Question

A 3-phase induction motor is being driven at a frequency of 80 Hz, and the motor speed is 1000 rpm. How many poles does the motor probably have and what is the slip? (10 points)

Answer 1

8 poles

Slip= 16.67%

Step-by-step explanation:

We use the formula for synchronous speed,
`N_s=\frac {120 f}{p}`

Where f is the frequency and p is the number of poles

Since frequency is given as 80 Hz and the motor speed also given as 1000 rpm then making p the subject of the above formula we get

`p=\frac {120 f}{N_s}`

By substitution

`p=\frac {120 * 80}{1000}=9.6\approx 8 poles`

To find the slip, we now use the actual
`N_s` and since we know that
`N_s=\frac {120 f}{p}` and p=8 then

`N_s=\frac {120 * 80}{8}=1200`

The motor speed is now
`N_r=1000 rpm`

To find slip, we use the formula

`Slip=\frac {N_s- N_r}{N_s} * 100`

Then by substitution

`Slip=\frac {1200-1000}{1200}* 100=0.166666667* 100\approx 16.67`

Therefore, slip=16.67%