back EMF re load, I, PD, A and electron density (1 Viewer)

[superhubert]

I understand the concept of Back EMF, but i know that if a motor slows down under a load, the motor can burn out. I also know that this causes changes to the level of back EMF and current, voltage etc, but can't find out what exactly. Is there anyone out there who can tell me? Also, how does the electron density of a conductor change the drift velocity?

 

[BlackJack]

electron density should decrease drift velocity..., but why do you need it?
(personal logic: same voltage applied to more electrons, then each electron is pushed less.)

For the motors, all I can tell you is only some types of motors burn out... I think AC normal...
Back EMF depends on how fast the coil is moving through the mag field, so back EMF decreases..

 

[McLake]

Um, BJ that's cause the AC motors are the only ones producing back EMF. Check old test, they tell you the relationship between V, I and EMF (can't remember off the top of my head ....)

 

[BlackJack]

ML, wouldn't all moving conductors in a magnetic field induce current? That current is back EMF.

 

[superhubert]

they do, if you remember, industry uses DC for the high power, and they have to put resistance in series when starting up the motors to stop the current burning the motor out at start-up. once they're running though a back EMF exists and the resistance is turned off.

 

[McLake]

Oops, I was thinking of eddy currents. I obviuosly need to study physics more ...

 

[BlackJack]

Obviously you've got something more importatn to study for right now...
Good luck.

 

[Wintermute]

When a motor is moving, it contitutes a conductor moving through a field. As a result a back emf is induced (Lenz' law). This emf opposes the original emf, and the motor speeds up until the two emf are balanced. Now when you place a large load on the motor, there is less back emf induced because the coil cuts the magnet field less over the same period of time. As a result more current is allowed to pass in the motor which creates heat which can burn it out.

The only things that affect drift velocity are the strength of the electric field and some constant to do with the type of metal.

 

[superhubert]

the motor cannot speed up as a result of the back EMF, or energy would be created in the system!!! The back EMF opposes the motion so the motor must slow down until there is no motion (relative). THis is why the back EMF increases when the motor is spinning faster. Thus the current is higher at low speeds (more back EMF) and the motor burns out.

ps was it only me who just found out that induced currents flow in the opposite direction of the right hand rule?
god.... i'm still waiting for that meteor.....

 

[Wintermute]

No. I only realised that at lecture i attended a couple of weeks ago.

 

[superhubert]

just to change the subject....... does anyone know how to re-arrange the Lorentz transformations formula to make v the subject? i can calculate the length contraction from velocity but can't do it the other way around. ie

if Lo= 300m
and Lv= 72.9m
what's it's velocity.

any ideas how to rearrange?, (maybe i shoulda done maths........)

ps any q's, i feel kinda bad 'bout asking more q's than answering.....

 

[Wintermute]

v = square root of : [ c^2 x (1 - lv^2/lo^2) ]

 

[superhubert]

cheers!

 

[Wintermute]

No worries mate.