Electrics Problem – Lathe

[SillyOldDufferModerator @sillyoldduffer]

160uF vs 100uF is unlikely to be the problem.

I suspect the centrifugal switch – the fault depends on motor direction, and electrically there's no difference. The start winding gives the rotor a twirl in either direction.

The click is almost certainly the centrifugal switch. I suspect it's not making and breaking contact correctly. It's supposed to connect power to the start winding until the motor picks up speed and then disconnect. If if doesn't disconnect the winding overheats. I'm not sure what the motor will do if the switch operates intermittently and connects and disconnects the start winding randomly, but it can't be good! It's possible the switch nearly works properly in forward, but is worse in reverse.

I'd take the end off the motor and have a good look at the centrifugal switch. I think it will be mechanically deranged, have dirty or spark eroded contacts, or maybe loose wiring.

Dave

[Dave HalfordParticipant @davehalford22513]

Posted by Justin Thyme on 29/04/2023 11:10:29:

the one that is on is 160µF could that be an issue ?

When the motor starts I hear a click as it picks up speed, and also hear it at similar RPM as it is slowing down after switching off – I guess this is the centrifugal switch ?

Yes, but it may not be working all the time and clicking doesn't mean it's conducting power as SOD says. With that coil dead the motor should sit there and just hum to it's self. No idea why it tries to run unless the start winding is shorting neutral somewhere.

[Justin ThymeParticipant @justinthyme24678]

I am hoping this is the problem, contacts are a bit of a mess, pressing them together doesn't always make an electrical connection.

Not sure why they are open ? presumably when assembled they must be touching. it couldn't have possibly been starting this far open?

should the the contact have a hole in, or do you think there is something missing ?

i have now been able to measure the starting winding at 15 ohms, this compares to 5.8 for the main winding – does that sound about right ?

[Dave HalfordParticipant @davehalford22513]

Nasty flash burns. I don't think anything is missing, back in the day cars points used to have a similar setup to reduce arcing.

Can you clean the two contacts?

[Justin ThymeParticipant @justinthyme24678]

I was just going to pull some emry paper between them until they worked

Any other suggestions?

[SillyOldDufferModerator @sillyoldduffer]

Looks like the problem has been identified!

New contacts I think.

Normally emery paper isn't recommended because it removes any anti-corrosion plating that keeps the contact clean. The plating is thin because it will something expensive like Gold or Palladium, and it's easily damaged by abrasives. However, those are so badly damaged there's nothing to lose! Go for it.

I wonder what caused arcing in the first place? Maybe the contact distance should be adjusted between firm closing and definite opening, and this one wasn't? I'm at the limit of my centrifugal switch knowledge and a single-phase motor expert is needed. Another possibility is previous cleaning with Emery paper – removing the plating caused sparking, resulting in the damage seem. Or maybe someone carried on using the motor when the contacts first misbehaved, and now they're cooked.

I think the thing with a hole in it is just a backstop, and nothing is missing.

Dave

[Dave HalfordParticipant @davehalford22513]

Any plating must be well zapped by now.

One contact goes inside the other so you need to clean inside as well.

Brass brush?

[MacolmParticipant @macolm]

Surely the contacts should be closed when the motor is stopped? Also, are you sure the actual contact piece on the moving part hasn`t broken off? It seems to be there on one view, and not on the other.

Edited By Macolm on 03/05/2023 21:13:17

[Oven ManParticipant @ovenman]

Posted by Macolm on 03/05/2023 21:11:35:

Surely the contacts should be closed when the motor is stopped? Also, are you sure the actual contact piece on the moving part hasn`t broken off? It seems to be there on one view, and not on the other.

Edited By Macolm on 03/05/2023 21:13:17

Or do the contacts close and short out the start winding? I have always believed that the contacts opened to disconnect the start winding but perhaps that is not the case.

peter

Edited By Oven Man on 03/05/2023 22:32:12

[Justin ThymeParticipant @justinthyme24678]

Posted by Oven Man on 03/05/2023 22:27:14:

Posted by Macolm on 03/05/2023 21:11:35:

Surely the contacts should be closed when the motor is stopped? Also, are you sure the actual contact piece on the moving part hasn`t broken off? It seems to be there on one view, and not on the other.

Edited By Macolm on 03/05/2023 21:13:17

Or do the contacts close and short out the start winding? I have always believed that the contacts opened to disconnect the start winding but perhaps that is not the case.

peter

Indeed, I touched on that in my earlier post. They need to be closed to form the circuit, and since the motor does start (in a fashion) then I presume they are pushed together when it is assembled ? I am going to try and work that out when I next get a chance to work on it.

My intention, unless otherwise advised, is us to clean the contacts up as best I can, and then if it all works I can try and source a new set of contacts. So my question is – should it work, if only for a week or two (lets say 50 start up) if I just clean the contacts that are fitted up ?

[Dave HalfordParticipant @davehalford22513]

Maybe, we know the Cap was changed, the contacts are badly burnt and there's a chance of winding damage. I wouldn't spend money on it till I knew it wasn't drawing excess current.

[MacolmParticipant @macolm]

What you can do is bypass the centrifugal switch, and wire up to an external push button of enough current capacity instead. Switch on and immediately press for about a second. If it then starts and runs OK, the motor is at least worth checking further.

If you want to de-risk this, you can connect a one kilowatt or so element in series with the whole thing (motor not loaded), or a 500 watt or more old fashioned filament security light. This limits the fault current, and buys time if all is not well.

[noel shelleyParticipant @noelshelley55608]

This has gone on a bit ! If the windings ONLY have been tested properly then it is known wether they are good or bad ! What were the resistance readings ? Leakage ?Then there is the other bits, the cap, test for value, up to spec ?. The centrifugal switch could be failing to operate due to the rotating component dragging the contacts out of line ? Or simply be so badly burnt as to not conduct – clean ? Looking at the wiring diagram you do not show the centrifugal switch but say no connection – is this not the problem ? As Malcolm has said try wiring up by bringing out the wiring and use a NO push button switch to feed the start windings, in place of the internal one.

Please be aware that funerals are far more costly than electric motors and at risk of stating the obvious you are working with lethal voltages. That you don't know the history of this unit and mention smoke makes me think this is sadly a lost cause. With out suitable test equipment you will be struggling ! Good luck Noel.

[MacolmParticipant @macolm]

Clearly everyone must work within their competence, experience and comfort zone, with an adequate standard of security to avoid contact with anything live, and with adequate insulation and mechanical fastening even if temporary. Touch nothing of metal in a lashup! (except if unavoidable, the definitely earthed frame)

As regards the motor windings, the great unknown is the presence of shorted turns. These act as a shorted transformer winding, loading and unbalancing the magnetic circuit and thereby the electrical circuit also. Shorted turns in the start winding will potentially also unbalance the main winding, leading to peculiar symptoms as well as likely overheating, even when the start winding is not powered. Also be aware of a possible short circuit between main and start windings, though diagnosis in this case a simple matter of a resistance check. Likewise a possible short to frame should be checked for.

[Justin ThymeParticipant @justinthyme24678]

Posted by noel shelley on 04/05/2023 13:44:56:

This has gone on a bit ! If the windings ONLY have been tested properly then it is known wether they are good or bad ! What were the resistance readings ? Leakage ?Then there is the other bits, the cap, test for value, up to spec ?. The centrifugal switch could be failing to operate due to the rotating component dragging the contacts out of line ? Or simply be so badly burnt as to not conduct – clean ? Looking at the wiring diagram you do not show the centrifugal switch but say no connection – is this not the problem ? As Malcolm has said try wiring up by bringing out the wiring and use a NO push button switch to feed the start windings, in place of the internal one.

Please be aware that funerals are far more costly than electric motors and at risk of stating the obvious you are working with lethal voltages. That you don't know the history of this unit and mention smoke makes me think this is sadly a lost cause. With out suitable test equipment you will be struggling ! Good luck Noel.

 

Yes, it has gone on a bit. Time of year, so much to do in the garden, plus other more urgent projects means the lathe is well down the list.

However today, the poor weather men't no garden and some old folks getting married on the TV seemed to occupy the wife……

I did give readings for the windings earlier, 15 ohms for the starting and 5.8 for the main.  lrakage to earth readings from windings were zero

And I do put a great deal of due diligence into stuff like this, I do also have a fair bit of experience with electrics, but this is the first time I have got so involved with an induction motor, changed a few capacitors on them over the years, but this is far more challenging, and interesting problem that I would like to understand and fix

I will take great care I promise.

Edited By Justin Thyme on 06/05/2023 23:05:46

Edited By Justin Thyme on 06/05/2023 23:10:04

[Justin ThymeParticipant @justinthyme24678]

Posted by Macolm on 04/05/2023 09:55:12:

What you can do is bypass the centrifugal switch, and wire up to an external push button of enough current capacity instead. Switch on and immediately press for about a second. If it then starts and runs OK, the motor is at least worth checking further.

If you want to de-risk this, you can connect a one kilowatt or so element in series with the whole thing (motor not loaded), or a 500 watt or more old fashioned filament security light. This limits the fault current, and buys time if all is not well.

Thankyou. I done this today, made an external switch to control the starting windings. Worked beautifully, both directions, even with all the belts on.

However I am still not convinced. on one occasion (running clockwise) I switched the starting windings off too early, and it tripped the fuse! I tried to recreate this anticlockwise and it won't trip the fuse. But let either direction get up to speed (2 seconds) and it seems to work well. It does seem to run a little slow, but its rated 1400 RPM is not that fast. I will see if I can put an exact speed on it.

I think I probably will replace this motor rather than fit new points in the centrifugal switch, but I would still like to know what is wrong with it. I much prefer fixing stuff than replacing.

Thanks for the suggestion.

[Martin CargillParticipant @martincargill50290]

The motor on my lathe is supposed to use a current sensing relay that disconnects the start winding once the start current drops off. when I got the motor (second hand) the relay was missing and a replacement was not to be found (the original relay was an American device). I have mine wired with a second set of contacts wired to the go pushbutton, pushing the button brings in the contactor for the main winding and powers the start winding. You simply hold the button in for a second or so until the start winding is no longer required and then release it.

I got the idea of operating it like this when came across an identical set up on a single phase Sedgwick saw bench with a 4hp motor, although in this case the run up time was about 10 seconds and a label instructed you to hold in the button until the saw reached full speed.

Martin

[SillyOldDufferModerator @sillyoldduffer]

Back in the good old days, single-phase motors were started with a rope. Common when it didn't matter if employees were sent home with missing fingers and a dislocated shoulder. (As is right and proper,employers had no responsibilities and employees didn't get sick pay.)

Wrap several turns of rope around the chuck, apply power, and spin the rotor by pulling hard on the rope. Perfectly safe with a bit of luck and the right technique. I recommend the method to anyone who believes H&S has gone mad, because personal agonising pain is a good teacher!

Everyone else should fix or replace the motor.

Dave

[MacolmParticipant @macolm]

It could well be explained by a small span of shorted turns in the start winding. This would mean that the start winding would draw additional current, thus burning the contacts, but once out of circuit, the effect on the main winding might be less if the short was nearly at right angles to the main winding axis. The motor would run a bit slow and heat up more than normal. Presumably there would need to be enough asymmetry to account for the difference with respect to rotation direction.

Unfortunately, getting much further with this would be difficult without suitable test gear. You could run the motor lightly loaded for half an hour or more and check how hot it gets, but even if that seems OK, using it may merely delay the inevitable of new motor.

[noel shelleyParticipant @noelshelley55608]

Dear Justin, as you will have realised we here all enjoy a challenge and will do our best to solve a problem ! But diagnosis at a remote location with limited test gear is always going to be difficult, that it runs in one direction better than another is very interesting ! The one thing that would give the game away is to read the AC amps drawn by the run windings if above 4 A then all is lost, as Malcolm has said running it for a while may show the state of things. The start Amps can be very high, 1200% FLA but only for a VERY short time, this is why a fuse will not protect a motor, had this motor been run on an overload device it may well have saved it ! Leakage should be tested with megger type instrument at 500v, a friendly electrician may be able to help,since you have the wiring out a clamp meter will read the Amps, both start and run. 10/10 for trying, like you I would be loath to scrap a motor that I might be able to save. Good Luck. Noel.

[Justin ThymeParticipant @justinthyme24678]

Sorry to drag this back up again, but today has been the first time I have got back to the machine.

AMPS; When running, the run winding is drawing 3.9 amps (rated at 4 amps), starts off at about 4.3 and after 20 seconds settles at 3.9

SPEED; it is rated at 1400 RPM.
No load 1500 RPM
Forwards with belts on 1440 RPM

TEMPERATURE. The lathe motor which appears to be of similar design but 550w. after 20 minutes running, the case was 31.5°C. However the suspect motor after 20 minutes running reached 48.9°C  (infra-red thermometer)

I have not been able to borrow a clamp meter, however we do have a pat testing thing, that tests with 500v DC. . I have been able to check both windings separately with this. Insulation resistance test for both is 19.99 mega ohms (a fig given for every thing that passes) And the Earth leakage on both windings is 0.1ma (both windings and the machine in general pass the pat testing with flying colours.)

Comment and opinions must welcome

————————-

As an aside, I did use the machine to mill some bright mild steel today, it seems to be very robust and works well, can take quite a bit of metal with each pass. not that I have much experience of mills, just a micky mouse one that looks like a small pillar drill that seems only capable of taking off tiny bits at a time. This much bigger warco lathe come mill, seems to be like a proper bit of equipment.

Edited By Justin Thyme on 18/05/2023 23:45:22

[noel shelleyParticipant @noelshelley55608]

Interesting readings ! The amps reading, is this loaded or light ? 4 A is the FULL load current. The temp is not far out. Insulation test should be done winding to frame and winding to winding, 20meg seems ok. It sounds as though the reverse switching is in some way suspect . You mentioned smoke way back, that is a worry though insulation is ok.? Good luck. Noel.

[Justin ThymeParticipant @justinthyme24678]

Loaded to the extent that it is driving the mill at he highest speed.

I think the little bit of smoke may have just been down to the fact the machine had not been used for at least 10 yeatrs (may be 15) it may have just been a few long deceased spiders burning off – there was no electrical burning smell.

How would I test winding to winding with a pat tester. pretend one winding is earth with the live and neutral going to the other?

Absolutely convinced switch is 100% OK

[noel shelleyParticipant @noelshelley55608]

If it draws 3.9 on no load just top speed, that may be your answer, there's nothing left to do any work, the result of poor design and spec.. There should be nothing hot enough to burn ANYTHING, though the centrifugal switch might have given a good arc on first start up. Are the bearings free, has the grease dried out causing drag ?

To test the windings for sound insulation connect the line (live) to the one end of a winding and the other (earth) to first the frame and then to the other winding. If the tester is of the older type intended for use by trades people it may have a meter, or digital readout of resistance to infinity. If of the modern type it may give a reading and say pass/fail. If your readings of 20 meg are correct then insulation is good.

It is cap start, get the cap tested for rated value or try a replacement of 100uf at 400V !

The centrifugal switch needs to be carefully inspected as to it's operation – in both rotations ! This may be damaged or badly worn and not be switching correctly !

IF all the above are sound then it should work ! BUT if the reversing switch is wrongly wired or damaged all bets are off ! Fitting a spare motor of the same type though may be of a different power to the reversing switch and see if this functions correctly ?

Myself and other members have I think covered just about every angle, IF still it will not work then either take it to motor repairer for test OR fit another motor. Good luck. Noel

[Author]

Posts