Can the original Motor Efficiency be maintained when it is rewound?
A common misconception is that a rewound motor will lose efficiency during reprocessing. In fact, the efficiency of the motor after being rewound is totally dependent on the procedures followed during the rewind process. It is critical that proper procedures are followed!
When properly processed, motor efficiency can be maintained after a rewind.
A recent study undertaken by EASA / AEMT looked at 10 motors ranging from 40 HP to 75 HP that were processed thru EASA AR100 Accredited shops and the motor efficiency was measured before and after being rewound.
Motor efficiency of the processed motors was found to be virtually unchanged as follows:
– Overall average efficiency loss of entire group of 10 motors was 0.1 Percent!
– Average efficiency change for the entire group of 10 motors fell within the range of accuracy for the test procedure (+/- 0.2%)
In this article we will focus on two rewinding processes that are crucial to maintaining motor efficiency – removal of the old winding from the stator (commonly done thru a burn out procedure) and the actual rewinding of the stator. Let’s take a closer look at these two processes.
In order to remove the existing winding it is heated by an external source to burn off the insulation to facilitate removal of the winding. In order to maintain proper magnetic properties of the stator laminations it must not be heated to temperatures above 700 Deg F . – this is crucial to maintaining motor efficiency.
Some motor rewind facilities will utilize a flame burn off of the insulation as shown below. Note that a gas fired flame can reach temperatures as high as 5000º F as seen in the table on the right, resulting in a major loss of efficiency.
Testing the winding is critical to making certain a rewind is as good as the original winding. This testing includes winding resistance, phase balance / surge comparison tests and a ground test.
Many motor shops will utilize a burn off oven to maintain better temperature control during burn off.
During the burn out procedure, as the winding insulation is heated it will ignite and give off additional heat (exothermic reaction). This can cause rapid heating of the stator subjecting it to temperatures well above 700 degrees.
To prevent this temperature run away a very accurate temperature control of the process is required. It utilizes a second thermocouple, placed on the stator being processed. This is connected to a second temperature controller that will utilize a water spray mist directed on the stator to keep the temperature within the proper range should the temperature run away.
However, not all burn off ovens have this feature. The burnout oven used by The Flolo Corporation, similar to the one pictured above, does have this feature
.Actual Rewinding of the Stator
To ensure the rewound stator maintains full efficiency the following steps must be taken to utilize the same wire gauge and number of turns as originally supplied (or utilize a combination that performs as well, or better than the original). And maintain copper cross sections throughout the stator slots and keep motor end turns as short as possible.
Testing the winding is critical to making certain a rewind is as good as the original winding. This testing includes winding resistance, phase balance / surge comparison tests and a ground test.
There are additional procedures undertaken to ensure a rewound motor performs as well as a new motor and maintains the same Efficiency rating.
Always look for a motor shop that carries EASA AR100 Accreditation verified by the logo on the right. This is your assurance that the motor shop follows EASA guidelines to ensure motor efficiency is maintained – and you have a motor rebuilt to the highest standards of the industry that maintains peak efficiency and reliability! If you have an interest in seeing first-hand our repair facility and Flolo repair procedures contact your sales representative to schedule a tour.
The in-house technical specialists at the Flolo Corporation have extensive knowledge of proper motor rewinding procedures and how to maintain motor efficiency. Additional information on how to best repair motors is covered in extensive detail in classes available thru Flolo Tech. Please contact your sales representative for additional details.
