Induced voltage in induction motor

[liny]

I am a mechanical engineer with a electrical question: Where does the induced current/voltage go on rotor in an induction motor since the rotor does not connect to anything? Does it go in circles and the energy dissipates in form of heat? Thank you for your answer.

 

[electricpete]

I would say the induced voltage creates current which flows in circles (loops). There is resistance in rotor bar circuit which creates I*2*R heating in the rotor.

Another question that might be asked is whether a portion of the product of Vrotor*Irotor*PFrotor goes to mechanical work. I believe the answer to that is no. There is a difference between Vstator*Istator*PFstator and Vrotor*Irotor*PFrotor. A portion of that difference goes to the mechanical work output.

 

[Lewish]

Part of the induced current ends up going into the motor frame thru the bearings and causes bearing fluting. A serious problem.

 

[UKpete]

Yes the induced current only flows within the rotor (the exception being a wound rotor induction motor which has slip ring connections to external resistances for starting and speed adjustment, rare these days because of VFDs).

The construction of a rotor is referred to as a squirrel cage (a set of axial bars joined by a circumferential ring at both ends), although this refers to the aluminium or copper conductors in the rotor which are embedded in the steel rotor laminations.

The pattern of currents is easiest to describe in a two pole machine - current flows along the bars in the axial direction (same direction in adjacent bars), flows both ways around the end rings, and back in the opposite direction on the other side of the rotor. In a four pole motor the currents only flow around the end rings through 90degrees before returning through the bars; in a six pole, 60degs etc..

 

[GusD]

UKPETE.
About the rare WRMotors.
Most of the large conveyor belts used in the mining industry and possibly others are still using the old but "reliable" WRMotors.My firm is currently opening new mines that will have their conveyor belts powered by WRMs.
We do have the advantage of being able to recycle a lot of the old WRMs that we esu.I know that VFDs are making some inroads in the large HPower conveying applications,but still in a small number by comparison.


GusD

 

[UKpete]

GusD
I was looking at the bigger picture, but it's pleasing to see these machines still hanging on in there.

UKPete

 

[jbartos]

Suggestion: One must look at the motor as an energy converter, which for the mechanical engineer is a piece of cake. Simply, the induced currents in the motor rotor help establish the electromagnetic field that is electromagnetically locked with the stator rotating electromagnetic field. This tie over the electromagnetic forces causes the rotor to rotate and propel a load connected to the rotor shaft. This obviously consumes electrical energy supplied to the stator. So that the energy equations are satisfied. There is no perpetum mobile.

 

[swgrmfg]

Liny:

Just simply put, the induction motor is a rotating transformer.

There a rotating electro-magnetic 'B' field created by the 3-phase power supply and the geometry of the stator. Faraday's Law states something like in the presence of a magnetic field, a current carrying conductor is subject to a force F equaling to B.I.L.sin(alpha), B being the magnetic field, I the current in the conductor, L the length of the conductor upon which B is acting, and alpha, the angle between B and I. The conductor here is the rotor bar that develops a current due to the transformer effect between the stator and the squirrel cage rotor bars. F multipied by D, the rotor diameter, generates a torque, which in turn generates motion.
In order to understand the transformer effect one must think about the difference in speed, otherwise known as slip, between the rotating stator magnetic field, and the motor rotor itself. At standstill, the slip is 100% and the induced voltage is high, and at 100% speed (synchronous speed) the slip is 0% and the induced voltage is zero. In the case of an induction motor, the slip is 3 to 5%, and as long as there is slip, there is transformer effect, and thereby, torque.

GH