# DC Motors: what is the voltage limit?

• posted

I have a 7.2V Johnson Electric DC motor. I'm trying to incorporate it into a mobile platform I'm building. The only trouble is that I was planning on using a power supply which supplies 12V and 5V.

This has led me to a few problems which I can't resolve:

1) Can I put a resistor in series with a motor to lower the voltage a bit? I suspect not, because the internal windings of the motor are probably like 3 or 4 ohms, yeah? And a 4-ohm resistor would dissipate a lot of heat and waste energy. I saw a book where they tested motor idle current by inserting a series resistor, and it was this enormous one with the label "Behemoth" written in the caption.

2) What happens if I apply 12V to a motor rated at 7.2 nominal volts? Are the ratings there only to prevent the motor from overheating? And if so, then would I be safe running such a motor at 12V if i ensured that it never stalled? Im planning on running the motor at a about a 50:1 gear reduction, and its not carrying too much weight.

3) Assuming the two above ideas are bad ideas, (and as I've written this I've convinced myself that they probably are), what would you hobbyists out there do: find a 12V motor or find a 7V regulator?
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• posted

You'd be wasting loads of power, which for a mobile platform would be bad as you have to carry your power supply with you.

Eventually something might melt, but you'll probably just shorten the life a bit. The motor will likely be less efficient too.

I would be using PWM for regulating the speed, so I just wouldn't go above a 7.2/12 = 60% duty cycle for long. A couple of seconds at 12V is unlikely to do any damage, so you could keep anything above a 60% duty cycle as a "turbo boost" for use in exceptional circumstances (wires on the floor, kerbs, crushing cars - whatever is hard for your robot).

Tim

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• posted

No. An electric motor does not have a constant load resistance, it requires a low-impedance supply.

That's tough. 20-30% is commonplace with modeling enthusiasts, much more than that may shorten the service life dramatically.

Not only that, too much voltage will also destroy the commutator.

If the load is very light, you may get away with it. Check the current consumption and make sure it is significantly below the rated.

First choice.

Not good, a regulator will dissipate the excess power unless you use a switch mode regulator. You may use PWM to reduce the effective voltage.

Michael

• posted

Well the RPM will go up by a factor of 12/7.2 = x1.7 You might get away with this or you might throw a winding. It depends on what Kv (=rpm/volt) the original motor was wound for?

At the very least the motor efficiency will be reduced (and they are usually only 60% efficient to start with) so you need to reduce the load you apply to the motor to keep the overall power output down.

These motors are usually very cheap (

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