CMB16D and DC Motors

[quietdoc]

I have read in other posts that the CMB16D can be used to control a DC motor. My question is whether reducing the "intensity" will reduced the speed of the motor. I.E. by setting the channel to 50% intensity, will the motor run at about 1/2 speed? Or will it destroy the motor?

Thanks

RO

[LightORamaDan]

quietdoc wrote:

I have read in other posts that the CMB16D can be used to control a DC motor. My question is whether reducing the "intensity" will reduced the speed of the motor. I.E. by setting the channel to 50% intensity, will the motor run at about 1/2 speed? Or will it destroy the motor?

Thanks

RO

It should reduce the speed of the motor.

[Ponddude]

I can step in here.

I am using the CMD16D board to control my trolley. If you lower the voltage you will lower the speed. You just have to be careful with the voltage. For instance...with some motors, when you lower the intensity the voltage obviously lowers...that in turn will some times reduce the torque as well. So if you are depending on a motor with high torque and you need that torque, then you could have an issue lowering the voltage.

[-klb-]

Ponddude wrote:

I can step in here.

I am using the CMD16D board to control my trolley. If you lower the voltage you will lower the speed. You just have to be careful with the voltage. For instance...with some motors, when you lower the intensity the voltage obviously lowers...that in turn will some times reduce the torque as well. So if you are depending on a motor with high torque and you need that torque, then you could have an issue lowering the voltage.

Correct. The good news is that the CMD16D is a PWM card, and while many volt meters will show dimming as varying voltages, you are in fact only changing the percentage of the time that full voltage is applied. As a result most DC motors should generate more torque at half intensity than they would at half of the normal supply voltage. Virtually all DC motor speed controls are PWM, just for the gain in torque, as opposed to variable voltage.. Still, torque will virtually always go down as you lower speed.

I do also strongly recommend a fast diode rated for the full motor current be installed reverse biased across the motor. The reason for this is that to some extent, the windings of the motor form an inductor, and 400 times a second as the PWM state goes to off, the current through the inductor (motor) is shut off, and without any place for it to go, voltage spike is created. The reverse biased diode allows the current that would create this spike to dissipate back through the motor windings, instead of becoming the voltage spike that could damage the CMD16D.

- Kevin

[Steven]

-klb- wrote:

Virtually all DC motor speed controls are PWM, just for the gain in torque, as opposed to variable voltage..

Another reason for this is that using a transistor to lower the voltage would result in power wasted as the voltage drop across the transistor. At 50% voltage, such a controller would dissipate the same amount of power as that supplied to the motor.

"Power wasted" means "heat", which would mean you would need a much bigger transistor and heat sink than an equivalent PWM speed control.

[LightORamaDan]

-klb- wrote:

[snip]
I do also strongly recommend a fast diode rated for the full motor current be installed reverse biased across the motor. The reason for this is that to some extent, the windings of the motor form an inductor, and 400 times a second as the PWM state goes to off, the current through the inductor (motor) is shut off, and without any place for it to go, voltage spike is created. The reverse biased diode allows the current that would create this spike to dissipate back through the motor windings, instead of becoming the voltage spike that could damage the CMD16D.

Thanks klb, that is a very good point.