Help on Flux Measurement in IM

[BigLe]

Any one gets some experience on the measurement of magnatic flux position in an induction motor? I'm in the middle of a project that need such direct information instead of observers. the flux magnitude is not that important, just the position. Thanks a lot

 

[motorsdirect]

I have seen an experiment done using a magnetic compass in the stator of an induction motor,energised with a much reduced supply, and by moving the compass round the core,It indicated the magnetic pull,alternating between the north and south poles in the windings.
I hope this is the sort of thing you are after!!

Jeff

http://www.motors-direct.co.uk

 

[UKpete]

BigLe, what do you mean by observers?

I don't have any experience of doing this specifically with induction motors but one possibility is to use search coils - the voltage induced in which is given by:

v = 4.44*pi*f*N*phi - for a sinusoidal moving flux wave
(f=frequency, N=no.of turns, phi=peak flux in Webers)

Search coils can be wound with fine enamelled copper wire and can easily be fitted within the airgap, epoxy them to the stator core. You will only need a few turns, and can fit several within the airgap. Individual coils needn't extend along the full length of the core.

And it's cheap!

To detect the flux position you will need to display the coil voltage (sinusoidal) on an oscilloscope against a reference eg from a shaft encoder.

 

[BigLe]

Thanks, guys,

by observer, i mean to reconstruct the flux information from the measured stator currents and the knowledge of machine parameters. it's "calculated/estimated" flux, which is used in Indirect Field-oriented Control of IM motor.

Talking about coil, I do have one wound around a tooth on the stator, while the v(t) = diff(flux)/dt at realtime, the fomular v=v = 4.44*pi*f*N*phi may be the steady state relationship.

Sounds the coil idea might be the only choice, Thanks a lot.

 

[UKpete]

BigLe, yes the voltage is the RMS value. For the record, I slipped up in my original post - the shaft encoder isn't required as the coil voltage will go through zero as the peak of the flux wave passes the search coil.