Is My AC Rotor Bad?

Surge tester motor windings large

Can I Rule it In or Out as the Cause of Failure?

Rotor bar problems are infrequent, in most applications. But, when they occur, they are easy to overlook.  A lot of time is wasted on diagnosis when a rotor bar issue is not obvious.  The guesswork about rotor bars being good or bad can easily and quickly be eliminated for a motor that can run.

There are about 15 different ways to check for fractured or broken rotor bars. If the motor can run, the Electrom iTIG surge tester is the quickest and simplest way to detect a rotor issue.

Overview of  Rotor Bar Failures – Primary Impact on the Motor.

  • What are rotor bars:  Bars in an AC rotor carrying the current induced in the rotor by the stator.
  • Most common reasons why rotor bars fail: Frequent starts of a motor, especially under full or excessive load conditions,  which will draw very high currents.
  • Rotor Bar  failure progression:

High currents generate high heat which expands the bars, the  joint between a  bar and an end ring  fractures,  in cast rotors porous bars or end rings rotors can also fracture, the now resistive joint or fracture heats up more, and eventually cracks.

  • With a resistive fracture, currents are diverted to adjacent bars increasing the current in these bars and causing localized hot spots. Worst case it will cause the rotor to warp or more bars to crack.

Secondary Effects Created by Broken Rotor Bars That Impact a Motor.

  • Broken bars can cause sparking, a serious concern in hazardous areas.
  • If one or more rotor bars are broken, the healthy bars are forced to carry additional current which leads to rotor core damage because of the higher temperature
  • Broken bars cause torque and speed oscillations in the rotor, provoking premature wear of bearings.
  • Large air pockets in die-cast aluminum alloy rotor windings can cause non-uniform bar expansion  which leads to rotor bending and imbalance.
  • As the rotor rotates at high radial speed, broken rotor bars can lift out of the and strike against the stator winding causing a catastrophic motor failure.
  • Rotor asymmetry (the rotor rotating off-center), both static and dynamic, could cause the rotor to rub against the stator winding leading to rotor core damage.

The End Result:  The motor may lose torque, have torque and speed fluctuations, vibrate and produce higher noise, generate hot spots and overheat, even arc and damage rotor laminations.

surge testerSolution:

The Electrom iTIG surge tester/winding analyzer has an accessory called the Rotor Bar Clamp (RTR-03). It  is connected to one phase lead of a motor and to the front panel of the iTIG. With the motor running, the iTIG will display the sinusoidal 60Hz power wave.

A stable wave means the rotor is good. If the peaks of the wave move back and forth then a rotor bar issue has been detected.  This peak movement is due to fluctuations in current as the damaged bar passes by the poles.  If a report is needed, the wave can be captured at two different instances in time, meaning two different positions of the rotor.

surge testerThis is a screen picture of a rotor with a broken bar taken with an iTIG. The waves are automatically stored and can be included in reports.

Article by Jacob Beck

Test Performed by the Electrom iTIG

  • Megohm (resistance) Tests
  • Hipot Tests
  • Surge Tests – No need to turn the rotor on assembled motors
  • Rotor Bar Tests (Open or cracked AC Rotors)
  • Step Voltage Tests
  • Polarization Index (PI) Tests
  • Dielectric Absorption Tests
  • Armature Tests (DC Rotors)
  • Form Coil Tests (with up to 2X max rated power)
  • Transformer Tests

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JM Test Systems rents Electrom iTIG surge testers and sells Electrom iTIG surge testers for all of your motor service maintenance needs. Let our Reliability Product Manager Bryan Buhler assist you with your motor service issues. Call us today for a quote at 800-353-3411