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

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

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

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

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

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

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

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

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

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

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