Electric Car Motors: AC vs. DC

Jeff from Ohio wrote me the other day, "What, if any, are the advantages of AC electric car motors over DC motors?"

There's a debate worthy of a page of its own!

Metric Mind, a well-respected supplier of AC electric car motors drive systems for electric cars, says that for general drivability, the AC system is far superior to the DC system. They say your first clue that this is true is that when the major automakers - GM, Ford, Toyota - made an electric car, they ALL went with AC electric car motors. This is probably not a coincidence; )

These are the advantages of AC electric car motors that Metric Mind discusses on its website's FAQ, paraphrased liberally:

  • Regenerative braking without any extra stuff.
    Because the AC system is set up nicely to "take back" energy from braking as readily as it "gives out" energy under acceleration, you can recover a LOT of your battery power during the normal driving process. Metric Mind points out that a few DC systems can do this to some extent also, but they don't do it nearly as well and it always makes them more complex and expensive.
  • Favorable torque.
    In the capsule version, this means that AC electric car motors can be well matched to your gas-guzzler's torque curve so that you don't inadvertently murder your transmission with your normal driving habits. The gas-guzzler's engine is considerably different than an electric DC motor, and the drive system your donor was born with is not designed to withstand the low-end workout your series DC motor is capable of delivering. The AC electric car motors are much more...diplomatic, and your transmission will last longer. How does Metric Mind put it?..."broken gear teeth, stripped splines, twisted shafts, damaged CV joints....AC setups don't have these issues." Oh, the expensive carnage.

  • No motor brushes.
    A lot of people, including my Uncle Chuck, say that brushes last a long time with no problems. Have them serviced every 50K miles or so, and you're good to go.

    Me, I say they're just one more thing that can fail.

    Metric Mind seems to agree with me on this one: "There are all these issues with brush advancing, seating, commutator arcing and self-destroying at high RPM".

    Anyway, no matter what Uncle Chuck says, they're no good in a regenerative braking environment. Ditto for electric reverse (Metric Mind again: "If electric reverse is used, the requirements for brush advance for forward and reverse rotation direction are contradictory"). Not a problem for AC electric car motors, which have no brushes.
  • Programmability.
    Now, a DC controller COULD be programmable, but usually isn't. AC inverters are perfectly matched to the motor they're sold with, and you can set all the software parameters yourself to best fit your driving style and your batteries. Want to stretch out your range? Set the battery voltage to maximum for regen and minimum for driving. This is just a small sample of what the sophisticated AC inverter is capable of, all displayed on a laptop screen in real time as you drive.
  • Safety.
    Want to hear something terrifying? If a DC controller's power stage fails, entire pack voltage - all 120 or 140 volts or whatever - is applied to the motor. In Metric Mind's colorful prose: "...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."

    The worst that happens if an AC inverter fails is that you'll roll to a very boring stop and have to walk; )
  • Electric reverse is as easy as adding a small toggle switch on the dash.
    "All it takes for an inverter is to swap sequence of 2 phases, so the rotor runs in opposite direction," Metric Mind says. Bit more complicated with DC electric car motors.
  • Ease of installation.
    Contrary to popular opinion, AC systems are easier to install than DC systems...not harder.

    Yes, the AC inverter itself is a complicated instrument, but then, so is the DC controller. They're both using computers, but that's nothing unusual, since so is your typical gas-gulper. As to the wiring - Metric Mind says: "To wire a Siemens AC system you have to make 6 connections: 3 phases to the motor, 2 cables to the battery, and plug encoder cable. The rest is low voltage wiring: +12V DC-DC output - to 12V wiring system in the car, 3 wires to the ignition switch and 3 - to the throttle pot, 3 - to the direction switch, 2 - to start inhibit switch. All this wiring harness is pre-fabricated and included."

    On the other hand...

electrical boxes

"A typical setup for a DC system with series wound motor: 2 cables from the battery to the main contactors. 3 jumper cables for reversing contactors. 2 cables from the main contactors to the controller. 2 cables from the controller to the motor. One cable to jump the field and armature of the motor. 2 wires to the DC-DC converter input.

Then on the low voltage side...

3 wires to ignition switch. 2 wires to the throttle pot. 2 wires to the precharge contactor coil. 2 wires to the reversing contactors coils. 2 wires to the power resistor precharging capacitors in the controller. 2 wires for start inhibit switch. One heavy wire to ground DC-DC converter neg. side, and one - to connect its output to the 12V system in the car. 2 wires from the motor temp switch to the light on the dash.

Typically, no harness exists or is included; you come up with your own."

Where AC electric car motors AREN'T the best fit...

white zombie NEDRA

There are two situations where a DC drive system drive system is preferable to an AC system, though.

The first is the EV drag racer who wants to blow the doors off the ICE machine sitting next to him (or her), and won't be going more than a quarter-mile at a time...

...he'll probably prefer a DC motor. (The very quick Wrightspeed X-1 has an AC drive system from AC Propulsion, but most drag racers use DC motors.)

And the other is the "quick and dirty" hobbyist converter who just needs to get something running on batteries as cheaply as possible. There's no shame in this, a lot of the EVs on the road today belong to this guy.

I'm just saying there's something better out there, and the electric cars of the future will doubtless have 3-phase AC electric car motors. Eventually, we electric car enthusiasts will have take a deep breath and conquer our fear of the unknown. We'll be better off in the long run.

Come on in; the water's fine!

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