Week-6 Challenge: EV Drivetrain
1.Which types of power converter circuits are employed in electric and hybrid electric vehicle?
The electric components inside the vehicle, however, vary in their voltage requirements, with most running on a much lower voltage. This includes the radio, dashboard readouts, air conditioning, and in-built computers and displays.
The main components of an Electric Vehicle are DC-AC inverter, DC-DC converter, battery, and electric motor.
There are two major power electronic units in the general configuration
- DC-DC converter
- DC-AC inverter
Usually AC motors are used in HEVs or EVs for traction and they are fed by inverter and this inverter is fed by DC-DC converter
DC-DC converters
DC-DC converters in an electric vehicle may be classified into unidirectional and bidirectional converters. Unidirectional DC-DC converters cater to various onboard loads such as sensors, controls, entertainment, utility, and safety equipments. They are also used in DC motor drives and electric traction. Bidirectional DC-DC converters find applications in places where battery charging, regenerative braking, and backup power are required.
DC-AC converters (Inverters)
An inverter is a device that converts DC power from the battery to AC power used in an electric vehicle motor. The inverter can change the speed at which the motor rotates by adjusting the frequency of the alternating current.
2. An Electric Vehicle's Drivetrain 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 is2'. The other parameters of the electric vehicle is given below,
Motor and Controller Parameters:
Rated Armature voltage= 72 V
Rated armature current= 400 A
Ra= 0.5Ω, KΦ= 0.7 V-s
Chopper Switching frequency= 400 Hz
The vehicle characteristics are given by the equation
TTR=24.7+0.0051w2
What is EV steady state speed if duty cycle is 70%?
Solution:
Equations to be used:
It is know that the voltage at 70% duty ratio is Vm= Vb.d
Vm = 72*0.7 = 50.4 V
Substituting the above value and given data to speed torque equation of motor
T = 70.5 – 0.98
The vehicle characteristics are given by the equation
TTR=24.7+0.0051w2
Equating vehicle characteristics and speed torque equation of motor
0.0051w2+0.98w-45.8=0
By solving the above characteristic equation
We get the value of EV steady state speed as 38.9 rad/s
3. Refer to the blog on below topic:
Induction Versus DC brushless motors by Wally Rippel, Tesla
Explain in brief about author’s perspective.
Induction and DC brushless motors
Earlier there were lead acid batteries, DC brush motors and contact controllers. Lead was then replaced by lithium and DC by either DC brushless or induction.
DC brushless motors:
- The rotor includes 2 or more permanent magnets that generate a DC magnetic field
- Stator core (a core made up of thin, stacked laminations)
Rotor generates the magnetic field; this field enters the stator core and interacts with current flowing within the windings to produce a torque interaction between the stator and rotor. Magnitude and polarity of the stator current can be varied and the conversion of electrical to mechanical energy is efficient. The device that provides the current control is called an inverter.
Induction Motors:
In this case, the rotor contains stack steel laminations with peripheral conductors that form a shorted structure. Current flowing in the stator windings produces a rotating magnetic field that enters the rotor. The frequency of this magnetic field is equated to the different between the applied electrical frequency and the rotational frequency of the rotor itself. In response to this voltage, currents are produced that are proportional to the voltage. Finally, these currents interact with the original magnetic field to produce forces.
Induction motor can start under load and no inverter is needed. DC motor produces no starting torque when directly connected to fixed frequency utility power. They need the aid of an inverter whose phase is maintained in step with the angular position of the rotor.
Limitations: Induction motor cannot separate from DC; AC is a must. Shaft speed is proportionate to line frequency when used within utility power, they are constant speed machines. When operated from utility power, they have limited starting torque and limited running peak torque capabilities when compared to DC type machine.
The above issue can be overcome by adding an inverter and it becomes possible to power an induction machine from a battery or other DC sources. Variable frequency is also possible by adjusting the inverter frequency. Both DC brushless and induction drives use motor having similar stator. Both drives use 3 phase modulating inverters. The only difference are the rotors and the inverter controls.
Induction motor have no magnets and magnetic field are adjustable since B is proportional to voltage to frequency ratio. This means that at light loads the inverter can reduce voltage such that the magnetic losses are reduced, and efficiency is maximized. Thus, the induction motor when operated with a smart inverter has an advantage over a DC brushless machine. Induction motor is forward approach where high performance is needed. Peak efficiency will be lesser than with DC brushless, but average efficiency may be better.
Permanent magnet is expensive and difficult to handle due to large force acting. Induction motor will likely retain a cost advantage over Permanent magnet. Induction machines are difficult to control due to its complexity and difficult to understand. Achieving stability over the entire torque-speed range and over temperature is more difficult with Induction than with DC brushless.