Using a brushed DC motor as a mechanical (variable) load... 1

[Iamanewbie]

Basically I'm interested in the pros/cons/implications of using a DC motor as a mechanical load... if anyone has experience in this area?

One option is a passive approach, which would be to just apply a resistor across the terminals to load the motor when it starts to spin (driven by an external source). That isn't enough for my application.

I guess specifically I'm interested in the following... Trying to drive a motor in one direction while it is spinning in the other!

Is this worse than being stalled. The motor will draw a LOT of current, compared to what it is rated for??
If I had a constant current source would that be useful?

Will this approach damage the motor controller/driver
(I guess it is the equivalent of spinning full speed (100%) in one direction then switching to reverse at 100%)

Advice on selecting a motor controller, sizing, protections etc.

I think I'll build up a test jig with some small motors to see what happens...

Advice/feedback/insights muchly appreciated! Thanks bye!!

 

[Skogsgurra]

What you describe is a typical motor test bench, aka Dyno, the way they used to be built for around 40 years. And some still are. ASIMs and VFDs are more common today.

You need a four-quadrant thyristor (SCR) drive and a DC motor with either permanent or external excitation.

The drive works with positive current when motoring (can do it both ways) and negative current (also possible in both CW and CCW).

You set speed with the Engine Under Test and the load torque with the Current set-point, usually known as current limit, in the SCR drive.

I have a couple of those in my shack. Mainly for educational puposes (Vintage Drives do still exist) but I also use them for some testing. You can find both motors and drives on eBay. Or sometimes in a scrap-yard. For low power, you can use 12 or 24 V DC motors from any car or truck.

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[waross]

A cheaper approach may be to connect a suitable resistor across the brushes and vary the field strength. However the torque will vary with the motor speed. If you need to accommodate varying speeds you will probably need Gunnar's solution.
Of course this method won't work with a permanent magnet motor so most automotive motors will not be usable.
Cheap but very limited capability.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[sreid]

Use a current mode servo amplifier to drive whatever torque you want into the load motor. This is done all the time for motor testing. Current one direction put a load on the motor under test. Current the other direction tries to "Over Run" the motor under test.

 

[Iamanewbie]

I tried on my bench two small 12v motors with the shaft connected - and it worked fine!!
One motor had the P/S (CH1) at 12V, the second was run in constant current mode (CH2), by varying the current in the second motor the speed (of the common shaft) slowed down. I guess this is obvious really!

My project has actually taken another direction, since the motor to be tested is a brushless hub motor. If found out from the customer that the torque is large (i.e. 10s of Nm) so I am probably going to use an electromagnetic powder brake instead.

Thanks anyway