Wiring Newman motor on Myford ML7

Hi all

My ML7 has a Newman motor which runs from the single phase mains via a Mem Auto Memota direct on line (DOL) starter. The motor does not have a capacitor, so far as I can see, and only operates in one direction. I would like to be able to run the lathe backwards (mainly to help when cutting metric threads), by fitting a reversing switch (possibly of the Dewhurst drum type). Unfortunately, the terminals within the motor aren't marked. Can anyone tell me what the terminals are from the photograph?

Is there a diagram under the cover plate? Normally is

Yes thats what I expected, but there isn't one.

Why would you need to run the lathe backwards when cutting metric threads as opposed to whitworth ones? Just wondering !

.

I would think that he has an imperial leadscrew. - As such one does not disengage the leadscrew in the normal way between passes so you have to reverse to clear the job before dialing in a deeper cut.

The same would be true if it was the other way round. i.e. cutting an imperial thread with a metric leadscrew.

Someone with a better command of maths than I have will probably explain better.

Nick

Edited By Nick_G on 01/08/2017 13:28:09

You do not need the maths, simply remember, do not disengage leadscrew when screwcutting metric on imperial or imperial on metric machine.

Yes that's exactly it, because the leadscrew has 8tpi, and most metric threads don't have an exact whole number of teeth per inch, the thread dial indicator doesn't guarantee that you rejoin the thread at the same point following disengagement of the leadscrew. Hence it is necessary to keep the leadscrew engaged. I find it hard work manually turning the lathe backward to the start point of the thread, to take successive cuts.

Hi Max the wires in the connection box are blue neutral to end of two coils in motor and live to two coils in motor via link strip not forgetting the earth connection. One coil is a starting coil which switches off as the motor comes up to speed via a centrifugal switch. The other coil is the run coil and is always connected. In order to reverse the motor you have to reverse the start coil feed and to do that you need a reversing switch and replace the motor cable with a 5 core cable and a no volt on off switch.

David

TURN OFF THE POWER! It appears from the pic that the two topmost wires are the start winding, and the lower pair are the run windings. As Dave has said, you need to reverse the polarity of the start winding to reverse the motor. You need another four cores from the motor to the reversing switch, one pair from the start winding, and one pair back to the start winding terminals on the connector plate. How you connect the reversing switch depends on what type it is. Always stop the motor completely before reversing it!! You may need to take all the wires off the connector plate after labelling them and the terminals 1 2 3 4 . Label the top ones 1 and 2, and the lower three and four. draw a diagram to show how they are connected at the moment.. then remove 1 and 2, and confirm with a multimeter that there is continuity between them, but nothing to three or four. Now see if you can operate the centrifugal switch manually with a peice of wire or a screwdriver etc , If the switch operates between the centre left terminal in the pic and neutral then 1 and 2 are the start winding ( which I "think" they are ) Basically, again as said by Dave, you need to find the winding that is switched off by the centrifugal switch, and reverse the connections to reverse the motor.

Hope that is understandable!

Phil

If, after you have sorted out the connections to the motor, you need the wiring for a Dewhurst Switch, please PM me.

Howard

Thanks Gentlemen

Your help has enabled me to proceed.

I started as Phil suggested by removing wiring and labeling the posts as per photograph below. As David mentioned, the removable link between posts 2 and 4 would not be required for reversing operation, so I took it out:

At this stage I didn't bother to number the top left post as I established it had no connection and therefore apparently no function.

I measured resistances between all the posts and discovered all were open circuit save for posts 1 to 3, which went closed circuit when the centrifugal switch was in the start position.

I measured the resistances between wires corresponding to posts in the starting position, and got 16.5 Ohms between 1 and 2, and 6.6 Ohms between 3 and 4. I know the resistance of the starter winding should be greater than the running winding, so 1 to 2 is the starter, and 3 to 4 is the running winding.

With this knowledge I was able to transcribe the circuit as Figure 1 below:

This is when I realised what the un-numbered post was for. I needed to be able to put the starting winding in series with the centrifugal switch. So, I rearranged the connections from the windings to the posts as per Figure 2 below:

To test this circuit, I lashed up a reversing switch arrangement using chocolate block connectors and tried it. It now works well and I can change the direction of the motor by reversing the polarity of the power connections at posts 1 and 3. Just need to wait for my reversing switch to arrive (I found one on eBay for about £9) and also the 4 core plus earth cable, then I can do the installation properly.

Thanks again to everyone for their help and support. I maybe could've worked it out by myself, but it would have taken quite a few days!

Cheers

Max

Hi there, Max,

Is the boss on the right-hand side of the terminal chamber tapped to take a conduit thread? If so, I suggest you fit a cable gland and route the cable in to the terminal chamber that way rather than through the slot as shown in your initial photo. The gland will provide strain relief and chafe protection as well as sealing. (If the motor is old, you might need an ¾" BSC Imperial to 20 mm Metric conduit thread adaptor.)

I'd reserve the slot for the earth wire to find a suitable connection point on the outside of the motor.

Best regards,

Swarf, Mostly!

Edited By Swarf, Mostly! on 02/08/2017 16:54:23

Yes I agree, and will be putting a gland in. Given the lathe is fifty years old this year (I've had it for 5) and to be honest the motor looks older, I'm surprised the cable hadn't worn through.

You have almost got it Max, but to reverse, you need to interchange the two ends of the start winding, not the incoming supply, hence the need for 4 extra cores. I have drawn you a diagram, which is in Hermetics album, but for some reason, although the pic is there, and can be opened, when I try to insert it in the post, the title does not appear in the list?? I will try again to post it later, unless the mods can do summat with it!

Phil

I have PM'd it to you, let me know you got it ok!

Phil

Max another thing to remember is that you need to let the motor stop or at least until you hear the centrifugal switch drop out before reversing the direction other wise it will continue to run the same way.

H

Thanks Phil, got the PM. I was using two supplies one to posts 2 and 4 for run, the other, interchangeable in the chocolate block to the unmarked post and 3. So (in effect) 4 wires plus earth.

Harry, yes I guessed that would be the case. You can actually hear the centrifugal switch drop back to the starting position as the speed winds down. That's with it on the bench, once back on the lathe probably won't be able to hear it. With the tumblers engaged, and the clasp nut engaged, the lathe actually stops fairly quickly, so I expect it will be difficult to throw the reversing lever fast enough to cause it to continue running in the same (wrong) direction, but I will hesitate at 0 to allow it to stop.

M