Using supercapacitors on DC-DC converter output for high in-rush current

Question

I have a motor that draws a max continuous current of 60A, but has a peak in-rush draw over 120A for a few milliseconds. I want to power the motor using a DC-DC converter.

I purchased a converter that has a peak output of 60A and tested it with the motor. It worked, but there was a long wind-up before the motor achieved its required torque. Turns out the converter has a built-in mechanism to prevent in-rush current so we were not getting the instantaneous torque we wanted.

I was thinking of purchasing a supercapacitor bank and connecting it in parallel to the converter's output. My understanding is that the converter will slowly charge up the caps initially, and as the motors start-up, it will draw the in-rush current from the capacitor bank. Once the motor reaches its steady-state current draw, the DC-DC converter will be able to supply that and top-up the capacitors.

I am asking to see if my thought process is correct or is there something I am missing or have to be careful when designing/testing. With regards to the details, I will make sure that the capacitors and DC-DC converter are spec'ed properly to be able to handle the voltage and current draw.

Any advice will be appreciated!

Answer 1

Using a supercapacitor is reasonable. Just make sure your power supply doesn’t complain about:

• charging current during the initial start-up
• back-powering when PSU is off

Why the concern? The capacitors will discharge through the power supply when primary power is removed. Diode isolation can prevent this.

As far as charging current, a large supercap bank will demand much more current than the PSU can deliver, tripping the PSU overcurrent protect mode until the charging can finish.

In short, the cap bank needs to be treated like a battery, with current-limited charge control and back-power blocking to protect the PSU.

Answer 2

I was thinking of purchasing a supercapacitor bank and connecting it in parallel to the converter's output. My understanding is that the converter will slowly charge up the caps initially, and once the motors gain momentum, it will draw the in-rush current from the capacitor bank.

Once the motor starts turning, a back-EMF is generated in the motor due to which an inrush current does not flow into the motor. The reason why, initially, an inrush current flows into your motor during startup is because it takes some time for the motor to spool up and develop a back-EMF before it draws a steady state current.

So adding super capacitors at the output and starting the motor as the DC DC converter turns on is of no use, as the DC DC converter will not charge up the super caps instantly and will go into current limit mode as it did with just the motor.

But what you can do is connect a super cap to the output of the DC DC converter and then connect the motor to the DC DC converter + supercap bank through a mechanical relay or a solid state relay. Then, turn on the DC DC converter and let its output voltage rise up. Then, close the relay. The supercaps, which are charged up to some voltage already, will take care of the motor's inrush current requirements.