Alt-BEAM Archive
Message #13903
To: alt-beam@yahoogroups.com
From: "John A. deVries II" zozzles@lanl.gov
Date: Fri, 13 Jul 2001 15:58:53 -0600
Subject: Stalled State (was: wheeled reverser etc.)
At 02:35 PM 7/13/01, Eric wrote:
>if i forced the motor to stop rotating, or sometimes it would stop on its
>own (when using a 74HC20 not 74AC240) - it
>would start 'squealing' (makeing high freq sound).
The rotor isn't turning any more.
Therefore there isn't any commutation going on.
For Newbies: Commutation is the action of brushes that rub on contacts on
the rotor spindle that make and break connections with the coil windings.
The coil windings which are wrapped around the armature that makes up the
bulk of the rotor. Commutation permits the coil windings, acting as
electromagnets, to "change poles". When they change, they are
attracted/repulsed by the permanent magnets in the motor Just In Time to
keep the rotor/shaft turning.
Usually.
In this case, though, the rotor acts as a plain inductor, although perhaps
one similar to a transformer with a loose plate -- that is, the inductance
can change because the rotor can still vibrate a teensy tiny little bit.
So, Wilf: what does the circuit look like if you don't think of the motor
as a motor anymore but as a simple inductor?
Zoz
References
D.C. motor definition
Newbie:
http://math.haifa.ac.il/robotics/dcmotor.htm
In-between:
http://www.mech.uq.edu.au/subjects/e4360/chap7/s2.htm
Past that, I figure you already know how D.C. motors work.
Ugly math but the model looks like pretty clean
http://mechatronics.me.vt.edu/book/Section3/motormodelling.html
it shows stuff like what happens to the output when the input isn't just
straight D.C.
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