# How to calculate SR (Speed Regulation), when no-load rpm isn't given???



## yuyii (Jul 5, 2011)

I'm currently working on a problem shown below. Bear with me, I need to go through my calculation first before I can ask my question...

"A generator provides 400 VAC at 500 Hz turning at 14,000 rpm under full load. Calculate (a) How many poles does it have? (b ) What is the speed regulation, SR(%)?"

Here's how I solved the problem:

(a) How many poles does it have?

sync rpm = 120 * f / p

Since sync rpm is not provided, I used the full-load rpm and solved for 'p' . Once solved, I rounded it down to the nearest multiple of 2.

14,000 = 120 * 500 / p

therefore, p = 4.28 = 4 (rounded down, since the number of pole is always a multiple of 2)

(b ) What is the speed regulation, SR(%)?

First, I calculate the sync rpm using the 'p' calculated in (a).

sync rpm = 120 * f / p

sync rpm = 120 * 500 / 4

sync rpm = 15000 rpm

Now, to solve the SR, I used the equation:

SR = (no-load rpm - full-load rpm) / full-load rpm

... now here's my question... since we were not given the no-load rpm in this problem, is it safe to assume that the sync rpm can be used instead? In other words, here's how I solved the SR

SR = (sync rpm - full-load rpm) / full-load rpm

SR = (15000 - 14000) / 14000

SR = 0.0714 = 7.1%

Is my SR answer correct? Or is it wrong because I used the sync rpm, not the no-load rpm, to calculate for SR. I understand that sync rpm is theoretically different from no-load rpm. But I'm not sure how to come up with the no-load rpm when it is not given in the problem.

Thank you. I appreciate any feedback you have for me.

yuyii


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## jeanbj2000 (Jul 8, 2011)

yuyii said:


> I'm currently working on a problem shown below. Bear with me, I need to go through my calculation first before I can ask my question...
> "A generator provides 400 VAC at 500 Hz turning at 14,000 rpm under full load. Calculate (a) How many poles does it have? (b ) What is the speed regulation, SR(%)?"
> 
> Here's how I solved the problem:
> ...


It is correct because the rotor field motor is always slightly less than the synchronous motor. Therefore, we p=4, you are able to calculate sync rpm which is sync rpm = 120 * 500 / 4

sync rpm = 15000 rpm.

I believe your approach is correct.


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## Phatso86 (Oct 10, 2016)

Ok, so the consensus is that any regulation is (NL - FL)/FL

I hate it when textbooks, especially review books, provide incorrect formulas


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## knight1fox3 (Oct 10, 2016)

Hey, way to revive a 5-yr old thread! :thumbs:


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## nukem2k5 (Oct 11, 2016)

Phatso86 said:


> Ok, so the consensus is that any regulation is (NL - FL)/FL
> 
> I hate it when textbooks, especially review books, provide incorrect formulas


Make sure you use absolute values.  One of the practice test questions asked a simple question about voltage regulation and the no load voltage was given with an angle.


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## Phatso86 (Oct 12, 2016)

oh yeah, I messed up a few times with that.

practice makes perfect


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## nukem2k5 (Oct 12, 2016)

Phatso86 said:


> oh yeah, I messed up a few times with that.
> 
> practice makes perfect


Is the reason that we neglect the angle of the voltage because: we only care about the difference in magnitudes, and the angle is only relevant to the power factor?


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