Two Speed Reverse Single Winding Motor

The new motor is in!

It was more of a fiddle than I'd imagined.

Yet again, I simply could not get the pulley stack off the shaft of the old motor, despite heat, cold, levers, gear pullers and (eventually) extreme violence. In the end, I had to cut the shaft of the old motor off: the part I really needed to keep being the base plate (to mate to the Bridgeport's head). That took a full day.

The next problem was that the motor I'd bought had a 1" diameter shaft, 1.5" long. I needed a 3.5" long shaft to take a pulley with an id of 3/4". I ended up making an adapter out of a length of 2" diameter steel. Turning that down, drilling through it and boring it out took the best part of another day. At least I got to use my 10" Sheldon lathe for a project.

Adapting the new motor's base plate was easy (angle grinder) as was cutting out the holder for the bearing on the old base plate and drilling it to fit the new motor. I would have used my Bridgeport but, of course... so, I had to resort to drilling free-hand and it went surprisingly well.

With the new motor in place, it became obvious that I'd made the shaft just a tad too long. My pulley wheel was catching on the final drive to the mill's spindle. Whereas the old motor was a squat, fat thing, my new motor is thinner and longer. This matters because my workshop has quite a low ceiling and the new motor just about touches the roof spar. Lifting up the motor to put a spacer washer underneath proved to be quite tricky.

The electricity was a doddle. The motor came with clear instructions (in the connexion box) for choosing either Wye or Delta and my only niggle was swapping wires to get it to run in the desired direction.

I used my hand-held rev counter and I seem to be getting about 39 rpm per Hz. I mean that at 60Hz I have a spindle speed of 2,340 rpm; at 80 - 3,120; at 30 - 1,170. I think it will be useful for the jobs that I'll be asking of it.

My only problem now is that the belt is a bit slack, even with the tension set to maximum. Under load the spindle has a tendency to stall while the motor keeps spinning. I think the solution is to fit a slightly smaller belt. Or, maybe, a new standard sized belt that hasn't stretched with years of use. Does any one know the dimensions of the standard belt? That would be useful...

Onwards and upwards!

Kind wishes,

Nick
 
Here we go: the internet can be useful at times!
Bridgeport BP 11192101 V-Belt, 34-5/8" Length x 1/2" Width
 
markba633csi said:
Your experience got me thinking that line filter chokes in each of the three legs between the VFD and the motor might be a good idea for
older motors- they would have to be high current chokes >10 amp but I don't know how much inductance would be required.
Mark
Click to expand...

Load (not line) chokes are usually specified by %, e.g. 3%, 5%. This is the percentage of line voltage dropped at full load amps so e.g a 5% choke will drop 12v on a 240v supply - 3% is typical for VFD/ motor protection, 5% if it's an irreplaceable item.
To get inductance from voltage drop and full load current at operating frequency the voltage drop divided by current gives the inductive impedance (Z), Z = 2 pi x frequency x inductance, rearranging that Z divided by (2 x pi. x frequency) = inductance - expect it to be in the millihenries.

Mine worked out at around 10 - 15 mH for a 6.5A full load current at 415v so...

I built a set on three identical 24v lighting/control transformer cores with the windings stripped, the E and I laminations rearranged to all-Es and all-Is separated by a PTFE shim and a hundred and some turns of 1.5 sq.mm emamelled wire in the original coil bobbin, they measure about 12 mH and appear to work so far...

Understand that the impedance increases with the frequency so a choke that drops e.g. 12v at 60 Hz drops substantially more at the harmonics of the VFD switching frequency (some kHz stretching to some 10s2or 100s of KHz) which make up the fast rising / falling edges that a VFD generates and which break down insulation in older motors, the current and voltage downstream of the chokes will be much closer to a sinewave (although a knobbly one).

Hope that helps, rather than confuses, YMMV etc.

Dave H. (the other one)
 
Great information there Steel, I see you've been knocking about with electrons for a bit
Mark
 
Wow, you have been put through the wringer on this Bridgeport motor. You should not need line reactors with inverter-rated motors, and the general experience with backyard shop use seems to be OK without them even on old motors. Obviously your experience was different. Don't know if discarded treadmills are common where you are, but they very often have a choke in them for smoothing the DC motor operation from a "chopper" type drive. Maybe the primary winding of a microwave oven transformer would work? Obviously you'd need three quite similar chokes. I don't see this being a two-speed motor. Are you sure the original switch isn't just for manual wye-delta starting?
 
Thanks,

The original motor was, in fact, wired for two speed operation on 415 V. Strictly, I suppose, four speed (two forwards and two reverse).

I wouldn't try to draw too many conclusions about using old three phase motors with modern VFDs from my experience. I sliced the outer case of the motor with an angle grinder, cut the wires to the windings and re-wired it to make it run on 240 V. Frankly, I think I was lucky to get a year out of it.

The new motor is "inverter rated" and I'm not going to bother with chokes and filters: I suppose it might have helped with the old motor, but we will never know. All I can say with any confidence is that my old, hacked-about motor suddenly started giving me ground-fault errors on the VFD and these disappeared when I fitted the new motor. It seems that something bad happened to the windings, but it was as likely to be me as the voltage spikes from the VFD.

Anyway, for the record, with a belt of the correct size the machine is now running smoothly again and I can carry on learning the basics of running a milling machine. I'm very grateful to everyone here who has given me advice: no-one has (yet) suggested that I might be an idiot.

Kind wishes,

Nick
 
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