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techdirections - March 2017

The Battle Of The Motors— Induction Vs. DC Brushless

Jeff Bogue 2017-03-04 00:51:08

INDUCTION vs. DC brushless is a debate that has been thrown around in engineering circles for a few years now. You might as well be talking Ford vs. Chevy as far as some of these guys are concerned. That being said, what are the advantages of one over the other and who will win? Well, if there was “One Best” of anything at doing all things, there would only be one car manufacturer, with one engine design. But no, there are numerous manufacturers and the number of engines and engine designs is mind boggling. This is because there is no one best at anything. Some people want to haul heavy things around, some people just want to get from point A to point B as cheaply as possible, and then there are people that want to get from point A to point B as quickly as possible. And so the induction vs. DC brushless debate goes. Let’s first discuss their differences. With the DC brushless motor, the rotor inside incorporates two or more permanent magnets that generate a DC magnetic field. This magnetic field enters the stator core and interacts with the voltage/current within the windings to produce a twisting force between the two (torque). As the rotor spins, the current within the stator has to have its magnitude and current constantly monitored and varied to optimize both torque and efficiency. This control unit is called an inverter and it is the drive brain of the motor. The induction motor is very different and at the same time very similar. Nikola Tesla invented the induction motor in the 1880s. His motor, when connected to three-phase AC power (as in every home in America, thank you Mr. Tesla), will spin at a constant speed. The primary difference is the absence of magnets within the motor. Both have rotors, both have stator cores, and both have distributed windings. The magnets within the rotor of the DC brushless motor are replaced with stacked laminations of steel or a squirrel cage rotor in the induction motor. Currents within the stator windings produce a rotating magnetic field and the rotor reacts accordingly by applying a rotational force. Of course there is a lot of engineering that goes into monitoring and application of current, voltage peak-to-peak, and frequency in these motors to maintain the optimal torque and power at any given speed. This brain is also called an inverter. They actually seem more alike than different, and they are, but the differences are there and they mean a lot depending on what you want the motor to do. In the DC brushless drive the magnetic field produced by the permanent magnets is constant. This puts the motor at a disadvantage where peak performance at all times is required. In the induction motor, this magnetic field is adjustable by changing voltage, frequency, and current to maintain optimal torque and performance at all speeds. That being said, the programming in the inverter brain of the induction motor is exponentially more complicated. Rare Earth Metals are another difference. The Rare Earth Metals are, in all actuality, not that rare. Some of these metals, like scandium and lanthanum, are just as abundant as nickel, copper, and zinc, but because they are usually accompanied with thorium and uranium, the extraction and refining processes is expensive, lengthy, and toxic. The last of these mining facilities in the United States was closed almost 20 years ago. Since then, China has taken control of the production of rare earth metals, now producing 97% of the world’s supply. The DC brushless motor absolutely depends on these and the induction motor uses absolutely none. Opting for copper and steel, the induction motor is controlled by a computer. Tesla Motors was the first to innovate with the introduction of the AC induction motor in automobiles and others have followed suit. Recently both BMW and Toyota have started experimentation, and utilization, of the induction motor so they could remove something from their cars that has a market that is both volatile and single sourced (probably not good for cost and availability). Now, with all of this information, you would think that the DC brushless motor would be falling behind, but currently there is no real advantage leader between the two. In the DC brushless motor it comes down to efficiency—it is the efficiency king. When this motor is running in its happy place, it can top 96% efficiency turning that applied electricity into electromotive force. When coupled with a multi-speed transmission to keep it in that happy place, it is the efficiency champ. Meanwhile, the induction motor’s big advantage—apart from its simplicity and ruggedness—has always been its ability to tolerate a wider range of temperatures and speeds. Providing adequate cooling for the Toyota Prius’ permanent-magnet motor adds significantly to the vehicle’s weight. An induction motor, by contrast, can be cooled passively— thereby dispensing with the hefty radiator, cooling fan, water pump, and associated plumbing. Better still, by being able to tolerate temperatures that cause permanent magnets to break down, an induction motor can be pushed (albeit briefly) to far higher levels of performance, like testing 0-60 times or climbing a steep hill. Hybrid vehicles like the Toyota Prius or the Chevrolet Volt have to use their gasoline engines to get extra zip. Pure electric vehicles such as the Nissan Leaf depend on gearboxes to generate the extra torque for arduous tasks. By contrast, the Tesla uses just one gear—such is the flexibility of its three-phase induction motor. When you take another look at how these motors actually work you can see how the similarities can lead to confusion. They both have rotors that are free to spin and align themselves with the electromagnetic forces when poles of the individually paired windings are electrified. In a three-pole motor, when pole pair A is excited, the rotor (either a magnetic material for induction motors, or permanent magnets as in a DC brushless motor) will align itself north and south with that pole. As that pole is turned off and the next pole is excited the rotor will rotate to align itself with the newly applied electromagnetic field, and so on in succession through the poles until 360° of rotation is accomplished. The difference here is that the induction motor applies AC voltage to each of the windings in succession, and the DC brushless motor applies pulsed DC to each of the windings in succession—the speed of the rotation depending on the frequency of the applied voltage to each of the poles—the higher the frequency the faster the rotor spins. When you look at Fig. 6, the similarities are very evident. The similarity of these motors is impossible to deny, as are their differences. This is also true when the motors are used for more than just driving the wheels. If the AC supply voltage of an induction motor is removed, the result is no drive force and no battery charging regeneration effect—the motor will freewheel. This is not true of the DC brushless motor. Both motors are driven by a variable speed frequency inverter (one AC and one DC). Regenerative braking is possible by reducing the supply frequency. The synchronous speed applied to the stators is less than the motor speed. During regenerative braking, the motor is generating more voltage than it is using and that excess voltage is fed back to the batteries. AC motors can be microprocessor controlled to a much finer point and thus can regenerate all the way down to a stop, whereas the brushless DC motors regeneration capabilities tend to fade off at lower speeds. The final advantage of the AC induction motor is in dynamic braking. This is done by reversing the rotating wave within the stators. By reversing the rotating wave within the motor, the motor wants to turn in the opposite direction. This does not regenerate power for the batteries but the magnetic field in the motor spinning in the opposite direction simply acts as a brake on the spinning rotor, and the excess energy is dissipated as heat. The DC brushless motor is capable of doing this, but excess heat build-up runs the possibility of damaging the expensive rare earth magnets within its rotor. All in all, both motors are quite good at doing what they were designed to do. The DC brushless motors have been around a little longer in this application and have a strong foothold in the industry, but that foothold is slipping as the technology to control the induction motor has improved. Tesla Motors has a policy of releasing its technology to other manufacturers so that they too can enter into the world of the plug-in electric vehicle. Jeff Bogue is an electronics specialist focused on research and development. He works at ATech Training as a product representative and contributor to ATech Educator News. This article is reprinted from the February 2017 issue of ATech Educator News.

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