Fitting a Reversing Switch to a Motor on a Myford Lathe

i have a Myford Super 7 without its original motor or Dewhurst reversing switch. Instead, the motor is a single phase 3/4HP one made by AER Ltd. Ashford, wired through a simple no volt release (NVR) switch.

Having tried to contact AER Ltd., I have had no replies and they are not answering their telephone. I was wondering whether anyone has dealt with the company and whether anyone is familiar with these motors. I am hoping, however, that they are fairly standard. The connection terminals are as follows:

AZ - wired to the live wire from the NVR switch

A - wired to the neutral wire from the NVR switch

Apart from the earth, the other two motor terminals are labelled K and Z, but are not connected to any wires coming into the motor.

Can anyone tell me whether it is possible to wire the motor to a reversing switch like a Dewhurst, and if so, how I would connect this up?

Many thanks,

Mike

I suspect that you can't reverse that particular motor, but be careful what you wish for. I have a lathe with a screwed mandrel nose, a Boxford. The only scar on its bed is due to having a reversible motor.

Normally in A/Z nomenclature, you would have two windings, A and Z with the winding ends marked A1 and A2, and Z1 and Z2. the Z winding is the start winding, and to reverse the motor, you would swap round Z1 and Z2, or wire Z1 and Z2 to the reversing switch, and let the switch do the swapping. it sounds like your motor may be connected internally, possibly at the centrifugal switch if it has one, or it may be a non reversible motor, which are becoming annoyinly common today. can you post up a pic of the motor and the connection box (is there a diagram insode the cover?), and also the rating plate of the motor. It may be that the empty posts have been used to connect the windings on the other side of the terminal plate, so it may be fairly easily doable.

Phil

I only intend to use the reversing facility for returning the carriage to the start of a screw thread.

What connection terminals should I expect on the motor if was reversible?

Mike

A reversing motor would have more than two wires in the junction box, probably four wires. The issue of chucks unwinding as mentioned by Clive is probably a result of rapid acceleration to full speed when not using back gear speeds. Reversing when in back gear is unlikely to loosen the chuck when reversing for screw cutting. The biggest risk is selecting reverse with a high speed setup. One solution to this is to replace the motor you currently have with a VFD driven 3 phase motor. This setup allows soft starting in both forward and reverse so reduces the risk of chucks unscrewing. There are other good reasons to go 3 phase if you end up replacing the motor. Plenty of existing threads cover this.

Martin C

I’ve just added a couple of photos of the motor connections to my album, though I can’t work out how to add the photos to this message (I’m writing this on a very small-screen iPhone!)

Motor plate shows the following:

Ref. BPA56P42900 B56 Fr.

0.55KW 0.75HP 1425RPM

240V 1Ph 50Hz

5.6A Class B CONT Rtg.

No. ZBM02641 Amb. 40degC

Start Capacitor 120uF 275V

Run Capacitor (no details)

IND. MOTOR BS5000 Pt.11 CE

Apologies for the large font!

I assume a VFD with a new 3-phase motor would be around £400?? Also, do the VFDs all create that horrible high-pitched whine?

Mike

No whine on any I have used.

Martin C

The motor images are below:

I picked up a 2nd hand 1 hp 3 phase motor and a 0.75 kW VFD off E-Bay for under £150 for both. The bargains are there , just got to look and be patient if cost is important. I made the control box using parts from Maplins for around £20. I use CPC Farnell to get my parts now

There was no high pitch whine when running, a very controlled start and a quick stop.

Be aware that some VFD's can have problems tripping domestic RCD's with their inrush current when first turned on.

The combination has worked faultlessly for a couple of years. ........however do not try and run the Myford in Reverse at anything other than slow speed ...... the chucks do spin off ........ !

Steve

One of my vfds from about 15 years ago does have a whine, it's the cooling fan I think, but later one is silent. I think it's the MCB that trips under inrush current, you can get slow action ones that will cope. I have a normal 30A MCB supplying the workshop and a local distribution board with a 15A fuse for the VFD

3 phase is so much better, but If you do go down the route of reversing your single phase, don't use a Dewhurst switch to start/stop, keep the NVR and use the Dewhurst when it's stationary to change direction

From the ‘quick stop’ comment, it may have incorporated a braking circuit. Something not to be used with a screwed chuck - or the chuck could come adrift during the forward deceleration (as well as acceleration in the reverse direction).

The whine is a function of the carrier frequency and can usually be adjusted by selecting the correct parameter on the VFD . This **LINK**

explains it better than I can.

Rod

Mike, getting back to your question I believe the start winding ends are the blue on Z and yellow on R wires.
Before spending money you could try swapping those wires over to check if the motor reverses, if it does come back here and you can get a drawing of connections to the Dewhurst.

Emgee

Coming back to the OP's question, the "AZ" terminal appears to have 3 wires connected to it. There is also an A and a Z terminal. So it looks like both ends of each winding are brought out and terminated on the terminal board. So I think the motor should be reversible by the method that Phil describes.

But I can only reiterate that reversing a motor can be dodgy with a screw-on chuck. I assume that you are cutting metric threads so the threading indicator doesn't work? There are other ways of picking up the thread when you return which I have seen described.

Mike,

This scan from The Model Engineer's Handbook seems to show a similar situation to yours although this is a connection diagram for Crompton Parkinson motors.

HTH,

Rod

Mine whines all the time and continues to do so for several seconds even after the power is switched off, which is what made me think it was a cooling fan. Eventually the noise reduces in frequency and stops, and I can feel the cooling draft tailing off at the same time, still sticking to my fan theory! The newer one on the milling machine doesn't, it is completely silent

Edited By duncan webster on 01/02/2019 12:53:58

Yes, I agree with Emgee, those are the best candidates for being the start winding. swap them over and see if the motor reverses.

Phil

Rod, as far as I can see, that diagram id for a capacitor-start;capacitor-run motor.

Mike seems to have a capacitor-start induction motor, the nameplate give no run-capacitor value.

Mike needs to establish whether there is a centrifugal switch to work out where to go from here.

Dewhurst switches are great, but expensive and are likely overkill for reversing a single phase motor; they are more universal and are able to be used on 3 phase as well.

Mike has a photo of the original wiring, so disconnecting everything and measuring resistances seems the way to go, as we're not sure what a previous owner has been up to.

The start winding will have a higher DC resistance than the run winding, so it should be easy enough to ascertain the colour coding of the wires from that. It looks like there are two red/green wires, so I'm guessing they will go off to the start capacitor, or maybe a centrifugal switch if there is one.

See the second diagram on HERE for the Capacitor Start Induction Motor. Yours may or may not have a switch.
To reverse the motor, all you need to do is reverse the Start winding, which you will have already identified via your resistance meter.

This can be done with a simple Double Pole-Double Throw switch There's a nice easy diagram HERE but don't let the colours confuse you as they will be different to those in your motor.

A-AZ are likely to be the run windings. most motors I've seen have Z-T as the start windings, but this one omits T and has K instead, so I'm guessing that might be going off to one side of a centrifugal switch.

Bill

Edited By peak4 on 01/02/2019 13:29:09

Hi Mike

I fitted a replacement motor a couple of years ago from Beatson motors in Sheffield and they helped me with this diagram. Your photos indicate you will have plenty space for the 2 in-line connectors that are needed.

Regards