Challenge

Week-6 Challenge: EV Drivetrain

MMegashyam V9 Jan 2021

1. Types of power converter circuits are employed in electric and hybrid electric vehicle

Power converter:

The power semiconductor devices or power electronic converter fall
generally into six categories :
- AC to DC Converter (Controlled Rectifier)
- DC to DC Converter (DC Chopper)
- AC to AC Converter (AC voltage regulator)
- DC to AC Converter (Inverter)

-Rectifiers(AC to DC Converter):

The process of electrical rectification is where current from an ac supply is converted to an unidirectional form before being supplied to a load .The ac supply current remains bidirectional, while the load current is unidirectional. With resistive loads the load voltage polarity is fixed. With energy storage loads and alternating supply voltage the load current is unidirectional but pulsating, and the load voltage in series-connected load inductance elements may vary and alternate in polarity during the load current cycle.A diode rectifier circuit converts AC voltage into a fixed DC voltage. The input voltage to rectifier could be either single phase or three phase.

Inverter(DC to AC Converter):

The process of transferring power from a dc supply to an ac load is known as inversion, and the necessary form of converter is usually known as an inverter. Inverters can be classed according to their manner of commutation, that is, by the process by which conducting switches are extinguished.An AC to DC converter circuit can convert AC voltage into a DC voltage. The DC output voltage can be controlled by varying the firing angle of the thyristors. The AC input voltage could be a single phase or three phase.

-DC Chopper(DC to DC Converter):

DC to DC converter is very much needed nowadays as many industrial applications are dependent upon DC voltage source. The performance of these applications will be improved if we use a variable DC supply. It will help to improve controllability of the equipments also. Examples of such applications are subway cars, trolley buses, battery operated vehicles etc. We can control and vary a constant DC voltage with the help of a chopper.
Chopper is a basically static power electronics device which converts fixed DC voltage/power to variable DC voltage or power. It is nothing but a high speed switch which connects and disconnects the load from source at a high rate to get variable or chopped voltage at the output.

-AC voltage regulator(AC to AC Converter):

AC to AC converters is used for converting the AC waveforms with one particular frequency and magnitude to AC waveform with another frequency at another magnitude. This conversion is mainly required in case of speed controlling of machines, for low frequency and variable voltage magnitude applications as well. We know that there are different types of loads that work with different types of power supply like single-phase, three-phase supply, and the supplies can be differentiated based on the voltage and frequency range also.

-conclusion:

In the electric and hybrid electric vehicles from the above only three power coverters are used ,they are AC to DC Converter (Controlled Rectifier),DC to DC Converter (DC Chopper),DC to AC Converter (Inverter). This is because the battery source used in the electric and hybrid electric vehicles are DC in nature so that there is no use for AC voltage regulator(AC to AC Converter).

2.Given:

Motor and Controller Parameters:

Rated Armature voltage(Vin)= 72 V,

Rated armature current(Iin)= 400 A,

Armature resistance(Ra)= 0.5Ω,

Motor constant KΦ= 0.7 Volt second

Chopper Switching frequency= 400 Hz

The vehicle speed-torque characteristics are given by the below equation

As from the before sessions we know the torque from which we need to find out the speed in rad/sec,the equation is as follows

T=((V*k) /Ra)-((k^2)*w)/Ra)

where T-Torque,

w-Total Speed

Comparing the two torque equation of the vehicle and motor we can identify the total speed,

24.7+0.0051*w^2= ((V*k) /Ra)-((k^2)*w)/Ra)

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

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

w^2+192.15*w-8992.15=the above equation can be simplified by the quadratic equations method,

After simplfying the equation from the above formula we get and the speed can not be a negative quantity the negative value is neglected,

w=38.95 rad/sec.

3. Induction Versus DC brushless motors by Wally Rippel, Tesla

The author writes this blog to entail the differences between the two most commonly used motor drives in the automobile industry: the DC brushless motor drive and the induction motor drive. Rippel goes in to detail about the differences in construction, working, and cost between the two drive types. However, with what he has written, he has not arrived at the conclusion that one type is better than the other. In the last lines of the opening paragraph, Rippel writes that "there is no one best engine type" and that it is all a matter of personal preference when it comes to choosing an engine. It is in this same regard that he ends the blog stating that there is still 'no winner' when it comes to motor drive selection either. In spite of both motor drive types having similarities, they also have their benefits and drawbacks.

For DC Brushless drives, we are told that their construction implements 2 or more permanent magnets at the rotor to generate a DC magnetic field. It also consists of a stator core (made up of thin, stacked laminations) It also uses an inverter for the current control, meaning that it cannot run on a DC power supply as the current direction has to be

changed repeatedly in to maintain a constant torque.

For induction drives, there rotor has no magnets, but instead stacked steel laminations with buried peripheral conductors which form a 'shorted structure'. It is the current in the stator windings that generates a rotating magnetic field that interacts with the rotor.

Induction motors can be used directly with power from the mains and does not require an inverter to start the drive and generate the starting torque. It has the ability to start under the mains load applied. However, DC brushless drives cannot do the same. They cannot generate a starting torque when connected to a mains power supply. They require an inverter to do this.

Induction motor drives cannot run on a DC power supply and are solely dependent on an AC power supply. Also, when they are connected to a 3 phase mains supply, the rotor rotates at a constant speed and also they have a limited starting torque as well as peak torque. In order to rectify this, an inverter can be used to operate the induction drive from a DC power supply or a battery. Also, if the inverter frequency is varied, then the constant speed is no longer an issue - it can be varied.

DC Brushless drives use permanent magnets, and cannot change the strength of their magnetic fields. It would be desirable if they could, because when the maximum torque is needed by the rotor, the magnetic field strength should also be at a maximum level. This allows for the inverter an motor currents to be at a minimum, thereby reducing the power loss by heat dissipation. Similarly, when the torque required is minimum, the magnetic field strength should all be minimum to reduce hysteresis and eddy current losses. The permanent magnets used are also quite expensive costing $50 per kilogram and their magnetic fields are difficult to control if ferromagnetic material gets close enough to the field.

On the other hand, induction drives do not have permanent magnets and so they can vary their magnetic field strength. If used with a smart inverter, they can be operated with minimum hysteresis and eddy current losses and maximum efficiency as the magnetic field strength can be changed according to the requirement by the rotor. These losses do not increase if the size of the induction drive also increases but as a DC brushless drive gets bigger, its losses increase and their efficiency decreases. So induction drives have a

cost advantage compared to DC brushless drives.

Although, DC Brushless drives generate less heat at the rotors. So rotors more easily and have a higher peak point efficiency than induction drives. In addition, the DC brushless motor can operate at unity power factor i.e. 1, whereas induction drives can operate up to a power factor of approximately 85%.

-conclusion:

Rippel has elaborated the different aspects to compare the induction and DC brushless motor drives and has detailed the advantages and disadvantages of both types. As mentioned earlier, at the end of the day it depends on the preference of the user. If a user is seeking a high performance with comparatively lesser costs, then the induction drive would be better suited for those requirements. However, if it is high peak point efficiency, high power factor and easier control that a user seeks, the the DC brushless drive would be the smarter choice. Both types have their place in the automobile industry and will retain their relevance in their respective applications. Rippel believes that due to the increasing performance of vehicle one day both may be used side by side under the hood of a vehicle in order to get the best of both drives. This would result in a new kind of hybrid that combines the characteristics of an ordinary vehicle.

Week-6 Challenge: EV Drivetrain

Ship work like this
in your own course.

Talk to a career counsellor

Ship work like thisin your own course.

Talk to a career counsellor

Ship work like this
in your own course.