I was showing my Brammo Enertia at a motorcycle show in Kansas City a couple weekends ago (I was the only electric – go figure) and this guy started asking me questions about the electrical system. I’m never very good at answering these questions because, while I can produce a spec sheet, I don’t have the underlying electrical engineering-type knowledge for their sometimes very specific questions. I’ve been asked questions about the loss of efficiency in converting between AC and DC before, and the exchange usually goes something like this:
Him: I assume it’s a DC motor.
Me: Actually, it’s a permanent magnet, brushless AC motor.
Him: AC? So you charge the batteries with AC, the charger converts it to DC, then the (something) has to convert it back to AC for the motor? Sounds like the most inefficient electrical system imaginable.
Me: uh…. it’s fun to ride (or something like that).
So, I asked Roger, a senior advisor with the engineering staff at Brammo for some help (and asked him to explain it to me at my 6th grade level of understanding) and here’s what he said:
1. We start with the desire to store electrical energy. Since nobody has found a cost effective way of doing this, the best we can do is to instead store electro-chemical energy. Our best attempts to store electro-chemical energy result in batteries that produce direct current electrical energy or “DC”.
2. We have an electrical energy distribution system [in the U.S.] that provides alternating current or “AC”. There are interesting technical and political reasons why throughout the world AC and not DC electricity is distributed. Most people agree the most compelling reason for the choice of AC is that it is less deadly when accidental exposures occur.
3. Low voltage is not a health hazard to humans. High voltage is deadly.
4. Our motorcycles operate at 76 volts DC in order to provide acceptable high speed performance at the lowest, and therefore safest possible operating point.
5. Regardless of whether we choose to use a DC motor or an AC motor, we need a motor amplifier to control the speed of the motor and this forces us to convert the 76 volts from the battery into a variable voltage that drives the motor at various different speeds. This amplifier is known as the motor controller.
6. It mostly doesn’t matter whether we are converting between AC and DC since we are forced to convert from high voltage to low voltage to charge the battery. Then we are forced to convert from a fixed low voltage to a variable voltage based on speed and power control of the motorcycle. So you can look at it as 2 conversions: AC to DC for storage, then DC to AC for propulsion, and this looks foolish perhaps. If you instead look at it as 2 conversions: high voltage to low voltage for storage, then low voltage to variable voltage for propulsion, it might appear more sensible? Who cares whether we change the format between direct and alternating current, there’s no meaningful efficiency loss in doing this.
In another email, Roger said something that really resonated with me, and it reflects what I have come to understand as the part of Brammo’s mission that differentiates them from the rest of the pack:
Every successful consumer product should be a delight to use; none should be a harrowing challenge. This is also true of the high performance ICE sport bikes. The Enertia is not meant to compete against these high-performance sport bikes; it should be delightful to use in its intended application.
I don’t know if I’ll remember all of this the next time someone asks me a question about AC versus DC or voltage, but I know where to send them for an explanation. Thanks Roger!
(This post also appears on the Brammo Owners Forum)