04-23-2014, 04:17 AM
|
#6481 (permalink)
|
EV Connoisseur
Join Date: Aug 2010
Location: Amsterdam
Posts: 309
Thanks: 70
Thanked 109 Times in 90 Posts
|
Quote:
Originally Posted by MPaulHolmes
PLAN B: With 3 feet of 2 gauge cable, you get a voltage drop of 0.193v at 400amp, and a drop of 0.289v at 600amp.
|
I still don't get it.. .. i'm sure its me.. but..
Isn't the voltage drop only dependent on the resistance of the cable? and thus dependent on the length?
Have the cables different lengths?
|
|
|
Today
|
|
|
Other popular topics in this forum...
|
|
|
04-23-2014, 05:07 AM
|
#6482 (permalink)
|
PaulH
Join Date: Feb 2008
Location: Maricopa, AZ (sort of. Actually outside of town)
Posts: 3,832
Thanks: 1,362
Thanked 1,202 Times in 765 Posts
|
Same length, but different current. It basically turns each Igbt into A mosfet. Higher current? Fine! But you will have a higher voltage drop, which causes it to use less curtent. Heres the math:
V1 + i1*R = V2 + i2*R. The point above both igbts to the point where both M+ connect is the same drop in voltage. R is the same for both because we are using equal lengths of wire.
So,
V1 - V2 = i2*R - i1* R
V1-V2 = R*(i2-i1)
i2-i1= (V1-V2)/R
So, the difference in current between the 2 IGBTs is smaller the bigger R is. And, its small when V1-V2 is small. But we cant control V1-V2.
In our case R is about 0.00005 Ohms. So,
i2 - i1 = 2000*(v1-v2).
So, if they differ by, say, 0.01v in their drop, the igbts will be within 20 amps of each other. Without the resistance, that 0.01v difference could be catastrophic.
Last edited by MPaulHolmes; 04-23-2014 at 05:21 AM..
|
|
|
The Following User Says Thank You to MPaulHolmes For This Useful Post:
|
|
04-23-2014, 05:18 AM
|
#6483 (permalink)
|
Dreamer
Join Date: Nov 2013
Location: Australia
Posts: 350
Thanks: 95
Thanked 214 Times in 151 Posts
|
Quote:
Originally Posted by flores
I still don't get it.. .. i'm sure its me.. but..
Isn't the voltage drop only dependent on the resistance of the cable? and thus dependent on the length?
Have the cables different lengths?
|
Voltage, current and resistance are all related.
To get the Voltage(V) drop you need current(I) multiplied by resistance(R).
Increase the current or the resistance and the voltage increases.
|
|
|
The Following 2 Users Say Thank You to Astro For This Useful Post:
|
|
04-23-2014, 05:19 AM
|
#6484 (permalink)
|
EV Connoisseur
Join Date: Aug 2010
Location: Amsterdam
Posts: 309
Thanks: 70
Thanked 109 Times in 90 Posts
|
haaaaaaaaaaa.. I get it..
because: V = R * I and R is fixed and I varies V will also vary, and you calculated that 3 feet of 2 gauge wire is enough R to vary V enough at max I, right?
To make the picture complete.. where does this voltage drop occurs? Between the collector and the emitter of the IGBT?
|
|
|
The Following User Says Thank You to flores For This Useful Post:
|
|
04-23-2014, 05:29 AM
|
#6485 (permalink)
|
PaulH
Join Date: Feb 2008
Location: Maricopa, AZ (sort of. Actually outside of town)
Posts: 3,832
Thanks: 1,362
Thanked 1,202 Times in 765 Posts
|
From capacitor plus to motor plus is the same voltage drop for all paralleled IGBTs. (From point where all collectors are tied together to where all emitters are tied together). the differerences happen inside each igbt, the drop from each collector to emitter.
|
|
|
04-23-2014, 05:37 AM
|
#6486 (permalink)
|
EV Connoisseur
Join Date: Aug 2010
Location: Amsterdam
Posts: 309
Thanks: 70
Thanked 109 Times in 90 Posts
|
Quote:
Originally Posted by MPaulHolmes
From capacitor plus to motor plus is the same voltage drop for all paralleled IGBTs. (From point where all collectors are tied together to where all emitters are tied together). the differerences happen inside each igbt, the drop from each collector to emitter.
|
Ok..and what are real numbers? 0.9V and 1.0V, that's a marginal difference of 0.1V on a total of 150V, that can't create really large current differences.. right?
I'm betting I'm wrong again?
|
|
|
04-23-2014, 10:20 AM
|
#6487 (permalink)
|
PaulH
Join Date: Feb 2008
Location: Maricopa, AZ (sort of. Actually outside of town)
Posts: 3,832
Thanks: 1,362
Thanked 1,202 Times in 765 Posts
|
The largest diode variation I saw was 0.02 across all 12. My guess is that the igbt on voltage drop will be similar. 0.1v difference would be very unlikely I think.
|
|
|
The Following User Says Thank You to MPaulHolmes For This Useful Post:
|
|
04-23-2014, 10:30 AM
|
#6488 (permalink)
|
EcoModding Apprentice
Join Date: Oct 2010
Location: southland NZ
Posts: 153
Thanks: 38
Thanked 86 Times in 55 Posts
|
Quote:
Originally Posted by flores
Ok..and what are real numbers? 0.9V and 1.0V, that's a marginal difference of 0.1V on a total of 150V, that can't create really large current differences.. right?
I'm betting I'm wrong again?
|
Hi flores
My understanding is that the 150v is across the system BUT the total current goes across the IGBT's in parallel so the problem is one could do most of the work - overloading it and under-loading the other
This would not be a problem (0.1v in 150v) if we were driving separate motors - one per IGBT
Paul's neat idea is to separate them out using the resistance in the cables - so they have to play nice together
It sounds really neat
Paul - is this your idea? - or is it a known fix?
If it is your idea you should look at deleting all of these posts and getting a patent
|
|
|
The Following 2 Users Say Thank You to duncan For This Useful Post:
|
|
04-23-2014, 12:19 PM
|
#6489 (permalink)
|
PaulH
Join Date: Feb 2008
Location: Maricopa, AZ (sort of. Actually outside of town)
Posts: 3,832
Thanks: 1,362
Thanked 1,202 Times in 765 Posts
|
haha, no it's not my idea. Lee Hart on the EV Tech list and an application note both suggested adding a resistor. At first that seemed really impractical, since who makes resistors with that power rating?! The application note suggested adding resistance that has around 0.2v drop at the current you will be running it at. That worked out to 3 feet of 2 gauge cable, so I figured, good enough for government work!
|
|
|
04-24-2014, 01:14 AM
|
#6490 (permalink)
|
PLUGnGO
Join Date: Sep 2012
Location: Olympia Wa
Posts: 137
Thanks: 75
Thanked 82 Times in 54 Posts
|
Quote:
Originally Posted by MPaulHolmes
Lots of debuggIng this afternoon after work. It's all almost good to go as far as the software and circuit board goes.
|
Paul
I think you said in an earlier post that RTDexplorer will not be supported by this controller. Will there be some similar user interface for setting variables and monitoring the controller?
Jerald
|
|
|
|