Discussion in 'Homework Help' started by elecstudent, Nov 17, 2011.

1. ### elecstudent Thread Starter New Member

Nov 11, 2010
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This is the questions that I have been given;

1a) A 4 pole synchronous motor rotates at 3000 RPM. The torque on the motor is 20Nm and the efficiency is 90%. What is the frequency of the electrical supply? (Hz)

1b)
What is the load on the electrical supply? (Watts)

Can anyone help to start me off?

2. ### crutschow Expert

Mar 14, 2008
13,469
3,356
There is a direct relationship between motor speed, the number of poles, and the supply frequency. Google synchronous motors and you will find that equation.

The power generated by the motor is torgue times RPM. From that and the motor efficiency you can determine the motor input power.

3. ### elecstudent Thread Starter New Member

Nov 11, 2010
26
0
I calculated question one to be 100..

am I right?

4. ### crutschow Expert

Mar 14, 2008
13,469
3,356
That is correct.

5. ### elecstudent Thread Starter New Member

Nov 11, 2010
26
0
using this formula (if it's the correct 1)
HP = rpm x T(torque)
5252(constant)
I got;

11.9HP = 8519W

Is this correct? as the W value looks huge..

Nov 11, 2010
26
0

7. ### crutschow Expert

Mar 14, 2008
13,469
3,356
I believe your constant is not correct. That looks like the constant for conversion of torque in lb-ft to HP, not Nm.

So you either need a new constant or you must convert Nm to lb-ft.

8. ### elecstudent Thread Starter New Member

Nov 11, 2010
26
0
thanks this was really appreciated, as my calulator in build conversion can't handle this, I only realised.

cheers