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I have good working (mechanically) solution using multiple 4S LiPo batteries (in parallel, for higher current), 500 A contactor and brushed motor. Motor is working for <1.2 s only, but at very high power (multimeter at hold mode shows around 200 A, so 3 kW power at peak or even more).

Would like to ramp-up motor to full power in ~200 ms, instead of going "full power on" (avoiding "kick") (during first 1/5 second motor turns around ~25 times). Tried to buy 300 A commercial brushed controller from PwrSolution, burned on first test; then build another one with more than 1.5 kA mosfets in parallel - again, burned on first test.

I guess BEMF is killing them, not amperage (hard to use proper scope on such device, but cheap scope captured ~120 V spikes, while input voltage is closer to 16 V), so tried to add multiple TVS diodes, but this also did not help - mosfets still burning.

I checked how old elevators are controlled - but found only few mechanical contactors with resistive/inductive loads. This may work when we have lots of time to speed up/slow down, but when total load cycle is less than 1.2 second - switching with mechanical relays between rheostats is not good idea.

Wires are AWG8 multi-stranded not twisted, ~40 cm from motor to contactor, then 7-20 cm to batteries (multiple connectors over that range)

But somehow some industrial applications (some high-power air conditioning using brushed motors, water pumps, etc) have "smooth startup" option. So question - how this is implemented? What options do I have, if I want to smooth-start > 200 A (or even 500 A) brushed motor?

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  • \$\begingroup\$ Did you have diodes across the motor? What commercial controller did you try? \$\endgroup\$
    – Kevin White
    Commented Jun 8 at 19:31
  • \$\begingroup\$ Currently i do not have diodes (because i'm using contactor and LiPos are absorbing freewheeling power). When used custom built mosfet controller (just on/off) i used many TVS diodes (12x3KW if i remember correctly), did not prevent mosfets from burning. About commercial controller - IIRC it was HL50200. \$\endgroup\$
    – Andrius Narbutas
    Commented Jun 8 at 19:36
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    \$\begingroup\$ You will certainly need diodes when using a PWM controller (they may be included on the controller itself). You will also need to be extremely careful with inductance in the motor leads as that can cause large voltage spikes. That controller has a large amount of decoupling capacitance so inductance in the power leads should not be a problem. \$\endgroup\$
    – Kevin White
    Commented Jun 8 at 22:02
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    \$\begingroup\$ Understand the difference between "free-wheeling" voltage also known as back-emf due to rotation of the armature and "inductive-reaction" voltage due to \$di_L/dt\$. The latter is the reason for the diode across the motor. It must be able to handle the maximum motor current. \$\endgroup\$
    – user319836
    Commented Jun 8 at 23:42
  • \$\begingroup\$ have good working (mechanically) solution a pity, rather - if it was lacking, it would be easier to seek a solution storing&absorbing mechanical energy. \$\endgroup\$
    – greybeard
    Commented Jun 9 at 5:32

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Why not arrange your contactors to start the motor on say 2S and see how lurchy it is? If things are too lurchy then try 1S. Now you can series/parallel switch the LiPo to step control the motor. It would now be easy to arrange a simple timer to step through the steps.

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  • \$\begingroup\$ This would require to completely re-work power setup, so instead of pack of 4S LiPos connected in series (for more current) i would need also pack of 3S LiPo's in series, 2S and so on. Not possible, due to weight/space limitations. Probably it's possible to re-wire only part of cells from current 4S setup, but again, this would limit flexibility a lot. But again, mechanical high-current contactors i tried are not so fast, so switching between sources in tens of milliseconds if probably hard task, too (not for mosfets). \$\endgroup\$
    – Andrius Narbutas
    Commented Jun 11 at 8:02

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