Electric car motor, peak torque at zero rpm!
It's true. We are not used to it as internal combustion engines, whether diesel or gasoline, whether powered by LPG or CNG, whether turbocharged or naturally aspirated, first need a minimum engine speed in order to run. Moreover, in all cases, the maximum torque speed is always higher than the idle speed. The sportiest engines can have maximum torque even at high rpm, up to 5,000 rpm to be precise, whereas the more "elastic" engines can have torque at rpms even below 1,500 rpm. For example, many turbo diesel or turbo gasoline engines with twin-scroll turbochargers are this way.
As a result of the architecture and operation of electric engines, they do not need to have a minimum RPM. Furthermore, they can deliver maximum torque even at zero RPM, practically from a standstill! Sounds strange, doesn't it? Yet it is, and to understand it, we need to know how an electric motor works for automotive purposes, which in many ways is not very different from an electric RC model engine.
The electric engine is constructed in a manner that can create infinite revolutions of what happens with a normal electromagnet. An electromagnet is a coil made from a conductor (usually almost pure copper), which generates a magnetic field when an electric current is passed through it. This magnetic field can be used to attract or repel a magnet or a second coil energized by an electric current. The force that the system generates in a rotating electric motor is represented by the driving thrust (or torque).
The electric engine is constructed so that it can infinitely reproduce (rotate) the same phenomenon that a linear electromagnet is capable of generating. The linear electromagnet, however, has a start stroke and an end stroke. The electric engine instead rotates by continually changing the phases with which the various conducting coils are energized, generating driving torque. Simpler motors, such as those used in toys or electric RC models, operate on direct current and require a collector inside them with at least three sectors and associated brushes that supply electrical currents to the coils. Usually, these motors use permanent magnets on the stator, while the collector has the function of selecting which coils to power as the motor rotates. These engines can be very powerful but have some drawbacks. These include (but are not limited to) poor durability of the brushes sweeping over the collector and the generation of strong electromagnetic disturbances when the brushes are switched over the collector.
Brushless Electric Engines
There is a preference to use more refined brushless motors on electric cars, but they require complex control electronics called inverters. The brushless engine has no brushes and typically uses a rotor composed of a permanent magnet that reacts (rotates) to the magnetic fields induced by the three sets of coils located on the stator. To generate the motion, the three sets must be supplied with a 3-phase sinusoidal voltage, a task entrusted to the inverter.
The inverter for electric engines used in cars
The inverter is a complex electronic circuit consisting of a logic part, i.e., a microprocessor. It oversees the operation of the whole apparatus and generates the PWM signals that create sinusoidal feeds (where in reality, the sinusoidal shape is not given by the voltage trend, which is always the same, but by time through a continuous variation of the duty-cycle), a driver stage, and the output stage. Drivers are electrical circuits that act as an interface between the digital signals coming from the logic part (which is also connected to the accelerator pedal) and the output stage made with solid-state semiconductors, e.g., Mosfet, Hexfet, or IGBT. The control logic generates a three-phase supply voltage with a sinusoidal pattern (frequency directly related to the electric motor revolutions) by dosing the driving torque through variation of the aforementioned duty-cycle. The duty-cycle represents the percentage of ON and OFF in each unit of time. When the torque/power demand is zero, the value of duty-cycle is 0%; conversely, its average value during the generation (simulation) of the sinusoidal engine power wave will reach its maximum value.
The inverter is powered by the power line supplied by the batteries (called a BUS line) and at its output has three large power cables running to the engine. The three lines, which can be likened to the 3 R S T phases of a normal 400V industrial line, are connected to various groups of coils inside the motor ( at least three coils) that generate the driving force in a rotary pattern. The brushless motor is synchronous, so its rotational speed is closely related to the rotational speed of the three supply phases. As the title of this article states, this type of engine can generate maximum torque even at zero speed, that is, from a standstill. It means that by fully depressing the car's accelerator pedal, the engine can provide maximum thrust already in the first few inches of forward motion (unless limited by vehicle management software). Obviously, this results in very (very) effective standing-start accelerations, as there is no need to wait for the engine to reach maximum torque, and there is no turbo-lag in the delivery. The electric motor can instantly deliver maximum torque at any speed and even from a standstill.
In this article, we have introduced the function of a normal DC electric motor and made a distinction between a brushless and a brush-type engine like those used in the automotive field. We have also very quickly touched on inverter operation (the writer has also worked on designing inverter systems for electric engines from scratch) by covering the logic, interfacing, and output stages. If you want to learn more about electric motors with special reference to propulsion engines, keep following us. We will discuss these topics again, touching on other points related to performance, autonomy, and much more...
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