Reaction time of eddy currents to change in magnetic flux

[soundara]

Hello there,
I have a question on finding the reaction time on eddy currents.
I have an electromagnet(iron core with coils wounded in slots on it). Its contained in a rotating part. The magnetising signal is a current ramp.When the magnetising current is increased at a very fast rate (rate of flux change is high)and decreased at the same fast rate. The maximum magnetic effect seems to happen at a point where the current is already decreasing.
the eddy currents increase as the rate of change of flux increases. Could you please tell me how can I relate the timing of change in the flux to the action of the eddy currents(It would be nice if i can predict when the maximum magnetic effect will happen for a particular rate of change of flux). Please refer figure thanks.

 

[prex]

You have a moving and changing magnet field. It is a complex phenomenon that is normally not treated afaik in terms of a time constant.
When you have a moving (rotating or sliding) magnet field in face of a conductor, the resisting force or moment initially increases with speed, reaches a maximum, then decreases and goes asymptotically to zero. A repelling force is also generated that steadily increases with speed and asymptotically goes to a maximum value. That's why this phenomenon is the basis for a concept of magnetically levitated high speed trains.
The changing flux in the electromagnet adds a variation to this phenomenon, that stays basically the same.
If you do a literature search for maglev trains and magnetic brakes, you should be able to find something useful.

prex

http://www.xcalcs.com

: Online engineering calculations

http://www.megamag.it

: Magnetic brakes for fun rides

http://www.levitans.com

: Air bearing pads

 

[soundara]

Hello,

Could you please look into the attached picture and tell me what are the possible reasons for the Torque to reach its maximum long after current ? ...Thanks

 

[BobM3]

Can you post a picture of your device? I envision something accelerating with an applied torque but then how do you measure the torque?

 

[prex]

There is no simple answer to your question, at least that I know of.
Take first your device with a constant current supplied to the electromagnet (perhaps you can test this). You'll notice that there's a rotating speed for which the resisting moment is maximum: it's lower at lower speeds, but also at higher speeds.
Your variable currents (with constant speed) create somehow an effect of changing speed. I'm not able to predict or explain quantitatively anything: a detailed calculation by FEM or an in depth analysis based onto the principles I recalled in the former post is required to explain.

prex

http://www.xcalcs.com

: Online engineering calculations

http://www.megamag.it

: Magnetic brakes for fun rides

http://www.levitans.com

: Air bearing pads

 

[soundara]

Hi Bob,
The torque is measured using a torque sensor which measures the torsion on the shaft.the shaft has a rotating cylinder inside which the electromagnet is contained. I cannot send a pic of the device as the work is confidential.

 

[BobM3]

The rotating part and its acceleration is what I'm getting at. If you sum the torques on the rotating part don't you get:

Applied torque (from magnetics) = Inertial torque (from acceleration) + measured torque?

So then, when accelerating, applied torque does not equal measured torque.

 

[BobM3]

Another possibility is ratio of magnetizing current to load current. The magnetizing current (the part that spins the rotor) will lag the load current. Can you measure the real and imaginary parts of the current?

 

[soundara]

some more info on the device. The cylinder shaft arrangement actually simulates the drive train of a vehicle. The cylinder contains a planetary gearset. For torque transmission, the two parts of the planetary gear set are held together by magnetising. The electromagnet also contained within the cylinder in a suitable arrangement provides the flux for this magnetisation.
I would like to know the relation between the rate of change of flux and the rate of change of the measured torqe...So indirectly the eddy currents come into picture. thanks