Week-6 Challenge: EV Drivetrain
1.Types of power converter circuits are employed in electric and hybrid electric vehicle.
- Types of converters used in Electric vehicle.
- The non-isolated converters type is generally used where the voltage needs to be stepped up or down by a relatively small ratio.
- There are five main types of converter in this non-isolated group, usually called the buck, boost, buck-boost, Cuk and charge-pump converters.
- The buck converter is used for voltage step-down, while the boost converter is used for voltage step-up. The buck-boost and Cuk converters can be used for either step-down or step-up. The charge-pump converter is used for either voltage step-up or voltage inversion, but only in relatively low power applications.
- Usually, in this type of converters a high frequency transformer is used. In the applications where the output needs to be completely isolated from the input, an isolated converter is necessary.
- There are many types of converters in this group such as Half-Bridge, Full-Bridge, Fly-back, Forward and Push-Pull DC/DC converters All of these converters can be used as bi-directional converters and the ratio of stepping down or stepping up the voltage is high.
- In case of interfacing the Fuel Cell, the DC/DC converter is used to boost the Fuel Cell voltage and to regulate the DC-link voltage. However, a reversible DC/DC converter is needed to interface the SCs module. A wide variety of DC-DC converters topologies, including structures with direct energy conversion, structures with intermediate storage components (with or without transformer coupling), have been published.
- Boost Converter( Rectifier): To Charge the battery of vehicle by converting AC supply to DC
- Bridge rectifier is used to convert AC supply to output DC.
There are Two modes of charging : 1. Voltage Mode
2. Current mode
Voltage Mode: The voltage is increased to the desired output in the voltage control mode to charge the battery.
current control mode: set the energy to the level you want to power your On Board Charger. There is a PFC stage rectifier on one side and an active CLL rectifier on the other side. It is a double rectifier coupled that boosts up the circuits in the automobiles system
There is an isolating transformer that connects the two rectifiers and passes 800 V.
- During this rectification Process, Losses will be more due to diode operations.
- Power converters are used in such a way that to reduce losses and optimize efficiency.
2. Finding EV steady state speed if duty cycle(d) is 0.7? for following parameters.
Battery Voltage(`V_b`)=72v
Motor Armature Resistance(`R_a`)= 0.5`Omega`
motor Constant(`kphi`)=0.7=k
Vehicle speed-Torque equation(`tau_v`)=24.7+0.0051`omega^2`
Motor speed-Torque equation(`tau_m`)=`(kV_o)/R_a-k^2/R_aomega`--------------1
At 70% duty cycle (`V_o`)=`V_b*d`=72*0.7
`V_o=54.2`
By substituting all values in equation----1, we get as
`tau_m=(0.7*54.2)/0.5-0.7^2/0.5omega`
`tau_m=70.56-0.98omega`
At Steady State; `tau_m=tau_v`
By this :
`24.7+0.0051omega^2=70.56-0.98omega`
We get an quadratic equation:
`0.0051omega^2+0.98omega-45.86=0`
By solving above quadratic equation we get real roots
`omega=-231.07 & omega=38.91`
Negative value is not valid
Hence
Steady State Speed of EV at 70% duty cycle is
`omega=38.91 (rad)/sec`
3.Mathematical model of a DC Motor for the below equation using Simulink.
`omega=V/(k$)-R_a/(k$)^2tau`
`omega=`Angular speed of shaft.
`V=`Supply Voltage
`R_a=`Armature resistance
`k$=`Motor constant
`tau=`load torque
Simulink Model
4. Author Wally Rippel describes the advantages and disadvantages of BLDC and Induction motors. And tries to predict the which motor will be the best solution for EV's and HEV's.
- Here some of the points discussed on it.
| Brushless DC Motor | Induction motors |
|
1.Inverter is needed to Rotate the rotor with constant torque
2. High torque can be generated by varying supply voltage 3.Less heat is generated and easily cooled. 4. As motor size increases both Copper and iron losses also increases and due to this efficiency will decrease. 5.Unity PF can be achieved for small Range EV's. 6. Field strength( Flux) is not adustable as Permanent Magnets were used. 7. High Cost due to Permanent magnets are used. |
1. Inverter is needed to achieve variable frequency and to get supply from Battery.
2. Torque is limited to an extent due to heat produced by electro magnetic field in rotor. 3.Difficult to control Temperature and and Speed. 4. Losses can be controlled and attain maximum efficiency. 5. Maximum PF for an Induction motor is 0.85. 6.Field strength(Flux) is adjustable by varying voltage levels. 7. Low Cost compare to DC machines. |
- By this we cannot come to an conclusion which is best for EV's. Because at last end consumer requirement will be High Performance, Low cost and Maintainence.
- Both will be in use Side by side as there is no much difference between BLDC and Induction motors due to following factors.
- Both have Similar Stator construction.
- Both Requires Inverters.
- Both have Sensors For BLDC: Hall effect sensor Induction Motor: Speed Sensors
Conclusion: In future Era Both motors will play a key role in Development of EV's.