Week-6 Challenge: EV Drivetrain

Aim1:

Which type of power converter circuits are employed in electric and hybrid electric vehicles?

Ans:

An electric vehicle is a vehicle that uses a combination of different energy sources, fuel cells (FCs), batteries and supercapacitors (SCs) to power an electric drive system as shown in figure 1 below. In EV main energy is assisted by one or more energy storage devices. Thereby the system cost, mass and volume can be decreased and a significantly better performance can be obtained. Two often used energy storage devices are batteries and SCs. They can be connected to the fuel cell stack in many ways. A simple configuration is to directly connect two devices in parallel, (FCs/battery, FC/SC, or battery/SC). However, in this way the power drawn from each device cannot be controlled but is passively determined by the impedance of the devices. The impedance depends on many parameters, e.g. temperature, state-of-charge, health, and point of operation. Each device might therefore be operated in an inappropriate condition e.g. health and efficiency. The voltage characteristics also have to match perfectly of the two devices, and only a fraction of the range of operation of the devices can be utilized, e.g. in a fuel cell battery configuration the fuel cell must provide almost the same power all the time due to fixed voltage of the battery, and in a battery/supercapacitor configuration only a fraction of the energy exchange capability of the supercapacitor can be used. This is again due to the nearly constant voltage of the battery. By introducing DC/DC converters one can choose the voltage variation of the devices and the power of device can be controlled.

Fig-1: Electric Vehicle Drive System

In reference, 10 cases of combining the fuel cell with the battery, SCs or both are investigated. The system volume, mass, efficiency and battery lifetime were compared. It is concluded that when SCs are the only energy storage device the system becomes too big and heavy.  A fuel cell/battery/ supercapacitors hybrid provides the longest lifetime of the batteries. It can be noticed that the use of high-power DC/DC converters is necessary for EV power supply systems. The power of the DC/DC converter depends on the characteristics of the vehicle such as top speed, acceleration time from 0 to 100 km/h, weight, maximum torque and power profile. Generally, for passenger cars, the power of the converter is more than 20kW and it can go up to 100kW.

1. AC to DC converter
2. DC to DC converter

• DC to AC converter

1. AC to DC converter:

AC to DC converters is one of the most important in power electronics because there are lot of real applications are based on these conversions. The AC current to DC current conversion process is known as rectification. This rectifier converts the AC supply into the DC supply at the load end connection. Normally transformers are used to adjust the AC source to get a step-down transformer to reduce the voltage level to have a better operation range for DC supply. Normally this AC to DC converter converts low voltage values.

Uncontrolled converter converts fixed DC voltage, which means the value of output for a given value of AC voltage. A fully controlled converter converts adjustable DC voltage when the adjustable range can be adjusted in both the positive and negative sides. Semi-controlled converter also produces an adjustable DC voltage which positive side power.

All three converters have unidirectional power flow the AC side to the DC side. PWN converter can produce adjustable both positive importantly it allows power flow in either direction. Making it ideal application motor drives and power system interconnection with the power system.

All three converters have unidirectional power flow from the AC side to the DC side. PWN converter can produce adjustable both positive importantly it allows power flow in either direction. Uncontrolled rectifiers can also subdivide into several subcategories such as:

• Single phase half wave rectifier
• Single phase full wave bridge rectifier
• Single phase center tapped full wave
• Three phases half wave diode rectifier

Single phase half wave rectifier:

Single phase half wave rectifier simplest controlled rectifier uses for single devices. To produce variable DC voltage form fixed current thyristors are used. Following are the circuit arrangement of the single-phase half wave rectifiers. In here diode or thyristors are used. Where the application needs to fix DC voltage, the diode is used as a switching element. When the application needs variable voltage thyristors are used.

Single phase converters can mainly classify as Bridge Converter and Center Tapped Converter. This type of uncontrolled converter is mainly used for building supply only in practice. In power supply by applying a capacitor filter on the output in other cases which inductive load causes we use this directly or inductive filters.

Single phase full wave bridge rectifier

Another type of circuit that produces the same output waveform as the full wave rectifier circuit above, is that of the Full wave Bridge rectifier. This type of single-phase rectifier uses four individual rectifying diodes connected in a closed loop ‘bridge’ configuration to produce the desired output.

The main advantage of this bridge circuit is that it does not require a special center tapped transformer, thereby reducing its size and cost. The single secondary winding is connected to one side of the diode network and the load to the other side as shown below

The Diode Bridge Rectifier

The four diodes labelled D1 to D4 are arranged in ‘series pairs’ with only two diodes conducting current during each half cycle. During the positive half cycle of the supply, diodes D1 and D2 conduct in series while D3 and D4 are reverse biased and the current flows through the load as shown below.

The Positive Half-Cycle

During the negative half cycle of the supply, diodes D3 and D4 conduct in series, but diodes D1 and D2 switch ‘OFF’ as they are now reverse biased. The current flowing through the load is in the same direction as before

The Negative Half-Cycle

As the current flowing through the load is unidirectional, so the voltage developed across the load is also unidirectional the same as for the previous two diodes full-wave rectifier, therefore the average DC voltage across the load is 0.637 Vmax. However, in reality, during each half cycle, the current flows through two diodes instead of just one so the amplitude of the output voltage is two voltage drops (2*0.7 = 1.4 V) less than the input Vmax amplitude. The ripple frequency is now twice the supply frequency (e.g. 100Hz for a 50 Hz supply or 120Hz for a 60Hz supply)

Although we can use four individual power diodes to make a full-wave bridge rectifier, pre-made bridge rectifier components are available ‘off-the-shelf’ in a range of different voltage and current sizes that can be soldered directly into a PCB circuit board or be connected by spade connectors.

The image to the right shows a typical single-phase bridge rectifier with one corner cut off. This cut-off corner indicates that the terminal nearest to the corner is the positive or +ve output terminal or lead with the opposite lead being the negative or -ve output lead. The other two connecting leads are for the inputs alternating voltage from a transformer secondary winding.

Single phase center tapped full wave diode rectifier:

We have already discussed the full wave bridge rectifier which uses four diodes arranged as a bridge to convert the input alternating current (AC) in both half-cycles to direct current (DC). In the case of a center-tap full-wave rectifier only two diodes are used and are connected to the opposite ends of a center-tapped secondary transformer. The center tap is usually considered as the ground point or the zero-voltage reference point.

Working of Center-Tap full wave rectifier:

As shown in the figure, an AC input is applied to the primary coils of the transformer. This input makes the secondary ends P1 and P2 become positive and negative alternately. For the positive half of the AC signal, the secondary point D1 is positive, the GND point will have zero volts and the P2 will be negative. At this instant diode, D1 will be forward biased and diode D2 will be reverse biased. As explained in the theory behind P-N junction and characteristics of P-N junction diode, the diode D1 will conduct and D2 will not conduct during the positive half cycle. Thus, the current flow will be in the direction P1-D1-C-A-B-GND. Thus, the positive half cycle appears across the load resistance RLOAD. During the negative half-cycle, the secondary ends P1 becomes negative and P2 becomes positive. At this instant, the diode D1 will be negative and D2 will be positive with the zero-reference point being the ground, GND. Thus, the diode D2 will be forward biased and D1 will be reverse biased. The diode D2 will conduct and D1 will not conduct during the negative half cycle. The current flow will be in the direction P2-D2-C-A-B-GND

 When comparing the current flow in the positive and negative half-cycles, we can conclude that the direction of the current flow is the same (through the load resistance RLOAD). When compared to the Half-Wave Rectifier, both the half cycles are used to produce the corresponding output. The frequency of the rectified output voltage is twice the input frequency. The output that is rectified, consists of a DC component and a lot of AC components of minute amplitudes.

Three phase half wave diode rectifier:

Three Phase Waveform

The advantage here is that a three-phase alternating current (AC) supply can be used to provide electrical power directly to balanced loads and rectifiers. Since a 3-phase supply has a fixed voltage and frequency it can be used by a rectification circuit to produce a fixed voltage DC power which can be then filtered resulting in an output DC voltage with less ripple compared to a single-phase rectifying circuit.

Three phase rectification

Having seen that a 3-phase supply is just simply three single-phases combined together, we can use this multi-phase property to create 3-phase rectifier circuits.

As with single-phase rectification, three-phase rectification uses diodes, thyristors, transistors, or converters to create half-wave, full-wave, uncontrolled and fully-controlled rectifier circuits transforming a given three-phase supply into a constant DC output level. In most applications, a three-phase rectifier is supplied directly from the mains utility power grid form a three-phase transformer if a different DC output level is required by the connected load.

As with the previous single-phase rectifier, the most basic three-phase rectifier is that of a uncontrolled half-wave rectifier circuit which uses three semiconductors diodes, one diode per phase as shown

Half wave three phase rectification:

So how does this three-phase half wave rectifier circuit work? The anode of each diode is connected to one phase of the voltage supply with the cathodes of all three diodes connected together to the same positive point, effectively creating a diode-‘OR’ type arrangement. This common point becomes the positive (+) terminal for the load while the negative (-) terminal of the load is connected to the neutral (N) of the supply.

Assuming a phase rotation of Red-Yellow-Blue (V_A-V_B-V_C) and the red phase (V_A) starts at 0^o. the first diode to conduct will be diode 1 (D¹) as it will have a more positive voltage as its anode than diodes D₂ or D₃. Thus diode D₁ conducts for the positive half-cycle of V_A while D₂ and D₃ are in their reverse biased state. The neutral wire provides a return path for the load current back to the supply.

120 electrical degree later, diose 2 (D₂) starts to conduct for the positive half-cycle of V_B (yellow phase). Now its anode becomes more positive than diodes D₁ and D₃ which are both ‘OFF’ because they are reversed-biased. Similarly, 120^o later V_C (blue phase) starts to increase turning ‘ON’ diode 3 (D₃) as its anode becomes more positive, thus turning ‘OFF’ diodes D₁ and D₂

Then we can see that for three-phase rectification, which ever diode has a more positive voltage at its anode compared to the other two diodes it will automatically start to conduct, thereby giving a conduction pattern of D₁ D₂ D₃ as shown

Half Wave Three Phase Rectifier Conduction Waveform

From the above waveforms for a resistive load, we can see that for a half-wave rectifier each diode passes current for one-third of each cycle, with the output waveform being three times the input frequency of the AC supply. Therefore, there are three voltage peaks in a given cycle, so by increasing the number of phases from a single-phase to a three-phase supply, the rectification of the supply is improved, that is the output DC voltage is smoother.

For a three-phase half wave rectifier, the supply voltages V_A V_B and V_C are balanced but with a phase difference of 120^o giving:

V_A = V_p*sin (ωt – 0^o)

V_B = V_p*sin (ωt – 120^o)

V_C = V_p*sin (ωt – 240^o)

2. DC to DC Converter

DC to DC converters in an electric vehicle may be classified into unidirectional and bidirectional converters. Unidirectional DC-DC converters cater to various onboard loads such as sensors, controls, entertainment, utility, and safety equipment. They are also used in DC motor drives and electric traction. Bidirectional DC-DC converters find applications in places where battery charging, regenerative braking, and backup power are required. The power flow in a bidirectional converter is usually from a low voltage end such as a battery or a supercapacitor to a high voltage side and is referred to as boost operation.

During regenerative braking, power flows back to the voltage bus to recharge the battery. As a backup power system, the bidirectional DC-DC converter facilitates the safe operation of the vehicle when ICES or electric drives fail to drive the motor. Due to the above-mentioned reasons high power bidirectional DC-DC converters have gained a lot of importance in the recent past.

The converter topologies are classified as

• Buck Converter
• Boost Converter
• Buck-boost Converter

3. DC to AC Converter

DC to AC converters is mainly designed for changing a DC power supply to an AC power supply. Here, DC power supply is comparatively stable as well as positive voltage source whereas AC oscillates approximately a 0V base stage, typically in a sinusoidal or square or mode.

The common inverter technology used in electronics is to convert a voltage source from a battery into an AC signal. Generally, they operate with 12 volts and commonly used in applications like automotive, lead-acid technology, photovoltaic cells, etc.

A transformer coil system & a switch is the simple circuit used for an inverter. A typical transformer can be connected toward the DC signal’s input through a switch to oscillate back quickly. Due to the current flow in bi-directional in the primary coil of the transformer, an alternating current signal is an output throughout the secondary coils.

Aim2:

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

Tv = 24.7 + (0.0051) ω ²

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

ANS:

Vehicle Output Voltage (Vv) is given by

Vv = 72*0.70

Thus vehicle voltage = 50.04

Now equation for vehicle torque is given by

Tv = ((Vv – ω.K_ɸ). K_ɸ)/ Ra

T = vehicle torque (N-m)

V = Vehicle voltage (volts)

ω = steady state speed of the vehicle (rps)

K = Motor constant

Ra = Armature Resistance

Substituting all these values in the equation:

Tv = ((50.04 – 0.7. ω). 0.7)/0.5

Tv = 70.56 – 0.98. ω

Tv = 24.7 + 0.0051. ω²

Solving these two equations,

 70.56 – 0.98. ω = 24.7 + 0.0051. ω²

ax² + bx + c = 0

since we get the quadratic equation

0.0051 . ω² + 0.98 ω – 46.94 = 0

Therefore,

ω = 38.85 rad/sec and ω = -230.95 rad/sec

we have taken the positive value

steady state speed of the vehicle (rps) = 38.85 rad/sec

Aim3:

Develop a mathematical model of a DC motor for the below equation using Simulink

 ω = V/Kϕ -Ra/Kϕ^2 .T

Solution:

Let

V = 50.4 volts

K = 0.7

Ra = 0.5 ohms

Let us consider torque as a linearly increasing signalled that is equal to the ram block of the following Simulink model

Thus, we find that for the fixed voltage,

The speed of the motor is inversely proportional to load this means that increase in load torque will result in a decrease in speed.

Aim4:

Refer to the blog on below topic: 

 Induction Versus DC brushless motors by Wally Rippel, Tesla

 Explain in brief about author’s perspective. 

Ans:

• In the first place the creator portrays the predominance of Internal Combustion Engine (ICE) in the realm of the auto business and furthermore examine various sorts of motors. He clarifies that all motors were comparative and it was dependent upon the client to choose as indicated by their necessity. He additionally talks about enlistment engines and brushless DC engines and presents his view that these two innovations will be broadly utilized in the coming years.
• Then he clarifies the working of both a 3-stage enlistment engine and a brushless DC engine. Presently the creator has done a corelation between these two innovations and expressed a few benefits and burdens of each.
• Induction engine enjoys a few benefits like it has no magnets. The engine can turn over under load. No inverter is required. A few disservices of induction engines are: they can’t work from DC:AC is an absolute necessity. Shaft speed is proportionate to line recurrence. Consequently when utilized with utility force, they are steady speed machines. When worked from utility force, they have restricted beginning force and fairly restricted running pinnacle force abilities when contrasted with DC type machines.
• Brushless DC engine enjoys a few benefits like considerably less rotor heat is produced with the DC brushless drive. The pinnacle point energy productivity for a DC brushless drive will normally be a couple of rate focuses higher than for an acceptance drive. A few drawbacks of a brushless DC engine are: as machine size develops the attractive misfortunes increment proportionately and part load proficiency drops. Perpetual magnets are costly and hard to deal with because of extremely enormous powers that become an integral factor when anything ferromagnetic draws near to them.
• Finally the creator couldn’t pick which drive is superior to the next however clarifies that DC brushless drives will probably keep on overwhelming in the crossbreed and coming module half breed markets, and that enlistment drives will probably keep up with strength for the elite unadulterated electrics.