Basic Power Electronics Question

Thread Starter

raschemmel

Joined May 16, 2020
2
Given two dc sources, drawn as battery symbols in series, one being a 160V/220A EV battery, the other a 160V HV DC current limited power supply, with a load (INVERTER) drawn as a 1.6 ohm/80kW resistor across the two sources in series , is there any reason this wouldn’t work, given that the power supply , by itself can power a 360V/220A/80kW load by itself, without the battery ? The question is whether the two sources are incompatible and therefore not able to work in series or whether it makes no difference as long as both sources can handle the 220A current ? Is there anything that would prevent you from connecting these two very different dc sources in series ? For that matter, does it even make any difference what the voltages of the two sources are as long as they total 360V ? Is there any reason the current limiting wouldn’t work in this configuration ? (given that in order to source more current , the battery pack has to have a path to ground that allows more current and the power supply current limiting should prevent that) Not that it matters, the inverter’s load is a 3-phase EV motor, which it drives just fine with the dc supply by itself. The purpose of the question is to understand WHY this would not work, if that is the case. I believe it has something to do with the difference in the internal resistance of the two sources, which , while both dc sources drawn as equivalent dc sources, are , in actual fact, far from equivalent.
 

AlbertHall

Joined Jun 4, 2014
11,506
If the inverter enters current limiting then the battery will still be supplying current and driving that current in the reverse direction from normal through the inverter output. It depends on the inverter output circuitry just what that would do to the inverter. I wouldn't recommend it.
 

Thread Starter

raschemmel

Joined May 16, 2020
2
If the inverter enters current limiting then the battery will still be supplying current and driving that current in the reverse direction from normal through the inverter output. It depends on the inverter output circuitry just what that would do to the inverter. I wouldn't recommend it.
Unfortunately, the inverter doesn't have current limiting. The HV DC PS is a Sorenson and it has current limiting.
The inverter only has the dc input and the 3-phase output. If there is no fault , it issues a READY signal and waits
for a command. It has some voltage and current monitoring capability but it will not give a reason for issuing a
NOT READY. My theory is that it somehow is capable of comparing the dc input internal resistance with the 3-phase
motor winding impedance and makes a determination based on that. That is pure conjecture with no data to back it
up. It always works from the dc power supply regardless of which motor we use but doesn't always issue the ready
with the battery. I am exploring ideas to increase the internal resistance of the dc voltage
input by adding 1 to 10 ohm 1000W resistor(s) in series with the battery to see if it changes
it from NOT READY to READY, which would of course indicate there is some relationship
between the dc input resistance and the NOT READY. I wish the inverter were 'smarter' but
it is not.
 

AlbertHall

Joined Jun 4, 2014
11,506
I wrote 'inverter' when I meant DC power supply. If that supply goes into current limiting then its output will be subject to reverse voltage.
What is the model number of the Sorensen?
 

MisterBill2

Joined Jan 23, 2018
9,304
WRONG! If the power supply enters current limiting, not overcurrent shutdown, the total voltage will drop.
But both of those voltages are rather non-standard and so I am wondering if this is a real situation or a theoretical problem. And using two 12 volt automotive batteries in series with a 50 anp mains powered supply does work to power a 48 volt, 50 amp, hydraulic pump for a short test, about 1 minute.
 

AlbertHall

Joined Jun 4, 2014
11,506
WRONG! If the power supply enters current limiting, not overcurrent shutdown, the total voltage will drop.
Wrong only if the the current stays below the limit. If the battery can supply the load and that current is above the PSU current limit then the PSU will see reverse voltage.
 

AlbertHall

Joined Jun 4, 2014
11,506
The presumption is that first, it would not happen, and second, that if things did not go right, the TS would quickly switch things off.
Presumptions are dangerous things.
I used to repair Sorensen supplies and I have seen some well destroyed by folk not understanding how to use them properly.
 
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