WRIM

The rotor will turn because there is material where a current can flow in the rotor plates. When the motor was run before the open cct, it built up a small residual magnetism in the rotor plates.

If the rotor was only a solid piece of metal it would turn in a rotating magnetic field, however, eddy currents would make it inefficient.

The plates inside the rotor coils are designed to try and minimize these currents.

You would probably not get any significant speed or power from the motor.

"What are the dangers of runing it to synchronism and/or for some time?"

I for one don't understand this part of the question. Are you mixing up wound rotor induction motors with synchronous motors? They both use slip rings, but they are not the same.

Sorry,I meant runing it to full speed and/or for a long time.Sorry for the mix-up of words!

If the motor is rated for continuous usage and the load is not beyond its capabilities and it is a reasonable condition, insualtion testing shows no weaknesses there, there is reasonably cool air around to remove most heat generated (not shut inside a box), no worn brushes or bearings etc., it should run just fine......

What is residual magnetic field? During operation could the cores of the rotor coils become magnetized? Perhaps, given a high retention in the coil cores, the rotor could hold enough magnetizim to supply the torque needed to turn the motor when there is no load on it?

Gavilan

You don't need residual magnetism to get rotation.The rotor will behave just like an ordinary squirrel cage motor,with currents being induced in the laminations producing a weak magnetic field.In a squirrel cage motor these currents are small compared to the current in the aluminium or copper bars,being deliberately minimised by the laminated construction.but are always present.

Be careful, a high voltage will be at the rotor sliprings if the rotor is open circuit with the stator energized,being at a maximum when the rotor is at rest,and diminishing with speed.

your answer is pretty good but are you talking about eddy currents in the luminations?Do you think they are enough to set up sufficient field considering the inter lumination insulation.

Often if the motor/transformer whatever is laminated has seen some service in wet/tropical environments the very thin paper between the laminations deteriorates and the laminations are effectively connected as a mass,albeit with still a fairly high resistance,but because of the large areas in contact quite large eddy currents can flow.

It just occurred to me that perhaps our colleagues rotor has some shorted turns,which would also give rotation and may not be noticed in normal operation as after startup the coils are shorted anyway. I have seen shorted turns but only on motors with very heavy start loads,like rock crushers repeatedly trying to start with a large boulder stuck in the jaws. These motors had a high initial current and violent starting,eventually breaking jaws instead of rocks!

The laminations in anything electrical are there always to reduce eddy currents, otherwise the item may get very hot and just waste power.

In the type of motor (which I believe you are talking about) these currents are needed for operation and are developed what is generally termed a "Squirrel Cage" design is in the rotor, to make use of these currents in a positive manner, eg. cause rotation.....but with as little heating as possible.

Please checkout Wikipedia on:-

http://en.wikipedia.org/wiki/Squirrel_cage_motor

Always thought the term 'eddy current' was misleading as the varying magnetic field set up by the primary windings or stator creates currents in whatever it passes through,but these currents are referred to as eddy currents when undesirable,and induced currents when useful!

I wonder why achibuye open circuited the rotor? When building stator-rotor starters I always left the first stage resistor permanently connected as there was no need to switch it,and this prevented high voltages at the rotor terminals,(just like cts.)