AC vs. DC- safety

Hi, I am just starting my quest for information for my first EV conversion. I am going to convert my 2001 Hinda Civic Coupe. I have read alot about AC drives… they seem to hold a lot of advantages, but of course the apparent cost could be a problem (though i don’t know for sure what they cost but seems like i could end up spending $6000+)… One of the most important features for me is the “what if” the converter fails… on the DC drive it seems that full power will be applied to the drive thus fast acceleration forward. On an AC drive, should the inverter fail the power simply gets cut off and you go noware. Does anyone know if this is always the case, or do more modern DC controls or converters have a safety built in? The safety is worth the money to me, but if i can ensure safety on a DC drive then why not save some money!

yeah, for alot of the controllers out there, the failure mode is full on.

Some controllers fail off, but they are out there, just have to check with the manufacturer.

If you have a car, you’ve got a main contactor, and should always have an emergency disconnect.

I agree with frodus.
Whether you go AC or DC, you should definitely wire in a main power circuit breaker with a cable pull that is accessible in to the driver.

Cable Pull?! By the time you can pull the cable, someone could be killed!

Well, it is a cable pull, but it is on a circuit breaker. If there is a short, the breaker will trip. The pull cord is more or less a convenience.

An accelerator pedal with a built-in contactor switch seems to be the safest option to me, or a pedal that’s been outfitted with a pressure switch that trips the contactor would be a good modification for a conversion. You could use two contactors, one wired to the ignition switch which shuts the whole vehicle down, and one wired to the pedal and only kills power to the motor.

This way, when you’re sitting at a stop light with your foot off the gas, the controller and vehicle systems are operating (you still have A/C, meters, radio, etc) yet there’s no way for power to make it to the motor. Even if the controller failed while stopped and went to full throttle, the vehicle is not going to lurch forward and total a bunch of cars before you can reach for a pull cable or some other switch.

Now the issue becomes: What happens when the contactors seize closed and won’t kill power?

this is a bit from some great reading at

Well known fact that if a DC controller’s power stage fails, entire pack voltage is applied to the motor and you better have good circuit breakers, fuses, kill switches and good reaction time if partial failure happens to be at the intersection while you are waiting your green. In contrast, power stages of AC inverter are used to generate power for the motor, not regulate it, so in case of a failure AC generation just stops and so AC motor just looses power.

Seems to me that this is the absolute safest… no human involvement required, and a system that seems to be designed to not allow any chance of error…

Anyone that says that a AC system will “always fail open” does not understand electronics. Sure, if the inverter in a AC system fails, it will not produce AC, or will send full DC pack voltage to the motor… But no one knows where electronics will fail until they do… I would say, build what ever type system you want / can afford, but in both systems, don’t skimp on safety features… What happens in an ICE if the throttle sticks wide open? Turn the ign off, E-brake, shift into neutral… These same features should also work in a EV… Main contactor or 2 contactors controlled by the ign switch, Fuses, ect

Good point… Since whatever i convert to will be a standard transmission i would have the option of being in neutral at an intersection should i feel that i could not trust the electrical… I would be interested to hear from anyone who has an EV conversion with DC motor who may have had a failure… what was it like, how often does this actually happen etc…

I don’t think that’s likely unless the driver totally panics. Full power in most EVs does not produce neck-snapping acceleration, and I’ve never driven ANY vehicle that could over-power it’s brakes.

[QUOTE=rjstractor;2459]I don’t think that’s likely unless the driver totally panics. [/QUOTE]

Better to err on the side of safety.

and what if the phase of an ac fails on… then what? well, you lock up the motor. This can jerk you around pretty good.

Some guys use a sheer pin for lockup protection…

There needs to be a device that senses throttle position, and whether the car is accelerating and if the brake is on. If it is, then deenergize the contactor.

then again, what if the throttle on a car goes full for some reason…

you just take the keys out of the ignition, brake and slow down.