Driving a DC motor above or below the rated voltage?

Using Arduino Motors, Mechanics, Power and CNC
1

I'm looking at a robot chassis that has 6V DC motors. I'm thinking of using an UNO and the older adafruit motor shield. If I power the motors from the 6xAA battery pack, that would give them a voltage of somewhere between 7.2V-9V depending on whether I use rechargeable batteries or not. If I power them using the regulated output from the UNO (which worries me a little since it could potentially overload the regulator, even though the motors are only rated for 120mAh) then they would get 5V.

What are the consequences of driving a DC motor above or below the rated voltage?

2

I wrote a text about dc motors behavior just some minutes ago, here. What destroys a motor is heat generated by current in the winding resistance. As long as you don't load the the motor so that the current stays below the overheating threshold you will be fine. Also, it takes some time for the heat to build up so running a highly loaded motor for very short periods works. Do some test with limited duration and keep track of the temperature by touching the motor and watching out for that characteristic "hot winding smell"

3

And don't even THINK of powering motors from the Arduino 5v pin.

...R

4

nilton61:
I wrote a text about dc motors behavior just some minutes ago, here. What destroys a motor is heat generated by current in the winding resistance. As long as you don't load the the motor so that the current stays below the overheating threshold you will be fine. Also, it takes some time for the heat to build up so running a highly loaded motor for very short periods works. Do some test with limited duration and keep track of the temperature by touching the motor and watching out for that characteristic "hot winding smell"

If the motors use brushes for commutation, using a higher voltage will potentially shorten the life of the brushes and/or commutator, due to increased arcing; this becomes especially true for the cheap DC hobby motors that you typically see in these small robot chassis. They usually use a metal strip as a brush (something like nickel or something), which unlike graphite brushes typically arc more.

It should also be noted that the L293, since it is a bipolar transistor (not mosfet) driver, will typically drop around 2 volts (depends on current as well - see the datasheet); so with a 7.2 - 9V battery, expect around 5 - 7 volts at the motor. If you can, shoot for using the high end voltage battery, because the operation will be best if the motor voltage rail is much higher than the 5 volt logic rail.

5

Thanks for all of your replies. I didn't think about that fact that the motor driver would have a voltage drop, so that will probably actually work out just about right!

6

cr0sh:
this becomes especially true for the cheap DC hobby motors that you typically see in these small robot chassis. They usually use a metal strip as a brush (something like nickel or something),

beryllium copper? phosphor bronze? - something that's mainly copper, not a poor
conductor like nickel! Graphite brushes are stacked full of copper dust too I think.

Back to the question: You can compensate for the over-voltage supply by limiting the
max PWM duty-cycle, then it will run as if 6V (arcing aside)

Note you can burn out 6V motor at 6V if its mechanically overloaded the whole time,