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View Full Version : O.k.... now that I've gotten this madness to work... why does it work?



Lionstrike
21 December 2010, 1011
I am embarrassed to say that I have no idea why the circuit needs a contactor. John Bidwell's books don't seem to have a contactor in the circuit at all. Curious...

Why is it needed?

Also, I used a 36v circuit and a 24 contactor. Someone said that overvoltage on the contactor will weld it shut.... but if so.... "so what?"

I once got on the phone with Thunderstruck-EV during my first failed project. I asked them if I could bypass the contactor completely. They said that the first time it may work... the second time it may work... the third time the magic smoke might escape from the controller.

If I am correct... I think that the concern here was bypassing the precharge resistor, in which case, I agree with them... not good for the controller.

But... did I get lucky? Let's say my contactor contact welded from overvoltage... would that damage the controller? It's still wired to the pre-charge resistor and the voltage still must pass through that.

I am not pushing my luck here am I? I am still just testing...and not drawing any real current through the connections. I'll use the heavy gauge wire for the 48v circuit.

Thanks everyone!

EVcycle
21 December 2010, 1021
For Safety.


:)

Lionstrike
21 December 2010, 1027
**This is me putting on the dunce cap again**

How does it help safety exactly? ... and no damage to the controller with this setup right?

The last thing that I want is to invite some friends over to see this thing and **POP* **Fizzle* Controller death...

That would be made of FAIL.

EVcycle
21 December 2010, 1030
I have a thumb switch on the handle bars that goes to the coil of the contactor.
If something goes awry, I can shutdown with the flick of the switch.

teddillard
21 December 2010, 1030
To say a little more on that point... :) The contactor basically allows the controller to shut itself off if horrible things start to happen. That's why the coil voltage comes from the controller...

I did a whole post on that and the other parts in the system here:
http://evmc2.wordpress.com/2010/08/21/stuff-ive-learned-controllers-contactors-and-convertors-oh-my/

to wit:
"The Contactor

The contactor is your safety valve- not so much yours, but your controller’s. You can wire in a main power switch, but the contactor allows the controller to shut itself off from the main battery pack if something catastrophic is happening.

It’s a big relay that’s usually wired to the pack voltage, turning it on or off. The contactor has a threshold voltage usually, a voltage it needs to hold the contacts in position, so if the pack voltage drops, the contactors will pull apart. It’s usually set up on it’s own switch- a “kill” switch that has to be able to handle the high voltage of the pack (usually not the key switch that is on the bike- they’re not able to handle much more than the basic 12V system) and then wired to the controller so if the controller needs to shut off the incoming high pack power, it can.

You may be tempted to run the system without a contactor. It’s a bad idea."

EVcycle
21 December 2010, 1032
The same thing I said in 50 words or less. :):)

Lionstrike
21 December 2010, 1042
To say a little more on that point... :) The contactor basically allows the controller to shut itself off if horrible things start to happen. That's why the coil voltage comes from the controller...

I did a whole post on that and the other parts in the system here:
http://evmc2.wordpress.com/2010/08/21/stuff-ive-learned-controllers-contactors-and-convertors-oh-my/

to wit:
"The Contactor

The contactor is your safety valve- not so much yours, but your controller’s. You can wire in a main power switch, but the contactor allows the controller to shut itself off from the main battery pack if something catastrophic is happening.

It’s a big relay that’s usually wired to the pack voltage, turning it on or off. The contactor has a threshold voltage usually, a voltage it needs to hold the contacts in position, so if the pack voltage drops, the contactors will pull apart. It’s usually set up on it’s own switch- a “kill” switch that has to be able to handle the high voltage of the pack (usually not the key switch that is on the bike- they’re not able to handle much more than the basic 12V system) and then wired to the controller so if the controller needs to shut off the incoming high pack power, it can.

You may be tempted to run the system without a contactor. It’s a bad idea."

Ahhhhh GOT IT!!!

So... it's essentially an electromagnet. Voltage drops too low... it cuts the contactor connection. No more circuit.

But why would undervoltage be a problem for the controller? Overvoltage I understand, but undervoltage shouldn't be a problem right? The manual said it only needs a minimal amount of power to run.

O.k., so if I understand it right, it stops overvoltage through this mechanism too. Too much voltage... controller tries to break the connection to the contactor. If it doesn't exist or is welded shut... problems happen and the magic smoke escapes.

**Can I take off my dunce cap now?**

magicsmoke
21 December 2010, 1050
And by tapping in to the coil circuit it's a convenient way of inhibiting the high current / high voltage circuit with low current / low voltage switches such as kick stand down, kill switch, emergency stop, etc.

Rob

teddillard
21 December 2010, 1112
And by tapping in to the coil circuit it's a convenient way of inhibiting the high current / high voltage circuit with low current / low voltage switches such as kick stand down, kill switch, emergency stop, etc.

Rob

Back on the switches thing again I see? :D

I think I can safely say, that depends on your contactor. Right? I'm running a 72V contactor, so I'd need to run a 72V rated switch.

(The switches thing was beat to death, er, discussed, here: http://www.elmoto.net/showthread.php?193-kill-switch)

Lionstrike
21 December 2010, 1120
Got it. So I am probably o.k. for testing and such (and showing off to my friends **COUGH**) but for final application, 48v and a 48v contactor. I am not in any danger of blowing up a controller at 36v when it's rated for 48.

Oddly, my multimeter is MESSED UP BAD. It reads 56v on my 36v battery pack, and 15v on a KNOWN DEAD 9v battery. That's jacked up.

Thanks again.

DaveAK
21 December 2010, 1121
Be advised though that not all contactors/controllers operate in the same way, (at least so I believe). It seems as though Ted is describing a contactor that operates on pack voltage, where as mine operates on 24V from the controller. The purpose of the contactor is the same, but operation might be different.

Lionstrike
21 December 2010, 1132
I think that I should be o.k. Mine operates on pack voltage I believe and I am not drawing enough current to contact weld it or anything like that.

BaldBruce
21 December 2010, 1140
Good point Dave. This is a large source of confusion to a lot of builders. The ratings on a contactor have 4 distinct values. The main contacts have a voltage and current rating. This is so you don't overvoltage them and arc or don't over current them and melt the contacts. (Both very bad...)
The second set of ratings is for the control circuit. What voltage and current does it take to operate the relay. (sometimes expressed as a power requirement)
So a typical contactor (fancy term for a big honking relay) we might use would be rated at 72v and 500 amps. Note that this means the main contactor parts can handle this voltage and current. It does not tell you what voltage and current will puill in the contacts. As Dave mentioned, there are two common methods used to operate the contactor. 24VDC is common, but all other pack voltage coils are avialble also. I use a 72V contactor that draws about half an amp to operate.

Just to make things more complicated, some controllers are smart enough to cut back the voltage to a lower value to save energy after initial pull in and yet keep the coil energized. Some of the lower voltage contactors can do this job themselves to lower their power consumption.
Clear as mud????

frodus
21 December 2010, 1215
Well in a perfect system, you'd have a BMS looking at LVC and HVC. So if you're charging and the cell goes too high, it cuts out the charger (not the contactor). The only time you'd get HVC in a setup is if you have regen. I don't think you do, so it won't ever be an issue while driving.

When a cell hits LVC, the BMS should tell the contactor to shut down. If your controller senses a pack voltage that is too low, it'l say "hey, the batteries are too low, lets stop". Its not to protect the controller, its to protect the batteries from being overdischarged. LVC should be done on a cellular level with lifepo4. if its lead, you can get away with it a little easier, but should still keep an eye on battery voltages from time to time.

Also, when you have the controller hooked directly to the controller, there is a small parasitic drain on the batteries via the controller. Even if the controller isn't enabled with its enable line, it still draws a small amount. It'l drain batteries when you leave it.

Its also safety. If you get in a sticky situation, and your controller fails (Most PMDC and series wound controllers fail ON) you have no way of protecting from a runaway situation. The controller could fail completely on. If this happens, how are you planning on stopping? What if you want to show your bike and some little kid is playing with buttons. There is no protection from directing battery power to the motor if somehow the enable line is enabled.

Another thing, is how do you work on the HV connections on the controller safely? If you try to do that while the controller is hooked up, the caps inside carry a charge. If you disconnect the controller via the contactor, the caps will slowly drain. I recommend further discharging via a 100W lightbulb. A few friends have been shocked with a 144V pack voltage on a controller that wasn't discharged.


The precharge is only to keep the caps charged so they're not HAMMERED with current when you close the contactor. You always want to precharge the caps to ensure that they're at pack voltage when the contactor closes. This also extends the life of the contactor since there'd be no load on it when you switch since both are at the same potential. If they were uncharged, you have a HUGE inrush of current to the caps as they charge, which could weld contacts if the cap bank was large enough.

teddillard
21 December 2010, 1219
FYI,
HVC = High Voltage Cutoff.
LVC = Low Voltage Cutoff
FYI = Frequent Yogurt Investigations
HAMMERED = frodus + 2 beers
:D

frodus
21 December 2010, 1232
Haha, thanks ted.

I hope that helped a bit though.

Lionstrike
21 December 2010, 1903
Thanks everyone... it certainly does help... but it will take a while to digest. You all have EV powers far and beyond my fledgling skills.

Basically... some of you are wearing Master's ranks.

I MAYBE have a stripe at the edge of my white belt.

billmi
22 December 2010, 1112
To say a little more on that point... :) The contactor basically allows the controller to shut itself off if horrible things start to happen. That's why the coil voltage comes from the controller...

I do not believe this to be true of all controllers. The contactor allows the operator to cut power in case of a controller failure, but I don't know that all controllers are set up to detect their own failure and open the contactor.

I e-mailed Kelly Controller and asked specifically if I ran the contactor separate from the KD (brudhed PM) controller, if I would be bypassing any safety features such as the controller opening the contactor in case of failure and they said no. The reason I asked, is I wanted to run a contactor with a 12v coil (sidesteppiong that whole 72v switch issue) and they said that was fine.