Won't start in high range

I tried running lathe on converter the other day. Lathe has two speed motor, 4.5/5 kw converter built from 7.5 hp motor. Starts ok in low but no go in high. Bigger converter required or more capacitors?? BTW works fine on real three phase
Bob

It is hard to comment on the issue. Missing data is:

Motor HP and current ratings from motor nameplate
phase to phase converter voltages loaded and unloaded.

Based on the RPC size and the KW rating of the lathe, your problem is most likely to small of a RPC. The problem with your lathe is the high speed end of the lathe requires alot of inertia to get the gears and lathe head up to speed, because of this your getting a large voltage drop on the manufactured leg. You can try adding in more run caps, but I have to say the end result will be to increase the RPC to at least a 15hp. Kept us posted on what you find

bob said:

I tried running lathe on converter the other day. Lathe has two speed motor, 4.5/5 kw converter built from 7.5 hp motor. Starts ok in low but no go in high. Bigger converter required or more capacitors?? BTW works fine on real three phase
Bob

Click to expand...

Would this be a Chinese two speed motor? If it is you may be in for a interesting time. What brand of lathe? I have had some experience.

Doug

Not starting on high speed, or slow starting on high speed, and similar starting problems, is traceable to RPC systems having too high a source impedance for the load.

Obviously, utility three-phase doesn't have this problem. The source impedance of each of the three phases is the same, and is very low (exception for so-called "wild leg" systems, more correctly called "open ∆").

An RPC system simulates, to some degree, an open ∆ in that the source impedance of two phases, Phase A and Phase C, is low or very low, whereas the source impedance of the remaining phase, Phase B, is much higher. It is very much higher in the case of an RPC system, whereas is it just much higher in the case of utility open ∆ three-phase.

So, the solution to the high speed starting problem, much as the solution to starting an air compressor, is to provide an RPC system which has a much lower source impedance for the manufactured phase. (The source impedance for the remaining phase will remain very low).

This solution requires a much larger idler, and more balancing capacitors.

It may well be that you will have to go to 2 times the rating of the tool motor. Possibly more than two times.

This would make a very stiff manufactured phase, indeed.

The other solution is a Phase Perfect.

Actually this is a Chinese motor but I have seen the same problem with European stuff. A guy I know has a Harrison with same problem and I had an old Maho mill once with same trouble.

peterh5322 said:

This solution requires a much larger idler, and more balancing capacitors.

It may well be that you will have to go to 2 times the rating of the tool motor. Possibly more than two times.

This would make a very stiff manufactured phase, indeed.

The other solution is a Phase Perfect.

Click to expand...

I have a lathe that has motor equal to about a 4.5 hp motor on high range and my 15 hp RPC will not quite start it. And it is very well balanced with caps. Runs great on low range. I'm going try adding another, 7.5 hp idler and see if that will do it but otherwise I think a VFD would be the way to go when I feel the need to use the higher speeds.

Doug

"I'm going try adding another, 7.5 hp idler and see if that will do it but otherwise I think a VFD would be the way to go when I feel the need to use the higher speeds."

That would indeed be a unique solution: use an RPC for low range, but use a VFD for high range.

I suppose it IS possible, but the control system could get rather complicated.

That would indeed be a unique solution: use an RPC for low range, but use a VFD for high range.

I suppose it IS possible, but the control system could get rather complicated.[/quote]

Well, actually I would just use the VFD on the high range as I'm sure that in the lowest gears the motor would have lots of power still at half speed.
I am still hopeful that I can get the high speed to work with some tweaking. Like bob's it works perfect when 3 phase from a PTO generator is supplied but hums and turns slow on the RPC. I can get it to run with the RPC if I switch quickly from low speed to high speed. (being careful not to harm the drum switch)

If I had it to do over again however I think I would try to stay away from a two speed motor

"If I had it to do over again however I think I would try to stay away from a two speed motor"

At least any two-speed motor which was also a "consequent pole" motor.

Oliver patented a motor control system for four-speed two-winding "consequent pole" motors, which was constant HP, not the usual constant torque type.

This was a direct drive pattern maker's wood lathe.

As the control system disconnects the line before changing the poles, and as the heaters were placed in a unique configuration such that all four combinations of windings and poles received the appropriate protection, this suggested a solution was actually quite simple: place the VFD after the contacts which disconnected the line but before the contacts which selected the windings and poles, and incorporate the heater's N.C. contacts into the VFD's emergency stop function.