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South Bend starter transformer problem

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dshore

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#1
Hello All:
It has been a long while since I posted here. Hoping somebody can help on a problem that just popped up on my South Bend 10L. I suspect the answer might already be somewhere in these forums; if so, would appreciate being pointed in right direction.

I've been running this lathe reliably and smoothly for several years using a VFD. The other day, the VFD failed (no output), and I got a faint whiff of smoke coming either from the VFD or from within the Furnas control box on the lathe. Today, I hooked up my rotary phase converter (that I use on some bigger machines) to the South Bend. After a few seconds of having 3-phase supplied to the machine, smoke started coming from the Micron Impervitran transformer within the Furnas box. I also heard a bit of sizzling from the transformer. Not good. Switched power off ASAP -- system had been energized for 5 to 10 seconds. I took a heat gun and shined in on the transformer and its temperature had risen from about 65 F to about 110 F. Double not good. After things cooled down, I tried it again, but the thing stayed cool and had no output on the low voltage side -- conclusion; it's toast. The little fuse on the outlet side showed zero resistance (not open), so I assumed it's OK.

So, question #1. How do I figure out what caused the transformer failure? Was it the VFD that took it out, or some other electrical component in the Furnas controls or switch? If it was the VFD, then I simply get another transformer and rely on the rotary converter -- job done. But, my concern is that the cause was something else within and another transformer will also fail even if I run with the rotary.

Question #2 (and this might be a stupid one, but I have to ask anyway). The transformer drops the voltage from 240 to 120 volts. The 120 is used for the starter contractor, a couple other thingss within the Furnas control box, and the lathe's work light. But since 120 is wired into the box anyway (one of the 3-phase legs), why isn't it used as is to run the starter, etc., instead of having to rely on a transformer to drop from 240 to 120?

Thanks in advance for any and all help.
dshore
 

markba633csi

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#2
Need more info- can you post a diagram of the system (block diagram OK) so we can see what's what?
Mark
It sounds like you have two problems which just happened at roughly the same time- I don't think they are related
 
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benmychree

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#3
Some 3 phase services do not provide for 120 volts to neutral. What I have trouble understanding is why the magnetic starter needs 120 volts at all, it could be used with a line voltage coil, or simply without a mag starter at all, just using a drum switch; about the only reason for a mag starter is to protect against overload or a dropped phase, neither is of much importance with a 9" lathe. I run my 19" Regal lathe with a two speed drum switch -----
 

dshore

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#4
Attached is hand sketch of the wiring diagram pasted to the inside of the lid of the Furnas control box. Failed transformer is the component in lower right (or upper right if image is not rotated correctly). Connections labeled 1, 2,3, 4, and 5 are wired to the forward, reverse, stop button switch box above the lathe spindle.
dshore
 

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markba633csi

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#5
A couple of points: Using contactors to reverse is not necessary with VFDs and I believe is not recommended. This may have led to the failure
of your VFD. The transformer may also have been the culprit due to insulation breakdown from the spiky VFD waveforms
Secondly, I don't see a reason for the transformer to be powered from the three phase as it is. It should be powered from the single phase input.
I suspect this circuit was originally for an RPC system and the VFD was added later, but I don't think it represents current good practice-I could be wrong
If you are going to have to replace the VFD I would eliminate these contactors and the transformer too.
Mark
 
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Eddyde

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#6
Mark is correct. You shouldn't have a contractor between the motor and the VFD. The VFD output should be wired directly to the motor with no switches or controls in series. The control wires from the forward reverse switches should go directly to the VFD which will control those functions.
 

dshore

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#7
Thanks Mark and Eddy. The VFD is history; I will use Phase-a-matic RPC from now on. I'll also put a new transformer in since it not only powers the switch, but it is somehow involved in the operation of the overload relay (contact X2 on the transformer's secondary side). Please chime in again if I'm still missing something.
Thanks again, dshore
 

markba633csi

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#8
That's OK, with an RPC it should be alright.
best regards,
Mark
 

Silverbullet

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#9
I think a little better communication in those VFD set up manuals would have saved you the costs. I guess being the new toy in the box makes everyone want and need. There not real simple to just plug in and use . I'm no electric genius but I've done a powerful Hugh amount of wiring and repair work motor repairs house wiring and electric tools . But I don't understand Chinese or some,,, most,,, of the terminology related with those toy box items. Kind of glad I'm sticking with the RPC build. Or dc motor switch out. Transformers have worked forever I see no reason not to still trust them . Guess I'm old and old fashion but look how long the thing lasted before the new toys come out.
 

dshore

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#10
Yes, the manual for the VFD I had used attempts to be very comprehensive, but since it is not well written it only ends up being very confusing. No where in the manual is there a basic or clear instruction for setting up the device as merely a phase converter for an old machine tool. I guess this should be taken as a hint: Do not use the device merely as a phase converter for an old machine tool.

The RPC is now hard wired into the lathe in question. A new transformer is now on its way to me in the mail.

dshore
 

ChrisW

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#11
I agree with Mark and others as to the potential cause of the VFD and transformer failure.

As far as the need for a transformer....it has to do with what is considered "safe" and economical. I have worked on machines that have had 480 volt starter coils (long, long time ago), as well as 240, 120 and 24 volt coils. It has to do with what voltages you want at the start-stop station. (Those push button switches have to be rated for the voltages you are working with). These days a lot of machine tool manufacturers use 24 volt coils and push buttons. That means there are lower (safer) voltages where the human operator interacts with the machine.

To the OP.....just remember not to use the generated leg of an RPC to tie into the transformer.

My 2cw
Chris
 
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