Myford Capacitor problem?

Hi Chris,
It could also be the centrifugal switch on the motor or the contacts on the Dewhurst reversing switch if fitted. If you can photograph the connection box on the motor an also make a diagram of the connections from the cable to the motor and also which terminals the start capacitor is connected to. From that it should be possible to identify the terminals connected to the centrifugal switch.

Les.

Chris,

Your problem is almost certainly caused by the contacts in the Dewhurst switch that have been arcing and have eventually burned so much that they no longer function properly. I had a similar problem some time ago and found that two of the contacts had actually welded themselves together. Dewhurst switches are not suitable for frequent on/off operations and should only be used in conjunction with a direct online starter simply to provide a F/R function.

Eric

Dewhurst switches are not really up to the job as Eric has said, they should be used in conjunction with a direct on line starter which has overloads and no volt release. The Dewhurst should be used for altering the direction of rotation of the motor only. Select D.O.R of the motor then press the GREEN button. There are spare un used contacts on a Dewhurst which you may be able to utilise as replacements. Note how the links are positioned before dismantling, but how often do you or me want to run your lathe in reverse. If you have much trouble you may wish to consider obtaining a 3 phase motor and inverter, there are plenty of pre owned ones around, no need to buy an expensive new one. Like there are plenty of pre owned starter available .Ted

Disconnect the motor. Wire the motor to a length of flex with a plug on. Switch on.!

If the motor runs it eliminates a fault on the motor and points to the control side. - If it does not, then look for a problem at the motor side.

Try spinning the motor by hand with power on. (watch fingers.! - Probably best to take the belt off the pulley) 'If' it then runs this can sometimes tell you that the staring capacitor has gone south.

Nick

Hi Chris,
Using a multimeter on the ohms range will not give a definitive result. Before doing any tests short circuit the capacitor and disconnect at least one of its connections. (It might still have some stored charge and give you a shock.) When you connect a multimeter to a large value capacitor it should first show a fairly low resistance but the reading should increase with time. (Depending on the capacitor and the current from the meter.) It should eventually reach a reading of several meg ohms. Very often capacitors used for this sort of application have a resistor connected across the terminals to discharge them. This would mean that the reading would never reach several meg ohms. It would probably only reach about 470 K ohms. Even if your capacitor behaves like this it could still be faulty with a high internal resistance. If the corroded spade connector was directly on the capacitor it would indicate that the electrolyte was leaking from it. If your multimeter has an AC current range of about 10 amps you could connect it in series with the capacitor. On a WORKING motor you would expect to see a current reading for less than a second after applying power. Identifying the centrifugal switch connections would enable you to do a definitive test on it with you multimeter.

Les.

Reminds me of 'the good ole days' (30 years ago) when it was considered acceptable to discipline apprentices for doing something wrong. (something serious like forgetting to put sugar in the tea)

We would get a capacitor from an old flou fitting and clip out the bleed resistor. Then charge the cap with a 1000 volt Megger (insulation tester)

The next step I am sure you can guess.? But it resulted in squeaking and an improved ethos to get it right next time.

Can you imagine the kufuffle if that sort of thing was done today.? lol Back then it was considered 'normal'. But today I would imagine considered similar to walking into a bank brandishing a machine gun.!

Nick

To eliminate the possibility of a faulty capacitor simply disconnect the capacitor altogether and put a piece of wire in its place, the motor should run. Your local motor re winder will have a replacement capacitor should you need one or maybe a member of the local club will have a spare hanging about doing nothing. I seem to remember all sorts of dangerous practices taking place in the 1950/60 from people who should have known better, good sense now prevails in most cases.Ted

Chris,
I go back to my original request for a picture of the terminal block inside the motor connection box. With that information any of the members that have replied to your post will be able to suggest what tests to do using your multimeter. This could identify an open circuit winding, a short between the two windings or the centrifugal switch being faulty. I would not have dismantled the motor before doing the tests.

Les.

Chris,

Single phase current does not produce a rotating magnetic field from the run windings so a second set of start windings are provide at an angle to the main winding to produce a temporary two phase situation. The start winding must however be disconnected very quickly as it will burn out if energised for too long. The centrifugal switch at the back end of the motor cuts out the start winding at about 75% of full speed.

The buzzing and lack of rotation of your motor indicates the run winding is OK but the start winding is not operating.

From the results of your tests you have now isolated the problem to the start winding circuit. Either the winding itself, its terminations or the centrifugal switch. I suggest that you disconnect and the isolate the start winding (usually yellow and blue on earlier UK made motors) in the terminal box and test for continuity. The action of the centrifugal switch is a bit more difficult to check but this can be done by running the motor up with a second good motor and testing the action of the switch with a meter.

Eric

Here's a photo as requested:

Edited By Chris Trice on 06/05/2014 15:50:47

The pink and green wires at the top go to the capacitor. The live, neutral and earth wires come in through the lower right to U1, U2 and Earth. The two black wires which control motor direction and go to "R" and (I think) "P" (the lower terminal).

The markings of the terminals of single phase motors can be a bit confusing.

Modern European motors designate the main run windings U1 and U2 with the start windings marked Z1 and Z2.

Older motors use A1 & A2 for the main windings, and start windings can be marked Z & T

The start winding (if is OK) will have a higher resistance that the run winding.

Eric

Hi Chris,
Start off by measuring the resistance between "P" and "Z2". You should get a reading of a few ohms. This will be the centrifugal switch in series with the start winding. If you get a high reading either the start centrifugal switch or the start winding are open circuit. I think the blue wire connected to "Z2" and the yellow wire connected to "P" go inside the motor. One will go to the one side of the centrifugal switch, the other will go to one end of the start winding. The other end of the start winding will be connected to the other side of the centrifugal switch. If you got a high reading between "Z2" and "P" (The most likely fault.) we will need to get to the connection point between the start winding and the centrifugal switch to identify which is faulty. Assuming these tests are on the motor NOT connected to the lathe control box measure the resistance between "U1" and "P" and "U1" and "Z2" This reading should be very high. (At least 10 meg ohms.) If you get a low reading there is a short between the windings. (This is very unlikely.) Did you check the centrifugal switch with your multimeter when you had the motor apart or just visually inspect the contacts ?

Les.

Thanks guys. I'm learning as we go. I probably won't get a chance to get my hands dirty again until tomorrow but I'll try the practical tests you suggest Les. I only looked at the centrifugal switch on the motor shaft visually and operated it manually. It seemed to be in good mechanical order. The contacts it plays against are on the edge of the board photographed. I cleaned the contacts up but didn't try electrically testing them. They're pretty basic and any damage I imagine would be fairly self evident.

Hi Chris,
Another failure mode has occurred to me. If the main winding was open circuit the symptoms would appear to be the same. The start winding would be powered (Causing the motor to buzz.) but it would not rotate. So I suggest also checking the resistance of the main winding. To do this measure the resistance between U1 and U2. It should be a few ohms.

Les.

Hi Chris,
I will be at Harrogate tomorrow (Saturday) so If you are going you could bring your faulty motor and I could have a look at it for you.

Les.