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Euler Cheung
science forum beginner

Joined: 15 Jul 2006
Posts: 12

Posted: Sun Jul 16, 2006 4:44 pm    Post subject: No Back EMF in Magnet?

When we pass an electrical current through a Magnet, it would
accelerated and rotate faster. If we do that with existing motor, there
is ceiling which limit the maximum upper rotational speed due to Lenz's
Law. It appears that it is due to the effect of Back EMF, but why does
Back EMF doesn't affect a Magnet in rotation?
Don Kelly
science forum Guru Wannabe

Joined: 30 Apr 2005
Posts: 166

Posted: Sun Jul 16, 2006 11:48 pm    Post subject: Re: No Back EMF in Magnet?

----------------------------
"Euler Cheung" <eulercheung@gmail.com> wrote in message
news:1153068253.702723.279680@35g2000cwc.googlegroups.com...
 Quote: When we pass an electrical current through a Magnet, it would accelerated and rotate faster. If we do that with existing motor, there is ceiling which limit the maximum upper rotational speed due to Lenz's Law. It appears that it is due to the effect of Back EMF, but why does Back EMF doesn't affect a Magnet in rotation?

I assume that you are referring to a motor. Please note that torque is
dependent on current. It is the torque which causes acceleration making the
motor run faster. The back emf is a speed voltage opposing the applied
voltage so that it acts to reduce the current. When it exactly balances the
applied voltage, the current and torque are zero. In actual motors, there
are mechanical losses so the steady state speed is reached when the back
emf + IR drop = applied losses and the back emf * current = the mechanical
losses.
I suggest that you look up the relationships that occur in a motor. Look at
the equations. For a DC motor, for example:
V=I*R +Eb
Eb=K(phi)w
T=K(phi)Ia
Pmech =Eb*I= Tw
where w =speed (rad/sec) phi =magnetic flux per pole, K is a machine
dependent constant
Eb = back emf , T= torque (N-m) , V is the applied voltage, and I is the
armature current.
Play with these a bit for constant magnetic flux, then go on to such cases
as the series machine where flux is proportional to current.
After this, get a good text and carry on.
--

Don Kelly dhky@shawcross.ca
remove the X to answer

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