Electric Machinery DC Machines A.E. Fitzgerald Charles Kingsley, Jr. Sixth Edition A.E. Fitzgerald Charles Kingsley, Jr. Stephen D. Umans DC Machines

7.1 INTRODUCTION

7.1 INTRODUCTION

7.1 INTRODUCTION

7-4 7.1 INTRODUCTION Current in external armature circuit Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 7.1 INTRODUCTION Current in external armature circuit Constant determined by the design of windings Direct axis air-gap flux per pole 7-4

7.1 INTRODUCTION Rectified coil voltages and resultant voltage between brushes in a dc machine. Power Speed voltage Angular speed

Typical form of magnetization curves of a dc machine. 7.1 INTRODUCTION Typical form of magnetization curves of a dc machine.

Equivalent Circuit (Not in the textbook): 7.1 INTRODUCTION Equivalent Circuit (Not in the textbook): For steady-state, current is dc, therefore Lf and La can be neglected.

7.1 INTRODUCTION Field-circuit connections of dc machines: (a) separate excitation, (b) series, (c) shunt, (d) compound.

Volt-ampere characteristics of dc generators. 7.1 INTRODUCTION Volt-ampere characteristics of dc generators.

Speed-torque characteristics of dc motors. 7.1 INTRODUCTION Speed-torque characteristics of dc motors.

Ea Ra Rf Vt EXAMPLE (Final Exam 2006) : Assume that a 240 V self-excited shunt motor is supplied by a line current of 102.4 A when it is loaded with a full load at a speed of 1000 rpm. The armature-circuit resistance and the shunt-field circuit resistance of the motor are 0.1 ohm and 100 ohm, respectively. Assume that a breaking resistor of 1.05 ohm is used for dynamic braking (breaking means that voltage source is removed and immediately a resistor is connected to the terminals of the DC machine) and determine the following. a)The value of counter emf Ea. b)The full-load torque of the motor c)The value of the armature winding current at the time of initial breaking. d)The value of initial dynamic breaking (initial torque during breaking) Ea Ra Rf Vt

The End of This Chapter

Dc machine armature winding with commutator and brushes Dc machine armature winding with commutator and brushes. (a), (b) Current directions for two positions of the armature. Figure 7.7

Waveform of current in an armature coil with linear commutation. Figure 7.8

Armature-mmf and flux-density distribution with brushes on neutral and only the armature excited. Figure 7.9

Flux with only the armature excited and brushes on neutral. Figure 7.10

Armature, main-field, and resultant flux-density distributions with brushes on neutral. Figure 7.11

Motor or generator connection diagram with current directions. Figure 7.12

Short-shunt compound-generator connections. Figure 7.13

Magnetization curves for a 250-V 1200-r/min dc machine Magnetization curves for a 250-V 1200-r/min dc machine. Also shown are field-resistance lines for the discussion of self-excitation in Section 7.6.1. Figure 7.14

Equivalent circuit for analysis of voltage buildup in a self-excited dc generator. Figure 7.15

Cross section of a typical permanent-magnet motor Cross section of a typical permanent-magnet motor. Arrows indicate the direction of magnetization in the permanent magnets. Figure 7.17

(a) Dimension definitions for the motor of Fig. 7. 17 (a) Dimension definitions for the motor of Fig. 7.17. (b) approximate magnetic equivalent circuit. Figure 7.18

Equivalent circuit of a permanent-magnet dc motor. Figure 7.20

Section of dc machine showing compensating winding. Figure 7.22

Schematic connection diagram of a dc machine. Figure 7.24

Series-connected universal machine. Figure 7.25

Typical torque-speed characteristics of a series universal motor. Figure 7.26

1200 r/min magnetization curve for the dc generator of Problem 7.4. Figure 7.27

Series crane motor (Problem 7 Series crane motor (Problem 7.22): (a) hoisting connection and (b) lowering connection. Figure 7.28