I've got a motor controller for a Bldc and I'm in the process of tuning the controller with both PWM frequency, duty cycle and commutation frequency.

Anyway, this power supply is a 30v 10A supply and when I drive the motor, the voltage and current limits kick in. I start at 12V and allow 10A, but the supply sags to 9.5V @ 3.5A. Then, when I shut the motor inverter off, and disconnect the supply, the power supply voltage indicator climbs up to 50V! I have to short out the terminals to get it back under control.

So, two questions :

Why doesn't my supply current limit at 10A like its supposed to? If I short the terminals, I can get 10A, but the motor won't pull 10A.

And, why does the voltage climb after being disconnected?

Edit: Well, it seems like it's something more serious that this... The power supply is very laggy when changing voltages, the voltage is unable to go below 2.5V, and it ramps up and down when it is initially turned on.... This was after leaving it sit for over an hour and then turning it on, open circuit.

I think she's spent.

  • \$\begingroup\$ Link to a datasheet for the controller? A "30 V 10 A" power supply is usually not a promise to limit at 10 A, it's telling you if you draw more than 10 A you could damage the controller. \$\endgroup\$
    – The Photon
    Oct 16, 2017 at 1:23
  • \$\begingroup\$ This one has a current limit knob. And it's the controller is a ground up design with 6 fets and a 3 fet drivers \$\endgroup\$ Oct 16, 2017 at 1:32
  • 3
    \$\begingroup\$ If you give a link to the datasheet, we'll know enough about it to give you useful answers, without having to read your mind. \$\endgroup\$
    – The Photon
    Oct 16, 2017 at 1:34
  • \$\begingroup\$ I'm not next to it right now, but this is the product page for the power supply. thediyoutlet.com/products/… \$\endgroup\$ Oct 16, 2017 at 2:16
  • \$\begingroup\$ does your is flyback system backfeed into the power supply? \$\endgroup\$
    – Trevor_G
    Oct 16, 2017 at 3:19

1 Answer 1


This is only a guess, but I will propose the following hypotheses:

Hypothesis 1)

The supply is rated to deliver 10 "Shenzen Commercial Amps" which seems to equate to about 3.5 Real Amps as you noticed when the voltage sagged under load. Then something inside blew up. This seems to be confirmed by the fact the supply is now acting weird.

Hypothesis 2)

The supply's transient response is slow. When the current drawn from your load abruptly switches from several amps to zero, the pass transistor takes a while to turn off. If capacitance on the power rail is insufficient, this will result in a voltage spike on the output. It should not produce lasting damage however, so this wouldn't be my favorite hypothesis.

Hypothesis 3)

  • The PWM stops, and the FETs all turn off.
  • The FETs are off, thus they act as rectifier diodes.
  • The motor keeps turning due to inertia and acts as a generator.
  • The generated voltage is rectified by the FETs and fed back into the supply
  • The cheap chinese supply dislikes this and something inside blows up

Now with a 12V supply the motor would reach a cerrtain rpm, but it wouldn't go fast enough to generate a lot more than 12V... unless we consider its internal resistance, which will matter due to the high current. When it acts as a generator, you will get its unloaded output voltage, which can be more than the original supply voltage, because the current is now very low, so the internal resistance matters less...

You should manage the motor stop, maybe leave the bottom FETs turned ON until the motor has stopped completely, I'm not sure.


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