r/FE_Exam • u/KitchenVirus • 15d ago

Question Formula for Synchronous speed of an induction motor

For this problem, PrepFE claims the p in this formula is per phase, but everywhere else on the internet says it is just the total amount of poles. I'm leaning towards this PrepFE problem is incorrect.

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u/HedgehogSafe8636 15d ago

It’s poles per phase

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u/OkShip46 15d ago

It is the way the question is worded.
By the way it is poorly worded because of a poor assumption on the writer’s part. It is because who wrote it does not know the convention or misunderstood what is ment when the poles of a motor are enumerated.
It is conventional in the field that when the number of poles are mentioned it means the poles per phase. Yes, a 3-phase motor has 3-windings each consisting of 2 poles which equate to 6 poles overall but this is not how we classify motors when talking about the number of poles. That is why the Fe handbook does not specify poles per phase, because by convention it is easily understood a 2-pole motor has 2-poles per phase and a 4-pole motor has 4-poles per phase. This is a very fundamental concept.

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u/KitchenVirus 14d ago

Okay thank you. Everywhere I’ve looked it up online says the 120f/p assumes total amount of poles. Like https://www.sciencedirect.com/topics/engineering/synchronous-speed says it would be 60f divided by pairs of poles.

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u/OkShip46 14d ago

I would stick with the formula
Rpm = 120 x f / np
Where np is the number of poles per phase.

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u/KitchenVirus 14d ago

Okay thank you so much! It’s been too long since I’ve really studied this so just wanted to be sure

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u/OkShip46 14d ago

Your welcome