#### elecstudent

Joined Nov 11, 2010
26
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?

#### crutschow

Joined Mar 14, 2008
29,786
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.

#### elecstudent

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

am I right?

#### crutschow

Joined Mar 14, 2008
29,786
I calculated question one to be 100..

am I right?
That is correct.

#### elecstudent

Joined Nov 11, 2010
26
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..

#### elecstudent

Joined Nov 11, 2010
26

#### crutschow

Joined Mar 14, 2008
29,786
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.