Zilla FAQ23 Dec 2007 02:30 pm

The main contactor in a conversion is a critical safety device. In case of a controller failure, it is the only thing keeping the car from accelerating uncontrolled at full power. Experience has shown that most braking systems can not stop a modern high power conversion that is accelerating at full power for any reason. (I’ve heard of a couple of cases where the accelerator pedal got stuck, watch out for those loose carpet mats!) It is not hard to imagine how a contactor that was welded shut from too much current, or one that doesn’t break the circuit due to too high a system voltage can become a real problem. Fortunately I have not heard of any deaths due to this issue, but with some of the unsafe installations I’ve seen it is certainly possible that someone will pay the ultimate price due to some person’s lack of engineering.

In the past, most conversions had maximum system voltages of 144 volts and limited fault currents due to flooded batteries. Many people used the SW200 contactor and they seemed to handle breaking those reliably in fault conditions. Recently more and more people are running high voltage EV’s for the increased performance that it gives. Those high voltages combined with high power batteries like the Hawker and Orbitals have greatly increased the requirements for safe disconnects.

Unfortunately, good data on our commonly used contactors is hard to find. Albright and Cableform use ratings that are very reliable, but also may underestimate what could be safe in breaking capability in a situation such as ours where they only need to break a fault current once. Kilovac seems to be a master of specsmanship. They now rate high continuous carry currents by cooling the contacts using huge cables. On their current data sheets they neglect to list carry currents with smaller cables and at other time constants which would be appropriate for EV use. A bit surprising for a contactor name that starts with “EV”. This data may be available from the factory, it is just not listed on the standard data sheet for their contactors. I have a curve from the older and much larger EV250 and EV500 contactors and it doesn’t look so good for our high power conversions. Add to that the fact that it is impossible to visually inspect the Kilovac contactors and you can see why I’m not a fan of them.

Enough background, I’ll get to the point of this post:
Due to the low voltage ratings of many high current contactors, it is often desirable to wire two main contactors in series to increase their maximum voltage breaking capability. A few things should be kept in mind when doing this.

1) If your contactors are open frame, and you are planning to push the voltage beyond the rating (not that we suggest that!). Be sure to leave plenty of space around the area where the arc is supposed to blow out to avoid starting a plasma ball fire in case of a emergency shutdown under load. I don’t have any hard data on how much space is required, but I like to see one inch of clearance from the sides of the contactor on a SW200 series contactor when run in common installations at voltages of 156V and below. I have seen evidence of a arc reaching over 6″ from each side of a SW200 contactor that was inadvertently required to break a 370V battery pack under some load. In that case the contactor was destroyed but did break the circuit, twice! Please don’t ever rely on such a overloaded contactor for a main contactor, people could be killed by a runaway car that did not turn off. As far as I know, the SW200 series contactor is only rated to break 1500 Amps at 96V. I am not recommending exceeding that unless we can get further testing to verify the seemingly higher capacity of this model. I have seen some companines sell it as a 120V contactor. Maybe they have information that I don’t have.

2) In order to increase the voltage handling capability, the contactor power terminals should be wired in series. The polarity of the contactor should be observed (if the unit uses magnetic blowouts) with one positive terminal being connected to the battery pack positive. The negative terminal on the other contactor should be connected to the controller B+ terminal. The remaining two contactor power terminals should be connected together + to -. Having the correct polarity on the contactor insures that the arc from a fault current gets blown outward away from the contactor rather than into the center of the contactor mechanism.

3) The precharge wires from the Hairball to the power terminals of the contactor should be connected as indicated in the Hairball wiring diagram. The two contactor set should be treated as one large contactor with a internal connection. The precharge wires are unfortunately somewhat sensitive to electrical noise. For this reason dual main contactor installations should strive to minimize the length of power cable between the two contactors. Additionally the precharge wires should be lightly twisted together where possible.

4) The coils of the two contactors should be connected in parallel and wired to the Hairball in the usual way. One snubber diode can handle the inductive kick from the coils of two SW200 contactors, but having a diode on each does no harm. In the case of more or larger contactors such as the Cableform, one of our diodes should be used for each coil. Only two SW200s or one large Kilovac contactor can be driven without overloading the Hairball “Main Cont Coil” output. If you are running more you will need to implement a system such as a relay to buffer the contactor power output from the Hairball. Since a relay is another possible failure point in the critical safety chain, I suggest that all power to that relay also be switched by the ignition key.

Fuses are another subject that I hope to address in the near future (maybe on the new site when it’s up). In short I want to say that the fuse blow curve should be below the contactor carry curve in all cases. Each of these curves should be the ones that show carry current versus time. If the fuse is not below the contactor curve, then there is a risk that the contactor will weld on during operation. Often this can require running a much smaller fuse, such as a when using Kilovac 200 series contactors. In that case the controller battery current will need to be reduced to protect the fuse and contactor.

2 Responses to “How do I wire two contactors for a higher voltage use?”

  1. on 06 Aug 2008 at 1:21 am Doug Weathers


    I want to use two pairs of contactors. One series pair in the positive lead (main contactor) and one series pair in the negative lead (the safety contactor).

    I have started worrying about what happens if one of the contactors welds shut. I’m afraid that the remaining contactor will still open and close, so there will not be any obvious syptoms until it tries to break too much power.

    Should I worry about this?

    What can be done about it?



  2. on 07 Aug 2008 at 4:53 pm Otmar

    Thanks Doug, you bring up a good point.

    There is no system in the Hairball that could detect one of a set of contactors being welded shut.

    The first prevention is to design your vehicle system so that a contactor would never get hot enough to weld. This can be done by limiting the battery current, properly heatsinking the contactor terminals (with large cables and good tight connections) and checking temperature rise of the contacts under worst case loading.

    The fact that the contactors in a properly designed EV never have to make or break loads in normal operation makes it unusual for one to weld unless it is overloaded.

    Regular visual inspections are a good thing and standard practice in high power race cars that do push the limits. At lease in those cars that use visible contacts as I suggest.

    Microswitches that indicate contact position are also available for the popular SW200 series of contactors. Those could be connected to in a logical way to indicate any time the two contacts were not both on or both off. In my experience they work well at indicating when even one of the two contact connections has welded.

    I hope this helps,