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

Answer:

8 poles

Slip= 16.67%

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

$p=\frac {120 \times 80}{1000}=9.6\approx 8 poles$

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

$N_s=\frac {120 \times 80}{8}=1200$

The motor speed is now $N_r=1000 rpm$

To find slip, we use the formula

$Slip=\frac {N_s- N_r}{N_s} \times 100$

Then by substitution

$Slip=\frac {1200-1000}{1200}\times 100=0.166666667\times 100\approx 16.67$

Therefore, slip=16.67%