Speed control via PWM field winding voltage control

Hi...

I wish to control the speed (close-loop control with speed feedback) of a dc motor via PWM field winding voltage control (see attached figure). Do I need any diode in the circuit? Is the voltage seen at the terminals of field winding "ripple free"? Is the current flowing through the field winding "ripple free"?

Thanks.

for ripple free voltage at the terminal of winding depends on the PWM frequency + C value + field winding current, better to use a Diode in a reverse bias configration across the field winding to kill the unwanted inductive kicks,

The circuit has to be wired as a usual buck converter with a free-wheeling diode. You must not load a capacitor through a switch, because it would cause unlimited currents and either damage the IGBT or the capacitor, possibly both of them.

You should however consider, that field regulation is only suited to increase the motor speed above nominal operation point. For basic speed control, you have to vary the armature voltage.

Well... the spec of the dc machine is as follows:

Field winding: 210VDC/0.2A
Armature winding: 180VDC/1.6A
Speed: 3000rpm

I want the dc machine operates at 400rpm. At the moment no close-loop control is used to control the speed of the dc machine:
[1] I set the Vfield=50V
[2] Then, I slowly increase the Varmature
[3] By observing the hall-effect sensor signal using a scope I can determine the speed
[4] The Varmature is increased until the speed is close to 400rpm.
[5] Then, I adjust the Vfield to make it closer to 400rpm.
[6] This process takes a long time and the speed will drift away from 400rpm especially when the winding temperature increases.

Therefore, I wish to build a circuit to regulate the speed at 400rpm. However, I want a circuit that can give a variable voltage supply (or current) that has no ripple or minimum ripples. I'm afraid the ripples might introduce some vibrations to the system.