# Help with dc motor speed

1. Sep 18, 2008

### jjackson927

Hi everyone,

I'm new here and I'm electrically stupid. I am working on a mock up of a system designed to pump a variable amount of liquid. I was looking the OEM manual for a small 12v pump and it said that the motor may burn out if run at a voltage lower than 12v.

I understand that the way to change DC motor speed is to vary the voltage. So my idea was to run the motor inline with a resistor to give it 6v and to then be able to switch it to 12v to increase the volume pumped.

So I guess my question is why would the motor burn out if run at low voltage? Can I run it at 12v and up it to 24v (it doesn't say anything about running it at high voltage)? Am I completely off base?

I realize my extreme limitations in this area, I'm just trying to put together a small scale mock up as proof of concept. Any help in this area would be greatly appreciated.

Thanks,
Joe

2. Sep 18, 2008

### Chandra214

welcome to PF!!
no you are not completely off base.
i had used a 12VDC motor and we could up the speed from 7V to 14V.(60rpm tp 110rpm)
study the datasheet of the motor in hand and find out its electrical characteristics.
all in all its fun trying reliability.

3. Sep 18, 2008

### Pumblechook

Better to use a pulse width speed controller maybe.

4. Sep 18, 2008

### Pumblechook

Or drop the voltage with diodes which produce a fairly contstant voltage drop irrespective of current and loading of the motor.

5. Sep 18, 2008

### Staff: Mentor

Yes, you do not slow down a DC motor by providing it a lower voltage -- the torque falls off too fast with voltage for that. Instead, you pulse-width-modulate the 12V power source to slow the motor down. So you would chop the 12V power at a few hundred Hz, for example to slow the motor down and still maintain a good torque.

6. Sep 18, 2008

### jjackson927

Wow thanks, I'll look into that and post back with what i find.

7. Sep 20, 2008

### Wondermotor

What kind of motor is it that runs the pump? Series wound or PMDC? I hope you have a PMDC motor and a 12v PWM controller is definitely the way to go.

8. Sep 21, 2008

### Utwig

Hi,

A quick note of caution as you don’t mention if your liquid is volatile or not, DC motors have a horrible tendency to arc across the contacts internally so if you are trying to pump petrol or something, please proceed with extreme caution in a very well ventilated space, if you don’t your proof of concept may well be up for a darwin award.

Utwig