I happened to be at my friends machine shop and took a couple of pictures of this old motor starter on his shear... Next time I have the thing apart I will take pictures of the innards.. Oh yes, thats the motor it controls ( 12 HP).. Just can't think of the type of motor it is even though its on the tip of my tongue!
We used to do a lot of work on presses and shears in the long ago. If I remember correctly this type of starter had some very 'nasty' oil in it. I seem to recall a lot of health warnings about it and a possibility of the switch mechanism not fully disengaging.
TrimixLeccy: Yes you are correct about the oil but this one had the oil changed out to something a lot less hazardous.. I THINK its transformer oil that you would find in every distrubution transformer manufactured now a days.. the old stuff probably was loaded with PCB's.. As for the switch mechanism not fully disengaging I have not had a problem with that if anything one of the contacts was bad for not engaging, thus single phasing the motor and tripping out the overload within seconds.. SO FAR.. so good ( fingers crossed).
Just guessing here of course, no way of knowing from pics, but it could be resistance-start. A wound-rotor induction motor has a stator like the squirrel cage induction motor, but a wound rotor, with insulated windings, brought out to slip rings and brushes. No power is applied to the slip rings. Their sole purpose is to allow a resistance to be placed in series with the rotor windings during startup. This resistance is shorted out once the motor is started; the rotor is then electrically similar to a squirrel cage type.
Why resistance? Squirrel cage induction motors draw 500% to over 1000% of full load current during starting. While this is not a severe problem for small motors, it is for large ones. Placing resistance in series with the rotor windings not only decreases starting current, but also increases starting torque. Breakdown torque peak is shifted toward zero speed by increasing rotor resistance. The resistance decreases the torque available at full running speed. But that resistance is shorted out by the time the rotor is at speed. A shorted rotor operates like a squirrel cage rotor. Heat generated during starting is mostly dissipated external to the motor in the starting resistance. The complication and maintenance associated with brushes and slip rings is a disadvantage of the wound rotor as compared to the simple squirrel cage rotor. This motor is suited for starting high inertial loads. A high starting resistance makes the high pull out torque available at zero speed. For comparison, a squirrel cage rotor only exhibits pull out (peak) torque at 80% of its synchronous speed.
Thats it! " Wound Rotor Induction Motor"! I couldnt think of the name of the type of motor it was Thank you Alan. You are also correct it is a resistance start where that handle controls what resistance is placed in series with the rotor winding.. That little tank is full of resistors and has the main contacts and everything is submerged in oil, well everything but the overload relays. And you are also correct in that type of motor being used for starting high inertial loads as this motor is coupled to a VERY heavy flywheel.