One of the four main components of an electric vehicle is the controller, which sends power to the motor as you request it with the throttle, instead of all at once. In the same way that fuel injectors spray metered blasts of fuel into your engine, instead of having the spark plugs mounted in the fuel tank. The way this is done today is with semiconductors, but it was once done with switches. Big switches which rearrange the battery pack into different voltage configurations to provide varying power to the motor. And those big switches are the part for the day.

Although modern electric vehicles are no longer controlled this way, these devices which are commonly known as contactors are still useful on modern EVs, for a couple reasons.

• Semiconductors are not life-or-death reliable. Semiconductors rarely fail, but they can under certain circumstances and when this happens, the usual result is that they become conductive. This means full power going to your motor, no longer under your control. Contactors present a safety mechanism to disconnect the pack in that situation, and also as a way of ensuring that when the car is switched off, it’s really off.
• Semiconductors are expensive; in some cases, it’s cheaper to use switches. Semiconductors are worth the price because they can switch quickly — many thousands of times per second. Where this speed isn’t needed, a switch is more cost effective. Examples of this are redirecting power for electric reverse for those like me who aren’t using transmissions, and something referred to as series-parallel switching, in which dual motors or dual coils on a single motor are connected in series for high torque and parallel for high speed to form a sort of two-speed electric transmission. This switching could be done with semiconductors inside the controller, but that would vastly increase its cost.

A contactor is really a fancy name for a relay, which some may be familiar with from car stereo installations or electronics. It’s a switch that’s actuated by an electromagnet. Instead of a flexible tab of metal common in small relays that is pulled to touch a contact to complete the circuit, a contactor typically has a large copper bar that is pulled down to bridge two contacts. Beyond that and the size difference, the two terms are the same. As a matter of fact, the manufacturer of the device in the picture refers to their products as “relays” not “contactors”.

The pictures of the contactor we’re looking at today, the Kilovac EV500 “Bubba”, were taken of one of four similar contactors that I currently have which I’d intended to use for this project. As usual though, things are a little complicated.

First, two of these contactors have auxiliary microswitches, and two do not. (I got these normally $800 contactors for a huge bargain, so I can’t complain.) What this means is that the latter two units are not able to communicate their status to the controller. This is a must-have feature — while switching from forward to reverse for example, if the controller doesn’t know that forward has not been properly disengaged and it applies the reverse configuration at the same time, the results would be catastrophic (at the very least I’d get to hear one of those fuses blow). I’ll be trying to work out a cheap way to provide this signal electronically, which promises to be challenging.

The second issue is how the contactor is made. This is an aerospace-grade electrical switch, and it’s no slouch — that 3000A disconnect rating is impressive, as is its continuous-carry rating of 750A at 25C. It’s equally impressive that they build them with a sealed, hydrogen-filled chamber; the gas acts as a dielectric to reduce arcing that can cause the contacts to weld. With all of this, the switch would seem ideal — and indeed some agree that it is. However, there’s one huge drawback that in many racers’ opinions is enough to justify avoiding this contactor. Its case is opaque and cannot be opened, so you can neither inspect nor replace the contacts.

For this reason, the device often used — way beyond its ratings I might add — is the traditional, reliable Albright SW200 series contactor. Make all the jokes you want about British-built electrical equipment in automobiles, this device is extremely popular among EV builders for its rugged construction, reliability, and inspectable, replaceable contacts. Though it has a continuous carry rating that’s a third that of the Bubba, and half the Bubba’s current break rating, the reality is that these contactors can, and routinely do, handle much higher voltages and currents than are specified. And if they do fail (this will almost certainly happen while racing, not on the street), the contact tips can be easily replaced. More importantly, damage that would predict such a failure can be seen and dealt with.

I will need 6 contactors in all to provide for series/parallel switching, electric reverse, and battery disconnect. The next two I buy will probably be the Albrights. At least I can use them as the main contactors, which would be the ones called upon in an emergency.

Link to today’s part.