Studying DC Machine characteristics using MATLAB

Objectives

1. Implement DC machine torque speed characteristics equations in MATLAB script file. It asks user to input data and plots speed (RPM) versus torque (Nm). Submit the .m file for evaluation and attach the screenshots of all the required plots
2. How can you distinguish between power cables and signal cables and also prepare a table where these cables are used in the wiring of an electric vehicle?
3. What voltage levels are considered as high & low for a battery pack of electric vehicles?

1. DC Machine

A DC Machine is an electro-mechanical energy conversion device. There are two types of DC machines; one is the DC generator, and another one is known as DC motor.

A DC generator converts mechanical power (ωT) into DC electrical power (EI), whereas, a DC motor converts d.c electrical power into mechanical power. The AC motor is invariably applied in the industry for conversion of electrical power into mechanical power, but at the places where the wide range of speeds and good speed regulation is required, like in electric traction system, a DC motor is used.

The construction of the dc motor and generator is nearly the same. The generator is employed in a very protected way. Hence there is an open construction type. But the motor is used in the location where they are exposed to dust and moisture, and hence it requires enclosures for example dirt proof, fireproof, etc. according to requirement.

Although the battery is an important source of DC electric power, it can only supply limited power to any machines. There are some applications where large quantities of DC power are required, such as electroplating, electrolysis, etc. Hence, at such places, DC generators are used to deliver power.

Basic Construction of DC Machine

The rotating electrical or DC machine has mainly two parts; one is Stator, and another one is Rotar. The stator and rotor are separated from each other by an air gap. The stator is the outer frame of the machine and is immovable. The rotor is free to move and is the inner part of the machine.

Both the stator and the rotor are made of ferromagnetic materials. Slots are cut on the inner periphery of the stator and the outer periphery of the rotor. Conductors are placed in the slots of the stator or rotor. They are interconnected to form windings.

The windings in which voltage is induced is called the Armature windings. The winding through which a current is passed to produce the main flux is called the Field windings. To provide main flux in some of the machine permanent magnets is also used.

The equivalent circuit of the armature of a DC generator

The equivalent circuit of the armature of a DC motor

Now to study the characteristics of DC machine by plotting Speed vs torque using MATLAB

`T=V*((Kphi)/(Ra))-(Kphi)^2/(Ra)*N`

Where,

V = Voltage

T = Torque

N = Motor speed

Ra = Armature Resistance

K = Motor constant

To calculate K

`Kphi = E_b*60/(2piN_s)`

Coding

clear all
close all
clc


V = input('Enter supply voltage(volts):');
Ra = input('Enter armature resistance(ohms):');
Speed = linspace(0,250,75); 
N = input('Enter speed of motor(RPM):');
% Back EMF
EMF = 1;

% To calculate motor constant
K = ((EMF*60)/(N*2*3.14))

% To calculate torque by using DC machine equation
T = ((V*K)/Ra)-(((K)^2/Ra)*Speed);

% Total current TC
TC = (V-EMF)/Ra

% To calculate torque 
T_max = K*TC

% Plotting
grid on
plot(Speed,T,'color','r','linewidth',3);
xlabel('Speed (RPM)');
ylabel('Torque (Nm)');
legend('DC Machine Characteristics');

Inputs and calculations

Output

2. Power and Signal cables

The cables are distinguished by their area of cross-section.

Power cables has larger c/s as they carry much higher current in them, for eaxample supply of power from battery to the motor, Ac supply for recharging EV etc

Signal cables has lesser c/s compared to power cables as they carry less current, for example supply to infotainment system, signals comming from diffent sensors installed in an automobile etc.

3. The high and low voltage requirments of HEV, plug-in HEV and EV are