What kind of motor is this?

[Richard Williams]

Old

 

[freddo]

Repulsion type,with brush gear and com, used on early type compressor needing good starting torque.

That reminded me that there was a large cast iron oil filled motor starter for a wound rotor motor in a local laundry basement that once fed a compressor, sadly the motor and compressor are long gone, but the air receiver sill remains. Still full of (probably hazardous) oil, had a manufacture date of 1949 IIRC. I wonder if it is still there. Had a little hand wheel to operate the variable resistors for starting, still turned ok. Fed from a large adjacent Memajor switchfuse all wired in poly butyl jute cables in 1 1/2 steel conduit.

How do I remember stupid details like that from years ago but can't remember important things that are happening right now?

 

[Santa]

One of my earliest jobs was working in a small brick factory in the Cotswolds. Every morning my first job was to start the motor that ran all the machinery through an overhead pulley system.

This was a hefty beast, from memory it was a least a couple of feet diameter and was controlled by a big cast steel switch box on the wall This had a lever with four positions, and I had to stand there while the rest of the crew sat around smoking and drinking tea, and move the lever through each position in turn.

There was no gauge or anything, I judged by ear when the revs were high enough to move up.

I wonder if anyone could tell me exactly what I was doing? This would have been in the early 60s.

 

[Simon47]

Could have been some combination of tap changing and star delta.
But in that era, the aforementioned slip ring induction motor with variable rotor resistance would be a good candidate - I'm assuming it was started unloaded but with a massive inertia of drive lines & pulleys to wind up.
With a large resistance across the brushes, the rotor current is limited - so torque is vastly reduced, but so is stator current.
So did the first notch set it slowly tunning up like there's all the time in the world ? Then the next notch add a bit of urgency ? Until it's up to speed and can go to the last notch where the rotor is shorted and it runs as a normal squirrel cage motor.
Presumably, you had to learn by ear how fast it needed to be running before you could go to the next step without blowing fuses ?

 

[Santa]

Could have been some combination of tap changing and star delta.
But in that era, the aforementioned slip ring induction motor with variable rotor resistance would be a good candidate - I'm assuming it was started unloaded but with a massive inertia of drive lines & pulleys to wind up.
With a large resistance across the brushes, the rotor current is limited - so torque is vastly reduced, but so is stator current.
So did the first notch set it slowly tunning up like there's all the time in the world ? Then the next notch add a bit of urgency ? Until it's up to speed and can go to the last notch where the rotor is shorted and it runs as a normal squirrel cage motor.
Presumably, you had to learn by ear how fast it needed to be running before you could go to the next step without blowing fuses ?

You got it exactly, although it didn't blow a fuse, it just cut out at the switch and I had to start again with much mockery from the others

. It was started as you say with the overhead shafts turning, but all the drive belts were disengaged. We just slipped the belt off the side of the pulley to stop a machine - no guards to get in the way.

 

[Simon47]

Standard setup with lineshafts is to have dual.pulleys on the driven end of each take off belt. One pulley is free running on it's shaft, the other drives it. So starting/stopping machines is a matter of throwing the belt from one pulley to the other - where it will stay on it's own thanks to the crowning on the two pulleys. In posh safety concious setups there may be a mechanism for doing it, otherwise it's done carefully with a stick to push on the side of the belt

 

[Lucien Nunes]

A.k.a 'fast and loose' pulleys, fast as in fixed rather than quick.
As Simon says there were a number of different arrangements for starting a motor manually. Resistance starters, such as a rotor resistance for an AC motor or armature resistance for DC, would often have more than two intermediate steps, so it seems more likely to have been an autotransformer starter and you were changing voltage taps. But there were certainly some resistance starters with just a couple of steps so anything is possible.

Another possibility is that it was a synchronous motor, which was a popular choice for larger drives to keep the power factor up (and even compensate for low power factor elsewhere, by running it over-excited). In that case, the steps might have been off, star, delta, synch, but something that sophisticated would probably have merited an ammeter.

I wish I could get one of these installations complete to rig up in the museum workshop. It's a way off. I have a nice big DC motor with 8" belt pulley, something like 50hp at 110V although the plate is missing. I think it served in a wood shop, it was packed with sawdust to the point that the armature appeared to be wound with MDF. Unfortunately no suitable starter for it, although I might be getting a couple of cast iron starting pillars - the kind with drum controllers with handles on the side, and glass windows - so if we are very lucky one might have suitable resistances and coil voltages. It would run nicely off one of the mercury arcs or DC gensets.