Motor test stand, RPM and torque

[buspor63]

I've been wanting to build this so I could just compare same brand of motors against each other. The disk is a Futaba servo wheel glued to a Carrera pinion and I a Dubro clevis pin and threaded rod for the push bar. Not shown is the digital optical tachometer available for around $12 and the digital scale bought from Harbour Freight years ago. I'm not going to go through and do the math like Robert Livingston did, I just want to compare one motor to another.

[HomeRacingWorld]

There you go, nice work.

[RV Racing]

Another excellent thread, gentlemen. You've kick-started my few remaining brain cells. I love this stuff.

Thanks,
Rick V.

[dr fabio]

Nice! The maths is easy. Just measure the radius from the centre of the disk to the centre of the pivot arm. Just multiply the radius (in cm) by the weight in grams. Example: If your radius is 2cm then multiply the weight 87g (in the pic you have). So torque is 87 x 2 = 174g.cm When you take your measurement make sure the pivot point is horizontal with the motor axle and not at some angle off horizontal. Adjust the weight of the scales with some packing material or make the pivot arm longer/shorter.

Make sure you rotate the armature and take at least 4 readings with the armature in different positions to get a good average. DC electric motors produce a different torque depending on the position of the armature with reference to the magnets. 4 readings rotating the arm 90 degrees is the minimum, 8 readings rotating the arm 45 degrees is better. Also better to do the measurements at 6V to avoid the motor heating up to much and just double the results.

[ddyke]

Excellent

[buspor63]

[dr fabio]

Yes you are correct that each "pull" will weaken the result. I note that your pic reads 2.3Amps. That's a hell off a lot of heat build up in the armature. This will lead to increased resistance in the arm and hence less torque. I do my measurements at half volts. Say 6V and try and limit the measurement to 1 or 2 seconds. Then rotate arm and measure again. If doing it at 6V just multiply up by a factor of 2 if you want to know the torque at 12V. Or back it off to 4V and multiply up by x3.

[McLiams]

I use a similar arrangement for testing motors. The tightness of the nuts used to secure the strap holding down the motor affects the results, on some motors at least. It seems that the cans are thin enough that anything more than the VERY bare minimum tightness will apparently deform the can enough to bind the armature.

Having made that discovery, and modifying the test bed somewhat; the practice is now to place the motor so that the strap lands on the end of the can. This way the vertical wall of the end of the can will help to support the load exerted by the hold down strap.

$0.02

Mc

[walker]

The same principle as usual, I had some perspex to spare and used it.



I drilled two holes in te bar, one for the motor shaft, one for pinions. So I can test the motor without removing it completely.



The discs are for different shaft diameters. And they are really small an thin so that the loss of speed is as low as possible.
The rpm checker is standard, adjustable for 2, 3 or 4 propeller blades.



Roland