Frequently Asked Questions

FAQ Subjects:
Squirrel Cage Rotor Rebar/Rebuilding
Root Cause Failure Analysis
Dynamometer Testing
Winding Varnish - VPI versus Trickle Method
EASA Trade Association

Squirrel Cage Rotor Rebuilding - more FAQs - click here.
What is important to a proper rebuild?
Analysis and duplication of material alloys, Duplication of the end ring cross sectional area, Duplication of the bar size and shape. Changes in any of these items will affect the conductivity and resulting rotor current, the starting and running torques, and the weight and the inertia of the rebuilt rotor. The best course of action for repair is to maintain the OEM’s design and performance characteristics.

Root Cause Failure Analysis
Why bother with a detailed analysis – why not just fix what is wrong?
Because understanding the method of failure will allow you to prevent it in the future. If the cause of failure is external to the motor, the repaired motor will not have a long life unless the problem is corrected. Darby personnel have been trained through the EASA root cause failure analysis course. We feel an obligation to tell our customers what went wrong, and why.

Dynamometer Testing
What is the importance of Dyno testing?
Putting a load on the motor under controlled conditions allows us to measure the motor’s speed, torque, current, and voltage to verify that the motor meets nameplate ratings after it is repaired. This prevents any “hidden” problems from appearing later. Many progressive customers are demanding this service, and Darby has four dynos in house to accommodate them.

Winding Varnish - VPI versus Trickle Method
What does it mean? Why use either one instead of the old “Dip and Bake” method?
VPI stands for Vacuum Pressure Impregnation – an insulating method whereby a new winding is preheated, placed in a vessel and put under vacuum to remove air and moisture. Resin is then allowed to cover the windings, (still under a vacuum) forcing trapped air bubbles to expand and pop. Finally, pressure is placed over the resin to reduce the size of any bubbles that escaped the first two steps. Ideally, this method will “wet” every bit of the winding surface with insulating varnish, eliminating voids to insure corona free motor operation. This method is commonly used on form-wound coils and very large random windings.Trickle Impregnation utilizes controlled AC electrical current to produce heat and vibration in a new winding while impregnating resin is poured over and through the winding from both ends. This method uses less varnish than the Dip and Bake method, and saves cleanup time, since no resin is applied to machined fits and threaded holes.

EASA Trade Association
What is EASA?
The Electrical Apparatus Service Association is an international association of motor repair firms dedicated to the sharing of ideas, technology, and resources among its members. Over two thousand members are represented from thirty two countries. Many industry specific vendors are included as “Associate” members, and bring their new products and technologies to EASA’s national and regional meetings.

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Squirrel Cage Rotor Rebar Rebuilding:

Why are there so many odd aluminum bar shapes?
One advantage of the aluminum cage is that it give the designer the ability to tailor complex bar shapes for desired speed-torque characteristics.

Is a copper cage better than an aluminum cage?
Actually the aluminum cage can exceed copper in operating characteristics such as efficiency, power factor, load acceleration and stall time capability. However, each material has advantages in certain applications.

Is it ok to machine the outside diameter of a rotor?
No. The total magnetizing current is closely related to the size and concentricity of the air gap and should never be changed.

What are the signs of a rotor with open bars?
When a rotor has open bars, the motor experiences a loss of speed under load, reduced starting torque, and an increase in vibration.

Are the number of rotor slots equal to the number of stator slots?
No. The number of rotor slots are never the same as the number of stator slots nor are they simple multiples of each other. This ensures that there are no dead points where the rotor cannot receive torque when the motor starts.

Why are some rotor slots skewed?
Rotor slots are sometimes skewed from end to end to reduce noise and to smooth out torque variations.

Does a rotor experience the same problem with eddy currents as a stator?
Because the rotor alternates at a much lower frequency than the stator, eddy current heating is usually not an issue for concern.