# Questions about servo motors

Discussion in 'Electronic Basics' started by Eric R Snow, Jun 6, 2004.

1. ### Eric R SnowGuest

To the motor folks,
I have a CNC mill from which the original motors were (by me) removed.
Now, because of control changes, these motors are going to be
re-mounted. I know which motor came off which axis but I am thinking
that the motors were put on the wrong axes by the previous owner. This
is because the axis which has the heaviest load had the smallest
motor. So, testing torque, and not physical size, seems the best way
to determine which motor goes where. Is the torque per watt output on
brushed linear motors linear? Can a lower voltage power supply be used
to determine which motor is strongest? If the same power supply is
used to power each motor and the torque measured is it safe to assume
that the motor with the highest torque at a low voltage will be the
one with the highest torque at the higher operating voltage?
Thank You,
Eric R Snow

2. ### John PopelishGuest

There is a factor you are missing. THe power output of the motor is
roughly proportional ot the mass of the motor, but the torque required
by a given axis is also related ot the gear ratio between the motor
and the load. So if the axis that you say is the heaviest load also
has a bigger gear down, it might do fine with the lower torque motor.
The power the motor has to provide is related to both the load forces
and the load speed (i.e. inches per second of travel). The motor must
match both the torque and speed requirements to be a good fit. The
whole system has to be taken into account.

3. ### Eric R SnowGuest

The motors are all geared to the screws the same and the lead screws
all have the same pitch. I should have included this.
ERS

4. ### John PopelishGuest

Then proceeding to your last question, yes, it is safe to assume that
a motor that produces more torque at low voltage will also provide
more torque at higher voltage. In general, the torque is proportional
to the current and the speed is proportional to the voltage.

5. ### Eric R SnowGuest

Thanks John,
I just finished testing the motors and I get three different torque
and amp readings. However, the stall amps were the same on two motors
while the running amps was high on one. But there is a big difference
in torque between the lowest torque and the mid torque. And a small
difference between the mid torque and the highest torque. I'm gonna
chalk the small difference up to my measuring technique.
ERS

6. ### John PopelishGuest

Were you regulating the current during stall, or just taking what ever
some supply could deliver?

To see if the two motors are designed for different speed torque
capabilities (assuming they are permanent magnet DC motors, not
stepper motors) you need to measure the torque they produce during
stall with a fixed current (or at least, a measured current) passing
through that is well below rated current, so you don't over heat them,
or degauss their magnets. That gives you their inch pounds (or some
other torque units) per amp. Then you spin them at a known RPM and
measure how much voltage they generate. This gives you the RPM per
volt rating. As I said, their power capability (amps times volts)
will be roughly proportional to their mass if they are of similar
construction but different sizes. But these two tests will tell you
if one is designed for more speed and the other for more torque.

7. ### Eric R SnowGuest

I used a lead acid lawnmower battery as the power source. I measured
the amps while testing. The highest, at stall, was 12 amps. I stalled
the motors for only a secnd or so. Probably less. I did this maybe 5
times on each motor. I sure hope I didn't harm the magnets. That would
really fry me if I did something that stupid.
ERS

8. ### Rich GriseGuest

If they're geared the same, put the big motor where it moves the
more stuff.

Have Fun!
Rich