AMA 250Vx750 fault

[Paul MercerParticipant @paulmercer13838]

Hello. After my lathe developed a fault earlier on today, I spent a few hours here reading some advice given to another member with a similar lathe issue.

I’ve had my lathe from new, about 15 years, and it gets light/medium use. Today, I was using it quite happily, and turned it off. When I restarted it, it tried to turn for a fraction of a second, then stopped and blew the circuit breaker. I reset the circuit breaker and upon trying to start it, the same thing happened. At that point to took to the internet, and this forum.

I thought it may be a Start Capacitor, so started to strip the lathe to find some specs for it. I was expecting to find a large capacitor near the motor, but found nothing, so presume it’s not that.

I found 2 circuit boards:
One is a DC motor speed controller ref: JYMC-220B-I. (I only know this because it says it on there.)

The second is an unknown circuit board. It has JD-013 REV B 070813 on it, and under that B43688. Research shows a similar lathe made by Grizzly who kindly have a similar part number and call this circuit board a Filter.

I don’t smell any burning on the circuit boards at all.

When I took the lathe apart, I closed the safety switches and tried to turn it on. The motor didn’t turn, but did make a buzzing noise, and the main chassis gave me a small shock. Figured that I’d mention that in case it made a different to anyone’s thoughts

Does anyone have any advice how I could start to find out how to repair this lathe?

Thanks in advance

[Paul MercerParticipant @paulmercer13838]

[JasonBModerator @jasonb]

I'd be looking for a loose wire, or swarf shorting something to earth.

[noel shelleyParticipant @noelshelley55608]

How ? Why did you get a shock ? Did you touch a point on the circuit board or is the earthing faulty ? If you have had the machine 15 years and not known it is a DC motor, what is your knowledge of matters electrical – should you be working on it or trying to repair it ? Since you expected to find a start capacitor can it be assumed that the motor is fixed speed – or is it variable ? PLEASE BE CAREFUL. Noel.

[mgnbukParticipant @mgnbuk]

Check the DC motor for an internal short to ground.

After prolonged use, carbon brush dust can build up & short the brush holders to the case or coat the brush holder insulating base & cause a short between brushes.

I experienced this several times on industrial installations. Usually poor maintenance of blower filters led to coolant or oil getting in to the motor & the brush dust stuck to it, though occasionally it was just a build-up of dry dust. Initial light shorts would sometimes burn off with a flash-over that the drive would ride out, or trip with an "instantaneous overcurrent" alarm that would not reappear after recycling power to the drive (industrial DC drives have much more sophisticated fault monitoring than hobby items). Eventually, though, a hard short would develop that would stop the machine & this required a clean-out of the brush area with a fast evaporating degreaser – a dirty & awkward job accomplished through access openings around the brushgear – and fitting a new air filter. The drives usually survived this though, occasionally, a drive would fail – handily protecting the expensive semiconductor fuses that were in circuit supposed to protect it !

Nigel B.

[Paul MercerParticipant @paulmercer13838]

Thank you for the replies.

I didn’t notice any swarf or loose wires, but will check again.

The shock came from the frame of the lathe itself – although all circuit boards were loose at the time, all earth cables were in situ. I wasn’t touching any electrical items at the time.

I’m ok ’ish’ with electrics, but don’t know much about motors or controllers. This is clearly beyond my knowledge though!

I don’t have an airline but will get hold of one of those pressurised air cans and give that a go. I’ll also get hold of some fast evaporating degreaser and give that a go.

Thanks for the advice, I’ll keep you updated on progress.

[John HinkleyParticipant @johnhinkley26699]

I don't know your lathe specifications but if you and others are referring to brushes – is it possible that they are simply worn down beyond their minimum length and require replacement?

John

[Les Jones 1Participant @lesjones1]

I think you have two faults. Getting a shock from the lathe metalwork indicates that it is not earthed properly. (Or you have become charged with static.) Check the resistance between the lathe metalwork and a known good earth.
For the motor fault connect an incandescent lamp (NOT a CFL or LED lamp.) of about 100 watts rating in place of the motor. The speed control should adjust the brightness of the lamp. Check that it works in the forward and reverse positions of the reversing switch if it has one. If this works without tripping an over current breaker or RCD then the fault is with the motor. If it still trips the breaker then the fault is probably on the speed control board.

Les.

[Paul MercerParticipant @paulmercer13838]

Here’s an update.

I double checked for swarf and loose wires, of which there were none.

I replaced the motor with a light bulb and the brightness changes with the speed controller which sounds good news! I tried it I’m reverse and again the lightbulb brightness changed.

I removed the brushes and gave it a good blasting with pressurised air. When turning the motor by hand, I noticed that there was a slight bit of darkening on one part of the commutator which reminded me of an arc burn. I cleaned up the brushes and reassembled. Upon starting the motor (outside of lathe) it ran, but sounded like it was sparking in the motor, looking up the rear end of it, confirmed that.
I took the motor apart and gave it a good blast of air, cleaning up the commutator and brushes. Upon reassembly, and starting the motor, it blew the circuit breaker again.

Electrical resistance on the motor was 0.9 ohms. Rotating the motor by hand the figure went up to 1.1 ohms momentarily every time it turned, the dropped back to 0.9 ohms.
Electrical resistance between each motor wire and the housing was 1 ohm.
Voltage on the motor wires increased when the motor was turned by hand.
Voltage between the motor wire and motor housing also increased when turned by hand.

I think I’ve (you’ve) narrowed the issue down to the motor. I couldn’t get and fast evaporating degreaser so that would be one last affront before I take the motor somewhere to be looked at.

Does that sound about right?

[JasonBModerator @jasonb]

How worn are your brushes after 15yrs use, the originals on my Warco 280 wore unevenly quite quickly, replacements as still going strong. Also check overheating has not softened a brush spring.

Worn overheated one on the right, ne won left

Edited By JasonB on 03/02/2023 17:08:32

[noel shelleyParticipant @noelshelley55608]

A dark area on the comm and arcing indicate worn/faulty brushes. The heat generated can melt or soften the plastic holding the brush holders – normally terminal and and out of round comm is likely to also be end game, though recutting the comm may work, again the heat ,arcing can destroy the insulation.

NON of this explains the shock – indicating poor or NO earth, unless as Les says it's static.

Where abouts are you ? Noel.

[JasonBModerator @jasonb]

"The shock came from the frame of the lathe itself – although all circuit boards were loose at the time"

If the boards were loose and flapping about part of them could have contacted the metal of the lathe

[mgnbukParticipant @mgnbuk]

Electrical resistance between each motor wire and the housing was 1 ohm.

That doesn't sound right – there should be no connection between either of the input connections and the motor frame. One of the usual tests carried out on a motor (completely disconnected from the drive & sat in isolation) is with an insulation tester & with a 500 or 1000v test I would expect a minimum of several mega ohms between input wires & motor frame. 1 ohm with a multimeter suggests a pretty hard short. You could try repeating the input wires to frame resistance check with the motor disconnected from the drive to eliminate the wiring.

It is usual for a DC permanent magnet motor to act as a generator when rotated. It is also usual to check the motor out with it disconnected from the machine. For further tests I would look to dismantle the motor, as it is difficult (or impossible) to access the armature, commutator & brush gear adequately with the motor assembled.

Probably cheaper to buy a replacement motor than take it to a specialist, who would probably not be able do do anything more than check it anyway – no spares other than brushes are likely to be available & a rewind would be prohibitive.

Nigel B.

[Paul MercerParticipant @paulmercer13838]

All electrical testing was done with the motor removed and disconnected. I’ll double check the resistance between the wires and the casing when I’m home from work tomorrow.

Jason, the spring on my brushes appears ok, but I’ll try and get some replacements. Current dimensions are 13.4mm X 5mm. The current length is 13.2mm. These appear to be quite large compared to replacement brushes I’ve seen online. Judging by the ration of others available, I’d guess these started off at 20-25mm

I’m based in Orpington, SE London/Kent borders.

Thanks

[KenLParticipant @kenl]

You may well have a short circuit in one of the rotor windings, this usually leads to sparking between brushes and commutator, impossible to repair economically. If you have a motor rewind shop nearby they might be willing to drop test the windings for you to detect any short circuits. Another sign of shorted windings is when the solder joints between winding and segments start to melt out due to heat buildup, you'll usually see spots of solder on the inside of the casing where it's spattered out as the rotor spins.

The first thing I would try though is to skim and undercut the commutator. As it's the lathe motor you probably won't be able to skim it unless you have access to another lathe. You could however just clean it off with some fine emery cloth and then undercut it by hand using a piece of broken off hacksaw blade whilst the rotor is mounted between centres. Pay particular attention to any areas of blackening between segments and make sure you cut the insulator down to clean material with no burning or blackening in the bottom of the slots between the segments.

There are videos on U-tube on how to do the job if you search.

[mgnbukParticipant @mgnbuk]

I have a Chester mini lathe on the bench, so did a couple of checks on that from a resistances POV as a comparison – not exactly the same motor as the OP's, but the same type & application.

This motor has not been used – I bought the machine at a Chester open day on clearance, mechanically complete but without it's electrical box fitted. It has sat on the bench ever since awaiting something doing with it – only around 20 years so far !

Resistance between the input wires was around 18 ohms measured with a Fluke 77 multimeter (which showed 0.3 Ohms with the leads shorted), which is probably more than a larger motor would be (IIRC the Chester machine is rated at "350 watts" ). Resistance from either input wire to the motor mounting stud (I can't get to the motor shaft where it is) with the Fluke was around 18 Mega Ohms.

As I also have an insulation tester (Center 360 autoranging meter) I checked resistance to ground with that at 250V (34 Mega Ohms) and 500V (33 Mega Ohms). I trust the insulation tester results more than the DVM, but they are in line with what I woudl expect for a "good" motor. Given that the motor has been sat for so long in a largely unheated environment, the readings may improve if the motor was warmed through.

I would be wary of just going at the commutator without confiming that the windings are good – largely a waste of effort if there is a short, as skimming the com won't fix that. While I have not had one of these specific motors apart, I did a repair to a similar motor from my sister-in-law's mobility scooter after my brother was a pillock and damaged it. The commutaor on this was decidedly "skinny", with thin segments that didn't give a lot to play with to skim up. Also the windings were staked, not soldered, into the segments. and neither were the windings impregnated. The whole thing just said "made down to a price" to me and, while I got it usable again, I recommended that he get another motor in hand for when this one inevitably fails. I don't see a budget lathe motor being made to much better standards & would best describe them as "disposable"

HTH

Nigel B.

[KenLParticipant @kenl]

Posted by mgnbuk on 04/02/2023 13:18:37:

I would be wary of just going at the commutator without confiming that the windings are good – largely a waste of effort if there is a short, as skimming the com won't fix that.

You're quite correct, skimming won't fix any shorts, but undercutting can.

Many years ago as an apprentice it was one of my jobs to regularly undercut very large DC motor commutators as part of the planned maintenance procedure. Reason was that carbon dust from the brushes would accumulate in the slots over time and sometimes cause a short between adjacent segments.Undercutting will remove the carbon debris and any "scorched" insulation so can often prevent short circuits and reduce sparking at the brushes, or at least that's what our foreman insisted. In any case it only takes half an hour on a small motor so well worth trying.

[Paul MercerParticipant @paulmercer13838]

I’ve been away for a few days so don’t now work on the lathe. I stripped the motor down and had another look at the commutator. The gaps on the commutator had a lot of dark residue built up, presumably carbon, and the copper edges seemed slightly burred over closing the gaps down at the surface. I gave the commutator surface a cleanup with a bit of fine emery and removed the burrs abs dark residue.

I checked the resistance and there appeared to be no shorts between any prt of the commutator. I put the motor together and using the old brushes checked resistance. No shorts between wires and housing which was good. I’m wondering if when I checked resistance before, I checked continuity between the housing and the wires??

I’ve spoke to the lathe producer and ordered another set of bushes. I’ll fit them when I’m home midweek, and give you an update.

[jimmy bParticipant @jimmyb]

I had a (slightly) similar issue with the DC motor on mill. 1 brush was total shot, but new brushes in and had a small firework display!

After stripping the motor, the gaps in the commutator were full of carbon and showed signs of heavy arcing.

Skimmed up and cleaned the gaps very carefully. (being honest it took 2 attempts to remove all the debris) and had the mill back to normal.

Good luck and keep us updated on this please.

Jim

[Paul MercerParticipant @paulmercer13838]

Good afternoon all.

back home now for a couple of days, and found a set of brushes on the doorstep. I’ve just read through the previous comments and noticed a couple of typos – apologies, but I use my phone more than my laptop.

I checked every wire in the lathe and did find that an earth to the chassis was loose, so tightened that. Hopefully that explains the shock.

Starting up the lathe with new brushes and she blew the breaker again. Stripped it down and started it with the motor separate to the lathe, and it worked. Fitted the motor and it still worked. Added all the panels and it still worked. Put on the drive belt and it blew the breaker again.

There doesn’t appear to be any excessive physical resistance when turning the lathe by hand.
Running the lathe with no belt, but adding physical resistance to the motor with a length of wood on the pulley once it’s running causes the motor to start sparking.

Why isn’t anything easy! Does anyone have any further thoughts?

Thanks.

Edited By Paul Mercer on 15/02/2023 16:55:32

[Paul MercerParticipant @paulmercer13838]

So I did some more investigation. I took the motor apart to see what if anything the new brushes had done to the newly cleaned up commutator

There were two distinct areas of burning on the commutator, with only 3 bars between them

First I did the “180 degree test”, I measured the resistance of the windings on commutator bars 180 degrees apart.
On most of the readings I got 7 ohms. On 4 pairs of bars I got 53K ohms.

Secondly I did the bar-to-bar test, which measures each individual loop.
On most of the readings I got 0.4-0.8 ohms. On 2 pairs I got 53K ohms.

The two individual loops around the burning, coincided with the 53K ohm readings.

With these more detailed measurements, I’m guessing that it’s a new motor needed.

[mgnbukParticipant @mgnbuk]

With these more detailed measurements, I’m guessing that it’s a new motor needed.

That does look rather terminal.

Depending on the cost of a replacement motor, it might be worth considering replacing the entire drive arrangement with a 3 phase motor + VFD. A bit more "engineering" to get that fitted, but probably a more reliable long term solution if done properly. It would be annoying to replace the DC motor & then have, say, the drive go down in a short time if it has been stressed by the current motor situation.

Nigel B.

[Paul MercerParticipant @paulmercer13838]

Gents.

I have a working lathe!

I bit the bullet and purchased a replacement motor. I stayed with the same DC motor for ease! Thank you for all of your help and advice. It was really appreciated.

Paul

[Author]

Posts