Knowledge Center

At Darby, we are committed to helping our customers reduce their total ownership costs and keep their equipment running longer without downtime. Our valuable training services ensure the customer’s maintenance team makes the needed repair at least cost and time. We invite you to browse through the frequently asked questions found below. If you cannot find an answer to your question, please do not hesitate to contact us as we are always available to help.

What is important to a proper rotor rebuild?

Analysis and duplication of material alloys, duplication of the end ring cross sectional area, and 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.

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.

What is the importance of Dynamometer 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 dynamometers in house to accommodate them.

What does VPI mean?

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. This method is commonly used on form-wound coils and very large random windings.

Why are there so many odd aluminum bar shapes?


Motor designers constantly tailor complex bar shapes in order to attain desired speed-torque characteristics.

Is a copper cage better than an aluminum cage?

Not always. An 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 the air gap 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.

Is the number of rotor slots equal to the number of stator slots?


No. The number of rotor slots is 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 to reduce noise and to smooth out torque variations.

Does a rotor experience the same problem with eddy currents as a stator?


The rotor alternates at a much lower frequency than the stator so eddy current heating is usually not an issue.