There are many ways that coils and insulation can become damaged in motors. Specific faults may differ between three-phase, single-phase and DC motors. Damaged insulation papers or bent coils can cause shorting between coils or to earth. Contaminants, abrasion or voltage surges may also cause grounding of the coils leading to burnt-out coils.
Overloading of a motor, or unbalanced voltage in three-phase motors, can lead to overheating and subsequent damage to insulation. The extreme imbalance that results from an open circuit on one of the phases can also cause severe damage to coils. This condition can be caused by a blown fuse, a loose connector or/and faulty wire.
A damaged motor will have a reduced efficiency, producing less mechanical power while consuming more electricity. It may also cause circuit breakers to trip due to current overload or residual current conditions. In the worst case it might not run at all.
MOTOR REWINDING NEEDED?
Before deciding whether to get a motor rewound, you need to establish that coil damage is the cause of motor performance issues. If your motor has completely stopped working or is nuisance tripping, there are a few basic checks to try first. (Note: these are guidelines and should not be used instead of training or advice from your supplier).
Start by giving the motor a quick look over. Check to see whether anything is obviously broken, whether there is any visible carbon deposits or other signs of burning and for any signs of contamination, such as water, oil, grease or rust. If possible, disengage the motor from its drive and try rotating the shaft by hand. At this stage, you should be checking if an object is jammed in the motor, preventing rotation, or whether the bearings have failed.
If these checks haven’t provided a clear diagnosis of the fault, the next stage is to disconnect the motor from the power supply and perform resistance measurements. Hopefully, the correct coil resistance will be written on the nameplate. If not, consult the manual or request it from the manufacturer. This can be checked by measuring the resistance across the motor’s main terminals. If the resistance is lower than expected, this indicates that the stator winding has a short. If the resistance is much higher than expected, then the winding may be broken.
The resistance of copper increases by about 0.4% per degree of temperature. Unless otherwise stated, the specified resistance will be given at 20°C. If your resistance measurement is taken at a significantly different temperature, you should correct it to give the resistance at the reference temperature. For example, if you measure the resistance at 10°C then the motor’s resistance at 20°C would be about 4% higher. Therefore, you should increase your measured value before comparing with the reference. If the motor has been running, the coil may be considerably hotter than the ambient environment. It is the temperature of the coil, not the environment, which affects the resistance.
Insulation resistance (IR) can also be measured, both between the windings and between the windings and earth. An insulation resistance of less than two mega ohm suggests that insulation is damaged. There are many causes of damaged insulation, such as broken wires, contamination, power surges, motor overloading and overheating. If the cause appears to be contamination, especially with water, then cleaning and drying out the motor in an oven may improve the IR. If these tests indicate damaged windings, then it might be worth considering a motor rewind.
THE REWINDING PROCESS
There are quite a few steps in rewinding a motor. These all need to be carried out carefully by a skilled technician, and good quality materials need to be used.
The first stage is to remove the old windings and clean up the motor ready for the new windings. Ties are cut and the old windings are pulled out. The old insulation papers are removed and carefully cleaned out. Finally, any remnants of insulation are burnt out of the housing using a gas flame or oven.
The new windings are then prepared away from the motor, on a spool. New insulation papers are inserted into the housing before the windings are carefully pressed into the housing by hand. The ends of the windings are tied off securely and the winding ends are soldered and insulated. 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. The complete process can be summarised as:
● Windings are removed
● Old insulation papers are removed and cleaned out
● Any remnants are burnt out of the housing using a gas flame or oven
● New windings are prepared on a spool
● New insulation papers are inserted into the housing
● The new windings are carefully pressed into the housing by hand
● The ends of the windings are tied off securely
● The winding ends are soldered and insulated
● The soldiered ends are tied off out of the way
● The windings are varnished
● The motor is reassembled and tested ready for use.
CHOOSING A PROVIDER
When choosing a supplier, price is always the most obvious consideration. However, all rewinds are not equal. Simply selecting the cheapest motor winder may not be the best course of action. In the long run, this could end up costing more in running costs.
It’s worth asking to see motors being rewound by the provider you are considering using. A good quality rewind will look tidy with the windings fitted well and the ends tied in properly. What is not so obvious from looking at the work is the quality of the materials used. You should discuss what materials will be used before work starts and compare this with what other providers are offering. You should expect a good quality rewind provider to be a member of the Association of Electrical and Mechanical Trades.
Nick Peacock of Rotamec says: “You should be able to tell the quality of a rewind by carrying out a visual inspection. The neatness of the insulation and the standard of the connections and tie-ins will also be apparent in a good-quality rewind. There are other differences that could affect performance, for example, the quality of the materials that are used could vary widely. However, these can’t really be checked or inspected – which is why it’s important to work with reliable companies.”
The decision on whether to get a motor rewound should be compared to the possibility of replacing the motor. Small, low-cost motors may be more cost-effective to simply replace, while for larger and more expensive motors there may be significant cost savings involved with rewinding. Lead time is also an important consideration, particularly if operations are halted by the faulty motor. It should be possible for smaller motors to be rewound within a day and this must be compared with the time to order a replacement motor. However, if the damaged motor is still functioning, then it may make more sense to continue using it while awaiting delivery of a replacement.