Week-6 Challenge: EV Drivetrain
1. Which types of power converter circuits are employed in electric and hybrid electric vehicle?
Power converters are the power electronic devices used in the hybrid electric vehicles and battery electric vehicles to process and control the flow of electric energy by supplying voltages and currents in a form that is optimally suited for user loads.
AC to DC converters are used during regenerative braking in EVs and HEVs where the AC motor is used to charge the battery. Battery can only be charged by DC current so AC to DC converter is used during regenerative braking.
DC to DC converters are also known as choppers. There are two types of DC to DC converters step up converters and step down converters. In a vehicle there are various electrical components like infotainment system, lighting system and several other systems. All the systems work at different voltage levels and its not technically feasible to have seperate electrical sources for each of the system. So a single electrical source is used and according to the requirement the voltage is increased (step up) or lowered (step down).
DC to AC converter is also known as inverter. Batteries have DC power and they cannot run AC motor so the DC current is converted into AC current using DC to AC converter.
2. An Electric Vehicle's powertrain with 72V battery pack in shown in the diagram below. The duty ratio for acceleration operation is 'd1' and for the braking operation the duty ratio is 'd2'. What is EV steady state speed if duty cycle is 70%?
Given Data:
Duty Ratio (d) = `(ON Time)/(ON Time + OFF Time)` = 0.7
Battery Voltage `(V_b)` = 72 V
Rated Armature Current = 400 A
Armature Resistance (`R_a`) = 0.5 ohms
`k*phi` = 0.7 volt second
Switching Frequency = 400 Hz
Torque Speed Relation `T_V = 24.7 + 0.0051*omega^2` (1)
`omega = V/(k*phi) - (R_aT)/(kphi)^2` (2)
Where V is the `V_(avg)` = `V_b*d` = 50.4 V (3)
Using equation (3) and (1) in equation (2)
`omega = 50.4/0.7 - (0.5(24.7 + 0.0051*omega^2))/0.7^2`
`omega = 38.914 (rad)/(sec)`
`omega = (2piN)/60`
EV steady state speed is N = 371.601 RPM
3. Refer to the blog on below topic: Induction Versus DC brushless motors by Wally Rippel, Tesla. Explain in brief about author’s perspective.
Wally Rippel in the blog says that no one type of motor will rule the impending golden era of HEVs and EVs. To further justify his statement he gave an analogy of engine head types. He goes on to say that initially there was flat head engine type and then different types of engine heads came into the market and till date all of them co-exist. He is pointing to fact that no one solutions fits for all.
The comparison here is in between two widely used motor types these days DC brushless motor and AC induction motor. DC brushless motor has a permanent magnet rotor whose DC magnetic field interacts with current in stator windings to produce a torque interaction between rotor and stator. To keep the motion continuous, stator current is varied continuously by current control device called inverter, without it brushless motors are useless.
AC induction motor and DC brushless motor have similar stator and the difference lies in the rotor. Induction motor doesn't have magnets in its rotor. They have stacked steel laminations with a buried conductor that form a shorted structure. Currents in the stator produce rotating magnetic field that enters the rotor. The voltage is proportional to speed difference between rotor and electrical frequency. In response to the voltage, current is produced within the rotor. These currents interact with the original magnetic field to produce forces, a component of which is the desired rotor torque. Torque is produced at the outset, can start under load and inverter is not needed.
AC induction motor can't be operated using DC power, the rotor produces a lot of heat so the peak efficiency is less than DC brushless motors. Shaft speed is proportional to line frequency so if we use utility power source (constant 50 Hz) the speed of the motor is constant and start time torque capcity is inferior to DC brushless motors.
We can use DC motor + inverter as an input for AC motor to vary speed (change inverter frequency).
Most HEVs and EVs use brushless DC motors with a few exceptions like Tesla Roadster and GM EV1.
Ideal DC motor should have maximum magnetic field initially to get maximum torque and less current to minimize the `I^2R` losses and less magnetic field later on when the torque requirement is less to reduce eddy and hysteresis losses but it is difficult to control the magnetic field of permanent magnets.
Furthermore, the permanent magnets are made of rare earth metals which are very expensive.
To overcome these we can use induction motor with a smart inverter which can reduce the voltage when the torque requirements are less to minimize the eddy and hysteresis losses. Also the magnetic field can be varied by varying voltage and frequency of the current. The control of AC induction motor is difficult adding to more costs.
So Wally Rippel concluded by saying that both motors have a lot of common parts and would co-exist in the HEVs and EVs era. With each motor having their unique traits. It is believed that AC induction motor would be used for electrical intensive applications and where high efficiency is required. Brushless DC motors might continue to dominate the HEV market.