Week-6 Challenge: EV Drivetrain
Q-1) Which types of power converter circuits are employed in electric and hybrid electric vehicle?
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
Overview of 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. 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.
During regenerative braking, the power flows back to the low voltage bus to recharge the battery (buck mode). As a backup power system, the bidirectional DC-DC converter facilitates the safe operation of the vehicle when ICEs or electric drives fail to drive the motor. Due to the aforementioned reasons, high power bidirectional DC-DC converters have gained a lot of importance in the recent past.
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.
Figure a: General Configuration Buck Converter
Figure b:General Configuration Boost Converter
Figure c: General Configuration Buck-Boost Converter
The amount of power flow between the input and the output can be controlled by adjusting the duty cycle (ratio of on/off time of the switch )
Usually, this is done to control the output voltage, the input current, the output current, or to maintain a constant power
.
Why is a DC/DC converter necessary?
While DC/DC converters can be based on a number of different designs, the underlying function remains the same. We might be talking about a step-up converter, which turns a low-voltage input into a high-voltage output, or a step-down converter, which does the opposite.
In electric vehicles which use a DC motor, running the motor can use up to three times the voltage provided by the battery. With the help of the right converter, we can bridge this gap without having to possibility to a larger, prohibitively heavy battery.
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.
Why DC/AC is necessary?
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.
Fig. 3rd generation inverter; Toyota
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.
The output voltage waveform of ideal inverters should be sinusoidal. However, the waveform of practical inverters are non-sinusoidal and contain certain harmonics.
Q-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%?
Q-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 is a long-time proponent of electric vehicles. He expresses his view on Motor used in Electric vehicle i.e Induction Versus DC Brushless Motors.
Working Principal of DC Brushless motor is includes two or more permanent magnets that generate a DC magnetic field. In turn, this magnetic field enters the stator core (a core made up of thin, stacked laminations) and interacts with currents flowing within the windings to produce a torque interaction between the rotor and stator. As the rotor rotates, it is necessary that the magnitude and polarity of the stator currents be continuously varied – and in just the right way - such that the torque remains constant and the conversion of electrical to mechanical energy is optimally efficient. The device that provides this current control is called an inverter. Without it, brushless motors are useless motors.
PROS of DC Brushless motor -
- In DC Brushless motor the strength of the magnetic field produced by the permanent magnets would be adjustable. When maximum torque is required, especially at low speeds, the magnetic field strength (B) should be maximum – so that inverter and motor currents are maintained at their lowest possible values.
- DC Brushless motor are relatively easier to control than Induction machines.
- Achieving stability over the entire torque-speed range and over temperature is more difficult with induction than with DC brushless
CONS of DC Brushless motor -
- A brushless DC motor produces no starting torque when directly connected to fixed frequency utility power. They really need the aid of an inverter whose “phase” is maintained in step with the angular position of the rotor.
- A brushless DC motor required current controler or Inverter.
- With DC brushless, as machine size grows, the magnetic losses increase proportionately and part load efficiency drops.
Working Principal of Induction Versus is that, the induction rotor has no magnets – just stacked steel laminations with buried peripheral conductors that form a “shorted structure.” Currents flowing in the stator windings produce a rotating magnetic field that enters the rotor. In turn, the frequency of this magnetic field as “seen” by the rotor is equal to the difference between the applied electrical frequency and the rotational “frequency” of the rotor itself. Accordingly, an induced voltage exists across the shorted structure that is proportionate to this speed difference between the rotor and electrical frequency. In response to this voltage, currents are produced within the rotor conductors that are approximately proportionate to the voltage, hence the speed difference. Finally, these currents interact with the original magnetic field to produce forces – a component of which is the desired rotor torque.
When a 3-phase induction motor is connected to utility type 3-phase power, torque is produced at the outset; the motor has the ability to start under load. No inverter is needed.
PROS of Induction motor -
- Induction motors are directly compatible with conventional utility power
- No inverter is needed
- induction machines have no magnets and B fields are “adjustable,” since B is proportionate to V/f (voltage to frequency). This means that at light loads the inverter can reduce voltage such that magnetic losses are reduced and efficiency is maximized.
- In Induction motor, magnetic and conduction losses can be traded so that efficiency can be optimized.
- With induction, as machine size grows, losses do not necessarily grow
CONS of Induction motor -
- Induction motor cannot operate from DC, AC is a must
- Limited running peak torque capabilities, when compared to DC type machine
- Induction machines are more difficult to control
Finally he conclude that DC brushless drives will likely continue to dominate in the hybrid and coming plug-in hybrid markets, and that induction drives will likely maintain dominance for the high-performance pure electrics.
Q-4) Develop a mathematical model of a DC Motor for the below equation using Simulink.
ω= V/Kϕϕ -Ra/Kϕϕ^2 .T
REFERENCE :