# Capacitors for ev inverter/ motor controller



## Siwastaja (Aug 1, 2012)

It's not only about the impedance of the capacitors, but the inductance of the wires between the caps and the power switches. This is why a laminated DC bus is used, and the best-quality (lowest impedance) film capacitors are mounted as close as possible to the transistors. Half-bridge modules allow direct mounting of the caps.

Electrolytics is the secondary power supply, and the battery pack is the 3rd. Electrolytics cannot be mounted directly to the power switch module (they won't fit), but they should be as close as possible; a few inches max, preferably directly on the laminated DC bus.

The idea of having enough electrolytic capacitors is that the wire length and routing to the battery pack would become meaningless. Of course you should always minimize battery cable length and run them next to each other, but it's not always so easy to do and the controller shouldn't blow up because of that.

The idea is similar to having two separate levels of cache in a computer CPU accessing the slow RAM (=battery pack); very small and very quick (= "snubber" film caps), and a larger but a bit slower (= electrolytics).


----------



## cts_casemod (Aug 23, 2012)

Siwastaja said:


> It's not only about the impedance of the capacitors, but the inductance of the wires between the caps and the power switches. This is why a laminated DC bus is used, and the best-quality (lowest impedance) film capacitors are mounted as close as possible to the transistors. Half-bridge modules allow direct mounting of the caps.
> 
> Electrolytics is the secondary power supply, and the battery pack is the 3rd. Electrolytics cannot be mounted directly to the power switch module (they won't fit), but they should be as close as possible; a few inches max, preferably directly on the laminated DC bus.
> 
> ...


 
Hi,

When designing my inverter I did try to fabricate the DC BUS in a way to reduce inductance to a maximun extent. Snubber capacitors were attached directly to the IGBT Terminals in order to reduce any voltage peaks caused by the inductance of the BUS.
The question is however based on the storage capacitors. I replaced the big Electrolitics with some metal film counterparts capable of ripple currents in the order of 600A @ 900V to cover the nominal current for my IGBTs.

While I do understand this would further reduce the effects of any inductance on the link I wonder if the capacity will be enough, since I dont even need a Pre-Charge circuit to charge those.

So what I am trying to know is the required capacity, ignoring the requird ripple current I should have, because the motor on its own nature will draw current with ripple, just like a 3 phase supply. At the same time there is a reactive current going from the battery to the inverter.

As with the cable, I dont really believe it would make a big diference on a car. Having, at most, 5 meters between the battery and the controller doesnt seem like an issue to me, unlike in factories, where the cables may be Kilometers away from the main distribuition point.


----------



## kennybobby (Aug 10, 2012)

A pack of large electrolytic caps holding up the DC buss is usually found in variable frequency motor drive inverters being powered from the mains--so the 'hold-up time' is on the order of 17 to 20 msec for one cycle of AC. 

i think you are right that there doesn't seem to be such a big need for them when using a battery pack. 

i've had to use a large 3-phase L-C reactor on the output of an inverter when using long cable runs to the motor.


----------



## cts_casemod (Aug 23, 2012)

kennybobby said:


> A pack of large electrolytic caps holding up the DC buss is usually found in variable frequency motor drive inverters being powered from the mains--so the 'hold-up time' is on the order of 17 to 20 msec for one cycle of AC.
> 
> i think you are right that there doesn't seem to be such a big need for them when using a battery pack.
> 
> i've had to use a large 3-phase L-C reactor on the output of an inverter when using long cable runs to the motor.


 
Well, to cope with 300Amp, which is only half of my inverter output I would need 12 Electolytics for a total of 25.000uF @ 800V and an average ripple current of just about 120Amp. Its both ridiculous in size and cost.

Also what many fail to realize, a standard electrolytic has an impedance of about 20mOhm, so we would need two in parallel as a minimum, just to mach the internal impedance of an average lithium battery.

Speaking about LC reactors, do you have any experience with DC-Line reactors? Not the output ones, the ones you attach to the DC BUS. I know they can help to considerably reduce the capacitor size and stress on the battery due to reactive power. Again, theres not much info I can find on the subject.


----------



## e*clipse (Aug 2, 2009)

I was wondering about that for my motor controller design.

I have found some app notes that are valuable for the subject; particularly from manufacturers of film caps. 

SBE make ring capacitors - their technical information is very helpful. 
http://www.sbelectronics.com/

Electronic Concepts also makes film capacitors. I found a paper published by them to be very helpful. The paper is called "Selecting Film Bus Link Capacitors For High Performance Inverter Applications"
http://www.ecicaps.com/technical-papers.php

Any additional help from folks around here would be welcome.


----------



## subcooledheatpump (Mar 5, 2012)

Don't know if you've considered this or not but, electrolytic capacitors won't last very long in high temperatures. Will your inverter be liquid cooled and/or sealed in a water proof container? I'm just thinking if you are going to go for the smallest possible capacitance, the ripple currents could heat the electrolytics, since as you said they have a fairly high ESR

I found this while researching yesterday...... 

http://www.illinoiscapacitor.com/tech-center/life-calculators.aspx


----------



## Siwastaja (Aug 1, 2012)

Good quality (non-counterfeit) electrolytic capacitors manage high temperatures just fine. They are used in power supplies where they run constantly at 60-70 deg C or more for YEARS 24/7. You never get such runtimes for anything in a car. Driving 300 000 km at 50 km/h is just 6000 hours = 0.7 years.

Use enough capacitors to make sure they are not overloaded by the ripple.


----------



## cts_casemod (Aug 23, 2012)

Electrolitics are false economy for an EV. 
By the time they are enough to handle the ripple capability, the inverter setup gets huge and price for a good quality electrolitic is not cheap either.

There are also other factors depending on what voltage it is required.

My experience: My battery temperatures have dropped since I increased my capacitor bank to 10.000uF @ 400V. But the average current I see on my setup is only 30Amps @ 307V. For an average 144V Conversion this current is at least doubled. High performance or heavy vehicles should have twice, so we all see where this is going.

For my future upgrade to 650V I would need a huge box of capacitors, but the same would apply for a setup drawing more amps, so I decided to go for high power metal film, capable of handling 300AMps RMS with 1/3rd of the space used for the electrolitics, that would only be good for 50-100Amps. 

There are also other factors like self discharge and the ability to clean inductive voltage spikes without damage.

My advice is to get a few high ripple capacitors for the inverter and if necessary get filtering (metal or electrolitic) with higher capacity to have close to the battery. 

As with temperature and capacitor life I agree with what has been said. A car is only being used for a few hours, however depending on the geographical location and the location of the inverter, (if they are inside a boot during a sunny day, temperatures in excess of 60C for a few hours may be the norm) this might not be ideal for everyone


----------

