1. Electro Mechanical System
Assignment 5
Page 95 : Problems 4-13, 4-14, 4-15 and 4.16
Due Date: Tuesday 8th March, 2011
Quiz No. Next Week
Quiz 3
Lecture 15
Electro Mechanical System

2. Equivalent Circuit Model
Circuit model development
Armature circuit
Windings containing a set of identical coils and possessing a certain resistance, which can be modeled as a series resistance w.r.t. the terminals
Total armature resistance RO is measured between the terminals when the machine is at rest
Resistance is in series with the induced voltage, which is represented by a voltage source, EO
Field winding circuit
Winding containing a set of identical coils in series
Total field resistance Rf
Lecture 15
Electro Mechanical System

3. Separately excited generator under load
Consider the generator operating under constant speed and field excitation
The exciting current is controlled by a potentiometer
The induced voltage EO is fixed
The voltage at the terminals E12
Equal to the induced voltage EO at no-load current condition, I = 0
Decreases as the load current increases E12 = EO – RO(I)
EO also decreases under load because of pole-tip saturation
Lecture 15
Electro Mechanical System

4. Shunt Generator Under Load
The terminal voltage of a self excited shunt generator falls off more sharply with increasing load than that of a separately excited generator
Field current in a separately excited generator remains constant under any load
The field current in a shunt generator is a function of the terminal voltage
Increased loading causes a drop in terminal voltage and consequently a drop in excitation current
For a self-excited shunt generator the voltage drop from no-load to full-load is 15% of the full-load voltage
For separately excited generators, it is less than 10%
Lecture 15
Electro Mechanical System

5. Electro Mechanical System
Compound Generator
The compound generator prevents the terminal voltage of a shunt generator from decreasing with increasing load
A compound generator is similar to a shunt generator except that it has additional field coils connected in series with the armature circuit
These series field coils are composed of a few turns of heavy gage wire for carrying the armature load current
The total resistance of the series coils is very small
Lecture 15
Electro Mechanical System

6. Electro Mechanical System
Compound Generator
At no-load, the current in the series coils is zero
The shunt coils carry the excitation current, IX to produce the field flux
As load increases
Terminal voltage tends to drop, but the load current IC now flows through the series field coils
The mmf developed by the series field coils acts in the same direction as the mmf of the shunt field coils
The flux increases under rising load
Over compound generator
Terminal voltage increases
Lecture 15
Electro Mechanical System

7. Differential Compound Generator
In a differential compound generator, the mmf of the series field acts opposite to the shunt field
Under load, the terminal voltage falls drastically with increasing load
The series field circuit is reversed in polarity to make a compound generator into a differential compound generator
Useful in welding applications
Limits short-circuit currents
Lecture 15
Electro Mechanical System

8. Loading Characteristics
loading characteristics of several generator types
Lecture 15
Electro Mechanical System

9. Chapter 5: Direct-Current Motors
Lecture 15
Electro Mechanical System

10. Electro Mechanical System
Introduction
DC motors transform electrical energy into mechanical energy
DC motors are seldom used in ordinary industry but are still found in many special applications
Motors drive many types of loads from fans and pumps to presses and conveyors
Many loads have a definite torque-speed characteristic
Other loads have a highly variable torque-speed characteristics
Motors must be adapted to the type of loads driven
Motor types
Shunt, series, and compound connections
Lecture 15
Electro Mechanical System

11. Electro Mechanical System
Motor Operation
DC motors are built the same way as generators
Armature of a motor connected to a dc power supply
Current flows through the armature winding
Armature is within a magnetic field
A force is exerted on the windings
The force causes a torque on the shaft
The shaft rotates
Lecture 15
Electro Mechanical System