When an electric motor starts making a horrible noise, or it just stops, what do you do. Is it broken, is it repairable?
This article sets out to show you how to easily fault finding & diagnosis electric motors, (without a test meter). What to look for, when identifying an issue. Why it’s stopped, why it’s making a horrible noise, or why it’s not going very fast… 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:
* Electricity can Kill *
As with any electrical device…
Make sure the power is disconnected before examining by:
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 can start to melt. Burnt varnish & plastic stinks. The smell is often dependant on how hot the electric motor has got, (and for how long). As a result, the insulation may have broken down and there maybe shorts in the electric wiring?
If something has completely burnt out, this can often mean it’s too expensive to repair. However, if it’s only a slight smell, it can still be worth checking, (Just in case the motor has got a little warm)?
An electric motor in a bad state, may be unsafe to use… The insulation may have partly broken down and may start to fail under load.
Firstly, is it safe to run the motor? Can it be rotated by hand (without disassembling the motor and with the power off!)?
If the motor rotates, does the device sound a bit rough? Sound is a good indicator of where a fault may lie…
If you see smoke (or feel heat) it can be dangerous! The motor could be about to catch fire..!
Can you hear a noise? Noise can come from a number of places (in no particular order)…
Can make noise due to:
Bearings can be cleaned, or flushed out with a thin spray oil, then more oil or grease applied. They can be replace with new, (info on replacing bearings can be found in the following article – How to Measure & Buy a Replacement Bearing ).
The noise can also come from…
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.
Check to see if the brushes are worn? Worn brushes can cause additional sparking, erratic running of the motor, juddering, etc. This can also make additional noise.
Check if the commutator is worn or dirty (usually carbon dust from the brushes, or from sparking). The lack of electrical contact, or a rough, bumpy running surface can cause:
Can be caused by any of the reasons above, but it 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.
Sometimes the rotating speed can create vibrations (especially fan blades).
Things to look out for:
Has something come undone. Is it rattling around inside the casing?
Something bent, or not assembled correctly? It’s not unknown for a motor casing not to be correctly re-assembled (after being taken apart for investigation). Especially with power tools and their plastic casings that can flex prior to assembly. A slight misalignment when screwing back together can mean something starts to rub, misalignment of the brushes, etc.
Carbon brushes fitted to an electric motor are designed to wear out, (more expensive motors may not have brushes fitted). Brushes are very cheap, in comparison to the cost of an armature. If the brushes were made of a harder material, the commutator would wear out first. This would result in a much more expensive repair and quite possibly, a terminal fault.
Therefore, it’s always worth checking the brushes (if fitted).
The main task of carbon brushes, is to pass electricity to the motor armature.
Please note that you can get more expensive (and often more reliable) brushless electric motors.
Therefore, carbon brushes aren’t always fitted.
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 towards the armature by the spring. This ensures there is continuous electrical contact.
Just remember that…
If it works, it’s only a temporary fix. Replace the brushes as soon as possible, or more damage may occur…
Important! If the carbon brushes are worn, they can…
If the electric motor is allowed to run in this state, it could start running on the brush springs, damaging the armature?
A double edged sword… Sometimes the brushes can damage the armature, sometimes the armature can damage 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).
Check to see if the armature commutator is worn or dirty:
(or, it’s a Brushless Electric Motor?)
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 motors own momentum will keep it going.
It can be a simple fix to replace the starting capacitor.
Please note that not all electric motors are fitted with Thermal protection...
Electric motors can be fitted with thermal trip to protect the motor from overheating. They will often be placed near to the motor windings, where heat is generated.
The are often two types of thermal Protection:
Resettable thermal trips break the supply to the electric motor. The resetting process often requires you to wait for the motor to cool.
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.
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.
With ‘Gizmo’s’, all you can do is check for the usual electrical faults. Note that, 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 you’re not sure what you need, sometimes it’s a good idea to have a look? The list below may give you an idea of what’s available, what to look for, and how much the part might cost…
Carbon Brushes, Bearings and Electric Motor Parts:
|Carbon Brushes||UK Prices|
|Test Meter||UK Prices|