Squirrel Cage Motor Rotors

National Electric Coil is experienced with the diagnosis and repair of squirrel cage rotors. NEC has the ability to perform all necessary tests and inspections, as well as manufacture needed replacement parts, including rotor bars and core laminations.

Broken rotor bars are by far the most common cause of failure for squirrel cage motor rotors, and this is directly proportional to the number of machine starts and time between them.

The three most common locations for failures are (1) inside the slot portion of the rotor; (2) at the point where the end ring joins the bar; and (3) at the point where the bar enters the core. The result is generally broken rotor bars.

These failures are the result of complex mechanical and electrical factors, but the thermal stresses almost always play a part. This is why the issue of motor starts and stops looms large, since thermal stresses can be a direct result of the number and frequency of the starts and stops.

These rotors are unique in that there is no infallible method to determine whether or not there are broken rotor bars with the machine assembled. However, after the rotor is pulled from the unit, it may be possible to visually spot cracks or breaks in the ends of the bars. Some units have retaining rings, and these must be removed before these visual inspections can be performed on the ends of the bars. If there has been arcing over a period of time, this will be evidenced by a discoloration of the core.

There are several ways to accomplish repairs to the windings of a squirrel cage motor rotor. Where a crack or break is found on the ends of just a few bars, brazing may provide a temporary repair until the unit can be fully rewound. However, once fatigue cracking is found in several bars, in is safe to assume that the others, which have undergone the same stresses, may soon show cracks or breaks. In that case, a full rewind is the only answer. The use of oxgen-free copper or silver bearing copper for the new winding, which will increase the life of the winding, may be appropriate, depending on the use of the motor.

National Electric Coil has the ability to redesign both the rotor bars and laminations, appropriate to the use of the motor and its condition of operation. Besides optimizing a motor's performance and enhancing its life, a redesign may also increase unit reliability and minimize future bar failures through the use of graded materials or other changes in the physical characteristics of the bars or laminations.