Stall current on dc motor

Question

I have a 250W dc brushed motor running off 24v. I controlled it with hbridge controllers rated for 40A, which should be enough... I thought. My reasoning being that 250W at 24v would limit it to 250W/24V=10.6A

This worked well enough, but when I prevented the motor from turning, the controllers blew. I then changed to building my own hbridge with 4 solid state relays rated at 80A, and when testing stalling the motor they died as well.

I measured about 1-2 Ohm resistance through the motor, when not connected, and with it being rated for 250W at 24v, that seemed reasonable enough, and should confirm the approximate maximal 10A current.

Obviously I am missing something here. Am I making too simplistic assumptions about the motor? I do not have any diodes connected to prevent back current, when motor is off, but spinning, and I did end all tests with letting it spin up to speed, so it could perhaps be the issue, but I was not expecting that to be greater than the stall current anyway.

My first large motor project, though, so I apparently need to understand something more...

Answer 1

The motor isn't going to limit its current, at least not in the way that you think.

A 240W, 24V motor means that if you run it at 24V at its rated torque, then it'll consume 10A (and 240W). It doesn't mean that if you ask for higher torque that it won't deliver it (and pull more current than it's rated for).

There's a very good chance that if you lock the rotor and apply 24V then you'll succeed in killing the motor, too. But maybe not, depending on the motor.

There's a lot of confounding factors, but in general you need to treat the motor right. Either through your motor driver or your mechanical design, you need to make sure that the motor doesn't suffer overload for more than whatever it's rated for*, and you need to make sure that whatever current the motor does pull doesn't burn up your electronics.

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* Many, but not all, DC motors are rated for short-duration overloads.

Answer 2

I measured about 1-2 Ohm resistance through the motor, ...

How?

Your indicated range of measurements has 100% variation. If this was measured with a DMM then

• The leads, zero error, contact resistance and meter resolution all limit you to measuring 'something between 0 and a few ohms'
• The brushes in a DC motor are non-linear, you will measure a different resistance at the mA of a DMM ohms measurement, and the amps of a running motor

The only way to measure the resistance of a motor properly is to lock the rotor, pass in the rated current, and measure the voltage drop.

Permanent magnet brushed DC motors are usually rated for a voltage. If you apply this voltage to a stationary motor, as you do every time you switch it on, the current it draws will not be sufficient to demagnetise the field magnets - quite an important consideration! That limit is in addition to the thermal limit of how much current you can pass for how long before the motor overheats.

The demagnetisation limit is usually fairly close, do not apply higher voltages to the motor without actively limiting the current with PWM on your H-bridge. It was quite common in the RobotWars community a few decades ago to run motors at a higher voltage for more speed (the life was going to be short anyway), but with a controlled current.