[dimitri]

Originally Posted by JRP3

Well I'm using a 120 volt AC engine block heater that I'll hook to my 108 volt pack, but the AC thermostat won't survive, as Dimitri found out

I'm still baffled why simple bi-metal thermostat cares whether its AC or DC current , is that because the contacts get welded shut by arcing while trying to interrupt the current? I looked at Digikey part that Tesseract mentioned, datasheet doesn't say it can't do DC, although current and voltage rating for DC is half of AC, so maybe that's the key here.

Funny thing about my experience with the heater is that it worked few times and then suddenly thermostat failed in closed position and heater element burned up within few seconds while violently boiling the water

[dimitri]

JRP3,

thinking back of my heater experience made me realize a simple solution. Assuming you already have a DC relay to energize the heater when the switch is pressed on a dashboard ( or however else you drive it ). You can open up the heater, remove power wires from the thermostat and connect them directly to the heater element, solder 2 new wires to the thermostat and route them back to the DC relay. This way thermostat only switches the relay and not the power directly. Thermostat will be in series with the switch to energize the relay and interrupt it when hot.

Hope this makes sense.

[Tesseract]

Originally Posted by dimitri

I'm still baffled why simple bi-metal thermostat cares whether its AC or DC current , is that because the contacts get welded shut by arcing while trying to interrupt the current?

AC passes through 0v 120 (or 100) times per second, DC doesn't, of course, and it is because of that that arcs tend to be self-extinguishing with AC and self-perpetuating with DC. It may take a kilovolt or two to initiate a arc but it might only take 30-40V to sustain it.

Originally Posted by dimitri

I looked at Digikey part that Tesseract mentioned, datasheet doesn't say it can't do DC, although current and voltage rating for DC is half of AC, so maybe that's the key here.

Download the datasheet. I seem to recall it is spec'ed up to 60V and 6A. Which seems a bit ambitious to me for that size switch. Might only have a cycle life in the thousands at that power level.

[dimitri]

Makes sense, that explains my results. BTW, when I opened the heater post mortem, the thermostat looks exactly like the part you pointed out at Digikey , so my suggestion of modifying the heater and separating thermostat circuit from power curcuit might work well and not require extra parts

[dimitri]

Originally Posted by major

The motor cares, so you might care too when the motor craps out

Thanks major, you seem to be pretty good at taking words out of context , I am glad you are following my progress and ready to jump in to get me out of trouble, I appreciate that

[Bowser330]

Originally Posted by dimitri

The air drag is the key, it takes 100A to cruise 40-45mph, but 300A to cruise 70mph. I think its somewhat normal from everything I read on the subject of air drag, which increases exponentially with speed.

I also had all 4 windows rolled down, which contributes to even more air drag.

AHHH I see, thank you.

So I guess its time to look for a glider thats got low drag...and or, make some aero-mods which help to lower the drag sort of like that one fellow did on his honda cvic hatchback...

[major]

Originally Posted by dimitri

I am glad you are following my progress and ready to jump in to get me out of trouble, I appreciate that

Hi dimitri,

You're absolutely right. I really haven't been following this thread. I was very interested in the other thread by Tesseract and was not aware of your beta test of the controller until recently. I do notice that you are using a Warp nine. And from some of the battery current values you posted, hitting it pretty hard. I am in the process of the installation of this same motor with a Zilla HV1K. And being a motor head (electric), I am keen on such things as to how the motor is doing. So, in the spirit of the forum, I am ready to jump in. Please continue to share what you feel appropriate. If things go well on my end, maybe we can compare some numbers in a month or so.

Regards,

major

[JRP3]

Originally Posted by dimitri

JRP3,

thinking back of my heater experience made me realize a simple solution. Assuming you already have a DC relay to energize the heater when the switch is pressed on a dashboard ( or however else you drive it ). You can open up the heater, remove power wires from the thermostat and connect them directly to the heater element, solder 2 new wires to the thermostat and route them back to the DC relay. This way thermostat only switches the relay and not the power directly. Thermostat will be in series with the switch to energize the relay and interrupt it when hot.

Hope this makes sense.

It does and it sounds like a nice solution. I have yet to do anything to the heater but I might implement what you've suggested instead of buying a DC thermostat.

[major]

Originally Posted by dimitri

I'm still baffled why simple bi-metal thermostat cares whether its AC or DC current ,Funny thing about my experience with the heater is that it worked few times and then suddenly thermostat failed in closed position and heater element burned up within few seconds while violently boiling the water

Hi dimitri,

At the risk of being out of context again, I'll see if I can help. I think AC heater cores depend on reactance to limit current. When you use an AC core on DC, you just get the resistive component of the reactance, so it draws much more current than on AC. This increased current contributes to switch failure and is the reason why it boiled away the water quickly. Sound like a reasonable theory?

Regards,

major

[blackpanther-st]

I am not sure about the ceramics, but most heater elements are purely resistive, so whether you feed it AC or DC doesn't matter, but Tesseract is absolutely correct about the switch. there is a big difference between switching AC and DC; what he mentioned about the zero crossing is just one factor.

Another issue is metal transfer between the contacts; with AC the current is balanced in both directions so the metal tends to stay put, with DC however the metal transfers from one contact to the other during the breaking arc. because the transfer is generally not flat across the surface, one of two things eventually happens; either one contact disappears completely and the switch will not close, or the growing contact forms a metal spike which reaches the other contact and shorts the switch closed.

The above is assuming that the switch originally had enough separation between the contacts to break the ark at all. if not the contacts will simply vaporize the first time you attempt to open them.

The principals here are the same as what makes an arc welder or plasma cutter work.

DC switches require a larger span between open contacts to break the same voltage, and the should open very quickly to minimize the arcing. they also require larger contacts for the same current to withstand the heat and ware from the arcing that does occur; that is why DC ratings are generally half the AC rating for the same switch.

On bimetallics; many of these thermal switches barely separate the contacts at all when open, and some don't even snap from one state to the other, but rather gradually move the contacts apart until continuity is broken. (these will not hold up to DC of any substantial current or voltage.

My choice for the electric hot water heater thermostat was for more than its ability to be set for a temperature in the ideal range; these switches are rated for at least 20A, AC, and they do snap from one state to the other. also the AC rating is for 240V rather than 120V so the separation should be greater. the sound they make when switching seems to indicate this as well. That being said, While I can not guaranty this, not having tested it; I would expect at least some serviceability out of them at 10A and 120V DC.