DC Motors: what is the voltage limit?

[Mark Haase]

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?

--
|\/| /| |2 |<
mehaase(at)sas(dot)upenn(dot)edu

 

[Tim Auton]

Mark Haase <mehaase@earthlink.net> wrote:

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

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.

2) What happens if I apply 12V to a motor rated at 7.2 nominal volts?

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

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?

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
--
My last .sig was rubbish too.

 

[Michael Hofmann]

Mark Haase wrote:

I have a 7.2V Johnson Electric DC motor.

1) Can I put a resistor in series with a motor to lower the voltage a
bit?

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

2) What happens if I apply 12V to a motor rated at 7.2 nominal volts?

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

Are the ratings there only to prevent the motor from overheating?

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

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.

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.

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

First choice.

or find a 7V regulator?

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

 

[CWatters]

"Mark Haase" <mehaase@earthlink.net> wrote in message
news:mehaase-E488A8.23301109062004@netnews.upenn.edu...

2) What happens if I apply 12V to a motor rated at 7.2 nominal volts?

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 (<$10) so I would go find a 12V model.