EV Drivetrain
Aim:-To get to know the types of converters used in HEV and EV vehicles.
-To find the speed using torque-speed relation.
- To get to know about induction and DC brushless motors.
Q1:-Which types of power converter circuits are employed in an electric and hybrid electric vehicle?
Ans:-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 an inverter and this inverter is fed by the DC-DC converter
-The power converter is made of high-power fast-acting semiconductors devices, such as a bipolar junction transistor. There are some other transistors such as MOSFET, IGBT, SCR, GTO, MCT. These devices are used as an on-off electronics switch to convert the fixed supply voltage into a variable voltage and variable frequency supply.
-All these devices have a control input gate or base through which the devices are turned on and off as per the command generated by the controller.
DC-DC converter:-
-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 equipment. 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. The power flow in a bidirectional converter is usually from a low voltage end such as a battery or a supercapacitor to a high voltage side and is referred to as boost operation.
-A DC/DC converter in its basic form converts unregulated DC input voltage at a certain level to regulated DC output voltage at a different level with very high conversion efficiency. These are operated at a very high frequency. The switches are turned ON and OFF by pulse width modulation(PWM).
The converter topologies are classified as:
- Buck Converter: In Figure a, a buck converter is shown. The buck converter is step down converter and produces a lower average output voltage than the dc input voltage.
- Boost converter: In Figure b, a boost converter is shown. The output voltage is always greater than the input voltage.
- Buck-Boost converter: In Figure c, a buck-boost converter is shown. The output voltage can be either higher or lower than the input voltage.
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.
-The use of Inverters can increase or decrease the power or torque of the motor by adjusting the amplitude of the signal.
-It plays a significant role in capturing energy from regenerative braking and feeding it back to the battery.
-The key component is that it has a direct impact on on-road performance, driving range and reliability of the vehicle also as a consequence of their weight and size.
Q2. 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 are 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 the duty cycle is 70%?
ANS:- As the torque formula of motor is given, we need to find the speed in rad/sec. Use the other expression available which is given below:-
T = torque
V = armature voltage
R = resistance
w = speed rad/sec
Compare the two equations:-
24.7 + 0.0051*w^2 = ((v*k)/R) - ((k^2)*w/R)
24.7 + 0.0051*w^2 = (72*0.7*0.7)/0.5 - (0.7^2/0.5)*w = 24.7 + 0.0051*w^2
70.56 - 0.98*w = 24.7 + 0.0051*W^2
w^2 + 192.15*w - 8992.15 = 0
In order to find the roots use the following formula
As the speed value can not be negative
w = 39 rad/sec
Q3. Explain in brief the author’s perspective for induction vs DC brushless motor.
Ans:- In This article, The author has mentioned about positives and negative sides of both the induction and DC motor.
-In the case of a DC brushless motor, permanent magnets are used which produces a rotating magnetic field. This magnetic field when induces a current in the conductor. This current results in force generation in the rotor and this results in rotation of the rotor.
- Whereas in the case of induction motor, the current is supplied to the stator which results in the generation of emf, when this emf comes in contact with the rotor which is generally a squirrel cage type, this results in current induced in the rotor. Unlike the DC motor, the rotor consists of conductors plates and placed in a cage-like structure. This also consists of bars around them in which currently get induced and the rotor starts rotating.
- One of the main differences is that much less rotor heat is generated with the DC brushless drive. Rotor cooling is easier and peak point efficiency is generally higher for this drive. It is mentioned that the DC brushless can drive at unit power factor, whereas the best power factor for the induction drive is about 85%.
- In the case of a DC brushless motor, the strength of the magnetic field is adjustable. When at low speed the maximum torque is required magnetic field should be maximum, i.e. inverter and motor current maintained at their lowest optimized efficiency. Whereas there is no such adjustment in the case of induction motor.
- As machine size grows, magnetic losses increase proportionally and part load efficiency drops. In the case of induction motor, as machine size grows, losses do not necessarily grow.
- The construction of a DC brushless motor is costly compared with an induction motor, due to the use of permanent magnets.
- However though the speed variation of the induction motor depends upon the frequency input, it is difficult to control.