Let's put our 'thinking-heads' on chaps!

During operation of an induction motor, the 3-ph stator windings create regularly rotating fields in a rotor placed within them, creating currents in the rotor's cage conductor bars. Variation in magnetic linkages, [ because of 'slip' ], induces current in these bars in a direction which tends to counter the stators' fields.

Now turn the motor off. All three supply phases are cut, the stators' fields collapse because no current is flowing in the windings. When it collapses, the rotor fields must collapse too, as they are induced in the rotor bars - hence the name "induction" motor.

If we now, during rundown, [ theoretically, of course, in the interests of Science! [Linked Image] ], re-arrange the motor connections by shorting a stator terminal to another stator terminal via a 'theoretical' electrician, what happens? Does he receive an electric shock?

Depends on the speed of the rotor, and whether any residual field is retained in the rotor, I think. Immediately after switch off, residual fields may exist in the rotor which could exite the stator fields to generate a voltage. The fields' rapid initial collapse means that new fields would have to form in the stator windings upon our theoretical electrician's shorting of the terminals. Without the regular poco 50-hertz supply creating the rotating field, what you could get is a rapid series of voltage spikes as each bar passes by each field winding, reducing in intensity as the speed falls away.

Not, I deign, a regular 50hz three-phase sinewave voltage, anyway.

Enough volts to shock? Don't know is the answer! -but probably yes if the residual motor speed is high enough and residual rotor magnetic fields actually exist.


Alan




[This message has been edited by Alan Belson (edited 08-18-2006).]

Wood work but can't!