Week-6 Challenge: EV Drivetrain

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

Solution:

Power converter: From below image power converter can be explained as main link between electrical energy source and electrical load. To enhance life and reliability of load it is necessary to give the power as per their requirements and these requirements gets fulfiled with the help of power converters.

• Control circuit helps for controlling the logic set by user to run any electric load.
• Power circuits includes SCR's, IGBT, MOSFET etc such kind of electronic components whose switching frequency is much more faster than normal transistion. These switching frequency preserves the load as well as power is not wasted.

There are main 4 types of Power converters are there based on conversion from AC and DC voltage:

1. AC to DC converter or Rectifier:
   • In this converter input AC source is converted to pulsating DC voltage, and with the help of filters it produces smooth DC output.
   • Diodes are used to rectify the voltage.
   • There are various types of rectifiers of which two are main under which remaining are listed: 1) Uncontrolled diode rectifiers. 2)Phase controlled rectifiers.

   

2. DC to AC converter or Inverter:

• This converter converts fixed DC voltage into variable/fixed voltage and variable/fixed frequency AC output. It is also known as inverters.
• Two types of inverters are available based on voltage classificiation which is Single phase and Three phase.
• Thyristors are used to invert the voltage level.

3. DC to DC converter or Chopper:

• This converter converts fixed DC input to variable/fixed DC output with change in magnitude. It is also knwn as Choppers.
• Step up chopper produces an average output level higher than input level where as Step down chopper prduces an average output level lesser than input.
• This converter uses thyristors in such a manner which controls the voltage level as per load requirements.

4. AC to AC converters:

• This converter controls the RMS value of output voltage at constant frequency.
• Common applications of these converters is starting of AC motors and controlling power to heaters.
• Here two thyristors in anti parallel fashion are used for controlling.

Electric vehicle drive system:

Two converters are mainly used in EV's or HEV's those are DC to DC converters and DC to AC converters.

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%?

Solution:

Comparing the given Speed torque characteristics with the general derived result which is: T = V*k/R - (k^2)*w/R

// T = Armature torque, V = Armature voltage, R = Armature resistance, w = Speed in rad/sec //

Thus, 24.7 + (0.0051)w^2 = V*k/R - (k^2)*w/R

Substituting the given values of voltage, resistance, constant in above equation. Here voltage will be 0.7*72 as we have to consider 70% duty cycle.

24.7 + (0.0051)w^2 = 72*0.7*0.7/0.5 - (0.7*0.7)/0.5*w

0.0051w^2 + 0.98w - 45.86 = 0

w^2 + 192.15w - 8992.15 = 0

Solving this quadratic equation as ax^2 + bx + c = 0 with solutions as

Therefore, from above equation two vaues of speed is obtain w1 = 38.91 rad/sec and w2 = -231.07 rad/sec

As value of speed cannot be in negative 38.91 is the correct solution.

So the steady state speed of EV with duty cycle 70% is 38.91 rad/sec

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

ω = V/Kϕ -Ra/Kϕ^2 *T

Solution:

Assuming values of parameters such as V, Kɸ, Ra and Ta

V = 50.4, Kɸ = 0.7, Ra = 0.5, T = ramp signal

All these values are consideed from above solved problem and model is prepared below with its result.

Model:

Result:

4. Refer to the blog on below topic: Induction Versus DC brushless motors by Wally Rippel, Tesla Explain in brief about author’s perspective.

Solution:

In the above topic, the author Walley Rippel initially described how the substituion of DC brushless motor and Induction motor in future can also be possible as evolution is going on as from his past experience nothing had stayed permanent. He has given the examples of Lead batteries replaced by Lithium, DC motor replaced by DC brushless or Induction motor.

Further discussing about each motor in detail starting with DC brushless motor he said:

• DC brushless machine has one or more permanent magnets in the rotor structure, which generates DC magnetic field and these field is linked with the currents carrying windings in stator body. To get output torque must be constant which is taken care by continuously varying magnitude and polarity of stator current.
• Constant torque brings out the application for DC brushless motor and these control is able to acheive by the help of inverter.

Induction motor:

• Construction of rotor is different than DC brushless motor as this motor does not includes permanent magnets, it has stacked steel laminations with shorted structure. Although stator has same three sets of distributed winding present.
• In this motor AC supply has given to stator which creates rotating magnetic field and is linked to rotor to produce torque.
• Induced voltage exists across the shorted structure that is proportionate to the speed difference between the rotor and electrical frequency.
• No inverter is required for Induction machine unlike DC brushless machine which has brought the much worth to Tesla.

Comaprison:

• Few comparisons are already stated above regarding construction besides that Induction motor can run only on AC power, shaft speed is proportionate to line frequency hence Induction machine operate at constant speed.
• Induction motor has abilith to start under no load, also compatible with conventional utility power. Where as DC machine requires Inverter
• Induction machines has limited starting and running torque capabilities compared to DC machine.
• By adding inverter to Induction machine can elimiate the two drawbacks of it those are constant speed characteristics and can power up from DC supply or battery but still torque performance is low although with these it can contribute to applications in Electric vehicles.
• Both of these machines use 3 phase modulating inverters, controls of inverter code is different.
• Major difference is that heat generated in DC brushless machines is less, cooling of rotor is quick and peak point efficiency is higher.
• Induction machine operates at 85% of power factor and cannot operate at unity power factor unlike DC machines. This means peak point efficiency is high in DC brushless machines compared to Induction.
• In ideal brushless machines magnetic fiels produced by permanent magents is adjustable, by keeping magnetic field at maximum inverter and rotor currents are maintained at lowest possible value. Ideally magnetic field is adjusted such that eddy, hystersis and I^2R losses minimised. But there is no easy way of changing B value with permanent magnets.
• In Induction machines magnetic fields are adjustable as B value is proportional to V/f. This improves the efficiency.
• Average efficiency can be said better with Induction machines when load is more, as for DC machine with increase in load size of machine grows and losses are also increased which is not happening much compared to Induction.
• Induction machines also has cost advantage over PM rotors as well as for higher ratings drives. But acheiving stability over torque speed curve and temperature is much more difficult with complex control laws.

Conclusion:

In this week challenge we concluded about 4 topics

• First we have seen the types of converters used in HEV and EV and also learnt very basic about all the converters.
• We have calculated the steady state speed of EV having some duty cycle.
• Derived a mathematical model of DC motor by asuming values.
• At last, explained in brief about the article written by author Walley Ripple in which comparison of DC brushless and Induction machine has made.