Adapter fails to power DC motor

From 'electrical Machinery 101':

The equivalent circuit for a DC motor with constant magnetization is a resistance in series with a DC generator. The resistance is mostly from the coils and commutator resistance.

The generator greates an opposing EMF proportional to the RPM of the motor and the motor torque is proportional to the feed current. The motor achieves such rotation speed that the feed voltage minus the back EMF creates suitable current in the circuit resistances to create sufficient torque to overcome mechanical friction and load torque.

A feeely spinning motor can appear as a capacitance, looked at sufficiently low frquency.

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-TV

Well, not false, just misleading. We use motors IN ROTATION, not braked to a total stall, so the current requirement is high when starting, and drops afterward, which is similar to capacitor loading (if there's a time-scale of milliseconds to seconds under consideration). At microsecond-to-millisecond time scales, a nonrotating motor is inductive.

I get what you are saying. A motor is an electromechnical device, not enti rely unlike a piezoelectric crystal. The mechanical effects become reflect ed in the electrical effects and vice versa. In fact, that is what makes i t work. The only question is which effect is dominant for any given aspect you are measuring. Sort of like the blind men describing the elephant.

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  Rick C. 

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Agreed. I said so already.

Jeroen Belleman

How low of a frequency?

low enough that the motor starts to move and back-emf of the moving coils exceeds the inductance of the electromagnet.

1Hz should be low enough for most easily portable motors. the input signal needs suficient amplitude to overcome static friction and magnetic cogging.

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  Jasen.

Low enough that the winding inductance isn't important. ;)

Seriously, we're talking ~100 Hz and below.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC / Hobbs ElectroOptics 
Optics, Electro-optics, Photonics, Analog Electronics 
Briarcliff Manor NY 10510 

http://electrooptical.net 
http://hobbs-eo.com

1Hz = 1 second. Most DC motors are already running by that time, and chopping the DC into pseudo AC pulses.

100Hz makes more sense than 1Hz.

Motors have long been run purely for their capacitive loading, though I don't recall them being pm.

NT

Free running synchronous AC motors can be made to appear capacitive or inductive, depending on the field excitation. That feature was used to provide power factor correction for certain industrial installations. These days they are largely obsolete.

Jeroen Belleman