Dynamic Analysis Of Shunt Wound D.C Motor

ABSTRACT

This work is on analysis of a shunt wound dc motor. Shunt DC Motors operate on direct current. As such, the field windings and armature are connected in a parallel combination, and in electrical terminology a parallel combination is known as a shunt. This type of motor is a “shunt-wound” DC Motor and the type of winding is called a shunt winding. The construction of a DC Shunt Motor is the same as any other DC motor. It contains all the fundamental parts, which include a stator (field windings), a rotor (also known as armature), and a commutator. This work focuses on the analysis of a Shunt wound DC Motor.

CHAPTER ONE

1.1                                                        INTRODUCTION

Almost every mechanical movement that we see around us is accomplished by an electric motor. Electric machines are a means of converting energy. Motors take electrical energy and produce mechanical energy. Electric motors are used to power hundreds of devices we use in everyday life. Motors come in various sizes. Huge motors that can take loads of 1000’s of Horsepowerare typically used in the industry. Some examples of large motor applications include elevators, electric trains, hoists, and heavy metal rolling mills. Examples of small motor applications include motors used in automobiles, robots, hand power tools and food blenders.

Micro-machines are electric machines with parts the size of red blood cells, and find many applications in medicine. Electric motors are broadly classified into two different categories: DC (Direct Current) and AC (Alternating Current). Within these categories are numerous types, each offering unique abilities that suit them well for specific applications. In most cases, regardless of type, electric motors consist of a stator (stationary field) and a rotor (the rotating field or armature) and operate through the interaction of magnetic flux and electric current to produce rotational speed and torque. DC motors are distinguished by their ability to operate from direct current.

A DC motor is any of a class of electrical machines that converts direct current electrical power into mechanical power. The most common types rely on the forces produced by magnetic fields. Nearly all types of DC motors have some internal mechanism, either electromechanical or electronic, to periodically change the direction of current flow in part of the motor. Most types produce rotary motion; a linear motor directly produces force and motion in a straight line.

DC motors were the first type widely used, since they could be powered from existing direct-current lighting power distribution systems. A DC motor’s speed can be controlled over a wide range, using either a variable supply voltage or by changing the strength of current in its field windings.

There are different kinds of D.C. motors, but they all work on the same principles. In this chapter, we will study their basic principle of operation and their characteristics shunt wound dc motor is analyzed. The objectives for this work are listed below.

1.2                                              OBJECTIVES OF THE STUDY

• Understand the basic principles of operation of a shunt wound DC motor.
• Understand the operation and basic characteristics of simple shunt wound DC motors.
• Compute electrical and mechanical quantities using the equivalent circuit. .
• Study some applications of shunt wound DC motors.

1.3                                            APPLICATION OF THE STUDY

The characteristics of a shunt-wound motor give it very good speed regulation, and it is classified as  a  constant  speed  motor,  even  though  the  speed  does  slightly  decrease  as  load  is  increased. Shunt-wound motors are used in industrial and automotive applications where precise control of speed and torque are required.

1.4                               CHARACTERISTICS OF DC SHUNT MOTORS

Torque vs. armature current (Ta-Ia)

In case of DC shunt motors, we can assume the field flux ɸ to be constant. Though at heavy loads, ɸ decreases in a small amount due to increased armature reaction. As we are neglecting the change in the flux ɸ, we can say that torque is proportional to armature current. Hence, the Ta-Ia characteristic for a dc shunt motor will be a straight line through the origin. Since heavy starting load needs heavy starting current, shunt motor should never be started on a heavy load.

Speed vs. armature current (N-Ia)

As flux is assumed to be constant, we can say N ∝ Eb. But, as back emf is also almost constant, the speed should remain constant. But practically, as well as Eb decreases with increase in load. Back emf Eb decreases slightly more than, therefore, the speed decreases slightly. Generally, the speed decreases only by 5 to 15% of full load speed. Therefore, a shunt motor can be assumed as a constant speed motor. In speed vs. armature current characteristic in the following figure, the straight horizontal line represents the ideal characteristic and the actual characteristic is shown by the dotted line.