What happens if a motor is towed by another at a rate faster than its maximum RPM? For example, Motor A’s output is 120RPM but is towed by Motor B and C, with output 240RPM.
The motors would stall after a while. It is best to have connected motors have the same rpm
I’m assuming you mean that motors B and C, the ones towing, would stall. I’m more concerned about what happens to motor A, perhaps from an electrical standpoint. Would it burn out a port, etc?
I’m not 100% sure but I would assume that all the motors will stall out. The ports should be fine, though
Kk, thanks for the help! I’m trying to use a motor to pull on an elastic. But there’s gonna be a point when the elastic makes the motor spin faster than max RPM. Hope nothing dies XD
Yeah I’m curious about this as well so if you could update to this thread once you do it and tell us what happens that’d be great
A rotating motor is a voltage generator, with the voltage proportional to the RPM. I don’t have the 393 Kv constant, but you can assume the back-emf at nominal speed to be around the system voltage, say 7V. If you turn it at twice the nominal speed, it would generate twice that, 14V. Now, MC29 FETs are 30V rated, IIRC.
Those open FETs (one diagonal of the H-bridge) could easily sustain 14V, while those with body diode biased into conduction (the other diagonal) would conduct into the system power bus, trying to raise it to 14V, and with no load (e.g. the MC29 disconnected from the Cortex), that would happen.
When connected, though, the current would flow from the motor through MC29 into whatever system load (other motors, battery) and the voltage difference would need to realize somewhere.
Assuming the system voltage bus is directly connected to the battery (which is a very hard voltage source), that leaves the motor winding, wires, PTC and the FET body diodes, heating them up. Since the current would be flowing, the motor would also oppose the rotation (brake).
At twice the rpm and the external voltage clamped to 7V, it would be a situation very comparable to full-on motor stall, so you could expect the PTC to trip soon and all the voltage difference being supported almost solely by the PTC. At this point, the current would go down significantly, the braking effect would diminish and the MC29 would be relatively safe again.
I have meet teams who have accidently ran different geared systems on their drive. For example a 1-2 sprocket ratio to a 1:1 high speed direct motor on the outer wheel. I do not believe it leads to fast stalling since it mechanically is forced to slow down the towed wheels speed. Turbo settings could have a stalling issue. The main point is that the entire system would run at least half the speed it should since the offset motor is forcing the netire systm down.
Short and sweet answer:
Don’t do it!
Long answer:
Bunch of calculations… = Don’t do it.
Anyways, with this kind of system, you will undoubtedly stall out the motors, and you will be damaging them. In the same way that constantly spinning a motor by hand damages it, pulling it past its maximum rpm would damage it and quickly destroy the motor.
Well, for all the practical purposes with the power provided by rubber bands (thus limited energy storage), the battery and diodes in the H-bridge would clamp the motor speed at a little over the nominal, since the PTC won’t have enough time to even trip.