How to Fault Find & Troubleshoot Electric Motors

Last modified on April 12th, 2022

If an electric motor starts making a horrible noise, or it just stops, what do you do. Is it broken, is it repairable?

The article below, sets out to describe easy electric motor fault finding & diagnosis (without a test meter). What to look for, when trying to identify an issue. Why it’s stopped, why it’s making a horrible noise, or why it’s not going very fast… So hopefully, it will help you decide if an item is repairable, or destined for the junkyard!

Easy Things you can Check:

We can break the motor diagnosis down into four distinct groups:

Topics:

• Does the Motor Smell?
• Does it Make a Horrible Noise?
• Have you Checked the Brushes..?
• If the Motor Doesn’t Smell & the Brushes are Fine, What do I do?

More info…

• Video:
• Parts Info:
• Related Articles:

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Video:

Video giving hints and tips on fault finding an electric motor:

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Does the Motor Smell?

If the motor smells horrible, it’s usually a bad sign. It’s often because the motor has got too hot. The windings (wire) in an electric motor are covered in a varnish, the insulation is often made of plastic and has started to melt. Burnt varnish & plastic stinks. Dependant on how hot the electric motor has got (and for how long), it may be too late! The insulation may have broken down and shorts in the wiring may have occurred?

If something has burnt out, this can often mean it’s too expensive to repair. However…  It may be worth checking, (Just in case the motor was switched off before it fully overheated)?

Things to Check?

If it’s safe to run the motor…

Does it sounds a bit rough? If the motor rotates, sound is a good indicator of where a fault may lie…

• Is the motor starting to seize or is something rubbing?
• Visually check for any heat damaged areas. Follow your nose?
• If you have a test meter, check the wiring. Is there anything obvious?

Notes:

• The residual smell will stay for quite some time, (for weeks)…

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Does the Motor Make a Horrible Noise?

The noise can come from a number of places (in no particular order)…

• Bearings (or bushes):
• Dust & Debris:
• Brushes:
• The commutator:
• Electrical noise:
• Frequency:
• Loose parts:
• Alignment:

Bearings (or bushes):

Can make noise due to:

• Lack of oil or grease.
• Dirt ingress.
• Due to wear and tear, use and old age!

They can be cleaned, or flushed out with a thin spray oil, then more oil or grease applied. Or, they can be replace with new, (info on replacing bearings can be found in the following article – How to Measure & Buy a Replacement Bearing ).

Notes:

• Spray oils are often too thin and don’t last!
• Before applying oil or grease, try and determine whether oil or grease was the original lubrication used in the bearing when new.

The noise can also come from…

Dust & Debris:

Dust and debris can get in and clog up all sorts of places. It can block airflow for cooling, get into switches and speed controllers and cause the motor to rub and bind against the casing. In the case of ventilation fans, it can stop the fan blades rotating.

Clean out and make sure the motor can breathe (cooling). Prevent obstructions that slow or stop the motor from turning.

Brushes:

If the brushes are worn, they can cause more sparking, erratic running of the motor, juddering, etc. This can make additional noise.

The commutator:

If the commutator is worn or dirty (usually carbon dust from the brushes, or from sparking). The lack of electrical contact, or rough a rough running surface can cause erratic running, slow speed and additional noise.

Electrical noise:

Can be caused by any of the reasons above, but may also be due to impending doom! If there are any faults, or the insulation is starting to break down, the motor may start misbehaving, drawing more current, making more noise.

Frequency:

Some times the rotating speed can create vibrations (especially fan blades).

Things to look out for:

• Lots of sparking and additional noise.
• Loss of power.

Loose parts:

Has something come undone. Is it rattling around inside the casing?

Alignment:

Something bent, or not assembled correctly? It’s not unknown for a motor casing not to be correctly re-assembled after being taken apart. Especially with power tools and their plastic casings that can flex prior to assembly. A slight misalignment when screwing back together could mean something starts to rub, misalignment of the brushes, etc.

Notes:

• If it starts humming, that’s bad news. Switch off immediately.
• The motor may have jammed.
• This can lead to smoke and fire!

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Have the Carbon Brushes Worn Out?

• Carbon Brushes:
• A Quick ‘Weird’ Temporary Fix..?
• Don’t Leave it Too Late!
• Brush Damage / Armature Damage?

Carbon Brushes:

Carbon brushes pass electricity to the motor armature.

Please note that you can get more expensive (and often more reliable) brushless electric motors.
So, carbon brushes aren’t always fitted.

1. Small springs are often used to push the carbon brushes against the motor armature to make electrical contact.
2. Carbon is cheap. So, carbon brushes are designed to wear away, (rather than the expensive armature).
3. No one explains that… Carbon brushes are a consumable item (dependant on hours run).

Electric motor carbon brushes are often housed in brush holders. These allow the brushes to slide in and out (with the aid of a spring). As the brushes wear, they are pushed by the spring against the armature to ensure there is electrical contact.

Notes:

• You can get lots of different shapes & sizes of carbon brush holders.
• Sometimes the brush holders are:
  • Removable, (by dismantling the motor housing).
  • Are fixed, but the brushes are removed, (by dismantling the motor housing).
  • Or, there is an external cover that’s removed to gain access.

A Quick ‘Weird’ Temporary Fix..?

1. Before taking the motor apart. Try tapping the motor with a piece of wood…
2. This can sometimes dislodge the brushes when they’re worn. Allowing electrical contact to be made, for just a little longer…
3. It doesn’t always work, but is an old trick that was used on vehicle starter motors when they wouldn’t run (just to get you home).

Just remember that…

If it works, it’s only a temporary fix. A repair must be made as soon as possible, or more damage may occur…

Don’t Leave it Too Late!

If the carbon brushes are worn, they can…

• Cause excessive sparking on the armature.
• The motor may:
  • just stop.
  • It may start making additional noise,
  • the motor may run erratically,
  • and there could be reduced power.

If the electric motor is run in this state, it could start running on the springs, damaging the armature?

Brush Damage / Armature Damage?

A double edged sword… Sometimes the brushes can damage the armature, sometimes the armature can damage the brushes!

The Brushes:

If the brushes wear away completely, the motor can end up running on the springs. The brush springs are a lot harder than carbon and and can wear the commutator. In most cases the springs can short out and blow the commutators, causing complete failure. Therefore, don’t run the motor on the springs, (you can usually tell by the noise and excessive arcing).

The Sequence…

1. The brush(es) wear away.
2. The motor runs on the spring(s).
3. Increased noise.
4. The commutators to blow!

The Armature:

If the armature commutator is worn or dirty:

• The armature can wear the brushes at a faster rate.
• Additional arcing can cause increased brush wear.

Notes:

• If the armatures blown, there’s not a lot you can do, other than fit a new armature.
• It’s important to replace the brushes before they completely wear out (and is a cheap solution).
• Sometimes the brushes just happen to be nearly worn out, and the fault can be somewhere else! Therefore, it’s worth checking for other faults?

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If the Motor Doesn’t Smell & the Brushes are Fine, What can I do?

(or, it’s a Brushless Electric Motor?)

• The Wiring Should be Checked:
• Starting Capacitors – Will the Motor Run if you Help it Spin?
• Thermal Protection:
• Switches, Speed Control & Gizmo’s:

The Wiring Should be Checked:

1. Look for open circuits, (broken wires, loose cables) in the motor circuits.
2. If a plug is fitted, check the fuse and check for external broken cables (such as extension leads).
3. Test the continuity.

Starting Capacitors – Will the Motor Run if you Help it Spin?

If yes, then it could be a defective starting capacitor. The capacitor gives the motor an additional boost to get it spinning. Once running, the motor has its own momentum to keep it going.

It can be a simple fix to replace the starting capacitor.

Thermal Protection:

Please note that not all electric motors are fitted with Thermal protection...

Some electric motors are fitted with thermal trip to protect the motor from overheating. They are often placed near to the motor windings, where heat can be generated.

The are often two types of thermal Protection:

• Resettable:
• Non-resettable:

Resettable:

Resettable thermal trips break the supply to the electric motor. The resetting process often requires you to wait for the motor to cool.

Notes:

• It’s often advisable to check the motor over to try and find out why the motor overheated?
• Make sure it’s not a ‘thermal fuse’ as a thermal fuse will need replacing…

Non-resettable (a fuse):

Non-resettable thermal fuses are often quite small, so it may not be immediately obvious that the motor has one fitted.

Notes:

• They often have an ‘amp’ rating like most fuses.
• If the thermal fuse has blown, the motor won’t run.
• A new thermal fuse will be needed.
• Make sure it’s not a ‘thermal trip’ as a reset may be required…

Switches, Speed Control & Gizmo’s:

I’m using the term ‘Gizmo’s’ for any additional items, bells and whistles that may be fitted to an electric motor…

Switches, speed control and gizmo’s can go defective. There can be a number of reasons why, and the list is not exhaustive.

• Broken wires.
• Burnt out.
• Component failure.
• Dust.
• Old age, wear and tear…

Generally, all you can do is check for the usual electrical faults. However, electric motors generate heat and often have a fan to draw in air for cooling. These fans pull in dust and debris which can cause issues. If the motor is used in a process that generates dust, then there may be even more of an issue?

• Like water, dust can get into all sorts of places you don’t expect.
• If the motor is ‘in something’, it’s always worth checking there is adequate air flow for cooling.
• In the case of powered hand tools, beware of blocking airways with your hands.

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Buying Parts for Electric Motors:

If you’re not sure what you need, sometime it’s a good idea to have a look? The list below may give you an idea of what’s what’s available, what to look for and how much it costs…

Carbon Brushes, Bearings and Electric Motor Parts:

Item	Link
Carbon Brushes	UK – https://ebay.us/cb4GSd
	USA – https://ebay.us/G76O4P
	AUS – https://ebay.us/99GlEP
Bearings	UK – https://ebay.us/M31arH
	USA – https://ebay.us/psNjrB
	AUS – https://ebay.us/gpFsei
Bushes	UK – https://ebay.us/I58idr
	USA – https://ebay.us/s3y4UM
	AUS – https://ebay.us/VvTMaU
Armature	UK – https://ebay.us/6mrh6j
	USA – https://ebay.us/WRQAny
	AUS – https://ebay.us/9ym1YY
Test Meter	UK – https://ebay.us/HuZBxX
	USA – https://ebay.us/olyfrh
	AUS – https://ebay.us/hW13GS

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Related Articles:

How to Measure & Buy a Replacement Bearing:

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