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Join Date: Jan 2016
Location: Bangalore,India
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VFD on Low Voltages

01/28/2016 12:52 AM

I would like to use a standard VFD for a small EV to run a 3 phase induction motor. I am constrained to use 60V or 72V DC battery banks.

I can design 3 phase 53V motors to be used for 72V DC input VFD. How can I use a standard VFD by modifying certain aspects in VFD like using DC DC converter to provide control voltage and turning off the undervoltage circuit or bypassing it!

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Guru
Engineering Fields - Electrical Engineering - Been there, done that, still doing it. Engineering Fields - Control Engineering - New Member

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#1

Re: VFD on Low Voltages

01/28/2016 8:05 AM

First, you must get a schematic diagram of the VFD. You might be lucky and get one from the VFD manufacturer but most likely you will have to reverse engineer the circuit from inspection and continuity testing. Next you will have to identify the AC to DC parts of the circuit (power supply) and determine if you can just bypass this stage with a connection to your battery bank. If the output voltage of the VFD does not match motor voltage then you may have to swap out the MOSFET or IGBT drivers for suitable parts. Make sure all of the pre-driver circuitry matches your choice of a new driver or you can destroy your new output transistors. Presumably the output transistors are configured in an H bridge configuration for each coil. In this case, you may have to reprogram the controller software for a different dead time interval. If you don't, all of your careful pre-driver redesign may be wasted by roasting output transistors.

Oh, there is no such thing as a "standard" VFD. That's why there are so many models from one manufacturer.

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Guru
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#2
In reply to #1

Re: VFD on Low Voltages

01/28/2016 8:10 AM

I almost forgot, you will have to add a steering diode to the batteries or the flyback voltage from your motor can damage your batteries.

I would instead just buy a properly designed and warrantied battery to motor controller.

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Guru

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#3

Re: VFD on Low Voltages

01/28/2016 9:11 AM

Being a firm believer in the KISS principle, I believe I would simply use a DC motor.

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Guru
Engineering Fields - Electrical Engineering - Been there, done that, still doing it. Engineering Fields - Control Engineering - New Member

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#4
In reply to #3

Re: VFD on Low Voltages

01/28/2016 11:29 AM

But it is so much more satisfying to build the whole thing from raw ores and hydrocarbons into preposterous configurations just to prove that you can.

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#5

Re: VFD on Low Voltages

01/28/2016 1:53 PM

Check with the VFD motor suppliers, chances are you can get a version of the drive that will work on such a low voltage DC and AC. They make them for the small vehicle EV market after all.

Alternatively you could look into changing your battery string arrangement to increase the available voltage (although you have probably already thought of this and are using a single series string of 12V batteries).

Do NOT try and modify the VFD! As someone who used to test and service these it is both unsafe and they are NOT designed for it.

A voltage step-up converter will be expensive, try a little more research first as I believe you can find the right VFD that will work for this application without having to modify one or add an expensive step-up converter.

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Guru

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#6

Re: VFD on Low Voltages

01/30/2016 2:01 PM

Kannansiyer,

Your post suggest a fairly good amount of knowledge. The advice to check with the VFD manufacturers, to see if they have a suitable model is good, but not likely to be successful.

  • I assume you are using a VFD that is built for DC input and does not have the rectifier head end.
  • The factory engineers may be able to help you find if parameters can be changed to allow the VFD to operate at a significantly reduced bus voltage. They usually have a minimum bus voltage setting below which they stop. Some manufacturers will be much more accessible than others.
  • You will have to supply the correct control voltage, probably from an external power supply. The smallest footprint option for this would be a custom-designed switch-mode power supply. Adding this will require identifying the power supply input and outputs in the VFD and isolating them.
  • A danger you will have to solve is how to handle the regenerative energy that comes from braking or going down hill. Above a certain level, this current flow will be greater than what the battery bank can safely absorb. At that point, your controls will need to use mechanical braking instead of regeneration to control the vehicle.

--JMM

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