effective or actual voltage a problem for motors?

jhankle

Let's say that I have a +3Vdc motor which I am speed controlling using PWM at about 500 Hz. Let's also say I have a source of +5Vdc.

If I never use a PWM duty cycle above 60%, is it OK to directly use the +5Vdc to supply the motor? The effective voltage would never be over 3V that way.

Or should I use a power resistor in series with the motor to prevent the voltage from going over 3V *ever*? I'd rather avoid that to avoid any extra power loss, but I don't know if the duty cycle approach is safe for the motor.

thanks,

jh

Phil Pilgrim (PhiPi)

It's quite okay to drive a motor at higher-than-rated voltage with a PWM. It's the current that matters most — not the voltage. Say the motor is rated at 3V and has a winding resistance of 6 ohms. Then the current rating will be 0.5A. When a motor is driven using PWM, the coil inductance will prevent saturation at high PWM frequencies. So at reduced duty cycles, the instantaneous and average current will be about the same. Okay, 500Hz is a pretty low frequency, so your peak current may exceed the motor's nominal rating. But I think you should still be okay.

-Phil

Bruce Bates

jhankle -

Let me follow up on what Phil said, since the health of your Stamp may presently be at risk. First, the current is indeed the more important issue, and if this is a direct-drive situation you may be putting your Stamp at risk. You can check the PBASIC Reference Guide (hardware section) to find out the maximum load you can put on any Stamp pin port based on the model of Stamp that you're using. This maximum current rating should not be exceeded.

Second, there is back EMF from the motor to consider. When a PM DC motor has power removed from it, it basically turn into a generator until it stops spinning. That produced voltage and current runs back up to the Stamp pin port, and it may too be a source of damage. The method to prevent this is to place a reverse biased diode across the power terminals of each motor. In this manner any current produced will be shunted (passed) directly to ground. During ordinary operation, this shunt will be ineffective as the diode will effectively be "invisible" in the circuit, due to the polarity.

What we like to see is: "No PBASIC Stamps were harmed during the production of this experiment"

Regards,

Bruce Bates

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jhankle

> What we like to see is: "No PBASIC Stamps were harmed during the production of this experiment"

No worries [noparse]:)[/noparse]. I'm running through a transistor to control the motor, so I don't think the stamp output pin current will get over 5 mA in my use, and it's rated for 20.

Also I have a diode in place already. I just posted my circuit diagram in the Basic Stamp thread because of a problem I'm having with motor EMF, but (as far as I know) the circuit should be safe for all the components.

Thanks for the reply!