Separately excited DC motors - control theory and applications

Thread Starter

Just Another Sparky

Joined Dec 8, 2019
228
Wasn't quite sure where to put this one.

Long story short, I stumbled across a listing for a 10 horsepower DC separately-excited motor for surprisingly cheap and it got me thinking. Given that these are largely considered to be antiquated technology nowadays, I've never really had reason to delve into DC motor theory too deeply.

My understanding is that shunt-type motors like this don't have the same starting torque as series motors, but do possess relatively flat torque-speed curves. That is to say, they are largely constant-speed motors, not unlike induction motors.

Just to tickle my intellect, how suited would one of these motors be to driving a variable torque load like an air compressor pump? What kind of locked-rotor currents are typical for them? Can they be D.O.L. started or is reduced voltage starting necessary?

Also, I've noticed that a 150 VDC field winding seems to be industry standard for them. Where exactly is this voltage typically derived from? A rectifier and a rheostat? A special transformer? Variac? Is it typical to control the speed of these motors by varying the field strength or are they intended to be used as single-speed motors?
 
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Delta Prime

Joined Nov 15, 2019
1,159
hello there.
A separately excited dc motor is a motor whose field circuit is supplied
from a separate constant-voltage power supply, while a shunt dc motor
is a motor whose field circuit gets its power directly across the armature
terminals of the motor. I'm thinking if the supply voltage to theese motors is assumed constant,there is no
practical difference in behaviour concerning torque and speed control between these two machines?
spead control using armature voltage control by AC-to-DC converters. Types of single phase DC motor drives are
Halfway converter drive,
Semi-Converter, full converter and double converter.
common methods of speed control is adjusting the field resistance RF (and thus the field flux φ).
Adjusting the terminal voltage applied to the armature.
A less common method is Inserting a resistor in series with the armature circuit. :)
 

MaxHeadRoom

Joined Jul 18, 2013
23,779
The typical method of obtaining higher rpm on CNC machines spindles for example, was to reduce the field current, with a consequent loss of torque.
They also had field loss protection that prevented catastrophic accidents if the field went open.
 

crutschow

Joined Mar 14, 2008
27,890
They also had field loss protection that prevented catastrophic accidents if the field went open.
Yes.
When I took an electric motor course my Freshman year in college, they had a picture on the wall of a shunt DC motor that had lost its field excitation.
It was totally blown apart!

And when we were doing some experiments with a shunt DC motor, I accidentally removed the excitation voltage.
It sounded like a jet plane taking off.
I quickly reconnected the winding and no damage was done.
But the sound caused everyone to look at me (the idiot). :oops:
 

crutschow

Joined Mar 14, 2008
27,890
I've noticed that a 150 VDC field winding seems to be industry standard for them. Where exactly is this voltage typically derived from? A rectifier and a rheostat? A special transformer? Variac? Is it typical to control the speed of these motors by varying the field strength or are they intended to be used as single-speed motors?
It could be derived from any of those methods that generate a steady DC current.
The value of the field resistance may put some limits on how you actually do it.

Normally there speed can indeed be varied by changing the field current.
Of course the speed varies inversely with the current.

How would you generate the high current DC for the armature?
 

Thread Starter

Just Another Sparky

Joined Dec 8, 2019
228
How would you generate the high current DC for the armature?
Full wave rectifier.

:p
It's an industrial motor with lots of inductance. Doesn't care about filtering or ground isolation. Better that it's *not* filtered to avoid needless complexity and additional failure points associated with voltage regulation, harmonics, etc. Also better that it it's on a solidly grounded system so a ground fault will open upstream OCPD and alert someone to the existence of a problem.

They used to power old DC railway traction lines the same way; A mercury arc rectifier (or five) right across the line. Modern permanent-magnet brush motors for gearbox service sometimes ship with a little bridge rectifier screwed right to one wall of the terminal box.
 
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