Re: Ohm's Law

[Robert Wilkinson <wilk@xxxxxxxxxxxxxxxxx>]

Mr. Bies wrote:

That the motor is largely "inductive" makes little difference in DC
analysis -- but the mechanical "load" on the motor is reflected as a
seemingly <lowered> resistance.

The running motor has the applied voltage on its input.  That voltage both
meets the electrical resistance -- and the mechanical load.  When running
no-load, the motor is <generating> a "back" voltage (essentially) equal --
it is, in effect, a motor and a generator at the same time -- acting as if
the electrical resistance were extremely high (little current flows).
Then as the mechanical load is increased, current flows to supply that
load -- the electrical resistance seeems to have dropped (when the motor
is completely stopped, to the resistance which would be measured by a
meter on the terminals of the stationary motor) the "back" voltage has
dropped to zero, and only the bare electrical resistance is present.

>r.m.bies

This is beautifully stated, but I must point out that the previous post,
which referred to inductance, was in fact correct.  Indeed, the phenomenon
that Mr. Bies describes is precisely the definition of inductance in this
context.  The rotation of the motor results in AC, not DC current being
involved, even though the input may be DC.  This is due to the commutator
"chopping" the DC before it is applied to the windings.  Within the motor,
"inductive reactance" is added to the windings' DC resistance as rotational
speed increases from zero.  This appears as increasing pure resistance
(with increasing speed) at the motor's DC input, thanks to the ingenious
rectifying-like properties of the mechanical commutator.  Note also that
only "series wound" or "universal AC/DC" motors (typically identified by
having brushes and a commutator) exhibit this behavior.  A good
reminder--I'm going to lube my wiper linkage this weekend.

Thanks to all for the helpful discussion.

Bob Wilkinson
72 Spider
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