Urgent Help - PM 940M down. Motor issue

[PNW_Guy]

It must be interesting doing tech support for a bunch of folks who dig in pretty deep before even contacting them.

 

[Rich Berg]

2023 mfg PM 940M

My Mill blew the run capacitor a couple days ago without warning - no symptoms.

I replaced it with a like capacitor, and after 2 uses, the mill started not turning up to full speed going rev (normal operation) for 2-5 seconds. It would spin right up going fwd, then switch back to rev and it would occasionally start right up, unless it was sitting for an hour or so, then right back to a slow start.

Today, it will not spin up at full speed in either direction.

I tested the capacitors and they are both still within spec. This leads me to think that either a winding in the motor has gone out, or the inertia board has failed.

Anybody know the specs (+/- xx.xxx ohms) for continuity testing the motor at the 4 wires - Rd [U1], Rd [U2], Gr/Y [V1], Wh [V2], and Or [W1].

Also need to know how to remove the motor. Is it just the 4 bolts on top of the head, or do I need to pop the top cover off, in which case, how?

Hi I have a PM mill with similar issues along with severe oil leaks, I'm familiar with capacitor start motors and changed the start cap, that was temporary. many times, I dissembled the motor I found that the centrifugal switch moved on the armature shaft. I shimmed the contacts closer to the armature, it worked for a while. I finally pinned it. that fixed that but then the capacitor blew up on startup, also the Forward- reverse switch became intermittent, PM sent me a replacement. I now believe that capacitor failure is due to the starts close together and not allowing the capacitor to cool off. this has been going on since new (about 5 years) very tired of this dance, I expected a long service from a machine that I paid around 7 grand for.
The latest thing for me now is replacing the motor with a 3-phase unit and a VFD. of course, the original motor is metric, so I'll have to adapt.
I have much more time working on this thing than my projects!
the people at Precision Mathews have tried to support me on all my issues.
Good Luck Rich

 

[Larry$]

The latest thing for me now is replacing the motor with a 3-phase unit and a VFD. of course, the original motor is metric, so I'll have to adapt.

I had a manufacturing business and learned to get 3 phase when ever possible. They run forever as opposed to single phase motors and their dammed centrifugal switches and capacitors. Try to get TEFC 3 phase motors!

 

[wrmiller]

Question if I may?

I've recently been looking at the offerings at PM and of course the PM940 was included. The website says the 940 motor is VFD controlled. My (limited) understanding would make this a 3-phase motor. Is this a relatively new situation, or am I misunderstanding the whole VFD thing?

Thanks, and sorry for the slight OT.

 

[mksj]

Hi Bill, that is correct the PM-932V and PM-940V are 3 phase TEBC motors or vector motor which are typically used to around 3X their base speed or around 200Hz.

 

[dabear3428]

Hi Bill, that is correct the PM-932V and PM-940V are 3 phase TEBC motors or vector motor which are typically used to around 3X their base speed or around 200Hz.

never heard of TEBC i know TEFC Totally Enclosed Fan Cooled, what does TEBC stand for?
is the information about the 940V motor being vector rated correct? did not know this, it makes the 940V even more interesting.

 

[wrmiller]

never heard of TEBC i know TEFC Totally Enclosed Fan Cooled, what does TEBC stand for?
is the information about the 940V motor being vector rated correct? did not know this, it makes the 940V even more interesting.

Totally Enclosed Blower Cooled. (just looked it up)

Edit: The description and manual for the 940 is sorely lacking. I'd volunteer to fix it, but I'm not much of a writer, nor do I have the tools (Adobe, etc...)

The TEFC design puts a fan on a fixed-speed motor. Design the proper fan for the BTU (heat) the motor develops and you're good. The TEBC uses an electric blower to generate the necessary CFM to cool the motor, independent of the motor's shaft RPM.

Had a TEBC on my Charter Oak mill (3HP 9x40). Noisy, but worked great.

 

[dabear3428]

Totally Enclosed Blower Cooled. (just looked it up)

Edit: The description and manual for the 940 is sorely lacking. I'd volunteer to fix it, but I'm not much of a writer, nor do I have the tools (Adobe, etc...)

The TEFC design puts a fan on a fixed-speed motor. Design the proper fan for the BTU (heat) the motor develops and you're good. The TEBC uses an electric blower to generate the necessary CFM to cool the motor, independent of the motor's shaft RPM.

Had a TEBC on my Charter Oak mill (3HP 9x40). Noisy, but worked great.

nice to know thanks. i also did not know that the 940V had a vector rated motor, this adds a lot to the decision between the 940M vs 940V decision a vector rated motor is a lot better than the cap start options.

 

[mksj]

nice to know thanks. i also did not know that the 940V had a vector rated motor, this adds a lot to the decision between the 940M vs 940V decision a vector rated motor is a lot better than the cap start options.

Yes. Vector motors are typically designed to cover a very wide operating speed and maintain full Hp above their base speed and full/constant torque below the base speed to almost 0 speed. As such they can typically cover a much wider variable speed range controlled by the VFD, and typically use a 2 or 3 speed transmission method. In the mills they either do this with a belt drive, and in some cases a 3 speed gearbox. Belt drives are much quieter, you also do not have the mechanical bits and pieces to wear as in a Reeves drives. The downside is primarily at low speeds below the base speed of the motor, you loose the mechanical advantage that you get using a gear head, multiple pulley head or a Reeves drive. Most mill VFD's systems typically operate at 20-200 Hz for conventional milling with a 3 phase vector motor (not high speed types or BLDC). Single phase motors is the easiest as they do not require 3 phase, but are more prone to failure, as well as other issues. In machines where there are frequent start/stop/reverse situations, a single phase motor is less than optimal if you have a choice/budget for 3 phase/BLDC.

 

[Sticks]

Hi I have a PM mill with similar issues along with severe oil leaks, I'm familiar with capacitor start motors and changed the start cap, that was temporary. many times, I dissembled the motor I found that the centrifugal switch moved on the armature shaft. I shimmed the contacts closer to the armature, it worked for a while. I finally pinned it. that fixed that but then the capacitor blew up on startup, also the Forward- reverse switch became intermittent, PM sent me a replacement. I now believe that capacitor failure is due to the starts close together and not allowing the capacitor to cool off. this has been going on since new (about 5 years) very tired of this dance, I expected a long service from a machine that I paid around 7 grand for.
The latest thing for me now is replacing the motor with a 3-phase unit and a VFD. of course, the original motor is metric, so I'll have to adapt.
I have much more time working on this thing than my projects!
the people at Precision Mathews have tried to support me on all my issues.
Good Luck Rich

To follow up, my problem returned after a couple weeks (did not use it much). PM support had me test every thing again, and determined that the centrifugal plate was in the wrong position on the shaft. Wanted me to move it.

At this point a insisted on a replacement motor. I had already replaced the start switch, as the other was now so pitted from arcing that it was not salvageable. I did try to move that plate but it was a press fit, and was going to require pulling the rotor. Been there, done that with plastic fans on the rear (was not coming off either) and them breaking.