NEIEP Mechanics Practice Exam

What is the percent of slip for a three-phase 60Hz motor that has 6 poles and rotates at 1157 RPM?

1. 1.75%

2. 3.58%

3. 5.00%

4. 7.25%

The correct answer is: 3.58%

To calculate the percent of slip for a three-phase motor, you first need to determine the synchronous speed of the motor. The synchronous speed (Ns) can be calculated using the formula: \[ Ns = \frac{120 \times f}{P} \] where - \( f \) is the frequency in hertz (Hz), - \( P \) is the number of poles. In this case, the motor operates at a frequency of 60 Hz and has 6 poles. Plugging in these values: \[ Ns = \frac{120 \times 60}{6} = \frac{7200}{6} = 1200 \text{ RPM} \] Next, you find the slip, which is the difference between the synchronous speed and the actual speed of the motor. The slip (S) can be calculated using the formula: \[ S = Ns - N \] where - \( N \) is the actual speed of the motor. Thus, \[ S = 1200 \text{ RPM} - 1157 \text{ RPM} = 43 \text{ RPM} \] The percent slip can then be calculated using the formula: \[ \text{Percent Slip} = \