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Anonymous Poster

Synchronous Speed ns

11/26/2007 3:10 PM

A 1/2 hp, 120 VAC, 60 Hz single phase induction motor is rated at nr = 1175 RPM.

I know that s= (ns-nr)/ns, also ns= (120fs)/p, and n= ns-nr

Not knowing the numbers of poles P, as well as the slip s, how can I determine the synchronous speed ns?
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TVP45
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Join Date: Jul 2007
Posts: 4448
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#1

Re: Synchronous Speed ns

11/26/2007 7:39 PM

Poles come in integers. I'm guessing 1200 rpm.

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Doggoneit
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Join Date: Oct 2007
Location: Canning Vale Western Australia.
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#2

Re: Synchronous Speed ns

11/26/2007 11:52 PM

synchronis speed = (Hz*60)/ (# of poles/2)

ie

s+slip =((60*60)2)/poles

poles = 7200/ (1175 +slip)

poles =6.127. As poles are always an integer number (6) the error is due to slip.

therfore 7200/6 = synch speed =1200

It is easy to give the answer but always show your working out.

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hastingselectric
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#3

Re: Synchronous Speed ns

11/27/2007 7:10 PM

My brain must be tired. The motor is rated at 1175 rpm. It's an induction motor. Thats the speed. Yes, there is slip. It's not synchronous. (the armature rotates at a slightly slower speed than the synchronous speed, synchronous motors have no slip) If I just had a brain fart, let me know. I'm about to get something cold that makes a psssssst, when you open it.

Good luck,

James

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Doggoneit
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#4
In reply to #3

Re: Synchronous Speed ns

11/27/2007 9:18 PM

Yes you are spot on. It is not a synchronous motor. What we were looking at was the synchronous speed for the number of poles in the motor.

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