Electrical Apparatus - March 2011

Electrical Apparatus – March 2011

This is a summary of the Electrical Apparatus March 2011 featured technical article,  by Richard L. Nailen, P.E.     

With the advent of variable-frequency solid-state power supplies, speed variation can be provided by the more reliable, less expensive squirrel-cage motor. Users occasionally inquire as to the feasibility of converting an existing wound-rotor motor to operate as a squirrel-cage design. One way is to short-circuit the slip rings and eliminate the variable-resistance speed control in the external circuit.

Although easily done, this often causes application problems. Inrush current becomes high, with starting torque so low that the motor may be unable to accelerate its load. And the rotor winding remains a potential source of failure caused by the effects of heat, vibration, and centrifugal force.

A conversion is therefore best done by replacing the rotor-shaft assembly with a simple “deep bar” rotor, based on what would be used in a squirrel-cage design for the parts involved. Some increase in horsepower may be possible because of the elimination of the rotor slot space taken up by insulation. However, the stator winding temperature limit remains. A complete engineering analysis is needed to evaluate any uprating. To maintain at least the original output rating, the new rotor must provide suitable ventilating air circulation. Fan blades may be added to replace whatever effect the former rotor winding extensions may have provided.

How much accelerating torque will be needed depends upon the load. Unless that is coupled through a centrifugal clutch, or an adjustable-speed drive is added to provided high torque, acceleration needs careful evaluation. A possible alternative is either a high resistance bar or double-cage rotor design; however, these tend to increase losses and heating as well as requiring a design study.

To order a back issue with the full article, “Converting Wound-Rotor Motors to Squirrel-Cage,” call 312-321-9440 or visit our online webstore.

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