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I am running a 3KW motor on a big 24Vdc power supply (Meanwell RST-5000), through a frequency drive. When stopping the motor, however, I occasionally get a regenerative backcurrent that trips the internal overvoltage protection in my power supply. The voltage ramps up to about 28.5V. The supply is protected against this, but it does have to be manually reset and breaks the workflow.

What is a good solution to prevent this? Can I just hook up a big (+- 100Ah) Lead-acid battery in parallel to the power supply? Or will this give me other issues?

Please keep in mind that there are huge currents involved (+- 125 A)

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  • \$\begingroup\$ Is the motor operated uni-directional or bi-directional? \$\endgroup\$ – Oldfart May 2 '18 at 12:53
  • \$\begingroup\$ Unidirectional. It drives a hydraulic pump. \$\endgroup\$ – BasK May 2 '18 at 12:55
  • \$\begingroup\$ Sorry, I am not an expert on power motors but know a little bit about small ones: do you stop the voltage abruptly and if so, is there a fly-back diode? \$\endgroup\$ – Oldfart May 2 '18 at 13:23
  • \$\begingroup\$ Adding some kind of TVS diode or thyristor between your power supply and your VFD might do the job. But my experience with this stuff is all at lower power levels. \$\endgroup\$ – Hearth May 2 '18 at 13:26
  • \$\begingroup\$ I am not sure on the inside workings of the frequency drive. The supply doesn't trip on every motor power down, so I think there is some kind of flyback system in the drive, but it is not powerful enough. For the TVS: What specs do I need? - I guess a breakdown voltage above the supply voltage, but below the trip voltage? - They are rated in power dissipation. Is this current*forward voltage? But there is also a time factor involved... - Is just the TVS diode enough, or should I install a high power, low resistance resistor in series? \$\endgroup\$ – BasK May 2 '18 at 14:49
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I've implemented 3 solutions to this problem:

  1. A capacitor bank sized large enough to absorb the worst-case regen energy without tripping the OVP. In your case it might be expensive and huge, but has minimal losses.
  2. A resistor bank with a comparator. Also big, less expensive but you waste the energy. Easy to implement and the "standard" solution when a capacitor bank would be too big or expensive.
  3. For really large regen energy that happens often, a grid-tie inverter that takes the regen energy and puts it back into the grid. You may need a backup resistor bank in case the transfer switch is down (due to power failure) to avoid damage or shutdown from large regen energy. This is the most expensive and complicated solution but also has low losses.
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A braking resistor is the conventional answer, switched in by a mosfet and comparator arrangement when the bus voltage rises, but a 24V lead acid bank would probably work fine, just make sure the thing is fused and has a suitable float voltage set on the supply.

Note that if your emergency stop is on the mains side of the supply, you may wish to think about the implications of the battery when it comes to stopping the thing.

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  • \$\begingroup\$ A battery isn't a good idea. What do you do when the battery is full? \$\endgroup\$ – Jeroen3 May 2 '18 at 14:15
  • \$\begingroup\$ If you set the supply to the correct voltage the battery will sit at 90% full and if it goes above that (should be seldom, the excess energy is probably not that much) it will supply power in preference to the main power supply, until it discharges back to the float level and the main supply takes back over. Braking resistor is more compact, but providing you can set the float voltage appropriately, a battery may actually be simpler. \$\endgroup\$ – Dan Mills May 2 '18 at 15:29
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In the end, I went with a 140Ah Lead-Acid battery bank, because we had that in stock and it was easiest to mount for me. It is connected in parallel to the power supply, fused seperatly with a 200A slow blow fuse.

I let the system run on batteries only for a bit to drain them to +- 24V, and then switched on the power supply.

This has run for a full day without a single issue. The batteries don't appear to get hot.

Thank you for the great advice though. If I see an issue like this coming in the future, I will definitely consider the comperator/resistor aproach.

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