2HP 3 Phase motor shot?

[Braf1895]

After finally managing to haul a Sheldon lathe out of the basement it was in for 50 years, it's back together in my garage with some fresh pain (pics to come elsewhere after I clean the garage). I was able to play around with it a bit, but I have what I believe to be an electrical problem.

Setup: WM-56P with the original 2HP motor running off of a static phase converter on 220.

Problem: The overload is tripping left, right, and center...
Whenever I try to either run the lathe at a higher speed, say over 1200 RPM, I start to hear the SPC kick in and out rapidly until the overload trips. At first, I thought it was a belt tension problem. So, I went through all the belts to make sure they had plenty of slack, but I was stuck with the same problem. Anytime I'd try to get up to speed, the overload would trip. I checked with a clamp on ammeter and saw it drawing 9.5 amps on 220 volts with just the chuck spinning at 1000 RPM. I tried disconnecting the motor belt to see what the draw was with just the motor but I was still seeing ~6 amps. The nameplate of the motor says 6.8 amps at 208 volts. It may be worth noting that I see 240 volts on the line side of the contactor with the lathe off, but that quickly drops to 220 volts once running. I don't think that's an issue but that's beyond me.

This is where the extent of my knowledge leaves me. I don't know what step to take next to find the problem. I'm not sure if the motor is somehow worn out, if I should try searching for a bad wire, or jumping to my long-term plan of adding a VFD should become a Christmas present for myself.

 

[projectnut]

Do you have the wiring diagram? It was originally in the cover of the contractor box.

To start with make sure you have the right heaters and the points on the contractors are clean. Also check the connections inside the motor junction box

 

[Braf1895]

This is all I have for a wiring diagram as the original was lost to time before I was born. Probably going to need to fully tear the controls apart and give everything a good cleaning.


Heater appears to be the right one. An h27 for up to about 6.13 amps. Granted, it is tripping around 9 after running for a little while, but the motor shouldn't be pulling 9 amps to start with. I'll start ripping things apart and cleaning tonight.

 

[Ulma Doctor]

Yank the old control system out and start over , you’ll do a lot less tail chasing. There’s not much chance of you knowing what has been done to the poor machine.
Try firing the motor off outside of the machine and do the amp draw test, the controls may be part of the problem

 

[Braf1895]

Yank the old control system out and start over , you’ll do a lot less tail chasing. There’s not much chance of you knowing what has been done to the poor machine.
Try firing the motor off outside of the machine and do the amp draw test

Seeing how I'm looking into a VFD as a long term solution anyhow, may as well do just that. I think I have a spare good contactor around here somewhere....

 

[Ulma Doctor]

you will not need a contactor to operate a VFD.
The motor wires directly to the VFD output U,V,W
The controls are run through the DCM circuit

 

[Braf1895]

That I do understand. Went and pulled a spare contactor off the shelf (it was right where I left it), hooked it up to rule out the control box, and.... same deal. 6.5 amps on 220v no load. Going to work on getting the motor out of the cabinet to get a better look at it. Next to no resistance when turning by hand, but I suspect something either rattled lose in the *cough* gentle relocation *cough* or it's wired wrong.

 

[projectnut]

I bought my MW-56P from a gentleman that worked as a design engineer for Furnas. His job was to design and implement control systems for Sheldon machine tools. He had completely rebuilt my lathe above the drive cabinet, including grinding the bed and cross slide. When I purchased I rebuilt the Worthington drive.


He told me Furnas contactors are notorious for having the contactor points come loose on the arm resulting in high resistance. They also have a tendency to burn the points once they start to come loose. When I purchased the machine, he warned me that it would probably need new contactors and starters within a couple years. He said I would probably be money ahead to replace the entire system with Allen Bradley controls. To that end i purchased an NOS control box complete with reversing starters on eBay for less than $100.00. I was also lucky enough to find several sets of NOS Furnas contactors that fit my machine. I have now had my machine for over 8 years and so far, have changed out a couple heaters and a couple sets of contactors. The AB control box is still setting on the shelf in its original carton should I need it someday

If your machine uses the gearmotor to change spindle speeds I wouldn't go to a VFD unless absolutely necessary. Using a VFD will require rewiring all the control circuits. Note the control station with the "Forward", "Reverse", "Fast', "Slow", and "Stop" buttons. THis entire station will need to be rewired if you go to a VFD

Here are a few pictures of my machine for comparison. I'll also include the wiring diagrams that were inside the control box. Keep in mind the part numbers are from 1961. Most have been superseded. However, there are frequently Furnas parts of this era on eBay.

As a side note my machine has been running on a static converter (pic 401) for over 8 years with no problems. It has plenty of power and has run as many as 60 hours a week with no overheating

 

[Braf1895]

Thank you for the diagrams! I'll need to go through everything and give it the once over. I have the pogo stick instead of the gear motor to adjust the speed. I'm leaning more towards a motor/wiring issue after having skipped over the entire control box and still seeing FLA with no load on the motor. I might just need to put a few of the belts on to see if the story is the same there, and I'm not just seeing unloaded weirdness with the motor.

 

[Ulma Doctor]

The green wire going to the contactor scares me