Week-6 Challenge: EV Drivetrain

AIM:

1) To study various types of power converter circuits employed in electric and hybrid electric vehicle

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

to find EV steady state speed if duty cycle is 70%.

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

ω= V/Kϕ -Ra/Kϕ^2 .T

4. To Refer to the blog on below topic:

Induction Versus DC brushless motors by Wally Rippel, Tesla

and Explain in brief about author’s perspective.

1)

THEORY:

Types of power converter circuits employed in electric and hybrid electric vehicle

1)AC TO DC CONVERTER (EV CHARGER)

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

Uncontrolled converter converts fixed DC voltage, it means the value of output for a given value of AC voltage. Fully controlled converter convert adjustable DC voltage when the adjustable range can adjust in both positive and negative side. Semi-controlled converter also produce an adjustable DC voltage which positive side power.

All three converter have unidirectional power flow on AC side to 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 converter have unidirectional power flow on AC side to 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 rectifier
• Three phase half wave diode rectifier

a)Single phase half wave rectifier:

Single phase half wave rectifier is simplest controlled rectifier which uses for single devices. To produce variable dc voltage from fixed current thyristors are use. Following are the circuit arrangement of Single phase half wave rectifier.

In here diode or thyristors are used. Where the application needs fix d.c. Voltage, the diode is used as switching element. When the application need variable voltage thyristors are used.

Single phase converters can mainly classify as Bridge Converter and Center Tapped Converter. This type of uncontrolled converters is mainly used for building supply only in Patrice. 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.

b)Single phase full wave bridge rectifier

In the above half wave controlled rectifier is only used to transfer the positive half cycle only. In full wave bridge rectifier, the entire positive and negative half cycle can transfer.

There are so many different types of configurations are use to create this full bridge rectifiers and following are one type of full wave bridge rectifier.

c)Single phase center tapped full wave rectifier:

DC Power Supply, built with a center tapped rectifier with capacitor filter and regulator. Assuming regulator input current to be approximately same as I0

d)Three phase half wave diode rectifier:

Diode D1, D2, D3 Take the positive rectangular block of current where Io is constant. The current are transformer secondary current and the net dc component of the same act to curve a DC magnetization of the transformer core.

This is decremental because the transformer input current can then be heavily distorted due to excessive distorted and peachy measuring current. Therefore this arrangement of conversion is not suitable and not use in practice.

However, if we are using a half bridge converter the direct term 3 phase supply (ie. without a transformer) the resulting DC current on the AC side, which is only small value for the upside distribution transformer there may not be the serious issue.

If we insist one a half bridge converter with it own transformer we must go to DZ transformer (Delta Zero) then the problem can solve. Most of three-phase AC to DC converters are much popular for industrial applications. These converters are used widely for power generation from DC side.

If we insist one a half bridge converter with it own transformer we must go to DZ transformer (Delta Zero) then the problem can solve. Most of three-phase AC to DC converters are much popular for industrial applications. These converters are used widely for power generation from DC side.

this rectifier is used as EV charger in electric and hybrid eectric vehicle.

2)DC TO DC CONVERTER

DC-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 equipments. 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, the power flows back to the low voltage bus to recharge the battery (buck mode). 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 aforementioned 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: In Figure a ,a buck converter is shown .The buck converter is step down converter and produces a lower average output voltage than the dc input voltage.

the average output voltage is given by

• Boost converter: In Figure b ,a boost converter is shown. The output voltage is always greater than the input voltage.

The average output voltage is given by

• Buck-Boost converter: In Figure c ,a buck-boost converter is shown. The output voltage can be either higher or lower than the input voltage.

the average output voltage is given by

D can be varied from 0 to 1. When, D = 0; V_O = 0

When D = 0.5, V_O = V_S

When, D = 1, V_O = ∞.

Hence, in the interval 0 ≤ D ≤ 0.5, output voltage varies in the range 0 ≤ V_O < V_S and we get step down or Buck operation. Whereas, in the interval 0.5 ≤ D ≤ 1, output voltage varies in the range V_S ≤ V_O ≤ ∞ and we get step up or Boost operation.

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 a constant power

While DC/DC converters can be based on a number of different designs, the underlying function remains the same. We might be talking about a step-up converter, which turns a low-voltage input into a high-voltage output, or a step-down converter, which does the opposite.

In electric vehicles which use a DC motor, running the motor can use up to three times the voltage provided by the battery. With the help of the right converter, we can bridge this gap without having to possibility to a larger, prohibitively heavy battery.

auxiliary like radio headlights works on DC supply , thus this chopper is used to supply the auxiliary equipments through battery.

3)DC TO AC CONVERTER

An inverter is a device that converts DC power from the battery to AC power used in an electric vehicle motor. The inverter can change the speed at which the motor rotates by adjusting the frequency of the alternating current.

The use of Inverters can increase or decrease the power or torque of the motor by adjusting the amplitude of the signal.

It plays a significant role in capturing energy from regenerative braking and feeding it back to the battery.

The output voltage waveform of ideal inverters should be sinusoidal. However, the waveform of practical inverters are non-sinusoidal and contain certain harmonics.

2)

given:

Motor and Controller Parameters:

Rated Armature voltage(Vs) = 72 V

Rated armature current (Ia)= 400 A

Ra= 0.5Ω, KΦ= 0.7 Volt second

Chopper Switching frequency= 400 Hz

consider seperately excited dc motor as shown below,

given that supply rated voltage(Vs) =72 volts

duty ratio (d) =0.7

thus motor voltage that come across the motor= Va=Vs*d=72*0.7= 50.4 volts

the torque speed relationship is governed by equation

where T= motor torque

V=Va= motor voltage

Ra= armature resistance

w= angular speed of motor in rad/sec

Thus

thus we get

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

equating above two equation , we get

the solution for the quadratic equation

is given by

thus we get w=38.95 rad/sec and w=-231.08 rad /sec

thus EV steady state speed is 38.95 rad/sec

3)

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

ω= V/Kϕ -Ra/Kϕ^2 .T

let us draw a model for the above quation as shown below.

let V=50.4 volts

ko=0.7

Ra=0.5 ohms

and let us consider torque as linearly increasing signal.let us consider a ramp signal.

the scope of speed of motor is shown below

thus we find that , for the fixed voltage, the speed of the motor is inversely propotional to load

this means that increase in load torque will result in decrease in speed.

4)

In that odious world of gas powered vehicles, engines are not all alike. There are flat-heads, Hemis, straight, opposed, and V configurationsand so on. there were lead acid batteries, DC brush motors, and contactor controllers. Today, none of these remain . Lead has been replaced by lithium and DC by either DC brushless or induction.Contactors, meanwhile, have given way to modulating inverters. 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.

In induction motor drives, A forerunner of the 3-phase induction motor was invented by Nikola Tesla sometime before 1889. Curiously, 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. (Were an inverter needed, Tesla’s invention would have been useless until sometime in the 1960s.) The fact that induction motors are directly compatible with conventional utility power is the main reason for their success. In contrast, a brushless DC motor produces no starting torque when directly connected to fixed frequency utility power. They really need the aid of an inverter whose “phase” is maintained in step with the angular position of the rotor.

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.

comparison between brushless and induction motor.

Back in the 1990s all of the electric vehicles except one were powered by DC brushless drives. Today, all the hybrids are powered by DC brushless drives, with no exceptions.

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.

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.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.

Permanent magnets are expensive – something like $50 per kilogram. Permanent magnet (PM) rotors are also difficult to handle due to very large forces that come into play when anything ferromagnetic gets close to them. This means that induction motors will likely retain a cost advantage over PM machines. Also, due to the field weakening capabilities of induction machines, inverter ratings and costs appear to be lower, especially for high performance drives. Since spinning induction machines produce little or no voltage when de-excited, they are easier to protect.
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.

we can conclude that DC brushless drives will likely continue to dominate in the hybrid and coming plug-in hybrid markets, and that induction drives will likely maintain dominance for the high-performance pure electrics.

CONCLUSION:

1) the various power converter circuits used in EV and hybrid EV is studied

2) the EV steady state speed for the given data is calculated.

3) mathematical model for dc motor is developed.

4) difference between induction motor and brushless dc motor is studied

REFERENCES:

http://www.electricalpowerenergy.com/2017/04/ac-to-dc-converters/

https://www.tutorialspoint.com/power_electronics/power_electronics_choppers.htm