What is rotor bar frequency?

As a loose rotor bar passes a winding slot, the magnetic force causes it to momentarily lift and then drop. The frequency, then, is the number of windings slots x RPM (Winding Slot Pass Frequency or WSPF). The number of winding slots will be between about 25 and 100.

How is rotor bar frequency calculated?

Loose or open rotor bars in AC induction motors are indicated by a 2 X line frequency (2 FL) sidebands surrounding rotor bar pass frequency and/or its harmonics (rotor bar pass frequency = number of bars X rpm).

How do I check my rotor bars?

The single phase rotor test is looking to see if there is any alternation in AMP draw. A broken rotor bar will cause the stator current to increase or decrease. The best way to perform this test is to apply single phase power to your motor. Only use low voltage over the course of several amps.

What is rotor bar?

The rotor windings are made up of rotor bars passed through the rotor, from one end to the other, around the surface of the rotor. The bars protrude beyond the rotor and are connected together by a shorting ring at each end. The bars are usually made of aluminium or copper, but sometimes made of brass.

Can a rotor bar problem affect torque and current?

Incipient rotor bar problems in some motors will not contribute significantly to overall vibration. Even though the effect on torque and current is evident, the contribution to vibration is not large enough to be definitive. Contributions to 2x Line frequency and 2x and 4x running speed can be difficult to distinguish, especially in 2-pole motors.

When do you replace a broken rotor bar?

From the spectrum we can see 1x harmonic running speed with pole pass freq sideband; this indicates a rotor bar problem (broken or cracked rotor). Electric & mechanic team replaced motor on April 6th, 2016. We did re-alignment with thermal growth 0.25mm in the vertical level (motor set to +0.25 mm). Result: Vibration OK!

What does low amplitude mean on rotor bar?

This symptom is not unusual and at low amplitudes – below 0.1 ips or 2.5 mm/sec – often means little more than an imperfection in the machine. In fact, it usually has more to do with a potential winding problem than rotor bar looseness (see the next page). Vibration at 2 x WSPF and even 3 x WSPF w/ sidebands at 2 x FLine.

How can you tell if a rotor bar is cracked?

This test is the most accurate and reliable. VIBRATION SPECTRUM ANALYSIS – This test can detect cracked or loose bars while under load. Under load, the vibration will modulate at a rate equal to the number of poles times slip frequency. With a broken rotor bar, the amplitude of the beat increases with load.