The electric vehicle controller is the device which operates between the batteries and the motor to control speed and acceleration. The controller transforms the battery's DC current into alternating current for the AC motors or simply regulates current flow for DC motors. The controller can also reverse the field coils of the motor so that when in a braking mode, the motor becomes a generator and energy is put back into the batteries. This is known as regenerative braking and over the course of a single charge can return as high as 10% or more of the energy consumed by the drive system to the batteries.
One of the well publicized benefits of an EV is regenerative braking. Regen braking is common on nearly all vehicles now, yet few people seem to realize what happens. The following paragraph is an attempt to explain how it works.
How regen braking works:
In the circuit shown in attachment1 is an output pair of MOSFETs (Metal Oxide Semiconducting Field Effect Transistors), with the motor being driven. The output from the controller is a pure d.c. voltage. The motor will generate a back e.m.f. which is proportional to its speed of rotation. At zero load, or no acceleration, this back e.m.f. will rise to be equal to the output from the controller.
The MOSFET is a bi-directional switch which conducts resistively (when it is turned on) for both directions of current. So consider the situation when the current is zero and the controller's output is now reduced. The motor's back e.m.f. is now higher than the controller's output voltage - so the motor will try and feed current back into the controller. If it succeeds in so doing the motor will be braked - we will have regenerative braking.
This type of circuit (where hi-side is turned on when the low-side is off) is capable of sourcing current or sinking it. The way this works is that the reversed motor current is now a forward current to the flywheel MOSFET so when this is on it shorts out the motor - whose braking current rises during this period (arrow B, reversed). The Flywheel MOSFET now turns off, but this current must keep flowing - because of the motor's inductance. So it flows as reverse current through the drive MOSFET, recharging the battery as is does so. The extra voltage for this is derived from the energy stored in the motor's inductance. The process of switching from drive to braking is entirely automatic. Moreover it is done entirely by the motor's speed exceeding the drive voltage and without any change of state or switching within the controller. The regen braking is, if you like, a by-product of the design of the controller and almost a complete accident.
If the vehicle is driven down too steep a hill (or the demand speed is suddenly reduced so that very hard braking results) the current generated by the motor could exceed that which the MOSFETs can safely handle. Since this would blow the MOSFETs it must be protected against so all controllers that give regenerative braking are also fitted with a current limit to stop such failure.
In Hybrid Electric Vehicles this problem becomes even more complex because of the unused current from the auxiliary power source. Since the drive motors are not drawing current from the auxiliary power source, that current still must have some place to go. The motor controller should monitor and take into account the excess current from the auxiliary power source, so that in certain situations where too much current is present with regen and APU operating, the regen MOSFET must also be turned off. to protect the motor controller.
In early version of electric vehicles with DC motors, a simple variable resistor type controller governed the acceleration and speed of the vehicle. Full current and power was drawn from the battery all of the time. At lower speeds, when little power was needed, high resistance was used to reduce the current to the motor. This resulted in a large percentage of the battery's energy being wasted as heat dissipated by the resistor. Modern controllers adjust speed and acceleration by and electronic process called pulse width modulation(PWM). Switching devices such as IGBT's(very fast, high current rated transistors) rapidly interrupt, turning on or off as needed, the flow of electricity to the motors. High power is achieved when the intervals(time between pulses) are very short. By increasing the time between pulses, the current is limited.
Wheel Motor
The wheel motor shown in attachment2 ismanufactured by Technologies M4.
As mentioned above, one of the more interesting designs for motors is to integrate the motor directly into the wheel. These are called wheel motors and may very well become the norm someday as they remove a tremendous amount of mechanical devices from the vehicle by providing propulsion to the wheel...in the wheel!
The motor-wheel assembly is an elegant integration of an electric motor and other components into a package that fits inside a regular-size tire.
The motor-wheel assembly consists of a highly efficient electric motor, a Motor-Wheel Slave Controller (MWSC) including power and control electronics, a brake, wheel bearings, a steerable front suspension interface and a heat sink embedded in the stator. The configuration of the 3-phase synchronous motor consists of a central stator which supports the windings and the inverter, surrounded by an external rotor which supports the permanent magnets.
The wheel is directly mounted on the rotor for direct transmission of torque and enhanced freewheeling. The motor assembly is liquid-cooled to sustain high continuous power demands.
Bus manufacturers will appreciate the packaging advantages and interchangeability of the motor-wheel rear axle, which easily fits within existing wheel housings. The axle deep-offset cross member enables a wider floor aisle area in the low-floor configuration.
Selecting a motor for use in an EV involves many variables. No single type of motor can be considered as the best. When an EV is being designed, questioned must be answered before selecting a specific type of motor. How much power do you need, do you need variable speeds, what is the operating voltage of the battery system, what kind of torque do you need and at what speed, how much physical space can the motor occupy, how much can it cost, what type of environment will the motor be operating in? Once these questions are answer, you can make your motor selection. Once the motor has been identified, a control system must be designed to make the motor functional.
