Week-6 Challenge: EV Drivetrain

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

Solution:

There are 4 main types of power converter circuits are used in EV and HEV, all these circuits are called as power electronics circuits, they comprise of controlled diodes or high frequency switching devices, these devices can be power BJT, MOSFET, SCR or thyristor etc. Following are types:

1. AC – DC Converter (Rectifier)
2. DC – AC Converter (Inverter)
3. DC – DC Converter (Chopper)
4. AC-AC Converter

Let us discuss and understand each one of the above in brief

1. AC – DC Converter (Rectifier)

As the name suggests, this converter Alternating Current (AC) to Direct Current (DC). As we all know, diodes conduct only in one direction, the circuit is arranged in such a way that for the positive half cycle in put of AC, one diode conducts and for the negative half cycle of input AC< another diode, connected in opposite direction will conduct and give continuous positive waveform at the output. Following is generalised circuit arrangement of a full wave rectifier.

2. DC – AC Converter (Inverter)

This circuit converts DC input to alternating or pulsating output, and to do so, again, these circuits make use of controlled diodes. Working principle of this circuit is almost similar to that of rectifier circuit, major difference is input to this circuit is DC, diodes are arranged in such a manner that, for a given duty cycle, two diodes, on the opposite legs conduct simultaneously and other two diodes will conduct for the other half cycle. Result of this operation is, we can get pulsating output at the output, this output is not ideal to be fed to a load so it requires further tuning and modulation.

3. DC – DC Converter (Chopper)

This type of converter is similar to a transformer in that it can change the voltage from its current level to another level. It can take an input supply of voltage and increase it or do the exact opposite and decrease it similar to a step up or step-down transformer. This kind of a converter is called a chopper. The purpose of using this type of converter is to provide the necessary level of power to a particular component and for the safety of the appliance and the user.

It works by taking the DC supply and passing it through a switching element which converts the waveform to a square wave which is actually AC in nature. Then this AC is passed through a filter which will turn it back in to a DC supply for use in a load. In EVs, HEVs and conventional vehicles, this type of converter is used for various purposes. E.g. In the case of stepping down the voltage, the car radio may require a stepped down voltage of 12V to operate and would not need the full voltage of the car battery. In the case of stepping up the voltage, the motor of a car may require 500V to operate, but the vehicle may need more voltage than the battery pack can provide.

Following schematic shows 4 main types of DC to DC converters

4. AC to AC converters

Thyristor based single phase to single phase cycloconverter can be considered as a Simple AC to AC converter circuit diagram. The frequency of AC power can be changed using cycloconverters and these cycloconverters can also be used to change AC power magnitude in specific applications. Typically, AC to AC converter consists of DC link which increases the cost of system, reduces the efficiency in the conversion process. So, AC to AC converter without DC links such as cycloconverters are preferred for converting AC to AC power.

The working of simple AC to AC converter can be easily understood with below figure. The frequency of the input signal can be varied by varying the triggering pulse to the thyristors which are connected to form an AC to AC converter as shown in the figure below. The frequency of the signal can be stepped up or stepped down depending on the requirement using cycloconverters.

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%?

Let us find the speed in rad/sec. Use the other expression available which is given below:

T = [ (V.k) / R ]- [(K^2.w) / R]

Where,

T = torque N.m

V = armature voltage V

R = resistance Ohms

w = speed rad/sec

Comparing the given equation with above equation,

24.7 + 0.0051*w^2 = [(v*k)/R] – [(k^2)*w/R]

24.7 + 0.0051*w^2 = [(72*0.7*0.7)/0.5] – [(0.7^2/0.5)*w]

70.56 - 0.98*w = 24.7 + 0.0051*W^2

w^2 + 192.15*w - 8992.15 = 0

Finding the roots to solve above equation,

w = 38.95 rad/sec

The steady state speed at 70% duty cycle shall be 38.95 rad/sec

3. Develop a mathematical model of a DC Motor for the below equation using Simulink.

ω= V/Kϕ -Ra/Kϕ^2 .T

4. Refer to the blog on below topic:

Induction Versus DC brushless motors by Wally Rippel, Tesla

Explain in brief about author’s perspective.

In the blog of induction and DC brushless motor, the author has mentioned about positives and negative side of both the induction and DC motor. In the blog, the basic construction of both the induction and DC brushless motor is explained.

In case of DC brushless motor, permanent magnets are used which produces a rotating magnetic field. This magnetic field when induces a current in the conductor. This current results in force generation in the rotor and this results in rotation of the rotor.

Whereas in case of induction motor, the current is supplied to the stator which results in the generation of emf, when this emf comes in contact with the rotor which is generally a squirrel cage type, this results in current induced in the rotor. Unlike the DC motor, the rotor consists of conductors plates and placed in a cage-like structure. This also consists of bars around them in which current get induced and the rotor starts rotating.

One of the main differences is that much less rotor heat is generated with the DC brushless drive. Rotor cooling is easier and peak point efficiency is generally higher for this drive. It is mentioned that the DC brushless can drive at unit power factor, whereas the best power factor for the induction drive is about 85%.

In case of DC brushless motor, the strength of the magnetic field is adjustable. When at low speed the maximum torque is required, magnetic field should be maximum, i.e. inverter and motor current maintained at their lowest optimize efficiency. Whereas there is no such adjustment in case of induction motor.

As machine size grows, magnetic losses increases proportionally and part load efficiency drops. In case of induction motor, as machine size grows, losses do not necessarily grow.

The construction of DC brushless motor is costly compared with an induction motor, due to the use of permanent magnets.

However thought the speed variation of induction motor depends upon the frequency input, it is difficult to control.

Conclusion:

1. We have discussed in brief about various types of converters used in EVs along with their schematics and working.
2. We have calculated the speed of EV at the given parameters with the help of DC motor equation.
3. We have modelled the DC motor equation in Simulink and observed the corresponding output plot.
4. The take away points from the article are mentioned.