Myford motor wiring problems

My Myford came with a switch feeding its SEM .75hp motor. I recently bought a Dewhurst switch to replace the very tired MEM switch fuse. I followed the wiring diagram for a capacitor start motor and was treated to a big flash and blown circuit breaker. I reasoned that there must be a short. As I couldn’t see anything i lifted the connection board to find a shorting link between A2 and Z2. I removed this but same happened.

I am coming to the conclusion that the motor wiring may not be connected as it should be to the terminals. There are 6 wires coming up from the motor 2 each red black and green how do I figure out which is connected to which winding so I can check what is connected to which terminal?

Otherwise any ideas how I can get this correctly wired?

Kevan

Is the drawing you have dedicated to the Dewhurst switch ?
The 6 wires could be 2 for start winding, 2 for run winding and 2 from the centrifugal switch, check them with a low resistance ohm meter, the windings will have some resistance but the switch pair will be almost zero.

Emgee

That is the idea, how do you tell difference between start and run windings?

I think the first question should be have you run the motor/machine since you purchased it with the original switch? Then, where is this circuit breaker that blew. Away from the machine presumably?

Start winding will be 6 to 7 ohms. Run winding will be 12 to 15 ohms. Green should be earth. At least one green should go to motor frame. Do you mean there are 6 wires coming out of the motor to the terminal board or that the terminal board in the motor has 6 terminals, or both. Is there an 8 way terminal block on the back of the lathe stand if you have the stand, or is the switch wired directly to the motor? Don't know the SEM terminal markings but A1 - A2 are usually black and run. Such info is not gospel as different manufacturers used their own codes. You need to confirm all the terminal markings? A1 A2 Z1 Z2 seems likely plus any others?

Please don't blow your own fuse. Always switch off first.

Dennis

Hi Kevan. Took me ages to find these pics in all the paperwork i have. Hope this helps.

Steve.

Steviegtr, You seem to have a Crompton motor from your terminals at the motor. it would appear that Kevan may have a Brook or Gryphon motor from what he has said so far, which has completely different terminal designations at the motor. Your Dewhirst switch connections are OK for single phase. I hope you have an earth wire too that is not shown

Dennis

No sir. I have just been to the garage to check. The motor is indeed a Brook crompton 0.75 hp single phase. I took it off with the controls when i bought the Super 7 & fitted a 3 ph with inverter. Brook is Crompton ???. I believe this motor was the one fitted when new. 

Steve.

Edited By Steviegtr on 09/09/2020 00:14:05

Typical values for a Brook .25 hp motor would be of the order of about 10 ohms for the run winding and about 40 ohms for the start winding

For 0.75 hp would be a bit lower

The start capacitor in series with the start winding would be of the order of 40-50 microfarads

Hope this helps

Kevan says the motor is an SEM.

Mike

Unwise to make assumptions. Motor terminals markings are only loosely related to the maker. Crompton and Brook were separate companies until they merged 40 or 50 years ago. Both made a wide range of motors in different configurations. Then they made motors together. Since then, the group expanded to include AEI, English Electric, Newman and Hawker Siddley Electric Motors, all of whom also made different types of single-phase motor, which may or may not follow the same terminal marking schemes. More recently, the group merged with ATB and Wolong, also major manufacturers of motors. Not complete chaos, but not reliably ordered either.

Although there's a fair degree of commonality, it can't be assumed terminals line up with a Brand Name. To be sure, identify the model, windings, and whether or not there's a centrifugal switch or not etc. Steve's circuit identifies A, AZ, T and Z, whereas Kevan's post mentions A1 and Z1 making it certain Kevan has a different wiring requirement.

Can Kevan post a photo of his plate and terminal box? Photo Instructions here. I expect someone will recognise it. Otherwise there's a decent introduction to wiring single phase motors in Tubal Cain's Model Engineer's Handbook.

Dave

Edited By SillyOldDuffer on 09/09/2020 10:47:02

Did you check the Dewhurst first?

SOD. You are correct i never saw or read properly the bit about it being a SEM motor, so yes connections could be different..

Steve.

It works

As suspected the wires were not all on the correct posts and in effect there was a connection post missing to pick up the connection from the start switch and the start winding. Once that was solved it ran, but backwards. Swapping the feeds to the run coil fixed that so now it is all working as it should I think!

If anyone else has aSEM motor the pair of blacks are the start coil and pair of reds are the run coil, the pair of greens are the start switch and the red and black are quite obviously the capacitor.

Thanks for the help and moral support😃

Kevan

You now seem to have additional red an black wires that you say are the capacitor, please confirm.

BTW usual to change over the start winding connections to reverse the motor.

Emgee

Edited By Emgee on 10/09/2020 13:33:55

Emgee,

yes I have a red and black going to the capacitor. Feed comes from the Dewhurst switch on Z2 into the capacitor the output from the capacitor goes into the centrifugal switch the output from that connects to one end of the starter winding at the new connection post I added. Other end is on Z1 that connects back to the Dewhurst.

neither set of winding wires show any polarity the starter windings being black and running windings being red. Should I change polarity of both sets to get correct direction of running? It seems to be happy at the moment?

Kevan

If it's all running and the Dewhurst is providing reverse motion best left as is.
Next time you may want to change the start winding ends to reverse to match fwd/rev on the switch.

I guess you will be aware of not using the Dewhurst as a stop/start switch, only operate that when power is off otherwise at a later date the contacts will give problems.

Emgee

Glad you got it sorted. For the record a run winding can be connected either way. It is just that. If you push the motor forward it will go forward. The same backwards. The start winding is the only one that needs to be changed. But of course you know that now. So go & make lots of swarfe.

Steve.