Electrics Problem - Lathe

it's S1 rated which is continuous and it should go like that all day.

Class B says the windings can be 130C and they probably are, which means it can double as workshop heater.

I'm betting it draws more than the plated 4A, though why it only pops in one direction with the belt off I have no idea.

I could only get a reading out of one of the coils, and this is a diagram of the wiring. Does the centrifugal switch have to be engaged to get a reading from the other winding?

So a recap on what we have.

The forward Reverse switch seems OK and has now been rewired to work in the correct direction, it looks as though someone had clearly rewired it the wrong direction in the past!

With no belts on, the motor often runs well in both direction, but sometimes trips the fuse in anticlockwise (may be just when its hot)

With the belts attached it starts poorly and then runs well in clockwise, but mostly 95% of the time trips the fuse anticlockwise.

I am now wondering, if the capacitor has lost its umph (can that happen, I thought they just suddenly died), may be it can't get the motor to spin fast enough for the main winding to kick in ? (is that a silly theory?) this would explain it working better with belts off, but doesn't really explain the direction issue.

Anoteher thought on the capacitor. It is clearly not the original, real DIY soldering for connections. I am also wondering if it is of the correct size, if it was too small would that also cause starting problems?

What size of capacitor should a 370w motor such as this be using ?

The motor for the lathe works well but is 500w. could I try the cap off that on the smaller motor

The Cap size is on the motor plate in box C1 100

According to your drawing the start winding is open cct. Check the centrifugal switch inside the motor end cover is free to move and the contacts make

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

If the starting winding is open, why is it starting? I did double and triple check this winding with the cap removed and could not get a reading.

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 ?

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

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 ?

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?

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

Any other suggestions?

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

Any plating must be well zapped by now.

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

Brass brush?

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

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 ?

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.

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.

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.

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.

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

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.