PWM

Pulse width modulation varies the amount of voltage that is applied to a motor. For instance, if the peak voltage of the drive signal is 12V, and the pwm is 50%, then the average, or effective, voltage is 6V.

By sensing the actual speed of the motor, and feeding that back to the pwm circuit, you can accurately control the speed of the motor. There are a wide variety of motor controller chips that do this.

Have you done this bhankiii? How does one determine the best PWM frequency to use? You don't want the motor's inductance to undermine power transfer at high frequencies, but too low a frequency and the hum drives some people nuts (me, for example).

I could look into this myself, but I'm on winter break and enjoying it too much to trouble myself. I do that kind of stuff the rest of the year...

-e

Yes, I've done this before. Typically using a frequency on the order of 100KHz or so.

Would you please show here the technic of PWM in DC motor controll by a appropriate circuit

As always, google is your friend. Here's the frist circuit I came across after googling "pwm motor control":

http://www.solorb.com/elect/pwm/index.html

Check the websites for most major semiconductor companies, TI, etc., for ap sheets and reference designs.

Thanks bhankiii.

I shall try with this circuit

This is done with Model train controls. Some of PIC's microcontrollers have built in High frequency PWM outputs built in. (On the order of 16.8KHz if I remember correctly). Most are open loop controls but closed loop is becoming more popular. Closed loop is done by looking at the back EMF of the motor and adjusting the PWM duty cycle as required.

Shawn

...and in servos for model aircraft control since about 1970 or so.....

Mostly: by feeding the armature after fitering out the transients.

Field too can be- but only a small % of range

Assuming the peak of the PWM is fixed volt

...as far as I am aware, the major difference is that the servo has a feedback control to tell it where to stop. How fast it gets there is due to the difference in the width of the signal.

If the mechanical connection between servo and feedback mechanism (sometimes its a pot, sometimes its a cap) is taken out, the moror will turn at a speed relevant to the difference in the signal (width of the pulse), eg.speed control.....if the signal changes from positive to negative (or the opposite) the motor will reverse direction.

The reason for PWM is so that the motor always generates good torque, as it always receives full voltage, though for varying amounts of time, which give it good speed control!