# Weird speed issues with brushed DC motors in parallel?

So I have a simple system where I want to run eight large DC brushed motors (for wheels) in parallel, which should give them all the same voltage, and they should independently change rpm depending on the torque they face (from their torque/rpm curves). The circuit is just this power supply and these motors and some wire, nothing else (no controller beyond me fiddling with the main voltage):

But they're not showing the right voltage, and when facing different torques, they act VERY weird.

When I apply 5 volts (deliberately low so I don't cut my fingers off messing with the wheels), the motors all run the same speed when unloaded, but are each motor is only showing 3.2 V (as measured with a multimeter at the motor itself). If I raise or lower the voltage, there's always a lower reading at the motors. And the power supply voltage is correct (verified by multimeter) and in constant-voltage mode. Maybe back EMF, but this seems huge?

But this gets really weird if I grab a wheel to stall it. As expected, the current rises (still well within the limits of the supply), but all the other wheels spontaneously slow down too, and their voltage drops even further from the supply voltage (~2.5 V). Stalling two wheels further accentuates the problem.

So my questions are quite simply: why is this happening and how do I stop it? I could power each wheel independently, though I'd rather not given that there's eight of them.

• Please specify "some wire". Apr 3 at 13:51
• Wires have resistance Apr 3 at 14:05

Obviously, there is a high-resistance component between the supply and the motors.

For example, wires, connectors, switches, fuses, terminals.

Use your meter to discover where the missing volts are appearing. In your example, the supply is 5 V but the motor sees only 3.2 V. That means that 1.8 V is appearing across a wire, connector, switch, or fuse.

Once you discover which component is high-resistance, replace it with a low-resistance alternative. It may be multiple components.

• This seems to be the common answer, and it is pretty thin wire, so I just ordered some 10 AWG to see if that fixes it.
– MCA
Apr 3 at 15:18
• The thin wire is your problem. Wire Resistance is inversely proportional to area. Apr 3 at 18:00
• Wire topology also matters: you will get best results if you route independent wires to each motor in a "star" topology. Apr 4 at 9:13
• @MCA You don't have to guess at the gauge - like all things engineering, we calculate. Running at the rated 12V, your motor has a 5.5A stall current, so pick a gauge that doesn't exceed 2% voltage drop for the length you need. 10AWG will be overkill unless your wires are extremely long. You can probably do with 18AWG on a shorter run (~1m/3ft). 14AWG would be safe for up to about 3m/9ft. 10AWG would let you run 8m/25ft, and if you need cables that long I'd say you've selected the wrong motor for the application.
– J...
Apr 4 at 18:30
• @MCA Of course, those gauges would be per motor (ie: a pair of that gauge for each motor back to the supply). If you want to use the same feeder for both motors then cut the distances in half. Again, though, if you need cables that long then a low voltage DC motor is probably not the ideal solution - your cable is now starting to cost more than your motor!
– J...
Apr 4 at 18:34

What wire are you using, and exactly how are things connected? I am thinking voltage drop in way too small wiring here.

To the first issue: Do you have current limiting disabled on the power supply? If there are no high-resistance components in series with the motors, that's the first thing I'd check. Current limiting will drop the power supply voltage to keep current below some maximum. To confirm, check the voltage at the power supply output while the motors are running.

To the second issue: When you stall one motor, it reduces the load on that branch of the parallel circuit. Instead of power going to the motion of the motor and resistance of the windings, it instead goes to just the resistance of the windings. The effect is like a short. Most power goes to that one branch, routing power away from the other motors. Thus, all the motors slow down. The only reason they don't stop is that the stalled motor still has some voltage dropped over its windings.