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Old 04-30-2009, 04:18 PM   #1101 (permalink)
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What to hear a funny story? Well, I put a resistor in from gate to source of each mosfet of the power section a while back. 15kOhms... Or so I thought. I just tried it in the car, and the precharge resistor started sizzling. I took everything apart, and I discovered that I accidentally used 15 Ohm resistors from gate to source. That works out to be an overall resistance of 1.5 Ohms from gate to source. A little low... I'm going to see if I can go take care of that real quick.



Where's a red-headed step child when you really need one?

Even funnier story:

I just cut them all out, and they were 15 KOhm. I'm really really dumb I think.

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Old 04-30-2009, 05:03 PM   #1102 (permalink)
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hahaha...

so why did the precharge resister start sizzling? that kinda makes no sense, even if you DID have low resistance in the gate/src.

Isn't that frustrating?
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Old 04-30-2009, 05:14 PM   #1103 (permalink)
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You are right. It didn't make sense. I found out the problem, though! Half way through the power section, I changed my mind about which bus bar would be B- and which would be B+. I thought everything was fine, but I had already soldered on the capacitors assuming the other way, and did the mosfets/diodes assuming the new B- B+ orientation. Oops. Basically, I had a NEAR short circuit through 10 beefcake diodes from M- (which is connected to B+ through the motor) to ground. So... I had about 50 amps going through the pre-charge resistor for about 3 seconds. No problem for the diodes, but a problem for the pre-charge resistor.

I just unsoldered all the capacitors, and now I'm going to reverse all of them, put in NEW DANG DANG DANG resistors from gate to source and drive into the sunset! ANGER!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!1 STUPID IDIOT!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
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Old 04-30-2009, 05:16 PM   #1104 (permalink)
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Paul, maybe I'm confused, but shouldn't that precharge resister be designed to take pack voltage directly into a capacitor bank? What is its resistance and wattage rating?
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Old 04-30-2009, 05:55 PM   #1105 (permalink)
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It's designed to take pack voltage if the current falls off exponentially, but not continuous current. Filling up the capacitors for 5 time constants is no problem for it. It's a 40 watt 2.5 Ohm resistor.
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Old 04-30-2009, 06:01 PM   #1106 (permalink)
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lets see now... 144V - 1.2V (approximate diode drop, total guess, actually) = 142.8V... now... P=(V^2)/R.... 142.8*142.*/2.5 = 8.1kW... I'm thinking that's more kW than 0.040

So... how about that PWM pre-charge mosfet circuit?

edit: what's the capacitance and time constant, btw?
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Old 04-30-2009, 07:51 PM   #1107 (permalink)
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Quote:
Originally Posted by MPaulHolmes View Post
I spoke to the EV Tech list about the mechanical chopper, and I guess it's been around for quite a long time. Here's Lee Hart's comments:

This idea is the original PWM "chopper" of Tom Edison's time. Before
transistors, before SCRs, even before vacuum tubes there were still
switches.

Yes, you can build a PWM controller with mechanical switches. But this
crude version implies that the builder has no idea how a PWM works, and
never studied the old ones that really *did* work every day in the 1900s.

The fundamental problem is arcing. You can't just switch an inductive
load on and off; the inductance insists that the current *must* flow
somewhere, so you get hideous amounts of arcing that will rapidly
destroy the switch.

Nowdays, we put a freewheel diode across the motor to carry the current
while the switch is off. This at least keeps the switch voltage from
rising past the battery pack voltage.
I snipped the rest. This is nowadays, so putting a freewheeling diode bank (or a FRED or two) across the motor isn't a problem

Same issue with the caps. I'm not trying to build a 1890's replica. If it needs caps, then we can put them on.

As for the servo motors Paul, it would be absolutely no problem for you. Servo motors are driven by a PWM signal with a duration of between 1 and 2 ms in a 20 ms frame. So the duty cycle is between 5 and 10 percent.

Here's a brief tutorial:

Whats a servo: A quick tutorial

I'm thinking that my initial prototype won't need the servos. I plan to keep the bushes stationary and make the rotating shaft move on a sliding rack. This will keep the heavy power wires from having to move.

ga2500ev
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Old 04-30-2009, 10:53 PM   #1108 (permalink)
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OK. Let me see if I understand. You are going to have the capacitors and the diodes, and still need a micro controller to manage the servos. So the whole mechanical contraption only hakes the place of the MOSFETs.

Wow! MOSFETS look a whole lot simpler to me.

Seriously, the only advantage I can see is using this for drag racing when you want to handle thousands of amps for a 10 second run. It should be cheaper than IGBT modules. But for your average car, this seams like a really complicated way to go.
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Old 05-01-2009, 03:59 AM   #1109 (permalink)
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I need a larger precharge resistor I think. The total capacitance is:

0.01 F

The resistance is 2.5 Ohms. That works out to a time constant of:

25 msec. yikes. haha! So, Maybe I'll buy some larger resistance 10 watt resistors tomorrow. I got everything back together, with all the capacitors removed and turned around. I tested the control section while hooked up to the power section. I triple checked everything in the power section. It should be fine for testing tomorrow. The software is radically changed, so there may be bugs there. It's more complicated now. Watchdog timer, resetting it here and there. Interrupt driven, making sure the interrupts are disabled during important parts of the code. That's a recipe for bugs. A bug recipe. That sounds really gross. Like fly pie. (that rhymes).

It's supposed to rain at the alternative energy fair. Big surprise! By the way, if anyone wants to make the wiki, go ahead! I ain't a carin'. wiki wiki wiki. I have a turn table.
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Old 05-01-2009, 08:25 AM   #1110 (permalink)
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Quote:
Originally Posted by Roger Heuckeroth View Post
Ga2500ev

OK. Let me see if I understand. You are going to have the capacitors and the diodes,
It may be one cap and one FRED, like this one:

MEO450-12DA IXYS 1200V 450A Fast Recovery Epitaxial Diode Module at The Electrostore.com - Electronic Surplus Parts & Equipment

Quote:
and still need a micro controller to manage the servos.
Actually I said I was going to build one without it by using the accelerator cable to draw the assembly back and forth across the brushes. I was thinking out loud about the micro/servos as a current control mechanism.

Quote:
So the whole mechanical contraption only hakes the place of the MOSFETs.

Wow! MOSFETS look a whole lot simpler to me.
A prototype is buildable with no control electronics whatsoever.

The biggest problem with MOSFETs is that they blow up. You overvolt, overcurrent, or overheat a MOSFET bank and they go POOF!

The second problem is that mounting a MOSFET bank is a major undertaking, milling/etching/drilling/soldering a board.
It's one of the reasons that folks keep asking about IGBT's as an alternative.

Quote:
Seriously, the only advantage I can see is using this for drag racing when you want to handle thousands of amps for a 10 second run. It should be cheaper than IGBT modules. But for your average car, this seams like a really complicated way to go.
As wonderful as this thread is, it also has shown the pitfalls of putting together a reliable PWM MOSFET motor controller. There are significant design/programming challenges.

The alternative is simple, cheap, and easy to build with common household tools. More importantly it will be reliable because it isn't relying on a ton of interconnected complex parts to pull off.

ga2500ev

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