The Electric Motorcycle, Part 2

Electric-vehicle power supplies.

Energica DC charge

Grid power lines supply electric power as AC, alternating current, which reverses itself 60 times per second. But batteries supply, and must be charged with, DC, direct current, which flows steadily in one direction. The most durable and efficient electric motors are also AC. All this incompatibility means an electric vehicle must either charge from a dedicated high-current DC charge point or carry a power inverter to change AC at a household power plug (15-amp, 120V wall outlet or 30-amp 220V drier plug) to DC at the voltage needed for vehicle battery charging. In the ideal electric future, rapid, inexpensive DC charge points will prevail.

Once the battery is charged, the task changes to converting its DC output into the three-phase AC power required by the traction motor.

DC electric motors are okay for brief high-current applications like drag racing and for years have powered electric forklifts, but DC motors have carbon brushes and commutators, which wear. DC motors are not easily capable of the energy efficiency required for useful vehicle range.

DC is very easy to control with power transistors; at some chosen frequency, you just rapidly switch the power rapidly on and off, varying the on time of each cycle to control the load. For maximum power, the transistors are on 100 percent of the time. To reduce power, you just reduce the fraction of the time that the transistors are on, and increase their off time. The same “chopper” control system is used to vary the speed of small DC motors driving things like electric drills or die grinders.

Why do most electric traction motors use three-phase AC? The single-phase power in household use is continually varying in sine-wave fashion, rising above zero, peaking at ~ 155 volts, descending in voltage again, passing through zero, and producing a negative wave that is a mirror image of the preceding positive one 60 times a second. As the wave passes near and through zero, almost no power is being delivered (makes sense; near-zero voltage means little current equals little power). This means that much of the time, nothing, or close to it, is happening. So for higher power, three phases, separated by 120 degrees, are delivered separately by a three-wire system. This guarantees that power is always being delivered, for when one phase is crossing zero and delivering little power, the other two phases are either on their way up or down from peak voltage, and they are delivering power.

The problem is, how can you get three-phase AC power from a DC source, the battery? You use fast-acting high-current switches. A first approximation to AC is an alternating square wave, made by switching on with forward polarity and letting current flow for half a cycle, then switching off, reversing the polarity, and then switching on again for half a cycle. You control the power delivered by varying the on time of each pulse. By more rapid switching and adding some reactance to the circuit, you can more closely approximate a sine wave by generating many stepwise changes.

Energica Ego plug-in

Naturally, because the electric traction motor operates across a wide speed range from zero rpm to some maximum, its three-phase AC power supply has to be capable of varying its frequency to stay “in step.” The motor carries a rotary encoder that tells the power supply how fast it is turning, so the power frequency remains in step with the motor.

Until recently, the solid-state switch for this duty was the GTO, or gate turn-off thyristor, but it had the problem of limited efficiency, turning as much as 10 percent of the power it transmitted into heat. When they call these switching devices “semiconductors,” they mean it, because they don’t conduct with the very low resistance of metals like copper or silver. Since there is significant resistance even in the forward direction, there is resistive heating. Semiconductors don’t like being too hot, so we need a way to cool them.

Okay, you bolt your array of GTOs to a big finned heat sink (a giant version of the CPU cooler in your gaming computer), taking care to coat the mating surface of each device with thermal compound to improve heat conductivity to the sink. Then you add a fan to push air over the hot heat-sink fins. Want to get fancy? You can have the fan switch on only when a heat sensor says it’s needed, or you can even have a variable-speed fan to match the cooling to the heat load. At 90 percent efficiency, if you are sending 18,000 Watts of cruise power to an electric car’s traction motor, 10 percent of that, or 1800W, is going to the heat sink. That’s like having two toasters going.

Everyone was much happier when IGBTs replaced GTOs. Although IGBT sounds like heart-warming gender inclusivity, it actually stands for Insulated-Gate Bipolar Transistor. The power inverter people love these things because they are three to four times faster switching than GTOs and cut power loss by about half by being 95 percent efficient. Now you can downsize those heat sinks and their cooling fan(s).

Power supplies remain expensive and bulky, so we can anticipate the coming of more power-dense gallium nitride (GaN) switching devices. Every reduction in weight of non-battery components means increased ability to carry batteries, resulting in small increases in range. This development is hoped to hit the industry in the early 2020s. Industrial change is not instant; Rudolph Diesel first ran his engine in 1893 but it did not drive the steam locomotive out of production until 50 years later.

Much is made of electric vehicles’ ability to employ regenerative braking, thereby recovering some of the moving vehicle’s kinetic energy during braking (kinetic energy is proportional to mass, times the square of velocity) and storing it in the battery, available for reuse. This requires turning the traction motor momentarily into a generator to act as a vehicle brake, then rectifying its AC output (turning it into DC), and sending it to the battery at an appropriate charging voltage. As you would expect, this is useful only in driving that requires a lot of braking , and on average has been said to add only five percent to vehicle range. Energy conversion is no easy business, for each step has its own efficiency, and to get overall efficiency, all are multiplied together.

Energica Ego static side view

For example, the motor operating as a generator might be 90 percent efficient, the rectifier 95 percent, the battery charge-discharge cycle 85 percent, the conversion back to AC in the power supply 95 percent, and the traction motor 94 percent. Multiplied out, that is 65 percent, which has been mentioned as a possible recovery efficiency for future electric garbage trucks. In highway driving, this would offer no benefit because motor output is being consumed by aerodynamic drag and tire rolling resistance. In the kind of stop-and-go driving in which you constantly accelerate and brake (that is, not urban creeping in which only low speeds are attained), it could make a useful difference. As a specific example, this could be very useful for a city bus, making frequent stops then accelerating to traffic speed. Regenerative braking of this kind has been written into Formula 1 technical rules.

Depending on the electrode chemistry of the battery, there may have to be controls to prevent too-rapid discharge that can shorten battery life.

Racer Eric Bostrom has a lot of experience with Brammo electric motorcycles in electric-bike competition and was surprised at how easily their software writer was able to come up with a traction-control system that worked quite well, first try.

On combustion-powered bikes, the fastest-acting component of the traction-control system is ignition retard, which takes place at electronic speed. But for deep cuts in torque, we must wait while a stepper-motor on the throttle shaft cycles through the commanded number of steps for the desired torque reduction, which takes time. But with electric power, torque changes are limited only by how fast stator magnetization fades when coil current is cut off. Even rotor inertia can be canceled by reversing the torque appropriately. Electric drive is the ideal basis for accurate traction control.

This is a central point regarding electric propulsion, that electric motors inherently deliver the constant, smooth torque that all of today's complex sportbike electronics can only try to approximate. If racing ever comes down to who can better match torque to traction, electrics will win hands-down. We just won't be able to hear them coming.

The power supplies and the motors are ready, and they are good. All that's lacking is the ideal battery system. The hopeful await the "scheduled breakthrough," a battery with 10 times the present energy storage capacity, rapid, no-penalties charge/discharge, low self-discharge rate, low cost, acceptable safety, and easy recycle-ability. Some hope to read about the breakthrough on GizMag on Monday and find the product in shops by Friday.

The problem is cost. In the past, industries have adopted new technologies because they gave a market advantage, measurable as increased earnings. The present high cost of batteries has kept electrics from reaching a mass market, and limited range keeps them from acceptance as an only vehicle. Until the recent economic downturn, the expected solution was government subsidies—paying people to buy electrics. This is politically difficult because it amounts to paying the well to do to buy premium-priced products.