DC motor speed control

im using PWM for DC motor speed control, and it works, but please explain, why? with PWM im changing/decreasing armature current, Ia. Torque depends on Ia, and speed depends on armature voltage, Ua. Well, im decreasing Ia, and Ua is "jumping and falling". So i should change (decrease) torque but instead im decreasing speed, and this speed is constant, and it isnt jumping like Ua, why?

Torque=km*Ia
Speed=Ua/ke

The motor winding is an inductor, so if the PWM frequency is high enough, the applied PWM voltage does not change the current very much. Instead, the inductance of the motor windings act to average out the current, and the averaged value of the current is dependent on the duty cycle of the PWM. So the effect is to control the motor current using the duty cycle.

The motor cannot react instantly to changes in applied voltage. There are both electrical inductance and mechanical inertia to be overcome. Because of this, the effective applied voltage is equal to the average of the voltage over the PWM cycle. E.g., at a 75% duty cycle, the average voltage at the motor is equal to 75% of the source voltage.

As for the relationship between speed and torque, this doesn't change because of the PWM. At any given PWM duty cycle, the motor current will still vary according to the torque. A fixed duty cycle won't control the current drawn or the resulting torque.

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Michael Griffin
London, Ont. Canada
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If you look at a torque vs speed curve, it would suggest exactly what you were expecting, but really the power is constant when the curve is made. First data point is determined at no load, at max speed [x-axis], then the motor shaft is torqued until it stalls and this is the other extreme data point on the curve [y-axis].
power = I x V = force x distance or omega.
You would need to load your motor down to see the inverse relationship between current [torque] and voltage.

oj

dear Michael Griffin
London, Ont. Canada
thank you for your reply i hope u can give me your email so i can contact u personal if u don't mind
thank you,
sulistio utami(Indonesia)
([email protected])

If you have an I.R. adjustment try it, but you may have a jumprt with J1, J2 and J3 locations. If I remember right, J1 goes to low, J2 goes to med and J3 goes to speed. I hope that helps.