Week-6 Challenge: EV Drivetrain

Aim:

• To study types of power converter circuits are employed in electric and hybrid electric vehicle.
• Calculate steady state of an Electric Vehicle's Drivetrain and study about dc brushless and induction motor.

Objective:

Understanding the power converters in power electronics which are used in EV and hybrid vehicle to control the voltage and frequency. We calculate EV steady state speed, if duty cycle is 70% of and ev drivetrain. The Wally Rippel's view on both the dc brushless and induction motor uses and advantages.

Types of Power Converter Circuits are Employed in Electric and Hybrid Electric Vehicle:

To process and control the flow of electric energy by supplying voltages and currents in a form that is optimally suited for user loads is known as power electronics. The components used to perform such tasks is known as power convertor. They feed only that much is required to perform the task, which helps to save energy losses in terms of heat loss. It is based on switching technique.

There are mainly four types of power convertor circuits-

AC to DC:

The EV charger comes in this category. It has rectifier which takes AC supply and gives DC output. Rectifier circuits may be single phase or multi-phase. Most low power rectifiers for domestic equipment are single-phase, but three-phase rectification is very important for industrial applications. Single phase has 2 types half-wave rectification and full-wave rectifier

DC to AC:

This circuit converts the dc into ac with the inverters which gives variable and fixed voltages and frequency output. In this the power is produced by the dc source not by invertor. This power invertor can be completely electronic or may be combination of mechanical effects and electronics. It is used where high current and voltages are present whereas the present circuit in it uses very low voltage signals.

DC to DC:

The chopper is used to perform this task.

They are of two types step up and step down which depends on the requirement of voltage up or down. As we use in our car axillaries.

step down step up

AC to AC: They not used in EV's and they gives variable frequencies output and voltage at variable inputs. They are used in fan regulators.

An Electric Vehicle's Drivetrain with 72V battery pack

The duty ratio for acceleration operation the duty ratio is 'd1'

for braking operation, the duty ratio is 'd2'.

The other parameters of the electric vehicle is given below,

Motor and Controller Parameters:

Rated Armature voltage

`V_a= 72 V`

Voltage after 70% duty ratio = 72*0.7 = 50.4

Rated armature current

`I_r= 400 A`

Ra= 0.5Ω,

KΦ= 0.7 V-s

Chopper Switching frequency

`f = 400 Hz`

The vehicle characteristics are given by the equation

`T TR = 24.7+0.0051*w^2`---------(1)

The derived formula for motor speed characteristics-

`T = V *(Kphi) / R_a - (Kphi)^2 / R_a * w`

so after putting the above values

`T = (50.4 * 0.7)/.5 - (omega*0.7^2)/0.5`

`T = 70.56 - 0.98 * omega`--------(2)

Now after comparing both the motor characteristics equations

`0.0051*w^2 + 0.98* w - 45.89 = 0`

using quadratic equation formula we get 2 values of `omega`

`omega=`-231.07 rad/s and 38.92 rad/s

Here EV steady state speed, if duty cycle is 70% then -ve value of speed shows that the torque is transferred to the motor externally where the +ve value states that the motor generated torque reaches to the wheels.

Author’s Perspective:

As we understand the engines from their types based on their performance and price similarly, the EV drives also has different types of motor and drive train according to utility. So, it is difficult to predict exact future of current technology. The author's perspective is that between the dc brushless motor and induction motor there is no clear winner as they both have their individual capabilities and one size does not fit all.

According to him we should talk about utility of these motors rather than best one of them. DC brushless and Induction motors have so much of the hardware common like similar stators, both drives use 3-phase modulating inverters. The only differences are the rotors and the inverter controls and in the inverter these differences are relatively small. After comparing both he finally conclude with that the plug in and Hybrid vehicle market will be dominated by DC brushless and the induction motor will drive the high-performance pure electrics so they both will have their major significance in this coming golden era of hybrid and electric vehicles . Which seem almost true.

Now here we could talk about some advantages over each other to understand the differences by Wally Rippel's side.

Advantages of DC brushless over Induction Motor:

• Much less rotor heat is generated with the DC brushless drive.
• Brushless DC motor produces no starting torque when directly connected to fixed frequency utility power. They really need the aid of an inverter
• Rotor cooling is easier and peak point efficiency is generally higher for this drive.
• The DC brushless drive can also operate at unity power factor, whereas the best power factor for the induction drive is about 85 percent.
• The peak point energy efficiency for a DC brushless drive will typically be a few percentage points higher than for an induction drive.
• In an ideal brushless drive, the strength of the magnetic field produced by the permanent magnets would be adjustable. Unfortunately, there is no easy way of changing magnetic field with permanent magnets.
• Peak efficiency will be a little higher than with induction motor.
• Induction machines are more difficult to control. as the control laws are more complex and difficult to understand.

Advantages of Induction over DC brushless Motor:

• Induction motors are directly compatible with conventional utility power.
• They cannot operate from DC; AC is a must.
• When operated from utility power, they have limited starting torque and somewhat limited running peak torque capabilities, when compared to DC type machines.
• Torque performance is low compared with DC machines.
• Induction machines have no magnets and magnetic fields are adjustable.
• This means that at light loads the inverter can reduce voltage such that magnetic losses are reduced and efficiency is maximized.
• Thus, the induction machine when operated with a smart inverter has an advantage over a DC brushless machine.
• Magnetic and conduction losses can be traded such that efficiency is optimized. This advantage becomes increasingly important as performance is increased.
• DC brushless, as machine size grows, the magnetic losses increase proportionately and part load efficiency drops.
• With induction, as machine size grows, losses do not necessarily grow. Thus, induction drives may be the favored approach where high-performance is desired.
• Average efficiency may actually be better.
• Permanent magnets are expensive and permanent magnet (PM) rotors are also difficult to handle due to very large forces.
• Induction motors will likely retain a cost advantage over PM machines. Due to the field weakening capabilities of induction machines, inverter ratings and costs appear to be lower

Conclusion:

Hence, we have studied about various types of power convertor circuits in power electronics and their applications. We also calculated the steady state speed for an electric drive train and discuss about the Wally Rippel's view on Induction and DC brushless Motor along with their advantages over each other.