Electric Vehicle Drivetrain

AIM : To study and explore about theory behind Electric Vehicle Drivetrain.

OBJECTIVES :

• To study about types of power converter circuits which are employed in electric and hybrid electric vehicle.
• To calculate the EV steady state speed based on motor and controller parameters.
• To refer the blog on Induction Versus DC Brushless motors by Wally Rippel, Tesla and explain the author’s perspective.

CHALLENGES :

1. Which types of power converter circuits are employed in electric and hybrid electric vehicle?

Before we see the application of power converter circuits in electric and hybrid electric vehicles, we have to understand which type of current is used for operation of different electronic components in electric and hybrid electric vehicles.

Power Source/ Battery – This is one of the most important part. A battery always supply energy in DC form. Additional power source like Solar cell also providing energy is of DC form.

Headlights/ Indicator lights/ Interior lights – These components consumes DC supply. So they are directly connected to DC supply (battery) though step-down circuitry.

Electric Power Steering (EPS) – In EPS Brushless DC (BLDC) motors are used which can be supplied by DC supply.

Motor ­– This is the most important part of EV or HEV. In Electric Vehicle AC or DC, any kind of motor can be used. Both are having some advantages and some disadvantages.

Types of Power Converter Circuits in EV and HEV:

AC-DC Converter or Rectifier –

Plug in Hybrid Electric Vehicles or Plug in Electric Vehicles need to charge the battery from a charging station by plugging the charger to battery charging port. The current from commercial power system is AC current and vehicle on-board high voltage battery requires DC current. So AC-DC converter or rectifier is used in PHEV or EV.

DC-AC Converter or Inverter –

The main source of electrical power is the battery which is a DC source. Some EV or HEV components need to convert the battery DC current to AC current for their working. So the DC output of the battery is bucked or boosted according to the requirement and then converted into AC using DC-AC inverter. The function of an inverter is to change a DC input voltage to asymmetric AC output voltage of desired magnitude and frequency. Inverter is used by devices such as the traction motors and coolant pump.

DC-DC Converter or Chopper –

The batteries of an electric vehicle typically output several hundred volts of DC. The electric components inside the vehicle, however, vary in their voltage requirements, with most running on a much lower voltage. This includes the radio, dashboard readouts, air conditioning, and in-built computers and displays. In electric vehicles which use a DC motor, actually running the motor can use up to three times the voltage provided by the battery. That is why we can find DC-DC converter or chopper in EV or HEV. The main dc-dc converter changes dc power from an on-board 200-800V (HV) battery into lower dc voltages, like 48V or 12V, to power headlights, interior lights, wiper and window motors, fans, pumps, and many other systems. In some applications like an HEV, the dc-dc can be run in reverse to boost the power from a 12V domain to a 48V or high voltage domain, to assist in starting or to provide backup power.

2. 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

T_TR = 24.7+0.0051`omega`²

What is EV steady state speed if duty cycle is 70%?

SOLUTION :

Given –

Rated Armature voltage = 72 V

Rated armature current = 400 A

Armature resistance (R_a) = 0.5 Ω

Motor Constant (kΦ) = 0.7 V-s

Chopper Switching Frequency = 400 Hz

T_TR = 24.7+0.0051`omega^2`

Now, for 70% duty cycle,

Voltage = 72*70/100

= 5040/100

\ Voltage (V) = 50.4 volts

Now, according to the torque speed characteristics of the DC machine, the Torque equation is given as,

`T=V.(kphi)/R_a -omega.(kphi)^2/R_a`

Where,

T or T_TR = Torque;

V = Voltage;

`omega` = Motor Speed;

R_a = Armature Resistance;

kΦ = Motor Constant.

Substituting the known values in the above equation,

`T=50.4×0.7/0.5-omega×(0.7)^2/0.5`

`therefore` T or T_TR = 70.56 – 0.98`omega`

We also have, T_TR = 24.7+0.0051`omega^2`

Equating both the torque equations,

70.56 – 0.98`omega` = 24.7+0.0051`omega^2`

On solving this equation, we get,

`omega` = 38.85 rad/s or `omega` = -230.92 rad/s

As negative speed is impractical or unrealistic, we have `omega` = 38.85 rad/s.

3. Refer to the blog on below topic:

Induction Versus DC brushless motors by Wally Rippel, Tesla

Explain in brief about author’s perspective.

The author discusses about the application, working and difference between DC brushless motors and Induction motors. While comparing them, the author basically conveys that there is no hard and fast rule that one is better than another. The author explains the pros and cons and comes to a conclusion that both of them have their own uniqueness and depending on the requirement, dynamic operation, task and availability, the preferred motor can be used.

There are some points and differences which the author wants to convey,

• BLDC motors can perform well with AC current supply which is why they need inverter circuit or they are useless.
• The essential difference between the two machines is with the rotor wherein the DC brushless motors have magnet and induction motor has no magnets.
• When a 3-phase induction motor is connected to utility type 3-phase power, torque is produced at the outset and the motor has the ability to start under load. So, no inverter is needed under as in BLDC motors.
• Talking about the limitations of both the motors, the author says that a brushless DC motor produces no starting torque when directly connected to fixed frequency utility power whereas AC induction motor when operated from utility power have limited starting torque and somewhat limited running peak torque capabilities.
• DC brushless drives require an absolute position sensor, while induction drives require only a speed sensor; these control code differences are relatively small.
• The DC brushless drive can also operate at unity power factor, whereas the best power factor for the induction drive is about 85 percent. This means that the peak point energy efficiency for a DC brushless drive will typically be a few percentage points higher than for an induction drive.
• The magnetic field strength (B) in BLDC should be adjusted such that the sum of the eddy, hysteresis, and I²losses is minimized whereas the induction motors have no magnets and B fields are adjustable since B is proportionate to V/f (voltage to frequency).
• Induction drives may be the favoured approach where high-performance is desired; peak efficiency will be a little less than with DC brushless, but average efficiency may actually be better.
• The author states that achieving stability over the entire torque-speed range and over temperature is more difficult with induction than with DC brushless.

Finally the author concludes 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. Owing to this, due to the common hardware between them, we can see induction and DC brushless live and work side by side in the near future.