Get To Know The Understated Element Of A Motor: Field Coils

Knowing and understanding every component of your motor or generator is not easy; considering the many different parts like the stator and rotor.

Either way, whether you have or have not understood how the rest of the parts work – to begin your journey in knowing your equipment – you can start with field coils. If you are aware of the above-mentioned parts – there’s actually a connection between field coils and them (stator and rotor).

However, before we go into that – understand that field coils have the same importance as those parts. Which is why it’s necessary for you to be familiarised with the following pointers, so your equipment is always operating at its best condition.

How do field coils work

Known as an electromagnet where its purpose is to produce a magnetic field within an electro-magnetic equipment – usually referring to rotating equipment like a generator or motor.

The way field coils are created by winding insulated copper strip or wire all around laminated steel cores.

You can find various designs of field coils – for certain units like alternators and dynamos; their main task is to supply the excitation for a generator. Usually, field coils can either be found on the rotating component of the stationary part (stator) or the electrical machine (rotor).

For those smaller field coils (known as interpoles), they are usually placed halfway between a field coil and the following one, in the armatures neutral position – where their purpose is to smoothen the output. In summary, field coils come together with either the stator or rotor to help support the particular part and overall, the equipment.

Common faults behind field coil failure


To begin with, earthing is simply just the release of electrical energy from equipment into the earth using a low resistance wire.

In this case, the primary reason for earthing is when the taping surrounding the coil starts to wear – so the wire is left exposed and makes contact with the metal body of the equipment. As a result, electrical energy is supplied directly to the ground or turns negative.

One other way earthing happens is when there’s a buildup of carbon dust coming from carbon brush wear. However, with servicing like generator or electric motor overhauling – a thorough check will determine if only a steam cleaning is required to make the coils baked and bring up the IR (insulation resistance). Otherwise, the engineer will make the necessary arrangements to do a replacement.


Shorting is caused by the unsuccessful insulation between individual windings found within generator rotors.

Afterwards, there will be a large current flowing in the shorted turn. The heat produced due to this can cause the field coil to turn hot and burn out – or even melt the wire and break the circuit – breaking the current flow.


Though overheating can be caused by shorting – there are still several other reasons like vibration and abnormal noise.

At the end of either an occurrence of shorting or overheating – the same end-result applies; it is most unlikely for a repair to be possible. You will probably have to replace the coil completely.

Solution: Repair or rewind?

Only when the coil starts to heat up and melt, will you need to do a replacement.

Depending on the condition of the field coils and the severity of the damage, individual field coils can be taken out, rewound, repaired or replaced. Just let the engineer check thoroughly during the motor or electric generator servicing, so he’ll know which is the best course of action to take.

In most cases, generator or electrical motor rewinding will suffice if there’s no critical damage done to the equipment. Also, you should seek the necessary service by checking the brand of your equipment. For instance, there’s the Cummins and Siemens motor repair available – so, take a look at your equipment once more for the brand name.

That way, you’ll ensure that your equipment is receiving the best servicing option and it’ll be operating normally once more.

When you understand how one of the equipment parts work – it’ll make it much easier for you to spot any faults and schedule maintenance when it’s time. However, field coils are just one out of the many components – so it’ll take a lot more learning on your end to understand how your equipment really works.

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