# VFD as Charger



## Coulomb (Apr 22, 2009)

bacanal said:


> He says that i can use my vfd as charger, because i can control the voltage and the current with it.


It is possible, and we were going to do this on our MX-5 conversion. But we've stopped using an industrial controller, and are using a pair of standard chargers now.

It's quite tricky, and I don't know if we would have eventually gotten it working. You have to:

1) Have your pack voltage higher than the peak of the mains.
2) You have to arrange for the mains to appear in series with the motor somehow.
3) You need to do a "flying start" and get the inverter to synchronise exactly with the mains (absolutely zero slip).
4) You need to be able to adjust the amplitude to the same as the mains and track that amplitude, and control the phase of the output relative to the mains to control current. This is completely different to normal motor control, so you need *very* unusually good control of the controller. This seems unlikely with a "normal" industrial controller. We had a Control Techniques controller, and paid extra for a special software package that allowed more flexible control. We also had the ear of a controller expert that we could ask highly technical questions.

This is not going to be a quick solution, and may well not be a solution at all.



> I need to know something now, because i need to run my truck as soon as posible.


Then this is not a solution for you, I think.


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## PStechPaul (May 1, 2012)

I'm not sure how you can use a VFD as a charger, except that regen from the motor will feed back to the DC bus link which means current will be fed into the batteries and charge them. This would only recoup a small amount of energy from dynamic braking going downhill. Of course, if you have a means of turning the motor from another power source, such as a windmill or waterwheel, it could act as a generator and charge the batteries.

If you need to charge the batteries from the electrical power grid, it might be possible to apply the power to the three-phase input of the VFD, and then add a three-phase bridge rectifier to the output. Then you might be able to program the VFD to provide a certain amount of current to the load, which in this case would be a battery pack rather than a motor. It would be similar to DTC, or direct torque control, which adjusts the current to the load according to user setting. And then you may be able to set an output voltage limit to stop charging at that point.

Much depends on the battery chemistry as well. You should have some sort of BMS to protect the cells from imbalance and overcharging. This is more critical for LiFePO4 and Li-Ion. A simpler system may work for lead-acid. 

It's probably easier, safer, and possibly even less expensive, to build your own charger or purchase one designed for the job. The cost of the charger will be much less than that of the batteries, particularly LiFePO4, and you probably don't want to risk ruining the cells or causing a fire or other dangerous condition with a home-made contraption. Basically, if you don't already understand how a charger can be made, and how batteries must be charged for best performance, you should invest in a quality product that has already been engineered and comes with a guarantee.


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## Coulomb (Apr 22, 2009)

Yes, I forgot to mention that we only considered using the VFD as a charger through regen, i.e. we connect the mains to the output of the inverter (with great care, ensuring everything is synchronised and there is an inductance between the VFD output and the mains), and leave the DC bus of the inverter connected to the pack.

The other way is as PStechPaul says: rectify the mains to become the inverter's DC bus (possibly using the original diodes or SCRs of the VFD), generate square waves, and rectify the output of the inverter to charge the pack. This uses the VFD in "motor" mode rather than "regen" mode. I have not considered this method of connection.

Either way, you likely also need large, expensive inductors to filter PWM noise so it doesn't get into the mains. These filter inductors could be as expensive as a low power (say 4-8 hour empty to full) charger.

This last requirement was the final straw for us. The only advantage of the VFD charging is that it gives you the option of fast charging where high power AC is available (e.g. 3 phase). But DC fast charging is emerging as the method of choice for fast charging.

However, if you are out in the country, you're much more likely to find a 3-phase outlet (e.g. at a baker or garage) than a DC fast charge outlet. But even more likely is a standard general purpose outlet, for a standard (slow) charge.

In other words, the only time a VFD charger would come in handy is if you have access to 3-phase or other high power AC and need a quick charge and DC fast charging is not available. For this situation, you have to pay for the extra contactors, smoothing inductors, high power rectifiers (if needed) and a lot of fiddling around. Whether this cost is worth the chance of coming across this situation is of course a personal choice.


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## bacanal (Feb 9, 2013)

Thanks guys, and sorry if i'm late. I read your answers the next day you post, but i didn't have the time for a reply.

These things are dificult here in Argentina, because our government practically don't allow imports, except for popular articles. That's why i have to make this proyect with things i can buy in a supermarket.

I realize that i can't use my vfd as a charger. The only alternative i have is to buy a 12v charger and switch the battery bank from series to parallel to charge the bank as a 12v bank.

Thanks again, your posts have been very helpful.


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## bacanal (Feb 9, 2013)

Today I tested the system with the 220vac grid. It worked very well. If i'm lucky, tomorrow i will test all with the battery pack. I'll post the news.


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