Mechanical Engineering student here. I'm a part of a senior project team that trying to run a 40 HP 230 VAC three phase motor with a 40 HP VFD for a hydraulic pump. Our plan is to wire 28 Lead Acid batteries (12 V, 81 Ah) in series to get up to 326 V and connect it to the DC Common Bus of the VFD. Now, I think that this sounds unsafe and unsure if it will work, but the only issues that I have come up with are the danger of charging the batteries and the voltage drop when running. The EEs in our group countered with this:

  • Charging: They will create a box with a bunch of relays that will switch the batteries from series to parallel for charging.
  • Voltage Drop: Voltage input rating of the VFD is +10% to –15%, which means it needs anything from 195.5 VAC to 253 VAC. So theoretically, it needs 276.5 VDC to 357.8 VDC. Even if the batteries drop to 10 V, the VFD would still run.

However, I still don't feel safe with this set up. I can't find a DC to DC converter or a DC to three phase AC converter with our power and voltage requirements. Is there anything else that I am not thinking about?


  • \$\begingroup\$ it sounds kind of like an electric vehicle. do you absolutely need to use an induction motor? \$\endgroup\$ – Jasen Feb 9 '19 at 6:12
  • \$\begingroup\$ @Jasen we would like to because we already own it. We actually only need 10 hp. \$\endgroup\$ – Dexter Kenta Yanagisawa Feb 9 '19 at 6:33
  • \$\begingroup\$ how much of the above do you already own? \$\endgroup\$ – Jasen Feb 9 '19 at 6:43
  • \$\begingroup\$ @Jasen the VFD and the motor. They were purchased for another senior project about 8 years ago that never finished. \$\endgroup\$ – Dexter Kenta Yanagisawa Feb 9 '19 at 6:45
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    \$\begingroup\$ Please watch the episode of Mighty car mods named JDM electric turd where they did exactly this, DIDN’T use bypass diodes nor BMS and came very close to burning the car to the ground. Link: m.youtube.com/watch?v=PZuShe0gefY \$\endgroup\$ – winny Feb 9 '19 at 11:50

Wow. You have multiple problems.

A '12V' lead-acid battery can have 2.2V/cell (or more) = 13.2V at the end of a recharge cycle. 13.2 x 28 = 370Vdc, so you're over the maximum input. Whilst the freshly charged battery terminal voltage will drop quickly upon load, it will be >357V for a non-trivial period of time. I would drop back to perhaps 25 batteries.

It's a 40 HP motor & VFD, which will happily draw 100A DC at full power, and a few times that under more demanding circumstances (rapid acceleration, locked rotor, etc, as noted in the motor's datasheet). I note in a comment you say you only need 10 HP, so that's good - but ensure you program the VFD to be deliver no more than that 10 HP / 7.5kW, because 40 HP / 30kW = ~100A from a string of 81 AH batteries is not how you want to treat 81 AH batteries, at least not regularly. And several times that, which is multiples of your C-rating, would be quite unwise.

As for your EE's plan to make a bigarse box of relays to reconfigure them from 28 in series to 28 in parallel, that's... cute, but unwise, because:

(a) relays that will carry several 10s of Amps continuous aren't cheap

(b) that's a wiring nightmare of materials & cost inefficiency (you're engineers, not scientists!)

(c) 28 batteries aren't all going to age the same way over time, they will discharge to differing V levels (growing internal resistance), then when you slam them all in parallel they're all going to try to equalise with uncontrolled currents flowing between them, which is not good battery treatment policy

(d) if 1 battery fails prematurely for whatever reason, and gets replaced, it won't have the same discharge characteristics, it will likely have a lower internal resistance, which will manifest the (c) problem even more - the new battery will tend to dump even more power into its peers

(e) that uncontrolled current has to be handled by the relays, which dramatically increases their current-handling spec & cost. Note: DC relays of X Amps rating are much more expensive than AC relays of the same current rating (it's to do with DC arcing during switching events and the much greater mechanical complexity of quenching that arc, whereas AC has the benefit of crossing 0-amps frequently

(f) recharging 28 x 12V 81AH batteries in parallel over, say, 20 hours (i.e. the typical recommended C/20 recharge rate = 4 Amps), requires a charger capable of ~120 Amps, which is yet more thick wire and another expensive (battery recharger) component.

Lastly, high voltage DC is dangerous, way more than AC at the same voltage. If you touch it, it doesn't let go - your muscles clamp on to the live conductor. This is quite unlike AC where the 50/60Hz reversal gives you some opportunity to let go consciously. Arcing in the event of short circuits poses critical danger to those working on it. I'm talking gloves and perspex face-masks, and TRAINING. The fact your EEs even suggested this relay box idea tells me they're not trained for this, at all.

The problem of course is that recharging a bunch of batteries in series is no walk in the park, either! The need to keep ALL of the batteries at very close to the same stage-of-charge is very real. Think about it, if 1, or some, of them falls behind and reaches flat first, then you have a choice to make (a battery management policy to decide): stop the whole system and start a recharge and hope they all reach 100%, or let the prematurely low SoC battery(s) become critically over-discharged but get the full discharge capacity from the battery-bank. There's various ways to keep a bunch of batteries in series at the same SoC, but that's a different question :-), albeit with no simple or cheap answer.

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    \$\begingroup\$ ”reconfigure them from 28 in series to 28 in parallel, that's... cute” I was going to say fire hazard if anything fails, but yours is better! \$\endgroup\$ – winny Feb 9 '19 at 11:48
  1. The battery supply will need to be connected where the output of the 3-phase rectifier is connected in the VFD. Make sure that you don't bypass the capacitor charging circuit.

  2. Don't let anyone tell you that all of that DC bus capacitance is not needed for battery input. Those capacitors supply the reactive component of the motor current, exchanging energy with the magnetic fields in the motor. The batteries will need to supply only the real power.

  3. Look carefully at the current and torque limit adjustment ranges in the VFD. Those need to be adjusted to suit the mechanical requirements of the load. The VFD will reliably limit the motor current within its adjustment range. It will also reliably adjust the frequency to limit the motor low-slip operation. The locked-rotor and breakdown parts of the motor curve do not exist with VFD operation.

  4. Note that the motor is cooled by a separately-powered blower on the end of the motor. The blower appears to be driven by a 3-phase induction motor. It likely can be connected for 230 or 460 volts the same as the main motor. The best way to power that is likely going to be a small VFD.

  5. Re-connecting the batteries for charging can probably be accomplished. The system needs to be configured to prevent switching the connections after shutting down the load and before connecting the chargers. I would recommend 28 small chargers or a custom made charger with 28 controlled outputs.

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