Rewinding Of A 1.5HP Single Phase Induction Motor

ABSTRACT

When a motor fails, the user must decide whether to repair or replace it. To make a proper decision, one must consider the cost of the repair, the availability of a replacement, the age of the motor, the electrical design required for the application, any special mechanical features, and the urgency of returning the failed motor to service. Placing the driven equipment back into service is frequently the highest priority, and users often make their decision based on this criterion alone. Plant managers tend to be less concerned if the rewound motor is less efficient when their operation’s downtime is costing thousands

 TABLE OF CONTENTS

COVER PAGE

TITLE PAGE

APPROVAL PAGE

DEDICATION

ACKNOWLEDGEMENT

ABSTRACT

CHAPTER ONE

• INTRODUCTION
• BACKGROUND OF THE PROJECT
• AIM OF THE PROJECT
• OBJECTIVE OF THE PROJECT
• BENEFIT OF THE PROJECT
• SCOPE OF THE PROJECT
• PROBLEM OF THE PROJECT
• CAUSES OF FAULTS IN ELECTRIC MOTORS AND THEIR EFFECTS

CHAPTER TWO

LITERATURE REVIEW

• INTRODUCTION
• OVERVIEW OF AN INDUCTION MOTOR
• HISTORICAL BACKGROUND OF THE INDUCTION MOTOR
• PRINCIPLE OF OPERATION
• CONSTRUCTION REVIEW OF INDUCTION MOTOR
• EFFICIENCY OF AN INDUCTION MOTOR

CHAPTER THREE

METHODOLOGY

• MATERIALS USED
• TOOLS USED
• WINDING CONSIDERATION
• CONSTRUCTIONAL DETAILS
• REWINDING PROCESS

CHAPTER FOUR

4.0      TEST AND RESULT ANALYSIS

• CONSTRUCTION PROCEDURE AND TESTING ANALYSIS
• ASSEMBLING OF SECTIONS
• TESTING OF INDUCTION MOTOR
• RESULT

CHAPTER FIVE

• CONCLUSION
• RECOMMENDATION
• REFERENCES

 CHAPTER ONE

       1.0                                                         INTRODUCTION

AC induction Motors are everywhere, from household appliances such as water pumps and refrigerators to industry equipment such conveyor belts, right through to pumps on Navy warships. AC induction motors are electric motors which are driven by an alternating current (AC) as opposed to a direct current that drives DC Motors.

An AC induction motor generally consists of two parts – an outside stationary housing, with the stator being made up from coils in the form electromagnets arranged on the outside of the motor. This is supplied with an alternating current to produce a rotating magnetic field. The second rotating magnetic field in made up of an inside rotor, solid metal axle, loop of wire, coil and squirrel cage constructed of metal bars and interconnections attached to the output shaft.

AC induction motors have the potential for a very long life, due to the fact they have no common wear parts such as brushes and commutators – being fairly low maintenance, routine inspections are often neglected for AC induction motors.

If a motor does fail, companies may experience downtime on production – which can be costly. When faced with this problem, businesses must decide between repairing, rewinding or replacing their motor. In this work we  are focusing on the rewinding of a 1.5hp single phase induction motor.

• BACKGROUND OF THE PROJECT

An electric motor converts electrical energy into a mechanical energy which is then supplied to different types of loads. A.c. motors operate on an a.c. supply, and they are classified into synchronous, single phase and 3 phase induction, and special purpose motors. Out of all types, 3 phase induction motors are most widely used for industrial applications mainly because they do not require a starting device.

A 1- phase induction motor derives its name from the fact that the rotor current is induced by the magnetic field, instead of electrical connections. The operating principle of a 1 phase induction motor is based on the production of r.m.f.

The stator of an induction motor consists of a number of overlapping windings offset by an electrical angle of 120°. When the primary winding or stator is connected to a single phase alternating current supply, it establishes a rotating magnetic field which rotates at a synchronous speed.

The direction of rotation of the motor depends on the phase sequence of supply lines, and the order in which these lines are connected to the stator. Thus interchanging the connection of any two primary terminals to the supply will reverse the direction of rotation.

The number of poles and the frequency of the applied voltage determine the synchronous speed of rotation in the motor’s stator. Motors are commonly configured to have 2, 4, 6 or 8 poles. The synchronous speed, a term given to the speed at which the field produced by primary currents will rotate, is determined by the following expression.

Synchronous speed of rotation = (120* supply frequency) / Number of poles on the stator

A rotating magnetic field in the stator is the first part of operation. To produce a torque and thus rotate, the rotors must be carrying some current. In induction motors, this current comes from the rotor conductors. The revolving magnetic field produced in the stator cuts across the conductive bars of the rotor and induces an e.m.f.

The rotor windings in an induction motor are either closed through an external resistance or directly shorted. Therefore, the e.m.f induced in the rotor causes current to flow in a direction opposite to that of the revolving magnetic field in the stator, and leads to a twisting motion or torque in the rotor.

As a consequence, the rotor speed will not reach the synchronous speed of the r.m.f in the stator. If the speeds match, there would be no e.m.f. induced in the rotor, no current would be flowing, and therefore no torque would be generated. The difference between the stator (synchronous speed) and rotor speeds is called the slip.

The decision to rewind a motor or generator begins with an assessment of its performance and condition. The tools include testing that combines electrical measurements, such as megger, polarization index, hi-pot, surge and partial discharge levels, with a comprehensive visual assessment. Repair specialists collect these results and take into account the required reliability the customer needs before making a recommendation.

1.2                                    AIM OF THE PROJECT

The aim of this project is to rewind a 1.5hp single phase induction motor from a burnt single phase induction.

1.3                              OBJECTIVE OF THE PROJECT

At the end of this work student involved will:

1. Learn about mantling and dismantling of a burnt single phase induction motor.
2. Learn how to count number of turns of coil

1.4                                BENEFIT OF THE PROJECT

The importance of rewinding an induction motor is to reduce the cost of buying new motor, and in the course of rewinding the efficiency of the machine can be increased.

1.5                                  SCOPE OF THE PROJECT

The obvious best approach is to rewind the motor.  If the magnetic core of a failed motor is undamaged and appropriate procedures are followed, a rewound motor will retain its original efficiency.  Properly repaired, a “standard” efficiency motor will have its original “standard” efficiency, and an energy-efficient (EE) motor will have its original high efficiency.

On the other hand, those times when a motor has failed are also opportunities to upgrade motor efficiency.  Especially if the failed motor is 10 or more years old — perhaps with unknown efficiency, and possibly having been improperly rewound in the past — you will want to seriously consider all the options, and look into the economics of replacing it with a new motor.

1.6                               PROBLEM OF THE PROJECT

The problem or issues noticed in this work is difficulty with the bearings, balancing, insulation, and rotor/stator refurbishing of the rewound motor.

1.7      CAUSES OF FAULTS IN ELECTRIC MOTORS AND THEIR EFFECTS

There are six main areas where faults occur due to a variety of reasons. These areas are usually referred to as fault zones and include the power circuit, power quality, stator, rotor, insulation and air gap. There are also different factors that can lead to failures in the electric motors. These are likely to affect the above fault zones in one way or another. The main causes include;

Low insulation resistance in Electric motors: Low insulation resistance is one of the most common causes of motor failures and also one of the most difficult to handle. A low insulation resistance leads to leakages or short circuits in the coils and finally the motor malfunction and failure.

When the insulation becomes weak, it eventually breaks down and does not provide the required isolation between the conductors or motor windings. The initial resistance of the insulation of the windings is usually very high – in the order over one thousand mega ohms ( > 1MΩ). However, after some time, the insulation starts degrading due to overheating or other undesirable conditions such as corrosion, physical damage and others conditions.

To prevent such problems, the maintenance people should perform regular inspections of the insulation.

Electric motor faults due to over-Current: An electrical overload or over current condition occurs when excessive current flows inside the motor windings. This is usually more than the design current the motor winding can carry efficiently and safely. An over current may occur due to various reasons; in particular a low supply voltage will cause the motor to draw more current in an attempt to maintain its torque. Another reason is when there are short circuited conductors, excess voltage etc.

An overcurrent condition leads to overheating of the motor and damage to the insulation. And it is possible to minimize the risk of motor failures due to over current. This can be done by using reliable over current protection, to detect any over current condition, and interrupt the supply and hence stop the current.

Overheating problems in electric motors: Overheating in the motor occurs from a variety of reasons, the main one being bad power quality such as overvoltage or under voltage condition. If the supply voltage is higher that rated voltage, the excess voltage is dropped in the motor windings, resulting in to heat dissipation.

On the other hand an under voltage will lead to over current condition which results into more I2R losses in the windings. An over current condition may also happen due to shorted windings or other conditions inside the motor.

The other reason why overheating occurs in the motors is the overloading of the motor, or operating the in hot environments which are beyond the motor design or recommended temperature. A motor with excessive temperatures due power supply or operating environment will fail faster, especially if the rate of removing the heat away from the motor is low. Temperature will continue to increase as the heat generated remains in the motor, causing further increase in temperature and damage to the insulation. It is recommended to use ventilated areas, ventilation systems of cooling fans if the environment is likely to get hot.

An overheating condition existing for a prolonged period of time regardless of the cause will lead to insulation damage and damage to the motor.

Vibration in Electric motors: Vibrations can lead to several mechanical issues inside the motor and likely to happen when the motor is installed in an unstable surface.

In addition, other faults in the motor such as loose bearings, misalignments, and corrosion related issues like wear may cause the motor to have internal vibrations. This reduces the accuracy and efficiency while accelerating the tear and wear on the moving parts that are in contact with one another.

Moisture in AC electric motors: The moisture can cause a lot of problems to the motor by causing corrosion of various parts of the motor. In particular, the moisture will corrode the insulation, and lead short circuit between the windings, corrode the bearings, motor shaft and rotors. This will prevent the smooth rotation, decrease efficiency and lead to complete failure of the motor.

Faults in Electric motors due to dirt: Dirt such as dust and other debris can block the flow of air in the motor cooling fans, and lead to overheating.  In addition, dust particles and other small objects inside the motor may introduce some resistance that will slow down the motor, meaning that it will have to work harder to overcome this resistance.

The dirt particles may also be abrasive in a way to damage the insulation.