Myford motor failure?

I've just been to use my Speed 10 and it won't start, I have not used it for 6 months or so. If I remove the drive belt it will run forwards but not backwards. If I replace the drive belt it won't run again.

Is the motor broken or is there something else I can check?

The motor is a standard Brook Compton Parkinson 50Hz 250W(?), is there a suitable alternative at all? I want to keep it single phase.

Any help appreciated,

Derry.

Hi Derry I'm not sure if the brook Compton motor has a start capacitor or more like I can't remember!!!! You could start by checking if you have, actually it may be a run capacitor again I'm not sure but anyway worth a check. Please ignore my comments if I'm talking out the back of my AC theory

Hi Derry,
It could be the reversing switch rather than the motor. When you say that the motor will only run forward with no load do you mean that in the reverse position of the switch it does not rub at all or does it run forward in both positions of the reversing switch ? Do you have a multimeter and can you access the connection box on the motor. If you can get a good close up photograph of the connection box on the motor. and list all the wires going to all the terminals it would help to work out how the motor is wired. I suspect (But have no evidence for this theory.) that the contacts on the centrifugal switch may be open circuit. With the above information (And possibly more questions.) I hope to get you to test if the switch is working. It would only take a few minutes for someone with electrical knowledge to work out what is wrong but it is quite difficult to talk someone else through the tests to identify the problem..

Les.

Hello Derry,

Does it make sounds of distress? From your description it sounds as if there is no staring torque worth speaking about, which in turn means the start winding is not connecting as it should. By distress I mean heavy humming..

I imagine it is a standard capacitor start motor with an internal centrifugal switch that disconnects the start winding once it is running. The main winding won't start it under any effective loading without that assistance.

It is quite likely the contacts on that switch are dirty, They arc on separation and then oxidise, add dust or dirt from standing and the insulating effect may well be enough.

Try squirting switch cleaner through the cooling slots at the cable entry end onto the centrifugal switch as a quick fix. Failing that you will have to strip the motor down and clean the contacts..You only need to open the motor at the cable entry end; do be sure to isolate it electrically first. On reassembly, a drop or two of oil on the end of the motor shaft would be good.

A capacitor start motor is recognisable by the round housing for the capacitor on the outside of the motor casing,. It is most unlikely that the motor has failed just in standing unused, or for that matter something has burned out when it is not used.

Regards Brian.

.

Thanks for the feedback.

Les, with the drive belt removed the motor will only run with the switch in the forward position. When moving the switch to the reverse position the motor does not run at all.

I can't get my hands on the motor again until next Wednesday but I have a multimeter so I'll use your thoughts to make more checks then.

Hi Derry,
From the above information I do not think the fault is just the centrifugal switch. I think some testing is required to establish if power is being fed to the main winding and the start winding terminals (By start winding terminals in this context I mean the start winding itself plus the centrifugal switch plus the start capacitor.) from the reversing switch. With the switch in the reverse position was there any hum from the motor to suggest that power was getting to at least one winding and if you spun the motor in this condition did it continue to rotate ?

Brian, My original theory was the same as yours suspecting the contacts had oxidised with lack of use.

Les.

I did find this link for a strip-down. Be interesting to see if it is the same as mine.

http://www.instructables.com/id/Dismantling-a-Brook-Crompton-AC-motor-from-a-Myfo/?ALLSTEPS

The writer also quotes resistance values for me to compare.

Hi Derry,
If your motor is the same rating as the 1/3HP one in the link then the resistance values should be about the same. My approach to fault finding is not to make any assumptions about terminal labels or wire colours. I just work out how it is wired from resistance readings. When first taking readings in the motor terminal box have the reversing switch in the centre off position. (I am assuming it is a Dewhurst or similar switch.) (DO NOT FORGET TO UNPLUG THE MAINS PLUG BEFORE TESTING.) I would not recommend stripping the motor down until the fault has been identified. If you would like my phone number so I can talk you through the testing then PM me. If you only visit the site where the lathe is located once a week then it could take a long time with questions and answers on the forum.

Les.

It sounds to me that the centrifugal switch is not closed or there is a loose connection somewhere . Colin

I suggest you phone Les. It can't be the centrifugal switch as it is used in both directions, so are the start and run windings, so that almost says the motor is OK. More like the dreaded "Dewhirst" reversing switch burnt contacts, they are just not man enough to be used as an ON/OFF. When you have sorted the Dewhirst get your self a proper motor starter with green/red buttons, with No Volt release. John/Ted

Yes I am coming to agree about the switch. It seems odd that both forward and reverse contacts are U/S but to be honest I cannot remember EVER having run the motor in reverse previously. We'll see.

Hi Derry,
IF the motor has never worked in reverse AND it is a capacitor start capacitor run motor THEN an open circuit centrifugal switch would fit the symptoms exactly. Can you remember if the motor has one or two capacitors ?

Les.

Never seen a cap start/ cap run fitted from new, on a Myford lathe, very unusual. Usually just cap start is sufficient, after all they start on next to no load. On cap start/cap run motors, the centrifugal switch is used in BOTH directions, it open circuits the larger of the two caps (60 or 70 Micro farads depending some what on size of motor) once the motor is up to speed. Then the motor continues to run on the paper type run cap only. Cap start/cap run motors, usually there are TWO cylindrical (tubular) objects attached to the motor, one larger than the other. The larger is a special AC electrolytic type, which is only in circuit for a brief period in time(short time rated) when the motor starts up. The smaller is a paper type, continuously rated and is in series with the run winding, which of course is in use all the time the motor is actually running. An electrician using his 500volt DC megger will carry out a rudimentary test on the smaller one for you. An ex fluorescent light fitting power factor correction capacitor will be OK as a temporary measure for a quarter horse motor. After all of the above, most of which is irrelevant, I still say have a good look at the reversing switch and, using a Myford be careful regarding running the chuck off.Ted

Hi John,
It was Derry's comment that the motor may have never worked in the reverse direction that made me think of the suggestion in my last post. The fact that the motor starts by itself under no load suggests that there must be some phase shifted drive to the start (auxiliary ) winding. If it had a run capacitor it would probably start on no load without the centrifugal switch being closed. The only other explanation I can think of is the start capacitor has gone low in value or developed a high ESR. A high resistance but not totally open circuit centrifugal switch would also explain the symptoms.

Les.

I can report that my motor is now fixed. It was a broken wire on motor terminal Z.

I need to change the motor operation in that today it would run FWD and REV with the drive belt removed. I could also hear the centrifugal switch operating in both directions. However if the drive belt was refitted then all the motor would do on applying power in either direction was hum.

After talking to Les I measured the resistance between the live and neutral mains plug terminals. It was 8.4 Ohms. Referring back to here ...

http://www.instructables.com/id/Dismantling-a-Brook-Crompton-AC-motor-from-a-Myfo/step5/Electrical-resistance-readings/

It should have been 4.2 Ohms or thereabouts. Now referring back to the website above 10.2 and 7.4 Ohms in parallel comes to 4.2 so in my case it would appear that one element was missing, perhaps the start winding?

On removing the motor wiring terminal access plate there was a red wire floating free. To confirm I had the correct terminal I removed the clamp nut on Z and a piece of broken copper wire fell free.

Reconnecting the red wire to Z and refitting the drive belt the motor now operates as it should in both directions.

Many thanks to all for your help.

Finally I should add there is no capacitor on my motor.

Derry.