Week-6 Challenge: EV Drivetrain
1. Which types of power converter circuits are employed in electric and hybrid electric vehicle?
Power converters are circuits which are used to convert one level of voltage to another level of voltage. They can convert AC to DC, DC to AC or DC to DC. Power converters have high handling capacity with high efficiency. Power converter is an electro-mechanical device which can convert AC to DC and vice versa. They can change voltage pr frequency or both according to the requirement. Power converters used in EV and HEV are:
- AC to DC converter (Rectifier)
- DC to AC converter (Inverter)
- DC to DC converter (Chopper)
- AC to AC converter
AC to DC converter:
An AC to DC converter will convert AC voltage to DC voltage. This converter is also known as rectifier, since it changes the alternating current to direct current, or in other words it straightens the periodic signal of AC. The AC voltage is converted to a regulated DC voltage. Rectifiers are mostly used in EV charger. These are classifed as half-wave and full-wave rectifiers (Based on positive or negative part of signal for rectification), Controlled and Uncontrolled Rectifers (Based on controlled or uncontrolled power electronic device used for rectification), Centre-tapped and Bridge rectifier (Based on the use of center tapped transformer for conversion). Based on the supply voltage, it can be again classified as single phase and three-phase rectifiers.
The above figure shows Simple arrangement of diode in rectifier circuit for half-wave and full-wave, single phase and three phase rectifier.
The above figure shows Simple arrangement center-tapped and bridge rectifiers.
DC to AC converter:
A DC to AC converter converts the DC power to AC power. This circuit is called as Inverter circuit, since the Direct Current (DC) is converted to Alternating Current (AC) with certain frequency. Here the input will be pure DC voltage and it will be converted to an AC output with certain frequency. There will be a transformer with primary and secondary winding. The DC supply will be given to the primary winding, where the switches will work inorder to change the direction to create an AC waveform. Inverters are used to supply power to the motor.
This figure shows the basic idea of inverter, where the DC input is given to the transformer and the switch is used to produce the alternating current or AC at the output using the transformer and switching pulse oscillator which is used to give triggering pulses for switch to turn ON and OFF.
DC to DC Converter:
A DC to DC converter or chopper can convert the DC voltage from one level to another. These are of 3 types, Buck Converter, Boost Converter and Buck-Boost converter. Choppers are used to supply power to auxillary of the vehicle, like headlight, radio, etc.
Buck converter acts as step-down converter, which steps down the voltage to lower voltage level like 5V, taking the input from higher voltage, say 12V, 24V etc. These provide much higher power efficiency with low voltage ripples.
Boost converter acts as step-up converter, which step up the voltage, from supply voltage to the output voltage. Here the ouput voltage will be always greater than input voltage. These are highly nonlinear systems in nature.
Buck-Boost Converter is a type of converter that has output voltage magnitude less than or greater than the input voltage magnitude. They can operate as either step-up or step-down converter, depending on the duty cycle.
AC to AC Converter:
An AC to AC converter is used to convert the AC waveforms with one particular frequency and magnitude to another frequency and magnitude. These are used to obtain the desired power supply from the actual power supply.
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'.
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 Volt second
Chopper Switching frequency= 400 Hz
The vehicle speed-torque characteristics are given by the below equation
What is EV steady state speed if duty cycle is 70%?
Answer:
Rated armature Voltage, Va = 72 V
Rated armature current, Ia = 400A
Armature Resistance, Ra = 0.5 ohm
Kɸ = 0.7 V-sec
Chopper Switching frequency, f = 400 Hz
Duty cycle ,d = 70% = 0.7
Duty cycle, d = Vs/Va
i.e, Vs = Va * d = 72 * 0.7 = 50.4 V
Given vehicle speed-torque characteristics equation:
T = 24.7 + (0.0051)w², where w is speed. __eqn1
The formula for motor speed characteristics:
T = (Vs*Kɸ)/Ra - ((Kɸ)² * w)/Ra
Substituting the values:
T = (50.4*0.70)/0.5 - ((0.7)² *w)/0.5
T = (70.56 - 0.98w) __eqn2
Comparing eqn1 and eqn2;
24.7 + (0.0051)w² = (70.56 - 0.98w)
(0.0051)w² + 0.98w - 45.86 = 0 __eqn3
This can be considered as a quadratic equation. (ax² + bx + c = 0), where value of x can be,-b∓ √(b² - 4ac)/2a.
Comparing with eqn3 ;
w = -b∓ √(b² - 4ac)/2a
where, a = 0.0051
b = 0.98
c = -45.86
Substituting these values:
w = -0.98+√(0.98² - 4*0.0051*(-45.86))/2*0.0051= (-0.98+1.3769)/0.0102 = 38.911 rad/sec
w = -0.98-√(0.98² - 4*0.0051*(-45.86))/2*0.0051= (-0.98-1.3769)/0.0102 = -231.068 rad/sec
Considering the positive value, steady state speed when duty cycle is 70% will be 38.911 rad/sec.
3. Develop a mathematical model of a DC Motor for the below equation using Simulink.
w= v/kɸ - Ra/kɸ² . T
Answer:
To develop the model, we are using the below values and assuming the torque value as 50 Nm.
Armature Resistance, Ra = 0.5 ohm
Armature Voltage, V = 72 V
kɸ = 0.7
Torque, T = 50 Nm.
Using these values, we developed the MATLAB model for the equation, using constant block, add/multiply/divide blocks .
The ouput obtained, angular speed = 5092 rad/sec.
4. Refer to the blog on below topic:
Induction Versus DC brushless motors by Wally Rippel, Tesla
Explain in brief about author’s perspective.
Answer:
The author in the blog compares the 2 types of drives; Brushless DC motors and Induction motors. Starting with Brushless DC motor, there is stator and rotor, and rotor will have permanent magnets which produces the DC magnetic field. Stator core is made of thin, stacked laminations. When the magnetic field enters the stator core, it will come in contact with the current flowing through the windings.This produces torque and since the rotor rotates, the magnitude and polarity of statir current will be varied continuously, which makes the torque constant. The inverter in the brushless dc motors are responsible for the current control, whih makes the motors more efficient.
3-phase induction motor, also has a similar structure for the stator compared to that of the brushless dc motors. Both have distributed windings on the stator core. Unlike rotor of brushless dc motor, induction motor doesn't have permanent magnets on the rotor core. In the rotor of the induction motor, there will be steel laminations with peripheral conductors.This structure is named as shorted structure. When the current flow through the stator winding, a rotating magnetic field is produced, which enters the rotor structure and induces a voltage across the peripheral conductors on laminations of rotor. The frequency of the magnetic field will be the difference between actal applied frequency and rotational frequency of rotor. As a result of the induced voltage in the rotor, there will be current produced rotor conductors proprtional to voltage, which creates the speed difference. When the currents interact with the oroginal magnetic field, the desired rotor torqe is produced.
Comparison:
- 3phase induction motors can start the motor by itself when connected to a 3 phase supply and np inverter is needed. However, brushless DC motors produces no starting torque when connected to 3 pahse supply and need an inverter to get the desired DC input.
- Brushless DC motors can work on DC supply and AC supply with help of inverter, whereas induction motors always need AC supply.
- When inverter and feedback loops are added, it is possible to power an induction machine from DC supply and changes in frequency can help to change the speed.
- Both brushless DC and induction motors uses modulating inverters and have similar stator structure.
- When it comes to digital controlers on both, Brushless DC requires position sensor, whereas induction motor requires speed sensor.
- Brushless DC motors have lesser rotor heating and rotor cooling is much easier.
- Brushless DC motors can work at unity power factor, however, the best power factor for induction motor is 85%, thus brushless DC is much efficient than induction motor.
- For a brushless DC motor, presence of permanent magnet induces eddy current, hysterisis losses, since the magnetic field strength cannot be easily changed with permanent magnets, whereas in induction motor, there are no permanent magnets and hence magnetic field strength can be adjusted, thery by reducing losses and achieves maximum efficiency.
- Induction motors are much cost effective than brushless DC motors. Permanent magnets in the brushless DC motors are much costly.
- Induction motors are difficult to control compared to brushless DC, it is difficult to achieve the desired torque-speed range over temperature in induction motor.
The author concludes that for pure Electric vehicle with high performance, induction machines would be the best option whereas for Hybrid EV and plug-in Hybrid vehicle, the effective one woule be brushless DC motor.