December 2007

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.

Zilla FAQ23 Dec 2007 12:14 pm

Some people choose to use one contactor to break the B+ lead to the controller in the usual way plus a additional contactor to break the B- lead of the battery pack for extra redundancy or safety. By the way, this is one way to turn off your DC-DC if you choose to wire it to turn off with the key. A few things are important in this case:

1) The B+ contactor must be wired as shown in the Zilla diagram and must use a snubber diode. Preferably it would use the snubber diode that came with the Hairball.
2) The B- contactor must be on before you try to “start” the Zilla using the start input on the Hairball. I suggest doing this by having the B- contactor turn on with the “ignition on” power that also drives the Key input on the Hairball. Be sure that you do not use the Accessory lead on your ignition switch as that will not work correctly since it drops out when your ignition key is turned to the start position.
3) In this case you must wire the Hairball Start signal separately to your start wire. You must not use the option of wiring the Start in parallel with the Key input or it will keep the Hairball from performing the start sequence correctly.
4) You should have some form of snubber diode on all contactor coils to avoid damaging other parts of your electrical system. In this case the one on the B- contactor could be any type. Albright contactors often come with a diode and that one would be fine for this use.


Production Notes21 Dec 2007 11:49 pm

Just a quick update: On the 14th of December Cafe Electric commenced holiday shutdown.

While the production employees are on vacation, Mandi and I have been doing inventory and reviewing financial data from the past year. We will be analyzing the current profit/loss situation in light of this years higher production volumes with increasing material and labor costs. So far it looks like we’re hanging in there reasonably well with the increases efficiency of higher volume helping to offset the current high inflation and devaluation of the dollar. Still, I do expect to have to increase prices some in January but as usual I will try to minimize that. I really don’t like making conversions more expensive. We are also forecasting for the new year while making plans to reduce the backlogs further toward our goal of actually having controllers in stock. What a concept! To assist in that, we may once again be hiring more help. Let me be clear: We are not looking for engineers or engineers in training, nor part time workers. We are looking for extremely meticulous and reliable friendly Buddha-like people who can build controllers day in and day out. Someone smart who can get things done. Although a knack for how to best use a screwdriver can be helpful, no technical experience is required. Patience and good eyes are helpful. If you know someone appropriate, please have them email in their resume.
During the shutdown we are not accepting any new orders until the new prices are announced. I am currently performing a long overdue update to the website. I expect the new site to be live early January with much information which hopefully will be easier to find. With any luck new shopping options will also be up and running with the new price schedule.

That’s it for now,