Speed Torque characteristics of dc machine

AIM:

To study of speed torque characteristics of dc machine.

OBJECTIVE:

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.

STUDY 1:

A DC machine is a electro-mechanical device.There are two types of machines namely DC motor and DC generator.

A DC MOTOR is a device which converts the electrical energy into mechanical work.A DC Generator is a device which converts the mechanical energy into electrical energy.

The important parts of the DC machine are

STATOR:

The stator is the stationary part in DC motor. The stator frame, the stator core and stator winding are the parts of the stator. The frame supports the core of the stator and protects their three-phase winding. The stator core carries the rotating magnetic field which induces because of the three-phase supply.

ROTOR:

The rotor is the rotatory part in DC motor. The rotor is situated inside the core of the stator. Squirrel cage and the phase wound rotor are the types of the rotor. The rotor winding is excited by the DC supply. The field winding induces the constant magnetic field in the core of the rotor.

COMMUTATOR:

Commutator is made with hard drawn copper to reduce wear and tear. The main function of the commutator in the DC machine is to collect the current from the armature conductor as well as supplies the current to the load using brushes.

BRUSHES:

Brushes are made with copper or carbon for small machines and electro-graphite for large rating machines.The function of brushes is to collect the current or give the current to the armature conductors through the commutator segments.

WORKING PRINCIPLE:

The principle of dc motor is the whenever a current carrying conductor placed in the magnetic field an emf is induced in the conductor.The direction of force can be done by Fleming Left Hand Rule.

TORQUE SPEED CHARACTERISTICS

EQUATION USED

The voltage equation is V= Eb+Ia*Ra

The induced emf Eb = KØw

We know that T = (KØ) Ia

Ia = T/ KØ

Therefore V = (KØw)*Ia + (T/ KØ)*Ra

On solving equation w = (V/ KØ)- (Ra/ (KØ)^2)*T

for motor speed yields

Torque speed relation is

`T=V*(KPhi)/R_a-(KPhi)^2/R_a*w`

Where KØ = Eb*60/2*π*Ns

V = Terminal voltage

T = Torque of motor

Ns = speed constant of motor

w = motor speed

Eb = back emf

Ra = armature resistance

KØ = constant of proportionality

MATHLAB CODE:

clear all
close all
clc
%input the data
V=input('enter the value of terminal voltage=');
Ra=input('enter the value of armature resistance=');
w=input('enter the speed constant of motor=');
%back emf
Eb=1;
%flux constant
Kt=(Eb*60)/(2*pi*w);
%speed array range values
N=linspace(0,1500,50);
%Torque speed equation
T=((V*Kt/Ra)-(((Kt)^2/Ra)*N));
%Speed torque characteristics of dc machine
plot(N,T,'color','r');
xlabel('speed of motor in rpm');
ylabel('Torque of motor in Nm');
title('Speed torque characteristics curve');

INPUT VALUES:

OUTPUT:

• We observe that the Speed of DC motor increases the torque of motor decreases linearly .Torque is inversely proportional to the speed of motor.
• The speed torque curve begins from Yaxis where torque is maximum and speed is zero.This is called Holding Torque.
• The curve intersect at X axis where speed is maximum and torque is zero.This is called no load speed.

STUDY 2:

A power cable is an electric cable, an assembly of one or more electrical conductors, usually held together with an overall sheath. The assembly is used for transmission of electric power. Power cables may be installed as permanent wiring within buildings, buried in the ground, run overhead, or exposed.

A signal cable is a cable which sends signal to control the functioning of the equipment.These cables carry low power and voltage and are used in measuring and instrumentation application. These cables usually bear a PVC insulation that protects them from impacts and harsh climatic conditions.

POWER CABLE	SIGNAL CABLE
It is used for transmission of electric power	It is used to indicate signal
Thickness is more due to large cross section area	Thickness is less due to small cross section area
High power and current rated wires	Low power and current rated wires
Power cables often have a unique colour code system that distinguishes the high voltage carriers from their low voltage counterparts.	Control cables offer superior resistance to interference, which makes them ideal for sending low voltage signals.
It is used in transmission line,motor..etc	It is used for test and measuring application
Cost is high as it carries high voltage and used for large scale application	Cost is low as it carries only low voltage and used for small scale application

STUDY 3:

An electric vehicle battery is often composed of many hundreds of small, individual cells arranged in a series/parallel configuration to achieve the desired voltage and capacity in the final pack. A common pack is composed of blocks of 18-30 parallel cells in series to achieve a desired voltage.

The EV requires higher voltage level as it allows more power to be transmitted with less over current in the cable. The EV voltage ranges from 400-800V are considered as high voltage level.The value below 400V are considered as low voltage level.

Fig 1. Lithium battery in Tesla model

RESULT:

Thus the study of torque speed characteristics of dc machine has been done successfully.