Myford ML7 motor possible fault.

[Alan Reynolds]

Hi chaps. My ancient Myford started to refuse to run and just hummed. Sometimes it would go but if I turned it off, it sometimes would just hum when I turned it back on.
Anyway, I got fed up with it and pulled it out. I stripped the end out and found the start switch contacts were more than a bit burned.
I cleaned them up and reassembled it and out of curiosity, measured between earth and the four terminals on the motor.
Two measure zero ohms to the casework. Now, I didn’t measure it before I took it apart. It’s all back as was but does anyone happen to know if one end of thefield winding and run winding are grounded.
It’s a reversible, non start cap Brooke Crompton.
I didn’t want to try it in the dark and pull down the National Grid network and suffer the wrath of ‘er indoors!

 

[6ply]

I have tested many motors in the past and never found one with a grounded winding (apart when the insulation failed).

 

[Martin-Sy]

No, the windings should not have any connection to ground. They would trip a leakage trip if they did. Its usually a good test to check the insulation to ground on an unknown motor. Should be in the order of Meg-Ohms.
Are you sure you didn't trap a wire on re-assembly?
Its possible to renew the start contacts, but if the windings are shorted to ground it might be time for a new motor.

 

[25.4_mechanic]

Hi chaps. My ancient Myford started to refuse to run and just hummed. Sometimes it would go but if I turned it off, it sometimes would just hum when I turned it back on.
Anyway, I got fed up with it and pulled it out. I stripped the end out and found the start switch contacts were more than a bit burned.
I cleaned them up and reassembled it and out of curiosity, measured between earth and the four terminals on the motor.
Two measure zero ohms to the casework. Now, I didn’t measure it before I took it apart. It’s all back as was but does anyone happen to know if one end of thefield winding and run winding are grounded.
It’s a reversible, non start cap Brooke Crompton.

No, the windings should not have any connection to ground. They would trip a leakage trip if they did. Its usually a good test to check the insulation to ground on an unknown motor. Should be in the order of Meg-Ohms.
Are you sure you didn't trap a wire on re-assembly?
Its possible to renew the start contacts, but if the windings are shorted to ground it might be time for a new motor.

The low-resistance windings against ground do not sound good.

After checking the correct assembly and ruling out all possible errors, it is probably time for a new one.

Electric motors have become better and much cheaper, and a modern one would no longer have the open contacts in the oil and chip flight line of the ML7 and should have jacket cooling, no swarfs in the inner air stream.

Look for a Dahlander circuit for two speeds in both directions without external electronics (depending on your supply voltage), i.e. positions L2-L1-0-R1-R2.

You will love it, good success!



Carsten

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[Agroshield]

Please excuse the obvious question: what does the screen of your meter show when you hold one probe in the left hand and one in the right hand without them touching each other? What does it show when you put the probes one each end of a metal ruler? Can you find something of a non-zero resistance and see what it reads (e.g. 50m extension lead)? What does one end to the other of each winding read? The start winding I think should be a little less than the run winding.

I am trying to eliminate the possibility that the meter is reading 'open circuit' (my own one displays 0L when this is the case) and you are misinterpreting the display. Also my meter has a 'hold' button on it that has caused me confusion numerous times.

 

[Guineafowl]

I was once driven nearly round the bend on another forum, trying to help wire up a motor with no labels. The chap kept insisting there was 1 ohm, where there shouldn’t have been.

In the end, he took a picture - meter reading overlimit “1. “ was being read as “1”

^^^If not the case, then an earthed winding after a stripdown might be a long screw placed where there should be a short one, or a wire trapped on reassembly. Strip it down again and re-test.

 

[Pete.]

What size motor is it Alan? I have a small 5/8" shaft motor you can have if it gets you out of trouble.

 

[Alan Reynolds]

Please excuse the obvious question: what does the screen of your meter show when you hold one probe in the left hand and one in the right hand without them touching each other? What does it show when you put the probes one each end of a metal ruler? Can you find something of a non-zero resistance and see what it reads (e.g. 50m extension lead)? What does one end to the other of each winding read? The start winding I think should be a little less than the run winding.

I am trying to eliminate the possibility that the meter is reading 'open circuit' (my own one displays 0L when this is the case) and you are misinterpreting the display. Also my meter has a 'hold' button on it that has caused me confusion numerous times.

One of the reasons I dislike digital meters. I use an analogue meter as well if I’m unsure of spurious readings. I did go though all the ranges from 20meg to ohms on the digital meter.
I was wondering if the “short” that I see, would not exist when current is applied and the resistance rises with the inductance when the motor starts.
I’m going to start it and if it runs, put the old neon tester on the bed and see if it glows!

 

[Alan Reynolds]

What size motor is it Alan? I have a small 5/8" shaft motor you can have if it gets you out of trouble.

Ooh thanks. I’ll let you know if my second attempt results in the motor genie appearing!

 

[Guineafowl]

I was wondering if the “short” that I see, would not exist when current is applied and the resistance rises with the inductance when the motor starts.
I’m going to start it and if it runs, put the old neon tester on the bed and see if it glows!

If there’s a DC short, starting the motor won’t make it go away. I wouldn’t…

Another stripdown and check would be the way to go.

 

[MoreWellie]

refuse to run and just hummed

I am ignoring the resistance questions. The usual cause of a motor doing this is the start or run capacitors failing (some motors just have a single capacitor) If this is the problem and you can safely flick start it whilst it is humming then the motor usually runs but is more easily stalled than usual

 

[brightspark]

What size motor is it Alan? I have a small 5/8" shaft motor you can have if it gets you out of trouble.

there normaly fitted witha 3d hp motor from factory but you can fit a bit bigger

 

[Agroshield]

The usual cause of a motor doing this is the start or run capacitors failing (some motors just have a single capacitor)

His motor has no capacitor at all. From his OP: "It’s a reversible, non start cap Brooke Crompton."

 

[MoreWellie]

It’s a reversible, non start cap Brooke Crompton.

so he did, except I read the first bit, bounced up and down and said I know this one

 

[Guineafowl]

His motor has no capacitor at all. From his OP: "It’s a reversible, non start cap Brooke Crompton."

Resistance start. As I understand it, an oldish design, using an aux winding that is thinner (higher resistance) than the main, thereby reducing the Q and causing a phase shift. Wouldn’t be surprised if that had popped open.

 

[Alan Reynolds]

Well I stripped it down again. No wires trapped. The winding innermost on the first picture measure 15 ohms and do not connect to the casework. I presume this is the run winding.
The outer two, red and black, measure 7.5 ohms and each wire measures just over 3 ohms to the casework.
I find it difficult to believe that cleaning a dirty contact on the start switch (middle picture) would have caused a fault.

 

[Guineafowl]

Run winding would be the lower resistance (7.5 ohm) one.

Is there an RCD somewhere in the supply to this? It really should have tripped if so, assuming the machine is properly earthed.

I think the short to case is part of a general insulation fault that could have been the cause of the symptoms, and not as a result of your switch cleaning. Time to rewind or scrap the motor, I’d say.

 

[Alan Reynolds]

Run winding would be the lower resistance (7.5 ohm) one.

Is there an RCD somewhere in the supply to this? It really should have tripped if so, assuming the machine is properly earthed.

I think the short to case is part of a general insulation fault that could have been the cause of the symptoms, and not as a result of your switch cleaning. Time to rewind or scrap the motor, I’d say.

It is on a separate circuit with rcd. The contact was open circuit on the start switch, well over 20 megohm until I cleaned it.
That red wire (15 ohm) at the top goes to the other contact on the start switch and gets disconnected by the bib weights when the armature gets up to speed. So I presumed that was the start winding.
I’ll give it one lat try tomorrow. Hopefully won’t end up like the cat. If you don’t hear from me……………

 

[Agroshield]

I think the short to case is part of a general insulation fault...

I wonder if it is a combination of filth and damp that has exacerbated the problem. Clean the motor very well (hoover, blower, brake cleaner, blower again) and leave in the airing cupboard for a couple of days (or a 40 degree oven overnight). Test resistance again. If now OK, perhaps revarnish the windings.

 

[arther dailey]

if you use one of those plug in trips designed for use with lawn mowers etc you will only trip that rather than anything else , they are perfect for testing stuff