Field Coil Repairs and Rewinds

Making up the magnetic field within DC motors and generators, field coils are essential to the proper performance of rotating equipment – a motor will not run in the event of coil failure.

Mawdsleys provides a complete field coil repair service for all makes and models of DC motors and generators, our in-house workshop is also fully equipped to replace existing field coils with new ones should they be beyond repair.

Complete Field Coil Services

☏ 0117 955 2481

✉️ enquiries@mawdsleysber.com

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What Is a Field Coil?

A field coil is an electromagnet used to generate a magnetic field in rotating electrical machinery. Coils are used in pairs which are connected to give north and south poles around the circumference of the frame, these coils are made by winding insulated copper wire or strip around laminated steel cores.

The image above shows a DC motor with 4 main pole coils and 4 interpole coils, the larger main poles provide the magnetic flux and are connected to a field supply, the smaller interpoles then assist with the commutation of the motor and are connected in series with the armature.

Common Reasons for Field Coil Failure

Field coil failure or damage will inevitably lead to motor underperformance or failure. Common reasons for field coil failure within DC motors and generators include:

• Earthing – Earthing generally occurs when the condition of the taping around the coil becomes damaged, leaving the wire exposed to the metal body of the motor or generator. Another cause of earthing is carbon dust build up from carbon brush wear, in this scenario, a repair is possible as the unit can be steam cleaned and the coils can be baked in order to increase the IR (insulation resistance).
• Burn Outs – Overheating within a motor or generator can be caused by a number of factors including abnormal noise and vibration for example. When a field coil becomes too hot, it may burn out or the wires may melt, breaking the circuit and causing the motor to fail. In either of these circumstances, a repair is unlikely to be possible and the coil will need replacing.

Additionally, some faults may occur with the flexible leads that are used to connect the ends of the coils to each other and into the terminal box. These leads can be replaced if a clean connection can be made onto the coil without damaging the insulation.

Field Coil Rewind Process

In the instance that a field coil cannot be repaired, the motor or generator will need to undergo a rewind; this is essentially replacing the defective coils with new ones.

At Mawdsleys, we have a fully equipped state of the art motor winding workshop on site, with a winding department and purpose designed precision jugs enabling us to carry out rewinds to a high standard and with a fast turnaround on service. The rewinding process is as follows:

1. The coil is taken off the steel pole – the pole is retained unless damaged.
2. The new coil is wound onto either a former or directly onto the pole which as been pre-insulated. High grade enamelled copper wire or strip (sometimes called magnet wire) is used.
3. Our coil winding machine counts the turns of the wire which are wound under tension.
4. The coil is dipped in a suitable grade of motor winding resin to the correct class of insulation.
5. The coil and pole assembly is re-fitted into the frame and connected.
6. The coils are tested for resistance and insulation resistance before the motor is rebuilt.

Nationwide Field Coil Repair Experts

If your DC motor or generator is experiencing underperformance or complete failure, Mawdsleys can help. We provide a nationwide service to our clients, offering a fast turnaround on service. For more information, call one of our team today on 0117 955 2481.