HOW POLE PASS FREQUENCY IS SLIP SPEED TIMES THE NUMBER OF POLES?

In vibration analysis, pole pass frequency is one of the important terms used during analysis of induction motors.

Pole Pass Frequency = Slip Frequency X No. of Poles of Induction Motor

Generally, many have doubts on why Pole pass frequency is Slip frequency times the number of poles in an induction motor and not running frequency times the number of poles. The answer can be better understood through the induction motor working principle.

How induction motor works?

In an AC induction motor, which is generally used in industries, the alternating current is supplied only to the stator windings. This produces an alternating flux field which rotates at the synchronous speed (speed of magnetic flux). The flux produced by the stator induces a current in the rotor windings as per Faraday's law of electromagnetic induction. The induced current in the rotor will also produce a flux around the rotor coils which lags the stator flux. The induced current in rotor tries to oppose the cause of its production as per Lenz's law, which is the relative speed between the stator field and rotor. This makes the rotor or armature to rotate and to try and catch up with the Rotating Magnetic Field (RMF) of the stator. But once the armature speed matches the RMF, there is no relative speed between RMF and armature, thus decelerating the armature due to mechanical losses, which in turn develops the relative speed again and the same process continues. Hence, always the armature speed will be less than RMF or synchronous speed, giving rise to slip.

Now back to our topic,

If we consider rotor or armature as stationary, the RMF can be seen to be rotating at slip speed. Hence the poles will be passing armature at a slip speed and not the running speed.

Therefore, pole pass frequency is the number of poles times the slip speed.

Use of pole pass frequency

1. To detect eccentric rotor with varying air gap: Increased amplitude at 2x Line Frequency with pole pass sidebands can be seen along with 1x vibrations.

2. To detect cracked or broken rotor bars: Harmonics of 1x with pole pass sidebands are observed in spectrum along with beating.