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Help Needed - Reversing a 240 Volt Motor

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Tony Watson 517/06/2022 04:24:01
9 forum posts

Having decided that a reversing facility on my elderly Advance lathe I purchased at vast expense ($A15) a Chinese reversing switch. This switch is designated QS-15 and has six terminals. The wiring diagram supplied refers to three phase connection and the wiring information on the net is either absent or confusing (to me) in its use of American electrical jargon. Through a process of deduction and insertion of a bridge I managed to wire the switch so the lines in could be continued or reversed at the lines out.

The motor concerned is non-capacitor start split phase 240 volt . Terminals are numbered one to four. Terminals one and three are connected to the start winding and neutral and active respectively. The run winding is connected to terminal four and terminal two has no apparent connection.

It is easily reversed by swapping over the start winding connections. I therefore assumed that reversing the polarity of the lines in would produce the same effect. However, this is not so. The reversing switch is doing its bit but the motor does not reverse. Manually reversing the motor lines in similarly has no effect. What am I missing? Any help would be most appreciated.

Tony Watson 517/06/2022 04:27:29
9 forum posts

Small correction. The run winding is connected to terminals THREE and four.

Paul Lousick17/06/2022 04:58:13
2079 forum posts
728 photos

Do a search on MEW for revering motors. Lots of articles have already been posted. (Note.Not all motors can be made reversible)

Hopper17/06/2022 06:21:22
6725 forum posts
348 photos

If you post pics of the motor, its data plate and the connection terminals there are a few electrical gurus on here who may be able to help if searching previous threads does not provide what you need.

DiogenesII17/06/2022 06:55:26
589 forum posts
234 photos

Posted by Tony Watson 5 on 17/06/2022 04:24:01:.

.. ...It is easily reversed by swapping over the start winding connections. I therefore assumed that reversing the polarity of the lines in would produce the same effect... ..

You need to bring just the start-winding leads 'out' and use the switch to 'swap' them... ..leave the run side as is..

noel shelley17/06/2022 10:22:57
1451 forum posts
23 photos

Diogenese has beat me to it ! ONLY the start windings need to be reversed, NOT the run windings. DO NOT use a reversing switch to stop or start the motor, you need a switch before the reverser to start and stop the motor. Noel.

Tony Watson 518/06/2022 00:13:53
9 forum posts

Bear with me on this one. Do you mean that the the reversing switch should only be connected to the start winding leads? In this case I take it that the run winding will continue to receive current through the start winding after the centrifugal switch cuts it out.

BTW, I have used an extra switch, having little faith in the long term durability of the Chinese contacts.

John Haine18/06/2022 08:14:40
4718 forum posts
273 photos

One end of each winding is connected to a common. You will need to split that out to get separate connections to the windings, then reverse the power supplied to either but not both to reverse.

John Rudd18/06/2022 09:30:19
1456 forum posts
12 photos

Cap start with cs
Here’s a motor with a cap start with a centrifugal switch…,,


Reversible cap start
Here’s the wiring for the same but with a reversing switch added….the switch is a double pole type with 2 positions…it will have six connections  to it…Usually the centre connections are wired with the capacitor/one end of the start winding. The outer connections are cross connected, with the Live and Neutral at one set of connections..


Edited By John Rudd on 18/06/2022 09:33:32

Tony Watson 521/06/2022 05:08:56
9 forum posts

More help required! I've wired up as suggested above - with the addition of an on/off switch at the point marked AC. The inputs were connected to each end of the run winding and thence to the reversing switch. The reversing outputs were connected to each end of the start winding.

The desired reversing of the motor was achieved but the motor ran slightly rough and after about 10 seconds I was greeted with plumes of smoke. I can only think that the start winding has overheated as if the centrifugal switch has failed to open. Or is the run winding somehow causing the problem?

All suggestions/solutions gratefully received.

John Haine21/06/2022 06:33:42
4718 forum posts
273 photos

You should assume that the magic has escaped with the smoke and junk the motor, IMHO. Buy a new motor, either a proper capacitor start single phase or (much better but more expensive) a 3 phase + VFD which will give you reversing and variable speed and be much smoother.

SillyOldDuffer21/06/2022 07:37:47
8912 forum posts
2000 photos

Doesn't help, because I agree with John's answer. 'Plumes of smoke' mean the motor's insulation is done for - even if it still runs, its untrustworthy and a potential fire hazard. Cause maybe:

  • the motor was accidentally wired so it was running on the Start winding, and Starting with the run winding, or,
  • the centrifugal switch was bypassed by a wiring error or stuck after being disturbed.

On this design the Start winding is only connected for as long as it takes to get the motor moving from stopped. As the winding is only used for starting, it isn't particularly robust and overheats If left connected for too long.

I see single-phase motors as a clever compromise rather than good performers. They work from single-phase AC, which is highly convenient. But their performance is inferior and moderately unreliable. Fitting a three-phase motor and VFD will improve the lathe. You can buy VFD/motor/cabling kits that come with full instructions. The only disadvantage is the cost...



Tony Watson 521/06/2022 23:42:00
9 forum posts

Please bear in mind that this was a theoretical exercise to provide a screwcutting facility on one of my unused machines. The Advance is one which I refurbished many years ago as a project. With a fixed carriage nut and a leadscrew clutch it is capable of cutting multiples of eight TPI but needs a reversing facility to cut others. The total expense was envisaged not to exceed the $A15 for the switch. I'm otherwise well covered for lathes with my 9A Hercus, B Type Drummond, A Type Drummond and six mm Lorch.

I do have another motor for the Advance but really need to know what I did wrong before I fry that one as well.

Tony Watson 524/06/2022 04:42:16
9 forum posts

I've found the answer - and a warning to others. Pulling the start winding leads from their terminals has isolated the centrifugal switch. I'm now stuck with fried motor.

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