# Contactor failure



## onegreenev (May 18, 2012)

Was this being used to turn on and off your heater?


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## evmetro (Apr 9, 2012)

onegreenev said:


> Was this being used to turn on and off your heater?


Yea. I have two of these, one for each of my two ceramic heater elements, and each half element gets its own pole. Each of these contactors is a 12 volt relay coil with two poles, so each contactor turns on one element. The first heater core that I built to hold my two ceramic elements was built with windshield urethane, but it turns out that the urethane is very conductive and I built one big short. There are some pics in my Tevie2 build thread.


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## Russco (Dec 23, 2008)

evmetro said:


> Yea. I have two of these, one for each of my two ceramic heater elements, and each half element gets its own pole. Each of these contactors is a 12 volt relay with two poles, so each contactor turns on one element. The first heater core that I built to hold my two ceramic elements was built with windshield urethane, but it turns out that the urethane is very conductive and I built one big short. There are some pics in my Tevie2 build thread.


Does it have blow outs? Did you observe correct polarity?


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## z_power (Dec 17, 2011)

There's a lot of contactors from Prius and other Toyota hybrids at eBay. They're in $25-50 range, are rated for ~400VDC and ~100A. I use three of them to control heater supplied from 180V battery - work flawless 

example: ebay link


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## evmetro (Apr 9, 2012)

Russco said:


> Does it have blow outs? Did you observe correct polarity?


I observed correct polarity, but here is the problem. I used windshield urethane to mount my ceramic elements into the heater core.










I found out the hard way that the urethane is very conductive, so the heater core that I made was basically a junction block, creating one big short circuit.










I had to make a new heater core out of something non conductive. I always have Evercoat body filler around my shop, so I ran some heat tests and electrical tests with my meter on it and was pleased with the results. The filler stays the same up to around 500 F, and begins to deteriorate above that. My DVOM can't detect it either... Now I have a heater core made out of polyester.


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## Sunking (Aug 10, 2009)

Ouch! How expensive was your lesson?


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## evmetro (Apr 9, 2012)

Sunking said:


> Ouch! How expensive was your lesson?


Two contactors @ 25 bucks each, two more portable heaters @ 25 each. That's $100, plus some body filler for the new heater core, masking tape, and wiring.. This probably cost around $125 altogether. It could have cost me the whole car if I had not learned the importance of fuses earlier in life. This is why I wanted to share this experience here. It has helped me to read about other peoples mistakes, so here is mine...


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## Sunking (Aug 10, 2009)

$125 is not a lot and could have been a lot higher like you said. No better teacher than failure and loosing money.


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## electro wrks (Mar 5, 2012)

evmetro said:


> This is a JQX 82F 2C contactor that I recently toasted. I thought I would share it here in case anybody uses these kinds of contactors and wants to know what it takes to kill one. It is rated for 80 amps at 250 VAC and 28 VDC. This particular contactor hosted a dead short to 122 VDC, and my 20 amp Ferraz AT120 fuse is what stopped further damage. I had expected this type of contactor would have welded with excessive current, and I suppose it might have if the fuse had not done its job, but the contacts did not weld together. The arm that the contact is attached to did bend out of shape, so the two sets of contacts no longer contact at the same time. I was using this contactor to connect 750 watts each pole, at 122 VDC, so about 6 or 7 amps per pole. This does push the limilts of this contactor, but I feel pretty comfortable replacing this with another identical unit. Now I can see what to expect if there is a dead short.
> 
> Here is the contactor that I killed:
> 
> ...


If the contacts are only rated for 28VDC, there's no way they can handle 122VDC. They'll just sit there and arc over, causing this kind of damage. AC voltage is much easier to break because the voltage goes to 0V with each cycle. As Russco alluded to, high voltage DC relays typically have magnets and other features to blow out and quench the DC arc.

A lot of us have tried to fudge this one with under rated switches and relays, and it just does not work. It also can be very dangerous. High voltage DC relays are less common. I'm not sure where to send you. Maybe other folks have good sources?


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## evmetro (Apr 9, 2012)

Electro Wrks, I use a nice gigavac contactor in one of my conversions, but it cost quite a bit. I experimented with pushing these cheap ones in one of my other conversions though. I run these on my 144 volt pack EV to run the same heater set up, and to run the DC DC. I keep an eye on them, since, as you correctly stated, this is not what they are rated for. I suspect that using two poles to spread the load out helps a bit. So far, I have not had any problems with them, other than the two in this thread that were cooked because of a dead short. I am pretty interested in what z power was talking about with the Toyota contactors on ebay though...


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## Coley (Jul 26, 2007)

I may be wrong, but isn't the 28volt the coil voltage for the contacts, not the carrying voltage?

I yse 12volts for my 72 volt system, but it is coil voltage.


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## evmetro (Apr 9, 2012)

These contactors that I killed were 12 volt coils. I am pretty sure that the 28 VDC rating is referring to the big circuit. 80 amps at either 28 VDC, or 250 VAC.


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## electro wrks (Mar 5, 2012)

I've been admiring your careful and beautiful builds for some time. Seriously, I don't thing this is a place where you want to cheap out. Also I'm not getting a clear picture of how the contactors, elements, and fuse(s) are set up. Could you draw up a schematic?


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## Sunking (Aug 10, 2009)

evmetro said:


> These contactors that I killed were 12 volt coils. I am pretty sure that the 28 VDC rating is referring to the big circuit. 80 amps at either 28 VDC, or 250 VAC.


That sounds about right airgap switches and relays can handle much higher AC voltages then DC voltages because of the Zero voltage sequence of AC wave forms. High voltage DC breakers, fuses, switches, and contractors are expensive.


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## alvin (Jul 26, 2008)

I think KTA has relays for that.

http://www.kta-ev.com/RelaysAuxiliaryControls_s/1835.htm


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## jhuebner (Apr 30, 2010)

electro wrks said:


> If the contacts are only rated for 28VDC, there's no way they can handle 122VDC. They'll just sit there and arc over, causing this kind of damage. AC voltage is much easier to break because the voltage goes to 0V with each cycle. As Russco alluded to, high voltage DC relays typically have magnets and other features to blow out and quench the DC arc.


I did some tests with placing a 1µF high voltage film cap in parallel to the switch. Lab only, not in my car. Arcing disappeared. 
Relay was rated 30Vdc, I tested with 140Vdc. Without the cap there was constant arcing until I shut down the power supply 

I chickened out at actually using that setup because I couldn't find any information on it i.e. the limits of this method. Anyone actually doing this?


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## electro wrks (Mar 5, 2012)

OK, this is what is great about this forum-I learn something new everyday. The link that alvin refers to:http://www.kta-ev.com/KTA_Auxiliary_Switch_Kit_p/kta_relay_kit_250.htm has contactors with caps and bleed off resistors that go across the points (to reduce the arcing), the coil snubbing smaller diode ( to bleed off the induced current in the coil), and a large flyback diode. Where does the large diode go and what does it do?

Another learned bit of info: the DC current rating goes down as the voltage increases. And, the manufacturers usually print the most optimistic (lowest voltage) DC current rating on the name plate, not the rating associated with the rated DC voltage!


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## major (Apr 4, 2008)

electro wrks said:


> ... and a large flyback diode. Where does the large diode go and what does it do?


Actually called a Free Wheeling Diode (FWD). It is reversed biased normally but when the switch is opened, the stored energy in the coil (load) induces a voltage which forward biases the diode and allows the current to circulate in the coil and diode until it decays.


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## electro wrks (Mar 5, 2012)

So, it serves a similar function as the small diode on the relay coil? Is it needed for resistive loads, like with the heaters? Or, is it just a good idea to have it?


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## major (Apr 4, 2008)

electro wrks said:


> So, it serves a similar function as the small diode on the relay coil? Is it needed for resistive loads, like with the heaters? Or, is it just a good idea to have it?


I suspect the heater core has a large inductance. But, no, the FWD would not be needed on a pure resistance, if such a thing existed. Just the wires from the source and relay to the core will have enough inductance to cause some arc on the contact opening. I've seen AC heating elements burn out on DC voltage sources because the reactance no longer limits current as when on AC.


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## evmetro (Apr 9, 2012)

electro wrks said:


> I've been admiring your careful and beautiful builds for some time. Seriously, I don't thing this is a place where you want to cheap out. Also I'm not getting a clear picture of how the contactors, elements, and fuse(s) are set up. Could you draw up a schematic?


I don't have a schematic with me right now, but the circuit is simple. Here is a pic of a ceramic heater core that I am about to harvest from a wal mart space heater. There are five connections on it and they alternate hot, neutral, hot neutral, hot, counting from either side. I have my 120 VDC positive running to the three hot terminals, and my pack negative running to the two neutral terminals. I run two of these elements, each with its own contactor, and I have a 20 amp fuse for each one right near the heater box, on the positive lead. The negatives go from these ceramic elements through my contactors, and back to my pack negative terminal. As you can see in the pic, there are two negative terminals on each ceramic element, and they go through the two poles on the contactor. After my two 20 amp fuses, my positives join together into a larger cable that goes to my positive terminal on the pack, and I have an additional fuse (30 amp) right where it connects to my pack. Each element is 1500 watts, so each pole of the contactor is 750 watts.


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## evmetro (Apr 9, 2012)

Major, I am glad you dropped in on this thread, although I am a little embarrassed about my error. Why didn't the contacts weld together?


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## PStechPaul (May 1, 2012)

Contact welding usually occurs when the contacts have deteriorated due to previous arcing events, and when there is a high current through the higher resistance of the poor contacts. They will then stick when the deactivation of the coil tries to open them, and if they do break free, the sharp points on the contact surfaces encourages arcing and melting (welding). 

A capacitor across the contacts is a good way to avoid arcing, and you can determine a reasonable value by knowing that the contacts take about 30 mSec (2 cycles) to open, and making sure the voltage does not exceed the 28 volt DC rating during this time. If you want to break a current of, say, 20 amps, the formula to use is V=(1/C)*I*t, or C = I*t/V. Thus you might need 20*30,000/28 = 19,000 uF to be safe. 

But in reality, you mostly need to keep the contact voltage below the breakdown of air which is about 10,000 volts per inch. If the contacts have a spacing of 0.1", they will withstand 1000 volts when fully open, and if the movement is linear, they will open 0.01" in 3 mSec and can withstand 100 volts at that time. Thus the formula becomes 20*3000/100 = 600 uF. But that is still very high. This analysis may apply to a highly inductive load, but a resistive load is easier to handle, and a capacitor of perhaps 100 to 500 nF is probably sufficient. There should also be a resistor in series with the capacitor to limit the current when it discharges through the contacts when they close.

Another thing that is often not considered is the reduction of contact opening speed due to the use of a commutating diode across the coil. What this does is provide a current path for the inductive energy, and this maintains a significant amount of current in the coil which can keep it closed long after the applied voltage has been removed, and it will make the contacts open more slowly. Thus it may be better to use a resistor in series with the diode to dissipate the energy quickly and open the contacts faster.

The contactor pictured appears to be a DPDT general purpose relay, which has smaller contact spacing and weaker springs so that less power is needed for the coil. A better choice is a DPST contactor which has stronger springs and greater contact spacing and faster operation, but at the expense of higher coil power, or the need for economizer resistors or PWM drive to the coil once it is closed. A DC coil typically needs only 10-20% of the actuation current to hold it closed.


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