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2 speed motor of VFD - check my reasoning plz

Peter.

Titanium
Joined
Mar 28, 2007
Location
England UK
I have bought a machine which has a 3 phase 2 speed motor and I don't have 3 phase power but I also don't want to replace the motor - I want to power it from a VFD which outputs 240v 3 phase. I've got the wiring schematic and figured out that it's a pretty standard 6-wire setup that switches 3 of the wires to be a star point and convert the 4-pole motor to a 2-pole for high speed. Here's the layout:

fwd rev 2 speed layout.jpg

What I want to do is re-configure the motor to run at the slow speed from a VFD (which is 240v output instead of 415 original supply). Online research suggests that if I run the motor in delta mode same as the slow speed schematic above, but split the two windings and wire them in parallel instead of series, that will be the appropriate configuration.

My plan is to disconnect each pair of windings at the mid-point between phase inputs, one at a time so that I don't make any mistakes, and re-connect them in parallel like this:

2 speed modified for vfd.jpg

If I do one at a time I'm pretty certain that this will keep the winding orientations correct but what I'm clueless about is will it give me the 4-pole 240v motor that I'm expecting or will that not be so? I don't mind giving it the 'suck it and see' approach but I thought I ask right here in case there is something fundamental I am not considering.
BTW I get the machine home next weekend so right now I cannot pull the motor, I'm at planning stage for doing this work right now. I've got the VFD ready to go and the wiring is no problem for me.

What do we think? Is it just as simple as that?

Pete.
 

J_R_Thiele

Stainless
Joined
Jan 22, 2003
Location
Columbia Missouri
Try and make the first image larger. It too small for me see.

In a recent thread rootboy posted a link to a youtube video which may be helpful for you. It is very well done, and through enough that it ended up being 4 video's. It shows how to make a RPC which runs off of 240 volt single phase and generates 415 volt 3 phase. I know this is not what you are asking about, but it would allow you to use the lathe and motor as designed.
There also may be something in his description and diagrams which would answer your question.

Rotary phase converter - UK - Part 1, Intro. - YouTube
 

jim rozen

Diamond
Joined
Feb 26, 2004
Location
peekskill, NY
From the details of the switchgear, this is a consequent pole motor. The typical approach to running these from a VFD is to hard-wire them into the high speed configuration, and connect your drive directly to the motor leads. This means three leads will be shorted together and three will go directly to the drive. (green lines on your diagram are the shorting connections for high speed).

On edit: the original post was not clear, is this a 415 volt designed motor, or a 240 volt designed motor?
 

jim rozen

Diamond
Joined
Feb 26, 2004
Location
peekskill, NY
Your approach might work but I've never seen it done - the windings are basically a ring so you might have some trouble digging out the leads to put them in parallel. If it does not work of course you could always put it back. Any chance for a re-wind from motor shop?
 

Peter.

Titanium
Joined
Mar 28, 2007
Location
England UK
Your approach might work but I've never seen it done - the windings are basically a ring so you might have some trouble digging out the leads to put them in parallel. If it does not work of course you could always put it back. Any chance for a re-wind from motor shop?


Well thee's nothing special per se about the motor except it's housed within a machine and it's a slightly over-sized open frame motor of good quality, so if it doesn't work I always have the option of replacing it.

I will atempt to open it up and re-configure the windings, and report back with my results.

Thanks
Pete.
 

Peter.

Titanium
Joined
Mar 28, 2007
Location
England UK
Your approach might work but I've never seen it done - the windings are basically a ring so you might have some trouble digging out the leads to put them in parallel. If it does not work of course you could always put it back. Any chance for a re-wind from motor shop?

This turned out to be providential. The motor had one lead open-circuit and I found the break easily enough as it was only 2" from the terminal but the insulation is so delicate that the varnish was flaking even with gentle manipulation. I have abandoned the idea in favour of fitting a more modern motor on an adapter plate.

Thank you to everyone who offered their input.
 

Peter.

Titanium
Joined
Mar 28, 2007
Location
England UK
As a follow-up I'd like to report that I didn't give up on this motor. What looked like impregnable insulation was actually quite easy to break away from the widings, which remained undamaged. I found all of the ends easily accessible under the wrapping, brought them out and linked them as in the diagram in post #1. I re-bound the windings, gave it two coats of winding varnish and a few days to dry and it's now up and running on a cheapo inverter whilst I wait to get hold of a branded unit.

motor windings.jpg

Motor windings joined.jpg

windings bind and potted..jpg

50hz.jpg

100hz.jpg
 








 
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