Electronic – DC motor starting and stopping

currentcurrent-limitingdc motormotorusb

I'm powering a motor, and instead of rotating at a constant rate, it rotates roughly a third revolution every half second or so; the rotation keeps starting and stopping. There's no load on the motor, but I plan to add a gear pump fixture (en.wikipedia.org/wiki/Hydraulic_pump) onto it. Also, the LED on the wall charger that lights up when the charger is plugged in dims out when I connect the motor.

Gear Pump Fixture

I have a USB wall charger plugged into an outlet with a USB cable plugged into it. The other end of the USB cable is cut, and the red and black wires are going to the leads of the motor. This diagram demonstrates the setup.

Motor Setup

The wall charger is DC 5V 1A supply. The motor is a RS-360SH.

How can I get the motor to rotate at a constant rate?

I measured the amperage that the motor is using, and it was always below 0.5 A. I understand that the motor is drawing amperage from the power supply.

I know that current limiting resistors, other circuits, and PWM are an option, but I'm trying to keep this low cost. Using the formula I = V/R, I get 1A = 5V/R, R = 5 ohms. So my total resistance needs to be 5 ohms or greater to limit at or below the supply amperage of 1A. The resistor would need to dissipate 5W (P=A^2*R, P=1^2*5= 5 watts). I do not have any resistors capable of dissipating 5 watts.

What is the best solution to limit current so that my power supply is protected from burning out?

Best Answer

The average current of the motor may remain below 0.5 Amperes as noted, but transient currents could rise as high as 8.6 Amperes - the starting current of the motor would approximate the stall current rating.

Secondly, a DC motor such as the one listed, would generate significant back-EMF (voltage spikes of reverse polarity) from the coil commutation.

Each of those factors may trigger protection circuitry of the power supply. From the description of the intermittent operation of the motor, it appears that the initial starting current drawn by the motor is causing the power supply to either enter short-circuit protection, or over-temperature protection. The supply then recycles, and restarts the motor, and the cycle repeats.

While current limiting may be an option, operation will still be marginal. Possible mitigation options:

  • Use a large value capacitor across the motor's terminals, to even out the current surge at start. This is unlikely to work with a 1 Ampere supply driving a 8.6 Ampere stall-current motor, the gap needs to be reduced further
  • Use a lower current motor, no higher than 1 Ampere stall current.

Even if you could make a motor work within the 1 Ampere limit with no load, any increase in load would inherently increase the current drawn, so measuring the average current with no load on the motor has no value for the intended operation.

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