Big flash and the lights went out!

[Kevan]

After my success with the DRO yesterday, today has been a disaster! The Henry used as workshop vac died so I did an autopsy but could not find the fault. The grinder has been increasingly reluctant to start so I opened this up and can only see a capacitor which, of course I can’t get a replacement locally. As I was waiting around I started working on installing a Dewhurst switch on the Myford. I should have known better!

Following a wiring diagram sourced on google I reworded the motor connections to the Dewhurst very carefully! I plugged it in then switched to forward there was a flash from the switch and the lights went out!


Obviously I had done something wrong. Having checked my connections were all correct I thought something else must be wrong. Looking at the wiring diagram I became a bit confused as to what all 8 wires from the motor to the connections are.

Essentially there are 2reds 2blacks and 2greens coming up from the motor and red and black pair going to the capacitor. I also found a shorting link on the back of the connection board between A1 and Z1, removed this and tried again, and the lights went out again!

The motor is a 75 hp SEM does anyone know what the connections to the motor are? I suspect that they are not as they should be according to the wiring diagram;)

[Roger]

Hi Kevan,
Personally I'd go back to basics, remove all of the connections to give you access to all of the wires coming from the motor and measure the resistance between all of them. From there you'll be able to figure out which are the field windings and the starting windings. I presume there's centrifugal switch? Again, I'd want to figure out what wires go to that and the capacitor.
Once you've got all of that, you can lay out a diagram of what you've actually got rather than what you think you've got!
These things are a real pain.

[jackrae]

Just to clarify, the image of the Dewhurst switch connections at the top of your illustration are merely an indication of what goes on internally within each switch position. You do not make any link connections. You simply connect mains to 1 and 3 and connect motor to 2,5,6 and 7 In your photo of the motor terminal box, can you confirm the red and black rubber cable connects to an external capacitor. To what is the white cable connected and to what is the tape covered cable connected.

[Kevan]

Sept 8, 2020 9:03:28 GMT jackrae said:

Just to clarify, the image of the Dewhurst switch connections at the top of your illustration are merely an indication of what goes on internally within each switch position. You do not make any link connections. You simply connect mains to 1 and 3 and connect motor to 2,5,6 and 7 In your photo of the motor terminal box, can you confirm the red and black rubber cable connects to an external capacitor. To what is the white cable connected and to what is the tape covered cable connected.

Hi yup no links in the dewhurst. The taped up cables including white are spare, it is an 8 core cable running between the Dewhurst and the motor. The red and black exit the motor connection and go to a large capacitor shaped can on the side of the motor.

[jackrae]

So I take you have red, white, purple and orange connected to the motor terminals. Can you sketch out exactly what six motor terminals you have (with any ID marks or internal wire colours), which of the four colours connect to which motor terminal and which of the 4 colours connect to which terminal on the Dewhurst. I presume you removed the two 'direction' links from the motor terminal head before you connected the Dewhurst

[Kevan]

Jack

I think you have hit on the problem. My suspicion is that the lathe was originally connected to a Dewhurst type switch but that was changed by a previous owner and in the process the wires from the motor have been switched around on the terminals to make the motor run in one direction. I found one link on the underside of the terminal board which is removed so some wire/s have been moved about on the terminals for the other connection for single direction operation. I will try and figure out which wires connect to what next time I go to the workshop and let you all know how I get on😃

[Kevan]

It works😃
As suspected the wires from the starter coil were in the wrong places and there was a post missing to connect one end of the start coil to the centrifugal switch.

Once I had figured out what wire did what I added another post in the existing hole in the board hooked it back up to the Dewhurst and it ran albeit backwards so I swapped the feeds to the run coil and it is all working properly I think😃


I also replaced the capacitor in my grinder and it is also back from the dead!

That leaves Henry the shop vac. I took him to a proper repair shop so hopefully he will be back soon! Anyway I can get back to model engineering now!

[timb]

Kevan,

You could do with proper glands for those cables. I can send a couple of abs stuffing glands in the post if you would like?
Just let me know, no charge!

Tim

[Kevan]

Sept 10, 2020 14:23:30 GMT timb said:

Kevan,

You could do with proper glands for those cables. I can send a couple of abs stuffing glands in the post if you would like?
Just let me know, no charge!

Tim

Tim

Thanks for the offer . I did put cable ties around the cables inside the box to provide strain relief before closing the box, the multi core is about 13mm OD so I couldn’t get it through a gland that would go in The existing threaded hole, the capacitor cable hole is small and not threaded, the cable has a chunky rubber sheath and shows no sign of chaffing, I guess it has been like that for the last 40 or 50 years! I did drill the box for a proper Earth when I opened it up the Earth was just trapped under the lid😳 earths also properly connected in the Dewhurst box. The previous switch had the incoming Earth twisted round the box closing screw and the outgoing Earth on the proper Earth screw!

This is the first time in 10 years or so I have owned the lathe that I have opened the motor connections or the switch box! It had passed a PAT test in this state though🤣

Big lesson is to open up and take a good look at electrics in second hand machinery🤔

[timb]

No worries, if you change your mind.......

Tim