Motor calculation??????

BDKuhns

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• Nov 7, 2005

• #1

OK, I know I've only been out of school for 2 years. I am looking for the motor speed calculation for a 3-phase motor. I remember that the 1-phase (inductive) motor speed formula is S= 120xf/P. I remember that a 3-phase motor requires a "1.732" or ".707" factor in this formula. My school notes were lost in a flood, and I can not find this formula on searching the net. Can you help?

 

rsdoran

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• Nov 7, 2005

• #2

Here are the standard motor formulas: http://www.patchn.com/motorformula.htm

RPM= 120 x frequency divided by number of poles, same for single and 3 phase.

1.73 is used when calculating Amps, HP, KW or KVA for 3 phase motors.

 

Last edited: Nov 7, 2005

BDKuhns

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BAM- the minute you said that, It came back to me. Thanks!!

 

BDKuhns

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Wait....that still does not work. I have a 480V, 3phz, 1720rpm motor. The formula does not work here. S=480x60/6pole ends up being 4800 rpm. Where am I going wrong?

 

leitmotif

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• Nov 7, 2005

• #5

BDKuhns said:

Wait....that still does not work. I have a 480V, 3phz, 1720rpm motor. The formula does not work here. S=480x60/6pole ends up being 4800 rpm. Where am I going wrong?

Click to expand...

FORMULA
120 x F / Poles
So 120 x 60 = 7200
divided by 6 = 1200

As a check
2 pole is 3600 RPM
4 is 1800
6 is 1200
8 is 900

Dan Bentler

 

BDKuhns

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• Nov 7, 2005

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Sorry, I left out some data. This 3-phase motor has a face plate that states RPM=1720. I'snt most motors about this RPM?.

 

CroCop

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• Nov 7, 2005

• #7

You gotz yourself a 3 phase 4 pole motor

So

120 * 60 / 4 = 1800

Subtract 80 for slip and there you are.

 

rsdoran

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• Nov 7, 2005

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As Dan stated a 4 pole 60HZ motor is 1800 rpm BUT there is another factor, SLIP. The page I linked offers data on this. The nameplate for most motors includes SLIP factor..ie an 1800 rpm motor becomes a 1750 (or in your case 1720) rpm motor.

 

BDKuhns

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OK. Maybe I'm missing something here. I thought that the "120" in the formula meant 120V in the 1-phase formula. Instead of being a 120V (squirrel cage motor), I have a 480/3-phase motor. Is the formula the same no matter what the volt or phase is?

 

CroCop

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BDKuhns said:

OK. Maybe I'm missing something here. I thought that the "120" in the formula meant 120V in the 1-phase formula. Instead of being a 120V (squirrel cage motor), I have a 480/3-phase motor. Is the formula the same no matter what the volt or phase is?

Click to expand...

Yep. Poles and frequency be the driving factors.

 

BDKuhns

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OK-again! Maybe I'm taking this too far....but what does the 120 in the formula mean. Does it stand for the 120 Elec/degrees that each phase is out of synch? Or does it really stand for the Volts?

I'm in assimulation mode here!
Thanks-barry.

 

CroCop

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http://www.aosmithmotors.com/html/motorDoctor/Understandingmotorspeed.htm

The 120 is a constant. I'm not sure where it comes from, but it is independent of Voltage.

 

leitmotif

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• #13

The 120 is one of those "magic" handy conversion factors which has several engineering values hidden inside it.

Sorry I forgot to mention slip. Yep ideal "synchronous" speed for four pole induction is 1800. With slip it is less so your nameplate PLUS slip will equal synchronous.

1800 RPM 4 pole is a very commonly used motor design. Forgot this on original message.

Synchronous motors have no slip so they always run at sych speed but they still obey the same 120 x F / poles formula

Dan Bentler

 

Last edited: Nov 7, 2005

kamenges

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• Nov 7, 2005

• #14

The 120 is a combination of 60 seconds per minute, which gets you from Hertz (1/seconds) to revolutions / minute, and 2 poles per pole pair.


Keith

 

Tom Jenkins

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• Nov 8, 2005

• #15

BDKuhns said:

Or does it really stand for the Volts?

Click to expand...

No. The motor speed is independent of voltage. For example, many motors are designed to work with 460 AND 230 volts, some even allowing 208 volts on the same motor. But, because the number of poes doesn't change neither does the speed. That motor will have the same speed regardless of which voltage is hooked up. Note that the wiring connections are different for each voltage, and the current will change for a given horsepower at each voltage, but the speed will be the same.