# PWM signal for motors

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#### elecLear78

##### Member
I had to write a driver program to generate pwm signal to drive a motor. I don't know much about motors and the driver circuit, but why is that always a pwm signal is used for driving motors. Why cannot we use direct DC voltage. Please help.

#### Ian Rogers

##### User Extraordinaire
Forum Supporter
If you have a DC motor then of course you can use a DC voltage to drive it... However! The motor will spin at speed..

The reason for PWM is current control... So you can control the speed of the motor.. If you wanted to control a motor using a pot to limit the motor, you get into all sorts of problems as the motor is designed to run at a certain voltage.... With PWM the voltage is maintained, but in a controllable condition.. Pretty much the same as a soldering iron is temperature controlled.. PWM turns a motor into a servo motor... But! There are some motors out there that cannot be controlled via PWM...

##### Well-Known Member
If this is a DC motor then there is also another method using phase shift of the incoming AC, using a couple of SCR's in a bridge off of the AC power in.
Max.

#### spec

##### Well-Known Member
Hi EL,

Another approach is to use a switch mode power supply (SMPS) and control the output voltage of the SMPS.

As Ian implies, the problem with controlling a motor using a linear technique is power loss. If, for example, you reduced the voltage of a four horse power motor from 12V to 6V, the motor power would drop to one quarter (1HP) and the dissipation in the linear element dropping the voltage would also be 1HP or 750Watts.

With a switching technique, either pulse width modulation or SMPS, the power loss in the element dropping the voltage would theoretically be zero. In practice about 85W would be dissipated in the voltage dropping element.

If you used a simple resistor in series with the motor to control the speed, not only would a lot of power be lost in the resistor but also, due to the relatively high source impedance seem by the motor, the motor torque would be extremely limited.

spec

#### shortbus=

##### Well-Known Member
Hi EL,

Another approach is to use a switch mode power supply (SMPS) and control the output voltage of the SMPS.

As Ian implies, the problem with controlling a motor using a linear technique is power loss. If, for example, you reduced the voltage of a four horse power motor from 12V to 6V, the motor power would drop to one quarter (1HP) and the dissipation in the linear element dropping the voltage would also be 1HP or 750Watts.

With a switching technique, either pulse width modulation or SMPS, the power loss in the element dropping the voltage would theoretically be zero. In practice about 85W would be dissipated in the voltage dropping element.

If you used a simple resistor in series with the motor to control the speed, not only would a lot of power be lost in the resistor but also, due to the relatively high source impedance seem by the motor, the motor torque would be extremely limited.

spec
Never heard or knew of that one. Won't dropping the voltage due to a SMPS also reduce the motor torque significantly?