It’s no secret that regular maintenance can prolong the lifespan of any electric motor. But when it’s time for repairs, many still believe that rewinding an electric motor will decrease efficiency between 1% and 5%. The basis for this misconception can be traced to the test results from five empirical studies performed between 1984 and 1992. However, only 52 smaller electric motors (most ranging from 3-20 hp) were repaired and tested across the five studies.
With no comprehensive study for electric motors over 40 hp, the flawed data was repeated across the industry until it became accepted as fact for all electric motors. That is until the Electrical Apparatus Service Association (EASA) and the Association of Electrical and Mechanical Trades (AEMT) combined resources to determine how repairs impact the efficiency of larger electric motors.
The EASA/AEMT Study of Motor Repair Impact on Efficiency
The EASA/AEMT study was comprised of three stages and included the following objectives:
- Determine the impact of rewinding/repair on motor efficiency.
- Investigate the effect of repeated rewinds on several motors.
- Determine if the impact of rewinding on efficiency is similar for low- and medium-voltage motors.
- Identify and compile a set of rewind/repair procedures that maintain motor efficiency.
- Determine if specific rewind/repair procedures could improve motor efficiency.
- Identify and compile a set of best practices for motor rewinds.
- Determine the effect on motor efficiency if motors are rewound without specific controls on stripping and rewind procedures.
- Evaluate the impact that physical damage to the stator core has on efficiency.
- Correlate the results obtained with the running core loss test and static core loss tests.
- Determine the effect of over-greased bearings on efficiency.
- Determine the effect on efficiency when motors are rewound using different winding configurations and slot fills.
The study used electric motors ranging from 50 to 300 hp for four years. Efficiency tests were performed in accordance with IEEE 112 Method B, which is more stringent than IEC 600034-2.
After compiling and analyzing the data sets, the study determined that the best rewind/repair procedures maintain motor efficiency within ± 0.2%. The multiple rewind stage of the study found that average efficiency improved by 0.2% when following the best repair/rewinding practices. In other words, the claimed 1-5% efficiency loss after repairs is officially debunked!
As a result of the study, here are the five best repair practices for shops and technicians to follow to maintain the efficiency of the electric motors they service and repair.
- Tightly control the burnout process to avoid temperatures that are either too low or too high and could negatively impact the motor. Burning out below 680° F (360° C) may not break down the varnish completely, which could require more force to remove the coils. This additional force could result in more mechanical damage, increasing stray losses.
- Minimize stray losses. When the endwindings are cut with an air chisel, the end laminations tend to flare, increasing stray losses as the coils are removed from the slot.
- Keep coil extensions as short as possible to properly control the mean length of turn (MLT) and reduce stator I2R losses. If necessary, reduce the straight section of the coil where it exits the slot to the minimum length required for the tracking length or the slot liner cuff.
- Do not change the winding configuration. A winding change from two-layer to single-layer wiring reduces the number of coils connected by 50%. However, any configuration change could increase I2R losses and stray losses. All winding conversions should be controlled by suitably trained personnel only.
- Proper lubrication is essential. Too much lubrication shortens the life of the bearing by increasing friction losses. Mixing incompatible greases, due to the variety of additives used for specific applications, also negatively affects the bearing. You can learn more about lubrication for rotating equipment and electric motors
- Increase the relative slot fill. This is one of the easiest ways to improve the efficiency of electric motors that initially used low or moderate slot fills. And remember, all winding conversions should be controlled by suitably trained personnel.
The study’s results are straightforward and conclusive. Contrary to industry acceptance, electric motor efficiency can be maintained during repairs/rewinding if the shop and technician follow the abovementioned best practices. Additionally, the study determined that multiple rewinds have no cumulative effect on motor efficiency.
Here’s the EASA/AEMT study link if you’d like to review the accumulated data and testing details. If you have questions regarding what this new data means for electric motor usage, repairs, or preventative maintenance plans, email the Hi-Speed Electric Motor Experts or call us at 800-713-0103.