Can someone explain to me what slipping a pole pitch means in regards to generators?

The following paragraph is from some class notes I was revising:

'A generator could lose synchronism with the power system because of a severe system fault disturbance, or operation at a high load with a leading power factor. This shock may cause the rotor to oscillate with consequent variations of current, voltage and power factor. If the angular displacement of the rotor exceeds the stable limit, the rotor will slip a pole pitch.'

  • \$\begingroup\$ In a 2-pole generator, it would slip almost an entire revolution. In a 6 pole generator (3 cycles per revolution) it would slip 120 degrees. \$\endgroup\$ – Brian Drummond Nov 12 '15 at 12:36
  • \$\begingroup\$ This quote would be clearer if it had only said "slip a pole" instead of "slip a pole pitch". \$\endgroup\$ – Olin Lathrop Nov 12 '15 at 13:42

Imagine two magnets, sure you played with when kid. If they are oriented in same way they attract, in opposite they repel. Now, when you place both magnets in opposite orientation and leave, they will turn left or right to allign in same orientation, so this is how they slipped a pole pitch.
A generator will exert maximum torqe when rotor and stator fields are 90 degress out of phase (you can see the same with magnets), if the rotor passes beyond this limit the torque will reduce to zero at 180 degress (Torque=Tmax*sin(phi) ) and then the rotor will flip over, skip the pole with a very strong beat.


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