical rotors?


At 90 degrees.


What are the four possible load conditions of a synchronous motor?

No losses and no load;

Light load and leading power factor;

Rated load and unity power factor;

Overload and lagging power factor.


As the load on a synchronous motor increases, how is the torque angle affected?

It will increase.


What is normal excitation?

The amount required to operate the motor at unity power factor.


What is under excitation?

When excitation results in a lagging power factor.


What is the condition when the amount of excitation current results in a synchronous motor running at a leading power factor.

Over excitation.


Changing the field excitation of a synchronous motor changes the ...?

Power factor.


What is another name for a synchronous condenser?

A rotary capacitor.


What power factor are most synchronous motors operated at?

Unity, 0.9 lead or 0.8 lead.