Re: Novice - Stepper Motor questions

--- In CAD_CAM_EDM_DRO@egroups.com, JanRwl@A... wrote:

> In a message dated 22-June-00 14:40:07 Central Daylight Time,
> lgpcox@e... writes:
>
> << How could I test a surplus stepper motor for torque ? (oz inch)
>

The pulley and weight method works very well for measuring holding
torque and low speed torque, but what if you need to measure torque
at medium and high speeds?

There is an indirect method that can do this and get results
suprisingly close (within 5%) of dynamometer readings if it is done
carefully. Here's how:

1. Disconnect any load from the test motor shaft.
2. Place a digital ammeter betwwen the power supply and the drive.
3. Run the motor up to the speed you want to measure torque at.
4. Write down the ammeter reading.
5. Load the motor until it is just ready to stall.
6. Write down the ammeter reading just before stall.
7. Subtract the no-load reading from the full-load reading.
8. Multiply the result by the power supply voltage.
9. Multiply this result by 4506.
10. Divide this result by your test speed in full-steps per second.

The result from step 10 is your torque in inch-ounces at the test
speed.

Step 5 is the hardest and will take a little skill and practice to do
accurately. What I use is a pair of channel-lock pliers and a paper
towel. I fold the paper towel over repeatedly until it is at least 8-
ply and about 1/2 wide. This is then wrapped around the test motor
shaft 1/2 turn ("U" shaped). The channel-lock then is used to grab
the motor shaft with the paper towel acting as a brake-pad. Moisten
the paper slightly for larger motors; otherwise the towel will smoke
and possibly catch fire from the friction.

I then slowly bear down on the handles, all the while watching the
ammeter. When the motor stalls, I restart it and repeat the procedure
until I get the highest reading on the ammeter. This is the reading I
write down in step 6.

FYI I use a homemade computer controlled dyno to generate step motor
speed-torque curves automatically. It uses a DC servomotor, encoder
and a precision current sink to absorb motor power and measure
torque. It is calibrated to +/- 1 in-oz and can measure up to 300 in-
oz torque at a maximum speed of 10,000 full steps per second. If
anyone is interested, I can show you how to build one (not a one
night project).

Mariss Freimanis