Like anything that moves, motors occasionally need repair and refurbishment to keep them at their peak.
The Electrical Apparatus Service Association (EASA, www.easa.com) is an international trade organization of more than 2,150 electromechanical sales and service firms in 50 countries. Engineers from the association recently tested a series of motors in order to determine if rewinding motors could induce inefficiencies in the repaired motors and, if so, under what conditions. Their findings were compiled in a report titled “The Effect of Repair/ Rewinding on Energy Efficiency,” published in December 2003.
Five losses
The essential components of an AC induction motor are a stationary series of coils of wire. The stationary ones located inside the case are called stators, while the assembly of windings around an armature inserted through the center of the motor, and held in place by bearings, is called the rotor.
During testing, it was discovered that core friction and windage losses do not significantly change with motor load, provided the motor is operated from a fixed frequency. The power and stray load losses increase significantly as load is increased. Both core and power losses and particularly the rotor losses, may be higher when the motor is supplied from a variable-frequency inverter (also called a drive). In many cases, losses can be decreased during the repair process when good practice procedures are followed.
Core losses caused during repair can increase if excessive pressure is applied to the stator core (for example, by fitting a new stator frame with too small a bore). Damaging the interlaminar insulation—the very thin layer of insulation between each lamination in the stator and rotor core—can also increase core losses.
The stator power loss is often the largest component of loss. In motors of 45 horsepower (30 kilowatts) and above tested, the average stator power loss was 30 percent of the total loss. Consequently, anything that affects stator power loss can have a big impact on the efficiency of a repaired/rewound motor.
Stator power loss can be reduced by increasing the conductor cross-sectional area and/or decreasing the mean length of turn (MLT). Changing the winding configuration can also increase the stator power loss, although some changes, such as increasing the cross-sectional area, will reduce it.
Allowing the MLT to increase will increase stator power losses and therefore decrease motor efficiency. Conversely, decreasing the MLT, where possible, will reduce stator power losses to help maintain, or even improve, efficiency. The goal is to reduce the straight section of the coil to the minimum required to avoid mechanical strain. Whatever coil shape is used, make sure the coil end turns are no longer than those of the original winding.
Several factors can cause energy losses through friction. Among these are badly fitted bearings, excessive interference fits, use of incorrect seals, lack of seal lubrication, installing an incorrect replacement fan and/or over-greasing bearings.
EASA engineers also discovered that it is important to keep air passages clear—that is, the ducts and channels in the frame or core through or over which cooling air passes. Wholly or partially blocked ducts or channels may reduce friction and windage loss, but the reduced cooling effect will increase other losses—particularly stator power loss—much more. This can lead to early failure as well as reduced operating efficiency.
Doing a motor repair process well will maintain or even improve efficiency. A poor job will cost efficiency. It pays to know your repair company.
