When electrical motors burn out or become unreliable, it is often the coils and insulation between them that have actually been damaged. Although your first instinct may be to simply replace the damaged motor, it is often possible to have a motor rewound, replacing these damaged components. It may come as a surprise that this can be done for relatively small motors fitted to industrial machinery. The cost can be almost half of the purchase price for a new motor. This is a significant cost savings, but it can also take considerably longer, requiring either a temporary replacement or operations interruption.
Mechanical faults, such as damaged bearings or gears, are often responsible for faults in machinery. However, many faults can develop within motor windings, resulting in reduced efficiency or nuisance tripping. For example, short circuits can occur if the insulation papers between coils become damaged, contaminants are present or the coils themselves are bent. The shorting may be between the coils or between a coil and earth. If motors are overloaded, heat build-up can result in the damaged insulation papers. This can also be caused by unbalanced voltages in three-phase motors. The most extreme example of this effect is when a blown fuse or loose connection completely shuts off one of the phases.
Identifying Coil Damage
Motor rewinding is only a possible option if it is possible to conclusively demonstrate that a fault is caused by coil damage. Coil damage should be suspected when a motor has completely stopped or is causing trips due to current surges or residual current leaking to earth (RCD tripping).Initial checks to confirm coil damage should start with a simple visual inspection. Signs of coil damage include clearly broken coils, signs of burning such as carbon deposits or contamination from water, oil or rust. It may be that an object has simply gotten lodged in the motor, preventing rotation, or that the bearings are damaged. Disengaging the motor and turning it over by hand can be a useful way to identify this.
If a visual inspection is not conclusive, resistance checks can be used to identify any short circuits or broken coils. The motor must be disconnected from the power supply before performing these tests. Next, check the nameplate. If the nominal coil resistances are given, it will be useful. Otherwise, check the manual or call the manufacturer to obtain that information. The coil resistances can be measured across the main terminals of the motor. If the temperature is not approximately 20 °C, then you may need to correct for that. The resistance of copper increases by approximately 0.4 percent per degree C. If the resistance is low, the stator winding may have a short circuit. If it is very high, the winding may be broken. For example, if the coil temperature is 0 °C, the resistance would be 8 percent higher than specified.
Resistance measurements taken between the coils, and between the coils and earth, give an indication of how effectively the coils are insulated. Damaged insulation will typically have resistances of less than 2 megaohms. Possible causes of damaged insulation include contamination, power surges, motor overloading and overheating. If the fault is caused by water, drying the motor in an oven may restore the insulation without requiring rewinding.
The MotorRewinding Process
The motor rewinding process involves the following steps:
- Remove windings
- Remove insulation papers
- Clean housing
- Burn out remnants of insulation
- Prepare new windings on a spool
- Insert new insulation papers into the housing
- Press new windings into the housing
- Tie off the ends of the windings
- Solder and insulate winding ends
- Tie off soldered ends
- Varnish windings
- Reassemble motor
- Test
Motor rewinding starts by removing the old windings and cleaning the motor housing. This involves first cutting ties and ripping out the windings. Insulation papers are then pulled out. Any remaining insulation is burnt away, either using a gas flame or oven.
New windings are produced using copper wire wound onto a spool. The new insulation papers are fitted into the cleaned housing, and the windings are pressed into position.
The final steps in winding a motor are to twist the wires together and solder to connect the coils, tie them off and insulate. The soldered ends are tied off out of the way, and the windings are varnished. Finally, the motor is reassembled and tested ready for use.
It doesn’t always pay to select the cheapest rewinding service. Quality can vary in a number of ways. The quality of the work will be visually apparent from the neatness of the fitted windings and tied-off ends. Using good quality materials is also important but may not be so easy to spot.
Motor rewinding can save considerable money. However, it is unlikely to be cost effective for small and low-cost motors. For large motors, it can save as much as 40 percent of the cost of a replacement motor. Reduced copper use is also an environmental benefit. Although, even if a motor is replaced, the windings should ideally be recycled. The determining factor will often be lead time. If you have access to a good rewinding service, it may be possible to rewind a motor the same day, possibly even more quickly than obtaining a replacement. However, when a motor is still operational, it may be preferable to order a replacement and keep using it until it can be swapped out. At that stage, getting the old motor rewound and keeping it ready as a spare may be a good option.