How many rotor bars does a motor have?

Check the content and you will find the statement that typically the number of rotor bars falls between 45 to 55. In addition, on the last page you’ll find a chart of motor condition vs. sideband amplitude as compared to the fundamental (decibel).

What is a motor 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.

What causes broken rotor bars?

1) Natural Degradation

Motors rely on movement in order to work. All of that movement leads to friction within the components. That causes natural wear and tear in the different elements. With enough friction, the rotor bars eventually give way.

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.

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How is rotor bar frequency calculated?

Vibration Analysis

  1. Sync speed = 1495.
  2. Slip freq = Sync speed – Rpm actual = 1495 – 1488 = 7rpm or 0.1167 Hz.
  3. Pole pass freq = Slip freq * Number of poles = 0.1167 x 4 = 0.467 Hz (Match with sideband 0.46 Hz)


Why are rotor bars needed?

You may have heard of the different parts making up your typical electric motor – including the squirrel-cage rotor bars. … From preventing power losses to providing motion – it boosts the efficiency of a motor just like the other essential components within.

How do you check AC motor rotor?

HIGH CURRENT ROTOR TEST – This test is performed by applying high current through the shaft of the rotor (rotor out of stator) and thermally scanning (infrared or thermoscan) the rotor O.D. searching for shorted laminations. These shorted laminations cause localized hot spots which cause uneven heating of the rotor.

How do you check for broken rotor bars?

An estimate of the number of broken rotor bars can be determined by comparing the sideband amplitudes. VIBRATION SPECTRUM ANALYSIS – When you want to detect cracked or loose bars while under load, you can use this test. Under load, the vibration will modulate at a rate equal to the number of poles times slip frequency.

Why rotor is short circuited?

When we give supply to the stator of motor, flux produces due to stator current. This flux cuts the rotor conductors and an emf is induces in the rotor circuit. But for flowing the current, circuit must be completed. So we short circuit the rotor conductors.

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How do I test my rotors?

How to Test an Alternator Rotor

  1. Remove and disassemble the alternator as indicated in your vehicle’s maintenance manual. …
  2. Visually inspect rotor shaft, slip rings and windings. …
  3. Connect one probe of the ohmmeter to a slip ring and the other to the shaft.

What causes 2x line frequency?

The magnetic flux produced by current-carrying conductors in AC machines alternates at line frequency (FL). Therefore all AC motors produce a 2 X line frequency vibration.

What is the formula of the rotor bar current?

What is the formula of the end ring current? Explanation: The number of rotor slots and the bar current along with the number of poles is calculated. … The maximum value range of the ratio of the total cross section of rotor bars to the total stator copper section for main winding is 1.6. 12.

What are rotor windings?

rotor winding is of the squirrel-cage type where solid conductors in the slots are shorted together at each end of the rotor iron by conducting end rings. In such machines there is no need to insulate the conductors from the iron.

What is meant by cogging?

The phenomenon of Magnetic Locking between the stator and the rotor teeth is called Cogging or Teeth Locking. … This condition arises when the number of stator and rotor slots are either equal or have an integral ratio.

How many types of motor tests are there?

Most of these fall under one of two categories: online or offline testing, or static or dynamic testing. A good predictive maintenance program typically uses both. Online dynamic testing is done while the motor is running. It gives technicians data on the power quality and operating condition of the motor.

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What is the range of current density in rotor bar?

What is the range of current density in rotor bars? Explanation: The minimum value of the current density in the rotor bars is 4 A per mm2. The maximum value of the current density in the rotor bars is 6 A per mm2.

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