Week-6 Challenge: EV Drivetrain

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

Solution: The task of a power converter is to process and control the flow of electrical energy by supplying current and voltages that are optimally suited for the user load. The block diagram of a power electronic converter is given below:

This circuit consists of 4 parts:

• electrical energy source
• power electronic circuit
• control circuit
• electric load

This convertor changes one form of electrical energy to the other.

The power electronic circuit consists of both the power part and control part. The power part transfers the energy from source to load and it consists of power electronic switches (SCR or TRIAC), transformers, electric choke, capacitors, fuses, and sometimes resistors.

The control circuit regulates the elements in the power part of the convertor. This block is built with a complex low power electronic circuit that consists of either analog or digital circuit assembly.

Depending on the type of function performed, power electronic converters are categorized into the following types.

• AC-DC(rectifiers)
• DC-AC(inverters)
• AC-AC(cyclo converter)
• DC-DC(chopper)

RECTIFIERS:

A rectifier is an electrical device comprised of one or more diodes that allow the flow of current only in one direction. It basically converts alternating current into direct current. Rectifiers can be mold in several shapes as per necessity like semiconductor diodes, SCRs (silicon controlled rectifiers), vacuum tube diodes, mercury-arc valves, etc.

INVERTERS:

A power inverter, or inverter, is a power electronic device or circuitry that changes direct current (DC) to alternating current (AC). The input voltage, output voltage and frequency, and overall power handling depend on the design of the specific device or circuitry. The inverter does not produce any power; the power is provided by the DC source. A power inverter can be entirely electronic or maybe a combination of mechanical effects (such as a rotary apparatus) and electronic circuitry.

CHOPPER:

A chopper is basically a dc to dc converter whose main function/usage is to create adjustable dc voltage from fixed dc voltage sources through the use of semiconductors.

CYCLO CONVERTERS:

A cycloconverter (also known as a cycloinverter or CCV) converts a constant voltage, constant frequency AC waveform to another AC waveform of a different frequency. A cycloconverter achieves this by synthesizing the output waveform from segments of the AC supply (without an intermediate DC link).

TYPES OF POWER CONVERTERS EMPLOYED IN EV AND HEV

1. AC - DC convertor

1. The AC-DC converters or rectifiers are used to charge the battery from external sources.
2. They are also called as rectifiers.
3. The external power stations use an AC supply and only DC can be stored in the battery.
4. This is mainly used in plug-in hybrids where the battery can be charged externally.

2. DC-AC converter

1. the inverter supplies current to the battery pack for recharging during regenerative braking.
2. Provides electricity to the AC induction motor for propulsion.
3. The inverter unit choreographs how these divergent voltages and current types work together.

because of the use of transformers and semiconductors (and the accompanying resistance encountered), enormous amounts of heat are emitted by these devices. Adequate cooling and ventilation are paramount to keeping the components operational. For this reason, inverter/converter installations in hybrid vehicles have their own dedicated cooling systems, complete with pumps and radiators, that are entirely independent of the engine's cooling system.

3. DC-DC converters

• a DC to DC converter is a category of power converters and it is an electric circuit that converts a source of direct current (DC) from one voltage level to another, by storing the input energy temporarily and then releasing that energy to the output at a different voltage. The storage may be in either magnetic field storage components (inductors, transformers) or electric field storage components (capacitors)
• The amount of power flow between the input and the output can be controlled by adjusting the duty cycle (ratio of on/off time of the switch). Usually, this is done to control the output voltage, the input current, the output current, or to maintain constant power.
• The most commonly used DC-DC converters are:

1. Unidirectional Converters:

They cater to various onboard loads such as sensors, controls, entertainment, utility, and safety equipment.

2. Bidirectional Converters:

They are used in places of regenerative braking and battery charging. The power flow is from a low voltage end like a battery or a supercapacitor to a high voltage side which is referred to as boost operation.

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.

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

Solution:

The governing equation for dc motor speed vs torque characteristics is

Wm = V/((K∅) )-Ra/(k∅)^2 * T ------EQU (1)

Along with the given vehicle speed-torque characteristics equation

`Tv=24.7+(0.0051)w^2` ------EQU (2)

By substituting the below values in equ (1) we get equation for speed vs torque

Rated armature voltage with duty cycle 70% = 72*0.7 = 50.4v, Ra= 0.5Ω, KΦ= 0.7 Volt second

Wm=`50.4/0.7-0.5/(0.7)^2.T`

Wm=72−1.02T

T = 72/1.08 - 0.98 Wm

T=70.58 - 0.98 Wm----EQU(3)

From equations 2 & 3 the speed equation is

(0.0051) ω^2+0.98w−45.88=0

The speed values obtained from above quadratic equation are ω = 38.2 rad/sec.

The EV steady state speed with duty cycle 70 % is w=38.2 rad/sec assuming in the forward direction.

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

ω= V/Kϕ -Ra/Kϕ^2 .T

Solution:-

In order to obtain the given equation, constant block, ramp signal, adder block multiplication-division blocks are obtained from Simulink library.

Supply voltage is taken as 300V, Motor constant as 0.25 and Armature resistance as 0.1ohm for calculating speed. From equation we get speed in rps. To convert rps to rpm a factor of 9.5493(60/(2*pi)) is multiplied.

As in graph, we need to plot speed with respect to change in torque, torque is not considered as constant and took as a ramp signal.

On simulation, both Speed and Torque graphs are obtained in the scope block.

Scope:-

Scope:-

We observe that as torque is increased, speed of the motor decreases. We can conclude that speed is inversely proportional to the torque. We can also observe that obtained plot for speed is linear.

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:

Induction motor working principle and operation:

The stators for the 3-phase induction motor and the DC brushless motor are virtually identical. Both have three sets of “distributed windings” that are inserted within the stator core. The essential difference between the two machines is with the rotor.

Unlike the DC brushless rotor, the induction rotor has no magnets – just stacked steel laminations with buried peripheral conductors that form a “shorted structure.” Currents flowing in the stator windings produce a rotating magnetic field that enters the rotor. In turn, the frequency of this magnetic field as “seen” by the rotor is equal to the difference between the applied electrical frequency and the rotational “frequency” of the rotor itself. Accordingly, an induced voltage exists across the shorted structure that is proportionate to this speed difference between the rotor and electrical frequency. In response to this voltage, currents are produced within the rotor conductors that are approximately proportionate to the voltage, hence the speed difference. Finally, these currents interact with the original magnetic field to produce forces – a component of which is the desired rotor torque.

When a 3-phase induction motor is connected to utility type 3-phase power, torque is produced at the outset; the motor has the ability to start under load. No inverter is needed. The fact that induction motors are directly compatible with conventional utility power is the main reason for their success.

Limitations and drawbacks of AC Induction Motor:

While 3-phase induction motors have great utility, they also have some severe limitations. They cannot operate from DC; AC is a must. Shaft speed is proportionate to line frequency. Hence, when used with utility power, they are constant speed machines. Finally, when operated from utility power, they have limited starting torque and somewhat limited running peak torque capabilities, when compared to DC type machines.

Add an inverter (without any feedback control) and it becomes possible to power an induction machine from a battery or other DC source; variable speed also becomes possible simply by adjusting the inverter frequency. Still, torque performance is low compared with DC machines. Add some feedback loops such that the inverter produces the exact frequency that the motor “desires,” and the induction motor is now capable of competing with DC and DC brushless for vehicle applications.

I almost forgot: Induction machines are more difficult to control. The control laws are more complex and difficult to understand. Achieving stability over the entire torque-speed range and over temperature is more difficult with induction than with DC brushless. This means added development costs, but likely little or no recurring costs.

Induction motor losses and efficiency

In contrast, induction machines have no magnets and B fields are “adjustable,” since B is proportionate to V/f (voltage to frequency). 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. With 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; peak efficiency will be a little less than with DC brushless, but average efficiency may actually be better.

DC brushless motors working principle and operation:

With brushless machines, the rotor includes two or more permanent magnets that generate a DC magnetic field (as seen from the vantage point of the rotor). In turn, this magnetic field enters the stator core (a core made up of thin, stacked laminations) and interacts with currents flowing within the windings to produce a torque interaction between the rotor and stator. As the rotor rotates, it is necessary that the magnitude and polarity of the stator currents be continuously varied – and in just the right way - such that the torque remains constant and the conversion of electrical to mechanical energy is optimally efficient. The device that provides this current control is called an inverter. Without it, brushless motors are useless motors.

DC brushless motors losses and efficiency

In an ideal brushless drive, the strength of the magnetic field produced by the permanent magnets would be adjustable. When maximum torque is required, especially at low speeds, the magnetic field strength (B) should be maximum – so that inverter and motor currents are maintained at their lowest possible values. This minimizes the I² R (current² resistance) losses and thereby optimizes efficiency. Likewise, when torque levels are low, the B field should be reduced such that eddy and hysteresis losses due to B are also reduced. Ideally, B should be adjusted such that the sum of the eddy, hysteresis, and I² losses is minimized. Unfortunately, there is no easy way of changing B with permanent magnets.

Similarities and differences between AC Induction Motor & DC Brushless Motor:

Both DC brushless and induction drives use motors having similar stators. Both drives use 3-phase modulating inverters. The only differences are the rotors and the inverter controls. And with digital controllers, the only control differences are with control code. (DC brushless drives require an absolute position sensor, while induction drives require only a speed sensor; these differences are relatively small.)

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. 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.

CONCLUSION: Based on the construction,working principle and operation High performance pure electric vehicles require induction motor drives and hybrid electric vehicles requires brushless dc motor drives . I also agree with the authors perspective of using both induction motor and dc brushless motor in combination for future vehicle requirements because most of the hardware is common with both types.