Advice on Variable High Voltage DC Power Supply

[TongueTester]

Hi Guys,

I am trying to make a 0-340V 6A variable DC power supply to simulate the HV battery from an Electric Vehicle for the purposes of mucking around with the vehicles inverter.

I'm in 'Straya which has 50hz ~240V AC power.

So far I have a 10A rated 0-260V AC Variac (Auto Transformer) and 3 x 15000uF smoothing capacitors (totaling 45000uF). But at the current levels I am looking for the output ripple is too high.

So I was thinking that adding some inductance might be a good idea - but I'm not sure how to size it, or even where the best place is to put it!

I had a crack at modelling it in LT spice, but when I add an inductor it makes no difference to the ripple rejection, or worse it makes the output go mental (depending on the value of the inductor).

Any tips are appreciated and thank you I am aware of the dangers of electricity (that's what makes it so fun)

[coppercone2]

http://www.vias.org/crowhurstba/crowhurst_basic_audio_vol3_024.html

https://www.qsl.net/i0jx/supply.html

https://www.hammfg.com/electronics/transformers/rectifier

I think you should save up and buy a regulated modern DC power supply capable of the voltage. Car parts aint exactly cheap. IMO doing this with a variac and 45000uF of capacitance is dodge city. Its also going to be spectacular from a distance if you have a short circuit etc. A sorensen switching power supply will cut down on the madness.



maybe get this refurbished and maintain some sense of normality
https://www.ebay.com/itm/362608870065?chn=ps&srsltid=AfmBOorqV9sS-TjzXmBYDbvp0canpjxWOre3GlLVky6TCtNknaUVnb4kQ24

[johansen]

what is the output ripple you are getting?

capacitors in that size are not cheap, did you buy new or used? they may not actually be 15mF

You may be able to find a partially functioning 3 to 5 ton heat pump inverter board from the dumpsters at hvac companies. some of them have totally separate PFC sections and they run the pfc all the time even when the system is not running. i know this because i have one such board, IIRC it uses a fan9673 controller, I know for a fact its a 3 phase chip but the board is only 2 phase.

to get 340 volts you would need to reduce your 240vac down to about 200volts, then find where the PFC chip senses the actual output voltage and change the sense resistor. you can then get 340 volts out of it, regulated fairly well assuming you don't mess anything up.

a 3 ton board will easily handle the 2KW you need.

[CaptDon]

Sadly, a Sorenson for his voltage and current requirements will top $10,000.00 U.S.D.

[coppercone2]

https://www.ebay.com/itm/362608870065?chn=ps&srsltid=AfmBOorqV9sS-TjzXmBYDbvp0canpjxWOre3GlLVky6TCtNknaUVnb4kQ24

its close. you can get 400V ones for a bit more. You don't need to get it new. its still going to kick the shit out some tricked out variac

electric car parts aint exactly cheap either. you should expect test equipment to test a high power high voltage DC motor to be expensive. its not furnace blower


https://alltest.net/categories/products/SGA400-12C-Sorensen

[themadhippy]

things are gonna get a bit tingly if you brush against the wrong shiny bits on your inverter,might want to consider an isolation transformer

[TongueTester]

Great responses guys thanks! 

things are gonna get a bit tingly if you brush against the wrong shiny bits on your inverter, might want to consider an isolation transformer

Yes - noted. Components will be housed in a steel box with explosion directing vents. I considered an isolation transformer - but decided to put my faith in the RCD powering the AC circuit instead... (it doesn't hurt that much - ask me how I know)

https://www.ebay.com/itm/362608870065?chn=ps&srsltid=AfmBOorqV9sS-TjzXmBYDbvp0canpjxWOre3GlLVky6TCtNknaUVnb4kQ24

its close. you can get 400V ones for a bit more. You don't need to get it new. its still going to kick the shit out some tricked out variac

https://alltest.net/categories/products/SGA400-12C-Sorensen

Thank you - I had not seen a Sorensen before - it is still out of my price range but I will keep an eye on the local classifieds in case someone is getting rid of one and doesn't know what they are worth!

what is the output ripple you are getting?

capacitors in that size are not cheap, did you buy new or used? they may not actually be 15mF

to get 340 volts you would need to reduce your 240vac down to about 200volts, then find where the PFC chip senses the actual output voltage and change the sense resistor. you can then get 340 volts out of it, regulated fairly well assuming you don't mess anything up.

a 3 ton board will easily handle the 2KW you need.

Thanks for your input.

I'm getting 6V P-P (modelled only as I am still trying to work out an NTC circuit to limit inrush to the capacitors).

The capacitors sure are 15mF @ 450V. Pic attached - they feel good and girthy to hold in the hand...

Regarding the 3-Ton board I need to be able to vary the output voltage and I am guessing they can only regulate to a single voltage level - what are your thoughts on that?

http://www.vias.org/crowhurstba/crowhurst_basic_audio_vol3_024.html

https://www.qsl.net/i0jx/supply.html

https://www.hammfg.com/electronics/transformers/rectifier

I think you should save up and buy a regulated modern DC power supply capable of the voltage. Car parts aint exactly cheap. IMO doing this with a variac and 45000uF of capacitance is dodge city. Its also going to be spectacular from a distance if you have a short circuit etc. A sorensen switching power supply will cut down on the madness.

Thank you for the links - as I understand a choke filter, it will limit the output DC voltage to 0.9 x AC RMS voltage - so that's a no go for me. Although when I tried to model a choke filter (as in the LTSpice file in my first post) it did not have that effect for some reason - so I am sure I have got somethign wrong in the LTSpice model.

I was thinking a 'PI' filter would achieve something similar to the choke filter, but wouldn't reduce the output voltage so much? (because the capacitor is 'before' the inductor so to speak). This is really what I am wanting to implement.

Thank you for pointing out the Sorensen, but as above its probably too much $$.

Any tips on adding an inductor to the mix, maybe in a PI configuration to smooth things over?

[coppercone2]

what is a failed motor going to do on that kind of source? Just keep in mind instantaneous surge current. A power supply is current limited. For a test bench I don't think you want unlimited. a car battery has some protection circuits built in. Do you have these designed or on hand? Are they capable of dealing with a pure capacitor power source on a failure mode? Battery ESR is low, but capacitor ESR is nothing in comparison.

The price might be similarly outlandish once you factor these things in.

The switching supply has a small output capacitor (relatively), certainly no where near 45000uF @ 350VDC.

Also a 30A choke aint cheap either.


Then there is another game, that is float. A DC power supply will probobly have a floating output. a battery has a floating output. That variac is grounded. It might cause a problem with your test setup that you don't anticipate. You start following test procedures that assume a floating source with your own modifications you might be in trouble. I don't know enough about that though.

THen also power factor I think might be a problem. I don't deal with it, someone else can evaluate that.

[johansen]

 I need to be able to vary the output voltage and I am guessing they can only regulate to a single voltage level - what are your thoughts on that?

- you should be able to change the voltage with a potentiometer,  but it will always be higher than the ac oncoming line voltage.

Also im not sure they will do better than 6v ripple under load.

[coppercone2]

another thing too from when I used to deal with people trying to use variac for precise things they should not be used for

I measured line voltage with a data logger. I found the error throughout the day was 5% in this particular place on one particular day (enough for measurement fail). This building power was reputed to be high quality/ stabilized or whatever. It will vary. It will vary on load too because it sound residential. Like sags on loads, combined with random grid changes. It could be alot worse then 5% probobly. A SM PSU fixes this. And this was without capacitors and shit, just to power heaters. They thought for a long time they could just assume the grid was 120V and measure current. Nope. This was for only maybe <300W resistive AC load, so it probobly considered to be very well behaved (but still fail paperwork) compared to what you want. More then likely turning this thing on will cause your line voltage to drop and when it spins up it will rise again etc. Talk about unstable motor supply. And they still had technicians basically babying variacs with ammeters (despite not knowing line voltage changes! talk about labor!)

So you don't just have ripple, you also have random ass changes that depend on your entire county power grid. And permanent changes too that are not cyclical like new equipment or equipment use schedules being changed, seasonal, etc, then stuff related to device load.

Its a metrology nightmare.


And be careful about big inductors on high current high voltage DC, even playing around.

[TongueTester]

what is a failed motor going to do on that kind of source? Just keep in mind instantaneous surge current. A power supply is current limited. For a test bench I don't think you want unlimited. a car battery has some protection circuits built in. Do you have these designed or on hand? Are they capable of dealing with a pure capacitor power source on a failure mode? Battery ESR is low, but capacitor ESR is nothing in comparison.

The price might be similarly outlandish once you factor these things in.

Now your talking my language... I had the notion of adding an incandescent light bulb & a DC fuse in series with the load to handle any major capacitor discharge event.

Then there is another game, that is float. A DC power supply will probobly have a floating output. a battery has a floating output. That variac is grounded. It might cause a problem with your test setup that you don't anticipate. You start following test procedures that assume a floating source with your own modifications you might be in trouble. I don't know enough about that though.

My understanding on this (someone correct me if I am wrong). The DC side has to float, if I reference the DC -ve to AC ground the negative cycle of the rectifier circuit will be shorted to ground instead of going through the diodes (it will go bang).

Also regarding testing my understanding here is that I have to make sure any probes (eg oscilloscope probes) are not referenced to the AC ground for the same reason.

This is aptly demonstrated by this nerd

another thing too from when I used to deal with people trying to use variac for precise things they should not be used for

I measured line voltage with a data logger. I found the error throughout the day was 5% in this particular place on one particular day (enough for measurement fail). This building power was reputed to be high quality/ stabilized or whatever. It will vary. It will vary on load too because it sound residential. Like sags on loads, combined with random grid changes. It could be alot worse then 5% probobly.

The ripple is more of a concern to me than AC voltage fluctuations because if its effect on the electronics in the inverter.

[coppercone2]

the DC will only float if you isolate it with a isolation transformer. a variac does not float anything. not sure I understand you. The connection to ground is in the circuit box even if you don't have a wire. diode does not matter for the ground, its just a path. the concerns are very numerous and if you don't know about what effects floating can have then dear god do not hook up a scope to your circuit, that is battery DMM only level of knowledge for safe work. anything for diodes on ground (bad idea) means for safety reasons you need to treat the circuit like its grounded, some people do this but its not by any means safety isolated)

and I don't think a motor driver for a car motor is similar at all to a induction heater (its a brute device in comparison). Question all information you are using from any video about induction heaters when applied to inverter drivers or whatever you are doing that is not a induction heater

[coppercone2]

as for light bulbs on 45000uF @ 400V, don't know, don't trust it at all. Seems mad bootleg. save up for the source and be pro, otherwise maybe someone else can give you some advice because IMO this plan is pretty nuts

tons of other concerns too like over voltage on the grid, it happens. real power supply should have a OVP monitor etc and shut down if the grid is being funky

[TongueTester]

the DC will only float if you isolate it with a isolation transformer. a variac does not float anything. not sure I understand you. The connection to ground is in the circuit box even if you don't have a wire. diode does not matter for the ground, its just a path.

Yes - I think I agree.

Can you explain what happened to the bloke in the video linked above when he hooks his oscilloscope to his rectifier circuit then?

Did it go bang because he connected his oscilloscope probe ground to the DC +ve side of his rectifier thereby shorting out the DC?

I really need to get this right because blowing up an oscilloscope is not in the budget!

Your isolation transformer is starting to look like good idea....

[coppercone2]

yeah but also that means if you have a bad wire things go live instead of breaking a breaker. its about what procedures you follow, its a elevator shaft filled with worms

I assume you have some service manual etc you want to follow that has guide lines. you need to match what they have if you follow procedures. if you are just making your own it sounds like maybe high power and voltage is not the best idea for your knowledge level. the idea is if you are following a procedure you trust the person that wrote it for safety (assuming its 'official') and you don't make a system that has different parameters to save money on equipment. its really specific stuff

I don't know how its done, maybe the techs that do this use a regulated power supply (good idea) and then actually ground the negative of the supply on their motor test bench (its an option).

What I can recommend is that you make test panels with specific labeled hookup points. No alligator clips or probes. I have seen professional 'motor' labs and they use color coded labeled sockets for everything with specific test cables etc. It did not look like the stuff that we do when we screw around with microelectronic circuit boards, reminds me more of something like a electrical control room, its very industrial. But that is how you can get people professionally to deal with this kind of stuff for a entire career with out accidents.

It's also how digikey an friends sell green colored sockets for 10x the price of the red ones from the same series lol. that green pigment sure must be expensive  . However, color beats labels 100% for not cross wiring stuff

[Retirednerd2020]

I have some 7700uF, 450V caps if you are interested.  See my "Bunch of equipment for sale" recent thread.  In any calse, for god sakes, be careful!  A big DC fuse would be advisable and fused input of course.