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theorical problem with dc servo motor

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Newbie level 2
Dec 23, 2003
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Sorry to bother with long descriptions but I think it is necessary to understand the situation.
I have dc servomotor with separate excitation and armature winding. At constant field current and armature voltage of Va=10vDC the speed of shaft reaches 2760 rpm. Then I use a PWM to change the voltage of armature. With Duty Cycle of 50% and the switching voltage of 20v DC. There for the effective DC voltage at the motor armature must be 0.5*20=10V. At the switching frequency of 30Hz, the speed of the shaft reaches 4800 rpm? Then I have increased the frequency and the speed of the motor decreased until at 600Hz the speed reached 2760 rpm. After that by increasing the frequency, the speed decreases. In theory the effective dc= duty cycle*Vmax although I did not change the duty cycle and the Vmax but it seems the effective voltage is varying by the frequency. Decreasing the speed after 600Hz is comprehensive and if the speed of the motor was below the 2760rpm at low frequencies, I could explain it but working at higher speed than the pure dc is confusing me. I have two power supplies one for the field winding witch is completely separate and one for the armature winding. The switching circuit is a simple NPN transistor and the protective diode at motor terminals. The switch is working properly at all frequencies. I want to know Why this is happening?

Thank you.

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