Electrics Problem - Lathe

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

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

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.

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.

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

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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

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.

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

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