# Anyone ever tried to run a very low current DC motor from a signal Generator?

#### Rscott9399

Joined Jan 13, 2017
51
Anyone ever tried to run a very low current DC motor from a signal Generator?
I know the signal generator will not support much current but has anyone tried this?
Just for pure educational reasons.
Hook it up and try and get the speed to change based on frequency instead of voltage magnitude?

I tried and it wont work.
Anyone wanna explain why?
Square wave was 1k and 4v Peak
Should of kicked this little tiny motor i have on with ease
Also verified motor was working prior.

#### #12

Joined Nov 30, 2010
18,223
If the square wave was 4 volts peak, has a 50% duty cycle, and never crossed the zero voltage line, you could apply 0.04 amps through the internal 50 ohm resistor. If the square wave is centered on zero volts, you were telling the motor to reverse over and over, and the average current was zero.

#### cmartinez

Joined Jan 17, 2007
7,634
Scoping your setup would come in handy...

#### Rscott9399

Joined Jan 13, 2017
51
If the square wave was 4 volts peak, has a 50% duty cycle, and never crossed the zero voltage line, you could apply 0.04 amps through the internal 50 ohm resistor. If the square wave is centered on zero volts, you were telling the motor to reverse over and over, and the average current was zero.
isn't it impossible to do anything at 50% duty since the average is always zero?

#### #12

Joined Nov 30, 2010
18,223
isn't it impossible to do anything at 50% duty since the average is always zero?
That is so wrong that I would need a book to describe the exceptions. Can't you see that the square wave you spoke of is in one condition half the time and the other condition during the other half of the time?

If you turn a light switch on for an hour, then turn it off for an hour, does that mean the light never came on? On a more microscopic level, the power to your home is in one polarity half the time and the other polarity during the other half of the time. Does that nothing electrical has any power at your house?

In a mathematical sense, if a square wave is at positive 1 volt for a millisecond and zero for the next millisecond, is the average over two milliseconds equal to zero?

Last edited:

#### Rscott9399

Joined Jan 13, 2017
51
That is so wrong that I would need a book to describe the exceptions. Can't you see that the square wave you spoke of is in one condition half the time and the other condition during the other half of the time?

If you turn a light switch on for an hour, then turn it off for an hour, does that mean the light never came on? On a more microscopic level, the power to your home is in one polarity half the time and the other polarity during the other half of the time. Does that nothing electrical has any power at your house?

In a mathematical sense, if a square wave is at positive 1 volt for a millisecond and zero for the next millisecond, is the average over two milliseconds equal to zero?

ok so i tried it again
This time i can get the motor to spin but only under the following conditions
The amplitude does not matter. Anything between 4 and 10 volts is fine.
At 50% duty cycle the motor will not spin at all. You have to be over 50% duty cycle to get it to run.
If its 50 or less it wont turn. I tried this procedure with various frequencies from 1k to 1M and it has no effect
Also the frequency had no effect on the speed
Any thoughts?

#### LesJones

Joined Jan 8, 2017
3,702
So are you saying that PWM systems can not possibly work ???

Les

#### Alec_t

Joined Sep 17, 2013
12,746
Sounds like the generator can't source enough start-up current to drive the motor properly.
My concern would be that back-emf spikes could damage the generator.

#### Rscott9399

Joined Jan 13, 2017
51
So are you saying that PWM systems can not possibly work ???

Les
Where did i say that? Im simply explaining the results i have at moment and how to change them

Amplitude anywhere from 4-10 volts
Motor will not spin at 50% or less duty cycle

frequency has no effect at this time
Are the conditions

#### Rscott9399

Joined Jan 13, 2017
51
Sounds like the generator can't source enough start-up current to drive the motor properly.
Hi there
Thanks for the idea
The motor has been tested at and rated at .027 Amps
Signal generator says it supports .5 A

#### dannyf

Joined Sep 13, 2015
2,197
Anyone wanna explain why?
not much current from the signal generator.

#### RichardO

Joined May 4, 2013
2,271
The motor has been tested at and rated at .027 Amps
Signal generator says it supports .5 A
0.5 amps sounds unlikely. What is the make and model of your generator?

#### LesJones

Joined Jan 8, 2017
3,702
What you are doing is pulse width modulation. You have a fixed voltage and a fixed frequency but you are varying the duty cycle so it is pulse width modulation. You are saying with a duty cycle of 50% or less the motor will not run. It should run at about half the speed it runs with the duty cycle at near 100% This is based on the assumption that the square wave goes from zero to some positive voltage. If the square wave was truly symetrical about ground the motor should start to rotate in the opposite direction when the duty cycle was below 50% (Assuming a permanent magnet motor.) If the output of the signal generator was capacitor coupled the motor would not rotate for any value of duty cycle as there would be no DC component.

Les.

#### BobTPH

Joined Jun 5, 2013
4,785
You seem to think the frequency will control the speed of the motor. That is not correct.

Bob

#### Rscott9399

Joined Jan 13, 2017
51
You seem to think the frequency will control the speed of the motor. That is not correct.

Bob
Yes i was under that impression
Simply going off many tutorials
Apparently I'm mistaken?

#### LesJones

Joined Jan 8, 2017
3,702
Just out of interest I have had a look at the square wave output of my audio siganl generator (Hewlett Packard 209A) The output is truely symetrical about zero and 600 ohm output impedance. (This is a fixed 50% duty cycle. Being 600 ohm output impedence it would not drive any motor.)

Les.

#### BobTPH

Joined Jun 5, 2013
4,785
Yes you are mistaken. This would be true for an AC synchronous motor. Not for a DC permanent magnet motor.

PWM uses duty cycle, not frequency, to control not the speed, but the power to the motor. The speed depends on the power supplied and the mechanical load.

Bob