This week I also had a very exciting purchase of the Toyota MR2 1993 for $1.5K. We are planning to tow it here on the weekend so I can get to work on it quickly before I leave next week for tournament.
I then continued researching motor types and assessing their suitability for use in an electric car. Here is what I found:
Single/Three Phase AC Induction Motors:
Single and 3 phase AC induction motors a completely different design to DC motors.
They are designed to run off of a single or 3 phase alternating current which is typically found in household outlets (single phase AC) and industrial power (3 phase AC).
Single phase AC is simpler, having a neutral wire and single phase wire (and ground for household safety). The waveform from single phase line generally follows a clean sine wave curve alternating from positive to negative voltage, where 1 wavelength occurs per 360o rotation.
3 phase electricity is more complex and consists of multiple sine waves each spaced out 120o out of phase from each other.
3 phase power is more efficient and powerful as greater energy transfer can be achieved . This requires more wires for electricity transfer but is better for power output with additional phases.
Induction motors work from the principles of induction and electromagnetism, and were invented by Nikola Tesla. The field coils on the outer area of the motor have the input alternating current into them. The placement of these electromagnets in the outer stator generates a rotating magnetic field (around the rotor).
The rotor acts as a closed wire loop. A squirrel cage rotor (in common squirrel cage induction motors) is a cylinder of stacked steel laminations, with a conductive, non-ferromagnetic material in between such as aluminium or copper bars.
When the varying magnetic field rotates around the rotor (a closed conductor), Faraday’s law states an EMF (electromotive force) will be induced, causing the rotor to become a current carrying loop. When electric current flows in a loop, Lorentz force law shows a force will be applied to the rotor, causing it to rotate. The magnetic field rotates at a speed known as the synchronous speed, which is determined by the frequency of the power source. The rotor in an induction motor is always trying to catch up to the synchronous speed, but their will always be a slip (percentage less) speed. In a perfect world example with no energy losses, the rotor speed would equal synchronous speed, however there is always friction and resistance inefficiencies in motors, therefore the rotor will rotate slower than the rotating magnetic field (NROTOR < NSYNCHRONOUS SPEED). For example, slip amount could be 5%, but will increase greatly with more mechanical load. However with increased load there will be more torque produced from the rotor due to increased power/current flow provided to the field coils - this happens due to the EMF in to field coils being much greater than EMF back from rotor’s induced current, leading to a greater potential difference in voltage and a greater current being drawn into the motor’s field coils.
Speed (RPM) of an induction motor is therefore determined by the input frequency (determining synchronous speed), and will decrease with increased mechanical load on the rotor output shaft.
Induction motors are very versatile and are a favourite for industrial purposes because of their low maintenance (no brushes) no permanent magnets (less cost) and ease of control on single and 3 phase AC power. However, in order to achieve different speeds, a variable frequency drive (VFD) is required, which are more complex and costly to build or purchase, particularly a produce 3 phase producing controller.
Induction motors are also more difficult to use as generators compared to typical DC and BLDC permanent magnet motors, as the field coil must have an input synchronous speed slower than the rotor speed. But it is still possible to achieve power regeneration with proper electrical controls, which is an important feature for a more efficient vehicle braking system.
(Learn Engineering - YouTube, 31/08/17)
Brushless DC Motor (BLDC):
A brushless DC motor is similar to a DC motor, however the rotor consists of permanent magnets, which revolve around a stator with magnetic field generating coils.
The difference is instead of a brushed electromagnet setup in a DC motor, there are no brushes which a controller has to sense and apply current into each set of coils strategically. The coils are placed in a 3 phase arrangement. So when used as a generator, this type of motor will actually generate a 3 phase sine wave alternating current.
An efficient controller will actually use a 3 phase sine wave input but this has to be in time with the rotor.
If the coils were labelled as A, B and C, and the north pole of the permanent magnets were attracted when each coil were energised, a pattern occurs with the input current which is demonstrated in the graph below.
The controller is able to detect the rotor position by an effect of when magnets pass over coils called back EMF. When the controller detects this from a certain set of coils, it sends current to energise the next set of coils just before the rotor catches up to the previous set (last phase energised). When this pattern of electrical control continues, it is possible to achieve any desired speed of the rotor (assuming enough torque). As seen on the graph, the torque output is less even when a square wave signal is used. Therefore, by using a sine wave input current for these 3 phase coils, it is possible to achieve smooth acceleration and torque output of the rotor. This also generates less noise as the square signal typically clicks when each coil is energised.
A BLDC motor can also easily act as a generator to recharge batteries when braking. By rectifying the 3 phase current produced, the electricity can be sent back to the batteries to increase the efficiency of the vehicle.
Another positive is the efficiency is typically very good.
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