alternator frequency????

[lester mcmanaway]

So I've wasted entirely to much time trying to figure out where I am going wrong here.

An alternator containing eight poles per phase is turning at 1500 RPM. What is the output frequency?
Your Answer: 100 Hz
Help: Output frequency calculation for three-phase alternator


the formula I used to find the 100hz is Frequency=poles X speed/ 60x2
so 8x1500/120=100
Am I doing something wrong here??

[GavinK5]

Your answer looks correct to me.

[lester mcmanaway]

Your answer looks correct to me.

That's kind of what I was thinking, however there were 2 of these questions with different values and I missed both of them using this formula. Taking my 3 next week and would really like to be sure I am doing this correctly, last time I took the test I missed passing by 2 questions. I do not want to repeat that.

[GavinK5]

That's kind of what I was thinking, however there were 2 of these questions with different values and I missed both of them using this formula. Taking my 3 next week and would really like to be sure I am doing this correctly, last time I took the test I missed passing by 2 questions. I do not want to repeat that.

Dang, sorry to hear about that. Check out some of these formula examples:

156

Source: http://electrical-engineering-portal.com/few-words-about-three-phase-alternator

[lester mcmanaway]

Dang, sorry to hear about that. Check out some of these formula examples:

156

Source: http://electrical-engineering-portal.com/few-words-about-three-phase-alternator

appears to be right in line with what I was thinking, wondering why it was marked incorrect. The only other thing that trips me up a little is when " poles per phase" is mentioned. So are you assuming 3 phase at that point and using the given poles per phase x3?

[GavinK5]

appears to be right in line with what I was thinking, wondering why it was marked incorrect. The only other thing that trips me up a little is when " poles per phase" is mentioned. So are you assuming 3 phase at that point and using the given poles per phase x3?

Good question. I would assume that the number of poles given is the total for all three phases.

[baldscreen]

Good question. I would assume that the number of poles given is the total for all three phases.

There are always an even number of poles. One provides positive half of the wave form, the other is the negative half. Each revolution of the alternator then provides (# of poles/2) waves, or cycles. An 8 pole machine produces 4 full cycles per revolution. So, in the original problem:

Frequency = (8 poles x 1/2 cycle/pole) x 1500 rev/minute x 1min/60 seconds

Poles & minutes cancel, leaving 100 cycles per sec.

Each phase is separate, so number of phases doesn't affect the calculation.

[mwallace]

There are always an even number of poles. One provides positive half of the wave form, the other is the negative half. Each revolution of the alternator then provides (# of poles/2) waves, or cycles. An 8 pole machine produces 4 full cycles per revolution. So, in the original problem:

Frequency = (8 poles x 1/2 cycle/pole) x 1500 rev/minute x 1min/60 seconds

Poles & minutes cancel, leaving 100 cycles per sec.

Each phase is separate, so number of phases doesn't affect the calculation.


I think this question is stating that there are 8 poles PER PHASE or 24 total poles. The number of poles is the total count of magnetic poles placed in a stator to produce a rotating magnetic field. The required speed can be achieved by adjusting number of magnetic poles of stator of an AC synchronous motor.

Therefore; F = P x Ns / 120 or F = 24 x 1500 / 120 => F = 300Hz