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Strange Myford Motor Fault

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Neil Bennett 319/03/2019 15:27:07
3 forum posts

Hello,

My recently acquired Myford Super 7 is fitted with a MEZ motor, permanent capacitor (10µF 500 volts), 1/3 HP, 1400 rpm, with a Dewhurst reversing switch, which was all working fine until the other day. Now, drilling from the tailstock a 3 mm. hole in brass stalls the motor. The capacitor has been replaced, the Dewhurst contacts all cleaned up, and the wiring checked. The motor windings resistances measure 15 ohms (U1-U2) and 50 ohms (Z1-Z2). The problem is not due to belt slip, the motor itself stalls, stops turning, then starts again when the load is removed.

I'm mystified. What could cause that to happen?

Thanks, Neil.

JohnF19/03/2019 16:17:53
avatar
841 forum posts
95 photos

Hi I'm not an electrician and have only basic knowledge on these matters, however it sounds like the motor is running on the starter windings which suggests the centrifugal switch inside the motor is not working as it should. When you switch off the motor listen for a click as it slows down -- no click I guess means the switch has not triggered at startup.

The Super 7 was generally fitted with a 1/2HP motor, if it comes to the crunch and you need to replace it i would strongly suggest going to 3 phase from one of the suppliers herein -- I got mine from Transwave

Cheers John

Brian Wood19/03/2019 16:17:54
1930 forum posts
37 photos

Neil,

I know it sounds silly, but what depth of hole is it you are drilling?

Brass can grab on a dull drill and if the hole is deep and the grab well developed, that could slow down a motor. 1/3 HP is not very powerful, the motors Myford usually fitted to S 7 lathes were nearer 3/4 HP. To actually stall yours is unusual, at that drill size in particular and may be a sign that the motor is very tired and due for change

The resistances seen too high as well, starter windings are about 10-15 ohns with a main winding nearer 5 ohms

I hope that gives you a few pointers.

Brian

john fletcher 119/03/2019 16:45:49
515 forum posts

Hello Neil, by the test results you have some electric knowledge and the values are about right. So I think its the notorious Dewhirst. Can you SAFELY connect an extension lead direct to the motor using a choc block connector, by passing the notorious Dewhirst. That will prove the motor is OK or not.. Also was the capacitor a good one, it should be the correct type for continuous running. Definitely not a DC only electrolytic type. When you get thing up and running once again get yourself a proper motor starter with No-Volt release, it with have big contacts which can cope with the sudden inrush of current when a motor starts up. John

Simon Williams 319/03/2019 17:00:26
397 forum posts
65 photos

Sounds like the symptoms of a shorted turn within the windings, which is difficult to diagnose with a multimeter. Suggest it's time to take it to a rewind house and see what they make of it. If is a shorted turn it might be possible to detect excess current draw, but the no power out for whatever power input is a trip to the scrap pile.

It's likely beyond economic repair whatever it is, so buying a replacement is probably the cheapest - or at least the most cost effective - option.

Neil Bennett 319/03/2019 19:25:18
3 forum posts

Many thanks chaps.

The motor is a permanent capacitor type so doesn't have a centrifugal switch. If it comes to the bit I will certainly look at a 3/4 HP 3-phase kit from Transwave.

Tomorrow I will bypass the Dewhurst with a temporary extension lead and report back.

bricky19/03/2019 19:37:13
373 forum posts
47 photos

Check that the gear that is housed in the mandrel pulley has not come loose as it is only glued in.It will not slip until under load.I had the problem of my motor cutting out and back in again and this happened under high speeds and I thought it was belt slip but I noticed that it wouldn't hold whilst changing chucks with the lock in the pulley.I stripped out the mandrel and after cleaning up the gear I reglued it with locktite.That fixed my problem.Just a thought.

Frank

Jon Lawes19/03/2019 21:25:27
avatar
313 forum posts

Sometimes my ML7 would stall and it would turn out the contacts in the Dewhurst switch were grubby, and I think the increased current draw would cause a burn on the contacts. Cycling the switch usually cleared it. Cleaning up the contacts in the switch reduced it, and installing an inline switch to control the power and just using the Dewhurst for reversing cured it.

the artfull-codger19/03/2019 22:55:18
avatar
238 forum posts

Neil a 1/3 hp motor is too small for a super 7 it should be a 3/4 hp, there's nothing wrong with the dewhirst switches if they're used correctly, the problem arises when people use them to start & stop the lathe thus arcing the contacts instead of using it to put in forward/reverse first & then starting the lathe with the push button contactor,i've had mine for over 30 yrs with no problems at all,never even needed to clean the contacts.

Lambton20/03/2019 09:41:12
avatar
675 forum posts
2 photos

Neil,

You may not have a motor problem at all.

You may have some end float in the spindle allowing it to be pushed back into the front tapered bearing bush thus stalling the whole job. The design of the S7 headstock, when set up correctly, is to have absolutely no end float. but retaining free rotation.

Check by trying to push/ pull the chuck backwards and forwards preferably using a DTI to detect any movement.

Adjust the spindle very carefully according to pages 24 - 26 in the Myford handbook. It is essential to make the adjustments in the correct order. Study the instruction carefully before making any adjustments.

A correctly adjusted spindle should have zero end float with a very small clearance in the front bearing - just enough to allow free movement and for an oil film to form.

Eric

Neil Bennett 320/03/2019 14:31:39
3 forum posts

All your suggestions have been followed up with these results:

The drill is sharp, about 3 mm. in and not grabbing in the work piece. Just to be sure a running centre was fitted in the tailstock and gentle pressure on the brass had the same effect - stall.

Temporary wiring to the mains, bypassing the Dewehurst switch, yielded no improvement. The switch contacts were examined again to ensure no sign of arcing.

When the stall occurs the whole drive train stops, from chuck to motor, so the fault is not in the mandrel pulley gear.

A DTI on the chuck revealed 1.5 thou end float. Not excessive one would think, but there was a definite click-click as the chuck was heaved in and out. That was not expected when there should be an oil film over the conical bearing. The dash pot was full of oil. A little oil was drizzled down the back of the chuck whilst running after which the end float was a quiet 1 thou and the problem had disappeared. Thank you, Eric, for steering me towards that.

I bought the machine about three months ago after it had lain unused for a number of years. One thing that wasn't checked was the condition of the spindle bearing oil wick. Now that it's been removed it's apparent that whatever lubricant had previously been used had turned waxy so that my newly applied oil was not reaching the conical bearing which has dried out to the extent that the least inward pressure on the spindle caused it to lock up. A manipulative soak in thinners has dissolved the waxy deposit after which a soak in oil has restored the wick to pristine.

Thank you all for your interest and assistance.

Brian Wood20/03/2019 18:33:03
1930 forum posts
37 photos

Hello Neil,

I'm sure I'm not alone in thanking you for closing the circle and telling us what had caused your trouble---all too often such feedback is missing and we never hear the outcome of the collective 'wisdom' brought to bear on the problem

At the risk though of repeating myself, a motor upgrade to something more powerful would be a good move at some future date

Regards

Brian

Mikelkie20/03/2019 19:49:45
avatar
91 forum posts
12 photos

It sounds to me like the rotor lost one of it bars

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