Brushed DC Motor w/PWM Control 1

[Strider17]

Gentlemen:

This is not my field, but I have had to pick up
some of the load....

I have a brushed DC gearmotor, 300 in-lb torque, with output of 8 RPM. If the speed is reduced thru a PWM controler to 1 RPM, will there be any torque left
for the load? Any guess as to how much?

Thanks in advance

Terry

The more you know, the more you
know you don't know....

 

[Skogsgurra]

The speed of a DC motor is load dependent. You have probably noticed that speed drops when you apply load to the motor shaft.

As long as the speed is reasonably high, the speed drop will not result in zero speed. But, the closer you get to zero, the less load is needed to reduce speed to standstill.

The qualities that determine how the motor behaves is the droop and also the internal resistance of the voltage source driving the motor.

A couple of percent droop is normal for large motors and your motor is quite small (I take it that your 300 in-lb torque is after the gearbox) so you can expect a lot more droop. Impossible to say, really, but I wouldn't be surprised if you have something like 10 - 20 %.

At 10 % droop, your 1 RPM speed will drop to 2.5 % at full load (1/8 = 12,5 %, 12,5 % - 10 % = 2,5 %) and at 20 %, the motor will stop at a little more than half load.

That's the general picture. You have to do some measurements to get a better (more exact) answer.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[BobM3]

Does the motor use a fan for cooling? Slow speeds at rated torque will heat it up.

 

[VEBill]

I'm certainly not an expert on DC motors, but I thought that in general they had the highest torque at zero rpm.

Check wiki:

http://en.wikipedia.org/wiki/Electric_motor

"...the series-wound DC motor develops its highest torque at low speed..."

The other points already posted are excellent.

 

[Skogsgurra]

That's the series wound motor. And running off a constant voltage with neglible internal resistance. A shunt or PM motor has constant torque vs armature current up to base speed.



Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[sreid]

For high ratio gearboxes and small motors, the drive torque is often the limit on the output shaft/gears. If this is the case, you may get full torque at 1 RPM.