Explanation about Power Converters, Vehicle and Motor Characteristics Equation, and Induction vs DC Brushless Motor
AIM :
1. Types of Power Converter Circuits employed in Electric and Hybrid Electric Vehicle.
2. By using the given input parameters for electic vehicle to calculate the Steady state speed if duty cycle is 70%.
3. Brief Explanation about Induction versus DC Brushless motor by Wally Rippel,Tesla.
EXPLANATION :
STUDY - 1:
POWER CONVERTER :
To process and control the flow of electrical energy by supplying voltages and currents in a form that is optimally suited for user loads.The main aim of the converter is to produce conditioning power with respect to certain application.The converter changes from one form of electrical energy into another form of electrical energy.
Power Electronic converters perform various basic power conversion functions.This converter is a single power conversion stage that can perform any of the functions in AC and DC power conversion systems.
TYPES OF POWER CONVERTERS :
Depending on the types of function performed, Power electronic converters are categorized into following types.
- AC to DC (Rectifier) - An AC to DC Converter is known as Rectifier, which convert AC supply from main lines to DC supply for the load. For Example, In Electric Vehicle electrical network is in the form AC supply in nature, but battery is in form of DC, so AC to DC(Rectifier) is used to charge the battery for an Electirc Vehicle.
Fig : AC to DC Converter
- The Transformer adjust the primary AC source supply to the input of rectifier stage.
- Rectifier converts the low voltage AC supply into DC Supply.
- It comprises diode or thyristor based on the type of rectifier. The output of the rectifier is of Pulsed DC and hence it is filtered using filter circuit, which is usually made with a capacitor or choke.
- The control block controls the firing angle of thyristors in case of phase controlled rectifiers. Since the diode is not a controllable device, control block is not needed in case of diode rectifiers.
- DC to AC (Inverter) - Inverter is a circuit which transforms from one DC level to another AC voltage and frequency.
An Inverter is a static device, that converts fixed DC Supply voltage to Variable AC Voltage. Here the fixed DC voltage is obtained from batteries or by DC link in most power electronic converter. The output of the inverter can be variable/ fixed AC voltage with variable/fixed frequency
- DC to DC (Chopper) - It converts constant to variable DC or variable DC to constant DC. DC chopper is mainly used in DC drives, that is Electric vehicles and Hybrid electric vehicles. Some of the applications are subway cars, DC traction systems, Battery operated vehicles, trolley buses.
Fig : DC to DC Converter Chopper Circuit
- The Chopper circuit is connected between DC input Source and DC input load.
- The Chopper consists of power electronic switching devices such as thyristors which are connected in a such a way that produce required DC Voltage to the Load.
- The output voltage is controlled by adjusting the ON time of the thyristor(or switch) which turns changes the width of DC Voltage pulse of the output.
- The Output of chopper can be less or greater than the input and also it can be fixed or variable.
- DC Choppers are classified into three basic types based on the input and output voltage levels and are discussed below.
i) Step down chopper or Buck Converter :
It produces an average output voltage lower than the input DC voltage.
The Average output voltage of the converter is varied by controlling turn ON or OFF periods of thyristor. Diode is acts as a free wheeling diode that allows the load current to flow through it, when the thyristor is turned OFF.
ii) Step up chopper or Boost Converter :
It produces Ouptut Voltage is greater than the input voltage.
As similar to Buck converter, Diode is placed in series with the load that allows the load current to flow when the thyristor is turned off. When the thyristor is turned ON, diode is reverse biased which inductor charges to the maximum input voltage. When the Thyristor is turned OFF, load gets the voltage from input as well as from inductor.
iii) Buck/Boost Converter :
This Chopper can be used in both step-down and step-up mode continously by adjusting its duty cycle.
Fig : Buck-Boost Converter
When the thyristor is turned ON, the supply current flows to the inductor through the thyristor and induces the voltage in inductor.
When the thyristor is OFF, the current in the inductor tends to decrease with the induced emf reversing polarity. The output voltage of this converter remains constant as capacitor is connected across the load.
By varying the value of duty ratio, Output voltage is lower than input voltage is act as Buck Converter and Output Voltage greater than input voltage is act as Boost Converter.
- AC to AC (Cycloconverter) - One Voltage and frequency of AC Supply to another voltage and frequency of AC Supply. Fan Regulator is example for this AC-AC Converters.
There are two types of AC to AC Converters, are discussed below:
i) AC-AC Voltage Converters - it converts the rms value of the output voltage at a constant frequency.
ii) AC-AC Frequency Converters - it is mainly used for varying the frequency of the input source to the desired level of the load.
STUDY - 2 :
Given :
Motor and Controller Parameters:
Rated Armature voltage= 72 V
Rated armature current= 400 A
Ra= 0.5Ω, KΦ= 0.7 V-s
Chopper Switching frequency= 400 Hz
The vehicle characteristics are given by the equation
TTR=24.7+0.0051w2 -------------eqn(a)
To Find :
EV Steady State Speed, if duty cycle is 70%.
Solution :
Motor Speed-Torque Characteristics equation, we know that
Rearranging the above equation in terms of Torque, we get
Substituting the given values in above equation, we get
T=70.56−0.98ω -------------eqn(b)
Substitute the values of TTR & T, i.e., eqn(a) and eqn(b) we get
TTR = T
24.7+0.0051ω2 = 70.56−0.98ω
0.0051ω2+0.98ω−45.86=0
By solving this quadratic equation, we get
ω=−231.057&38.90 rad/sec.
Omega value must be in positive, so neglect the negative value
Therefore,
ω=38.90 rad/sec.
ω=39 rad/sec.
MATLAB Program for Finding EV Steady State Speed :
clear all
close all
clc
%Given
%Rated Armature Voltage
V = 72;
%Rated Armature Current
I = 400;
%Armature Resistance
R_a = 0.5;
%Constant Factor
K_phi = 0.7;
%Chopper switching frequency
f = 400;
omega = linspace(0,100,50);
%Vehicle Speed-Torque Characteristics Equation
TTR = 24.7+(0.0051*omega.^2);
%To find
%EV Steady state speed for duty cycle 70%
% V = 72*0.7 = 50.4 v
%Motor Speed-Torque Characteristics Equation
%omega = (V/K_phi)-((R_a*T)/K_phi);
%Rearranging the above equation in terms of Torque
T = ((V*K_phi)/R_a)-((omega*K_phi^2)/R_a);
T = ((50.4*0.7)/0.5)-((omega*0.7^2)/0.5);
T = 70.56-(0.98*omega);
%Substitute Vehicle & Motor Speed-Torque eqn in for loop to get EV Steady State Speed
for i=0:length(omega)
TTR = 24.7+(0.0051*(omega.^2));
T = 70.56-(0.98*omega);
end
plot(omega,TTR)
hold on
plot(omega,T)
axis([0 80 0 80]);
xlabel("Speed(rad/sec)");
ylabel("Torque(Nm)");
legend("Vehicle Speed-Torque Characteristics","Motor Speed-Torque Characteristics")
Result :
In the above plot x values refers Speed(rad/sec), y value refers Torque. So Steady State speed of EV is 39 rad/sec for the duty cycle 70%.
STUDY-3:
Induction and DC Brushless Motors Similarities :
- 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.
- 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.
DC Brushless Motor :
- With Brushless Machines, the rotor includes two or more permanent magnets that generate a DC magnetic field.
- In turn, this magnetic field enters the stator core 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 drives require an absolute position sensor.
- Less rotor heat is generated with the DC brushless drive.
- Rotor cooling is easier and peak point efficiency is generally higher for this drive.
- In an ideal brushless drive, the strength of the magnetic field produced by the permanent magnets would be adjustable.
- With DC brushless, as machine size grows, the magnetic losses increase proportionately and part load efficiency drops.
- 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.
- 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.
- Achieving stability over the entire torque-speed range and over temperature is easier with DC brushless than with Induction.
Induction Motor :
- Unlike the DC brushless rotor,Induction rotor has no magnets.
- 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.
- 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.
- Induction drives require only a speed sensor.
- Heat generation is more in Induction motor drive.
- induction machines have no magnets and B fields are “adjustable
- With induction, as machine size grows, losses do not necessarily grow.Peak efficiency must be higher in DC brushless drive but average efficiency must be better in Induction drive.
- 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.
- Induction machines are more difficult to control. The control laws are more complex and difficult to understand.
- 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.
- Achieving stability over the entire torque-speed range and over temperature is more difficult with induction than with DC brushless.
Conclusion :
In Author perspective Hybrid and Plug in Hybrid powered by DC brushless drives and pure electrics are powered by Induction Drives.
When hybrid become more electrically intensive and as their performance level increase, Induction motor type and Permanent Magnet DC brushless motor type will be used in Hybrid Electric Vehicle. The fact that so much of the hardware is common for both drives, so It gives better efficiency in Hybrid Electric Vehicles.