309A exam prep. question

[poutinepete]

Here's the 309A prep question and answer:
1. A four-pole 60hz SCIM has a full load speed of 1770 RPM at full load. Calculate the % slip of the motor at full-load.
Answer: 1.67%
Can someone explain to me how to figure this out because I have no idea.
Thanks.

[suemarkp]

60 cycles per second = 120 peaks per second if you look at the plus and minus voltage cycles. 120 cycles per second * 60 seconds = 7200 RPM if the motor had one pole.

7200 / 4 poles = 1800 RPM calculated RPM.

Actual RPM was 1770 / 1800 = 98.333% of theoretical speed. So slip is 100% - 98.33% = 1.67%.