The answer is probably academic because since these machines are quite well made, by the time you get problems such as you relate the machine will have 'done its life work ' other parts may be worn and it will not be worth the trouble to obtain parts + cost unless you can cannibalise another machine.
READ ALSO THE SAFETY WARNINGS FURTHER DOWN
On the other hand if you are very interested in what is actually wrong and what to do if you were to fix it - keep reading.
If the cutter turns freely when power is off and the motor hums when power is turned on, there are several possibilities - mechanical and electrical.
The bearings may be worn and the rotor of the motor is not in the centre of the stator so the magnetic field is pulling the rotor to one side rather than rotating as it should. This can be checked by putting a small wooden stick into the hex drive hole at the bottom of the machine with the power off - you should be able to turn the stick and the motor shaft - next turn the power on and try and turn the stick - if you cant turn it the problem is bearings. If you can turn the stick and shaft, the problem is electrical.
To repair this (replace the bearings ) you would have remove the disposer and open the machine by removing the 4 long tie rod screws equally displaced just within the bottom circumference of the motor end plate / shield / bell (word depends upon terminology in your geographaphical location) Hopefully the bottom bearing is the one requiring replacement and it may not be necessary to further dismantle the rest of the machine. Check the bearing very carefully - there must be no sloppiness of the motor shaft in the bearing - also check that the motor shaft is smooth and not scored.
With bad luck and unfortunately more likely, it will be the top bearing which is worn, which will require further dismantling - check also to see that there are no signs of water ingress into the motor - the seal may be leaking which is why the top bearing became worn in the first place.
As I mentioned above, these machine are quite well made - by the time - years - these parts - bearings and seals - become worn the cutters - stationary and rotating - will probably also be worn - this will be indicated by the machine taking longer to grind.
ELECTRICAL
The motor comprises essentially 2 'windings' (coils), a running winding which receives power whenever the machine is on and a starting winding which operates very briefly, usually for less than a second to bring the motor up to speed - power is supplied to the starting winding via a starting switch, usually centrifugally operated, so that when the motor reaches speed, the centrifugal weights, (normally held in by springs so that the starting contacts are together i.e. providing a circuit) open or break contact. Check the linkages and springs and also that the silver contacts are in good condition, i.e. not pitted, if so, try and minimally 'polish' with the finest emery, just to remove pitting caused by sparking, like on car old ignition contacts. These machines reversed each time they switch on (in order to reduce likelihood of jamming if something was left in when the machine was switched off and to increase life of blades - this is accomplished by a reversing switch -'dragged' into the starting position by a spring a snubbber inserted through a n aperture bored diametrically through the shaft and acting upon the reversing switch plate. Check that the spring, snubber, plate and contacts are all in good order and functioning.
There is also a starting capacitor within the motor, generally secured to the bottom end plate. It is cylindrical and boosts the starting torque of the motor. Power is supplied to the motor starting winding via the starting switch, the reversing switch and the starting capacitor - if either are not providing a circuit the motor will not start. To check the capacitor, take an analogue style multi meter, set on ohms scale, disconnect the wires from the capacitor and place the meter probes on the capacitor, caferefully observing the meter at the moment the connection is made. If the needle moves slightly and then falls back the capacitor is OK. (To repeat this test first ‘short’ the capacitor with a screwdriver across the contacts to discharge it after it was charged by the meter battery) If there is no movement of the meter needle the capacitor is open circuit, if the needle moves right across and stays there the capacitor has a short circuit - in either case these conditions would prevent the motor starting and nescesitate capacitor replacement.
The final possibility is that the motor windings themselves are faulty . If the motor smells or looks burnt that would be self explanatory – an electrical or mechanical fault may cause this – for example water leaking onto the winding, the starting switch not disconnecting e.g. welded contacts etc although the overload switch should have saved it – but not always.
If either the starting or running winding are open circuit the motor will not start – you could check this out with a meter, but a partly shorted winding will also cause problems / magnetic imbalance / non rotation.
It would be very hard for an armature to determine this - you would need to know the correct resistance of the windings and or have other instruments such as a ‘growler’ with which you could check the motor - even very few electricians would have this type of gear - usually only possessed by specialised electric motor repair workshops.
But as I stated in the beginning, much of this is academic as it would probably not be worth spending the time or money.
Much of the above applies to a variety of waste disposal units and motors used in many other machines, but not all motors are the same.
Some waste disposers have an external reversing switch - esperscially useful for un – jamming , they may also use a magnetic coil starting switch which may also incorporate a reversing switch and some machines also have an air operated on / off switch connected by a small tube to an operating button ( but failure of this would not cause hum and failure to start).
If you open the machine take care where a capacitor is fitted – there is a slight chance it may have retained a dangerous charge / voltage – ENSURE IT IS SAFE BY PLACING A SCREWDRIVER ACROSS THE CONTACTS TO DRAIN ANY REMAINING CHARGE.
Take similar care with the stick in the drive shaft test - do not use the hex key to check this – if the motor starts suddenly - you may receive a nasty WHACK or WORSE ! In some cases you may be able to start the motor by twisting the stick fast enough - like starting an old engine with a rope or pull starter - BUT TAKE GREAT CARE. If your machine does not have a hex or even a screwdriver slot in the bottom end shaft the only other way would be to try pushing the cutter with the power on – not recommended for an amateur.
There are other tests you could make with or without a meter but they generally involve working live – again not recommended for an amateur.
Other motors you may come across may be different . For example domestic refrigeration compressors generally have 3 pins protruding through the shell - You would need a meter to check for continuity . The pins are generally arranged in a triangle pointed up – sometimes the reverse i.e. down . The pin at the point will be polarity + or – (common) connected to power via an automatically resetting overload whilst of the other 2 pins (on the same a horizontal plane) , one will be the running winding and the other will be the starting winding – there must be electrical continuity between all three contacts. Identify by measuring the resistance between all three terminals. Highest resistance is between start and run , second highest is between common and start and lowest resistance is between common and run. To run the motor, connect a mains test wire to common and the other wire to run. Connect another (the same polarity as connected to the run winding ) to the start terminal , preferably via a switch . With this latter switch in the on position , apply power and be ready to switch off the start switch within a second or less – the windings will tolerate longer but not recommended. The motor should start if OK. A compressor motor that has just been run may not re-start for some time after it has been stopped due to pressure build-up. The motor may not have the power to start it. This will be longer in a new motor in good condition – even 20 minutes or so until the pressure has’ balance’ . Some larger domestic refrigerator motors may also have a starting capacitor in circuit / series with the start winding. Some other types of motors may even have a capacitor in series with the run winding too.
Many small motors, such as fan motors do not have starting windings whilst some other motors have run capacitors which provide a bit more running torque than a motor without a capacitor but not as much starting torque as a capacitor start motor – because they do not need it.
These motors are often used in appliances such as dryers and dishwashers . Resistance of the two windings is equal . Power is applied to the common terminal and one winding directly and the other winding via the capacitor . This will provide enough power for the running and starting requirements of the machine in which the motor is used. The motor direction may be reversed by applying power to the first mentioned winding via the capacitor and the second winding directly from the mains. This may be used for example in reverse rotation of a washing machine or tumble dryer, or to use one pump for wash and empty in a dishwasher.
If you are repairing a motor with a magnetic starting switch ensure that when testing the motor or completing the job, the switch is in the correct position i.e. the contacts must be ‘open ‘ when the power is off – if the switch is inverted gravity will keep the contacts closed when the motor starts and the motor will experience ‘sudden motor death’ – burn out.
There are other unusual motors and set-ups. Most motors reverse direction by changing the polarity of the starting winding connections, but some motors have been reversed by changing the polarity of the running winding - for a good reason.
Good luck and observe the safety warnings.
Stuart Fox, Bondi Beach Australia.
Stuart Fox, Bondi Beach, Australia
May 2009
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