775pro Motor Running Intermittently

I’m relatively new at this so please forgive me if I’m missing something blatant. I recently got a 775 pro motor and wanted to test it out just to see how it runs. The issue is that it runs intermittently. On for a few seconds and then off for a few seconds. I am just using some breadboard jumper wires and a 12V wall socket power supply so maybe there’s an issue there. I used the same method for a cnc motor and it seemed to work fine. Any suggestions? Thanks

I strongly suspect that both your power supply and wires are insufficient for the current draw of a 775Pro, even at no load. motors.vexpro.com lists the free current as 700 mA, but there’s going to be a much larger startup spike.

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Ok thanks. Any suggestions on what type of power supply I should be looking at? Right now mine lists as 3A. And I think I’ll probably use some 10, 12, or 14 awe wire.

In general, I recommend not using a power supply to run motors, unless you have a special case where you can guarantee that they’ll never be loaded much, or you have quite deep pockets. The stall current of a 775Pro is 134 A.

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For FRC you can have up to 40A @ 12V running through a motor, which is ~500W of pure wire-melting power. There’s a good reason that the minimum for that load is 12 gauge wire. Breadboards are a legitimate fire hazard here.

As much as it will hurt your wallet the right way to run a motor is with an inline fuse, motor controller (SparkMAX, Victor, Talon, etc, your team might have some old hardware unused), and a big fat Lead-acid battery (they’re unlikely to catch fire and can support a pretty large current draw, just don’t drop them).

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It’s even worse than that, actually, because circuit breakers and fuses have derating curves. For example, our 40 A breakers can handle 100 A for about a second.

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A cheap and easy power supply is a battery with a charger. Unless you’re doing dyno work where precise voltage matters, this works fine. This setup is what we use on our motor test stand (not a dyno) what we refer to as the SOB (System On Board). 3 examples of abusive tests are:

Tested a custom CIMile using a flywheel the size of a chainsaw flywheel and did 500 “RD slams” with 50A current limit set in the Talon. Each slam took 5 seconds (42 minutes total). Gearbox good, 775PRO copper brush springs glow red during direction change. Duty cycle was adjusted so the charger could keep up and the motor didn’t fry.

Tested printed swerve bevel gears with said flywheel in place of a wheel. 800 “RD slams”. Gear passed but failed during other tests.

Falcon temperature test. Some of these tests pulled 45 Amps for up to 4 minutes. Battery sagged to 11ish volts to high 10ish volts. Pretty pictures here.

https://www.chiefdelphi.com/uploads/short-url/eVYO5tVOYZecwq6Tl2kURlFZFgq.pdf

Again, nothing you’d want to use on a dyno but certainly good enough to see if a motor spins.

I’d recommend building your own SOB or a SIB (System In Box) so you can put all the safety stuff in it along with an easy way to hook up any motor (to an actual motor controller) and use any old FRC battery laying around. It could be a simple PWM controller with a small fuse block.

I’m sure other teams have something like this.

Ok, that sounds good. Battery, inline fuse, and motor controller. So theoretically we should be able to control the speed of the motors by connecting an Arduino to the controller and using a PWM signal? Previously we had been testing out doing this using a breadboard and L293, but obviously that’s not going to work now.

That would be one option for speed control. OTOH, there’s always the Thrifty Throttle.