Week-6 Challenge: EV Drivetrain
1.Which types of power converter circuits are employed in electric and hybrid electric vehicle?
Sol:
Electronic circuits typically require dc voltage sources in order to function. Ac voltages
are used for bulk power generation, transmission, and local distribution. Thus, an ac-dc
converter is used to interface the ac grid to the dc battery.
Dc-dc converters are power electronics circuits which convert a dc voltage to a different dc voltage level. For example, the voltage of a fuel cell can drop significantly with
increased power demand, and a dc-dc converter is required to step up the fuel cell voltage
to a higher level for a more efficient powertrain.
A device that increases or decreases the voltage (AC or DC) of a power source depending on application. A converter that increases voltage is called a step-up converter and a converter that decreases voltage is called a step-down converter. In EVs/HEVs step-up and step-down converters are combined into one unit. An application of a step-up converter is converting EV/HEV battery voltage to about 650 volts to power the traction motor. An advantage of using a converter to increase voltage from the battery is a smaller and less expensive battery may be used while still utilizing an efficient high voltage motor. An application of a step down inverter would be decreasing the high voltage direct current from the HEV/EV battery to low voltage (12-14 volts) DC that can be used to charge the 12 volt auxiliary battery and operate light load devices such as lighting, radio, and windows.
Voltage source inverters (VSIs) are used in hybrid vehicles to control the electric motors
and generators. Figure shows the power electronic circuit arrangement of a VSI
to control the induction motor, PM synchronous motors, and PM brushless motors. The
switches are usually IGBTs for high voltage, high-power hybrid configurations or MOSFETs for low voltage designs.
The output of the VSI is controlled through the means of pulse width-modulated (PWM)
signals to produce sinusoidal waveforms. Certain harmonics exists in such a switching
scheme. High switching frequency is used to move away the harmonics from the fundamental frequency
DC – DC Converters
The bidirectional DC–DC converter in a HEV is also sometimes called an energy management converter, or boost DC–DC converter. This DC–DC converter is a high-power
converter that links the HV battery at a lower voltage with the HV DC bus. The typical
voltage of a battery pack is designed at 300–400 V. The best operating voltage for a
motor and inverter is around 600 V. Therefore, this converter can be used to match the
voltages of the battery system and the motor system. Other functions of this DC–DC
converter include optimizing the operation of the powertrain system, reducing ripple current in the battery, and maintaining DC link voltage, hence the high-power operation of
the powertrain.
- 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%?
Sol:
By solving the above quadratic eqn we get the roots
Taking the positive value, the steady state speed = 59.26 rad/s.
Induction Versus DC brushless motors by Wally Rippel, Tesla:
The wally ripple explains that there is nothing best in engines as well as motors , it all depends on our convenience and affordability. At his time there were lead acid batteries , dc motors, later they have evolved to lithium batteries and dc brushless motors. In the same way the current trend dc brushless motor and induction motor will evolve into something, which will be more efficient than these two with improved performance and overcome the limitations these device have.
Both dc brushless motor and induction motor has its own limitations and advantages. DC brushless motor has the permanent magnet where as the induction motor has just stacked steel laminations with buried peripheral conductors. Both have three sets of “distributed windings” that are inserted within the stator core. Dc brushless motor has the permanent magnets that generated magnetic field, this magnetic field enters the stator core and interacts with currents flowing within the windings to produce a torque interaction between the rotor and stator. The induction motor has the windings in the stator when current flows through it magnetic field is generated that interacts with the rotor windings . the magnetic field frequency is equal to the rotational frequency and applied electrical frequency. Both use the inverters but in difference with their controllers. Dc brushless has the peak power efficiency of unity whereas the induction motor has the 85%. The dc brushless motor has no control over the magnetic field strength in low speeds where high torque requirement is there . where as the in induction motor magnetic field strength can be controlled according to the torque requirements. DC brushless motors will occupy the hybrid and plug in hybrid vehicles as induction motors will be in electric vehicle for a while until their evolution.