Seperately excited DC Motor as a Servo

Hi guys,
I wanted to convert a Permanent Magnet DC(PMDC) Motor into a servo motor. But now with more information about PMDC motors I have
some ideas and may be you can correct them.
a) Since PMDC Motors get very hot and try to self destruct them in continous operations, I thought why not use a seperately excited DC motor which has constant field flux and also linear characterstics like a PMDC motor.
b) What about using a Universl Motor(the ones that run on both AC & DC) as a servo motor.Did someone try anything like this.
c) Windshiled wiper motors seem to be available in both PMDC type and field winding type.What if I use the PMDC motor type for a servo.Will it be suitable for such operations and long runs.Or should I detach the field winding and give a seperate excitation so that it can run longer without the affects of loosing torque due to loss of magnetism in the Permanent magnets.
BTW I am ready to use either a linear servo amplifer(Hi-Fi Audio Amplifier) or a PWM Chopper controller.
Thank You, prizark.
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On Fri, 21 May 2004 12:20:30 -0700, prizark wrote:

Actually PM motors run cooler than wound field motors (no heat dissipation in the field windings)

Only works if you power the field winding separately from the armature otherwise you can only go one direction...

If you use a motor with a field winding you must power the field separately (unless you only want a simple unidirectional speed control)
You will only lose PM field strength if you put excessive current into the motor - this is what current limits are for.

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something else you should remember is that dc permag motors are linear, whereas many other kinds, such as series wound motors, are not. this can be important for servo control loop purposes.
-chris.
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