In the history of the electric motor, progress has been very slow. It has been over 100 years since the day of the dynamo and early installations of direct current lighting. The large, bulky dynamo of the early 20th century gave way to Tesla’s alternating current solution which gave relative freedom over distances from the source to deliver power and do work.
We tend to think of electric motors as being fairly large machines. Depending on the application, they are. In coal mining and cement plants, electric motors move tons of rocks over miles of distance. The drives required to do these jobs are huge. The work they do is almost impossible to imagine in terms of manual labor.
As one contemplates electric motors in the current era of energy conservation and renewed interest in electric transportation, the motor and battery become the crucial technology that must perform at very high levels of efficiency to produce the performance required in these applications. Whether the design is for an electric moped, motorcycle or car, the demanding nature of these applications pushes the technology at all levels.
The current generation of electric “mobility” solutions would probably not have been possible without the next step in energy storage. The limitations of lead acid technology have made the “battery mass fraction” the limiting factor. The major applications that have been successful have been things like golf carts and fork lifts. With the advent of lithium batteries, a four times increase in energy density has allowed the designer to consider applications with one-fourth the amount of battery weight required. This has been a huge step.
The electric motor has undergone similar transition. The 92 pound 3hp dc permanent magnet motor of the past was certainly not designed with weight or size constraints, rather, with low cost as it’s goal. But in a mobility application, it’s simply not practical. Enter the new generation of liquid cooled motors where energy density exceeds 1hp per pound. It’s a whole new world.
This performance is not merely the result of managing the heat better but a combination of several improvements. Motor winding techniques have changed from the complex wave winding of a 3 phase motor to winding of a single tooth as a “segmented” part of the stator and then assembling all the teeth together in a ring. The segmented approach reduces copper usage by 30-40% depending on the design and reduces end turn losses, the portion of the copper that is not torque producing, by similar amounts. This results in motor which are 30-40% smaller for the same torque output.
In addition, the high energy magnets of the last few decades are able to produce a great deal more torque from the same cross sectional area of the rotor. While the market price and availability of Neodymium Iron Boron magnets has gone up sharply in the last few years, the reduction in motor size for the same power requirement has largely offset the cost.
The optimization of electric motor operation has also benefited from low cost control technology and ongoing price reduction in power semiconductors. As with any semiconductor based technology, the IGBT and Mosfet devices used in electric motor controls continue to improve as prices decline. The biggest breakthrough in the last 5 years has been the low cost and high level computing capability that is available through embedded processors. The low cost and high performance has made it economical to manage all aspects of how voltage and current are applied to the motor at microsecond levels.
And there is still room for improvement.