ECE3300 Lecture 2-5 Magnetic Fields

ECE 3300 at the University of Utah. More info and the power point files are available at www.ece.utah.edu/~ece3300

Category:

Education

Tags:

• ECE3300
• electromagnetics
• magnetic field

[WallesWillerWalla]

I have one simple question, do the magnetic flux lines always stay the same in relation to the magnet (be it a permanent magnet or an electromagnet), even if the magnet has speed? If a magnet is spinning, for example, will the flux lines "lag behind", or will they always have the same direction in relation to the magnet? I'm thinking of speeds way below relativistic speeds, such as a magnet spinning at 8 krpm. Thank you!

[cfurse]

@WallesWillerWalla For the purposes of ordinary non-relativistic work, yes, the flux lines stay constant relative to the magnet even as it is moved.

[cfurse]

@cfurse

I have heard about this but don't have tremendous depth. As a motor reaches very high speed, which DC brushless motors are very capable of, the non-instantaneous rate of current through a coil can possibly lag the armature speed.

DC brushless motors have many stators, possibly 8 or more, so at 30K revs/min each coil would need to fire on the microsecond time scale (my math might be off here so check me).

[alabuzid]

thanks for the lecture. In minute 1:34 you said relative permetivity. Did you mean relative permeability?

thank you

[cfurse]

@alabuzid

Yes, correct.

[WallesWillerWalla]

@cfurse Thank you for your reply! We had a discussion about this at work, if "lag" in the magnetic field could affect the performance of a brushless DC motor. The theory someone had was that the hall effect sensors timing could be off at higher RPMs due to magnetic flux "lagging" behind after a change of material in the shaft (different permeability). Now I know for sure that's not the case! :)

[cfurse]

@EEUte Static just means that it is constant, not time varying. DC current is static.  An example would be current coming from a battery such as in a flashlight. If you have a static (not time varying) load, such as the resistor in a lightbulb, the same current will come from the battery all the time. This will go on 'forever' until of course the battery wears down and its voltage decreases or until you change something like switching off the flashlight.

[jpsant2006]

I don't think the magnetic field, H, becomes stronger due to the material if the equation is truly B (magnetic flux) = uH.So the statement H_iron > H_air, its actually the magnetic flux because of the ferromagnetic material. So the correct statement should have been B_iron > B_air

[tschakka144]

Not a big deal, but you could put units on mu naught, and mu