Motor starter / static phase converter repair

[Lucien Nunes]

We've had a few threads about phase converters recently. Here's one I repaired on Monday. It's a little box that takes in single-phase 230V and feeds a 3-phase motor wired for 230V delta. It includes a manual 'soft' starter and overload relay. Unlike the units in the other threads, this one dates from around WW2. More pics and video tomorrow!

 

[Lucien Nunes]

The starter was working, but the back of the box was warm to the touch when running, which it shouldn't be. The heat was coming from the large black box bottom left, which contains a 17µF paper-in-oil capacitor of 900V AC test voltage. Failed leadout seals have let in moisture, giving 20kΩ of leakage resistance (in capacitors, leakage doesn't mean earth leakage, just a resistance in parallel with the capacitance) which would soon have cooked it and perhaps caused a solid short. FWIW paper capacitors also develop leakage without being unsealed - the paper can degrade and release moisture and contaminants into the impregnation.

This was a field repair so I took along two 35µF 470V caps to connect in series, and as a precaution, a 10µF 450V type to replace the green cap bottom right (although that cap turned out to be in perfect condition electrically, like the two grey ones above). I'll do a permanent job when it gets back to the workshop, but for now, the three capacitors are strapped together and slotted into place with a suitable packing piece.

 

[marconi]

WW2 - German equipment - what was/is it for? Do we see a lens bottom right of the last image?

 

[Lucien Nunes]

No, you see a wired glass window. But there is a lens, further up. It's not actually of wartime manufacture, I don't have an exact date. The systoflex sleeving over the PVC-insulated wiring is very much of its time though. PVC was around and had new and interesting properties, but no-one was quite confident in its ultimate insulation capabilities. The very best rubber insulation is remarkably high, PVC could not match it and at first some considered it a poor substitute, for use only while rubber supplies were limited.

More later.

 

[Chris1234]

Interesting, I have a more modern one that likes to vibrate all its bolt loose.

What your opinion on single motor vs two motor versions?

 

[westward10]

Zeiss Ikon must surely point to something optical or photographic.

 

[Lucien Nunes]

Indeed it is. An Ernemann VIIb 35mm projector.

The starter is operated by steadily raising the handle at the side. This turns a small drum-switch that re-arranges the caps in a sequence that boosts torque at starting. It then latches in the run position with the ideal running cap values connected and the resistance out of circuit. When the manual stop button is pressed or the overload coil operates, the handle springs back to the stop position. There is no zero-volt release; if power is lost and restored with the handle latched in the run position, if the machine is still turning it will regain speed, which is preferable in this application. If it is stationary, it might start, or if not, the overload coil will trip.

The motor runs silently with reassuring voltage symmetry. After the repair it had something like 232, 236 and 237V on the three phases. It looks like any normal 3-phase induction motor but it is actually synchronous and runs at exactly 1500 rpm without slip, and would drive a clock with perfect accuracy. The rotor has a cage winding for starting, but also salient poles so that when it approaches synchronous speed, it pulls into step with the stator flux wave and runs as a reluctance motor. One limitation of the salient-pole synchronous motor (unlike a hysteresis type) is the definite maximum moment of inertia it can drive, because of the need to accelerate the load fast enough to pull-in within one half-cycle of slip. This motor is well matched to the machine and runs up smoothly in a second or so.

Cinematographical moving pictures for you:

 

[Lucien Nunes]

Interesting, I have a more modern one that likes to vibrate all its bolt loose.

I had some agro with vibration on a DP75 not long ago, where the chassis was resonating near 24Hz and vibrating the lens mount, enough to look like travel ghost (albeit horizontal) clearly visible on the squares on the RP91 test film. I detuned it by moving some panels, leaving the mech door open and various other fudges, after which it was fine and gave very good results.

What your opinion on single motor vs two motor versions?

As in electric vs. mechanical takeup drive? I do mainly preview work so tend to use rock-n-roll mechs with electric takeups top and bottom. Sondor Novas are flavour of the month which are 5-motor. If I'm only going forwards, I can live with mechanical takeup, although our tower-equipped DP75 which has no spindles at the moment is about to get an electric one.