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Rotor bar issues can and do occur in electric motors on occasion

June 23, 2005
(Motor Testing)

Rotor bar issues can and do occur in electric motors on occasion. Normally the issue relates to mis-application or improper operation of the electrical machine with rare instances of motor manufacturing defects. There are three types of rotor bar types: Cast aluminum-alloy; Cast copper-alloy; and, Copper-alloy bars. The variety of rotor bar shapes depend upon the application and design considerations for the motor, itself.

All rotor bars can fail due to extreme overloads, under voltage or too-frequent starts. The most common cause is frequent starting which causes extremely high electro-mechanical stresses. The heating and cooling causes thermal stress, the acceleration and deceleration causes inertia fractures and the magnetic fields cause mechanical stress. These stresses will eventually cause the rotor material to fracture or break. In addition, the expansion and contraction of materials can also cause looseness in the rotor slots resulting in noisy starting. Motors have number of start limits partly due to this situation.

Cast rotors have casting voids as a normal result of manufacturing. All cast rotors have some level of casting voids. (Note: The copper industry has announced that cast copper alloy rotors have fewer casting voids than aluminum alloy.) This condition has received a great amount of attention over the past several years as newer technologies are sensitive enough to identify casting voids easily. Only a small percentage of cast rotors are affected by this condition and dynamic balancing takes care of the primary concern of vibration. In severe cases, the casting void(s) may restrict or block current flow through rotor bars acting as a broken rotor bar. These are manufacturing defects that may, or may not, be honored by the manufacturer, depending on if the motor meets the nameplate of the motor.

Copper alloy rotor bars tend to break where the rotor bar exits the rotor laminations or at the joint between the rotor bars and shorting ring. These can occur due to mis-operation of the motor, such as too many starts or excessive loading and unloading, or due to poor braizing or welding of the bars.

The effect of a broken rotor bar is a reduction of torque and, in severe conditions, torsional pulses and vibration. Eventually, some broken bars may lift and damage the motor winding. However, the time from fault detection to failure can be a considerable amount of time. The detection of a broken rotor bar due to operation, as opposed to a casting void, should prompt a review of the operating conditions of the application.

Tip provided by Howard W Penrose, Ph.D. 

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