In article , MP Toolman says...

Shunt wound or permanent magnet DC motors are inherently constant torque
devices because the magnetic field is constant at or below base speed. Any way
to get variable armature voltage will produce variable speed at constant
torque.

This is true, but there are many cases where shunt wound motors
are run to keep the torque high at low speeds.

The hardinge feed motor setup is like this. Originally they would
have the armature voltage controlled with a variac, so the
voltage increased to speed the motor up, and at the same time
the field voltage was controlled with a large variable resistor,
ganged to the variac shaft, so the field current was decreased
as the arm. voltage went up. This way the field was at full
strength as the control was set to lowest speeds.

They ultimately changed this, and made the field be basically
PM by running it constant current, and using a fed-back
SCR control circuit to maintain torque at low speeds.
They adjust the firing angle of the scr based on the
back EMF of the armature as far as I can tell. I've
got the reverse engineered schematic, and I would be
interested to get a 'real' EE's take on it.

Jim

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