Carbon Brushes in Universal Motors
Why are the clothes in my washing machine still soaking wet when the program has finished?
When the motor in your washing machine stops rotating totally, on wash as well as spin, the first thing usually noticed by the person operating the machine is that the machine has failed to spin, and the clothes are still soaking wet, and, although replacing the carbon brushes does not always affect a repair, it is the least expensive option that you are able to carry out without the use of any test equipment whatsoever,see our help with fitting carbon brushes and if it doesn’t work then not too much financial expenditure is lost.
Carbon brushes are fitted in what are known as universal motors. They are known as universal motors not because they fit every situation but because they work on both DC and AC current electricity. This means they are ideally suited for use in washing machines because as such they are cheap to produce and the speed can be controlled easily and cheaply by electronic means. Carbon brushes come in many varied shapes and sizes sometimes they come as just the carbon brush or brushes and sometimes they come in plastic encapsulations which make them easier to fit and easier to change. The carbon brushes themselves are made from carbon and copper in various mixtures and proportions, the amount of carbon in proportion to the amount of copper determines the hardness and therefore the durability of the carbon brushes. The more copper the harder the brush, the more carbon the softer the brush, so the more carbon the quicker the carbon brushes wear away, the more copper the longer the brushes last. The amount of copper cannot be too excessive as this would cause the commutator of the motor to wear out faster so a balance has to be struck and this is down to the designer of the motor.
It could be that the motor is burned out or it could have indeed gone short-circuit which means one copper turn on the armature has connected itself to an adjacent copper turn due to a breakdown of the insulation on the copper windings of the motor armature. In this scenario, the motor is not going to last very long before it burns out completely, either that or the thermal safety fuse which is usually embedded in the field or stator windings of the motor is going to blow and protect the motor from actually burning. In this situation, the motor usually rotates, but with an arcing effect around the commutator, very similar to the arc of an electric arc welding torch, but this situation is not reversible and the motor needs to treated as if it were indeed burned out. None of these can be determined without specialist equipment, unless of course, the motor smells of burning, or, by taking the motor to someone who has this sort of test equipment in order to get it checked out elsewhere.