MODELLING AND STEADY STATE PERFORMANCE OF DUAL WINDING INDUCTION MOTOR

ABSTRACT

Dual winding induction motors have aroused quite a lot of interest and controversy over the past years. The question of whether dual winding induction motors have improved performance characteristics over  their  single  winding counterpart has  yielded varying answers.

In this research work, the equivalent circuit of the dual winding induction motor for steady- state operation is obtained through standard induction motor analysis methods. The equations derived are used to simulate the  motor performance in MATLAB/Simulink environment and comparison is made between the dual winding induction motor and an equivalent single winding induction motor.

The simulation results are presented and discussed. The results of the study show that the dual winding induction motor has a higher starting current than that of the single winding equivalent which is unattractive considering that high starting current is one of the drawbacks of the conventional motors. The high starting current is however responsible for the rather impressive starting torque in the dual winding induction motor which is desired. The starting and maximum torque of the dual winding motor are 18.4Nm and 36Nm respectively which is an improvement over those of the single winding motor (3.2Nm and

12.1Nm respectively).

There is a marked improvement in other performance characteristics of the motor under study. The power factor is at 0.9 compared to 0.7 of the single winding motor and the efficiency of the motor also improved from 78% for the single winding motor to 88% of the dual winding induction motor. This shows a 12.8% improvement in motor efficiency, and translates to reduction in energy demand by this motor.

CHAPTER ONE

INTRODUCTION

1.1      Introduction

Induction machines are perhaps the most widely used of all electric motors [1]. They are called induction machines because the rotor voltage (which produces the rotor current and the rotor magnetic field) is induced in the rotor windings rather than being physically connected by wires [2]. In normal operation, the stator is excited by alternating voltage (We consider here only poly-phase machines). The stator excitation creates a magnetic field in the form of a rotating, or travelling wave, which induces current in the circuits of the rotor. The current in turn interacts with the travelling wave to produce torque [1].

Induction machines are more robust, require less maintenance and are cheaper compared to other types of motors of equal kilowatt and speed ratings [3]. Consequently, they are used  in  a  wide  variety of  applications as  a  means  of  converting electric  power  to mechanical work. It is without doubt the workhorse of the electric power industry [4]. Although it is possible to use an induction machine as either a motor or a generator, it has many disadvantages as a generator and so is rarely used in that manner. For this reason, induction machines are usually referred to as induction motors [2]. Pumps, steel mills, hoist drives, conveyers, fans, compressors, crushers and punch presses are but a few applications  of  induction  motors,  ranging  from  small  single  phase  ones  to  large multiphase induction motors [3, 4].

A crude type of three phase induction motor was first introduced by Tesla in 1891. Subsequently, the construction of the motor was improved by using distributed stator winding and cage type of rotor [4]. An induction motor consists mainly of two parts:

a.  A stator and b.  A rotor

The stator of an induction motor is, in principle, the same as that of a synchronous motor or generator. It is made up of a number of stampings, which are slotted to receive the windings. The stator carries a 3-phase winding and is fed from a 3-phase supply. It is wound for a definite number of poles, the exact number of poles being determined by the requirements of speed with less speed requiring greater number of poles and vice versa [5].

There are two different types of induction motor rotors which can be placed inside the stator. One is called a cage rotor, while the other is called a wound rotor. A cage induction motor rotor consists of a series of conducting bars laid into slots carved in the face of the rotor and shorted at either end by large short-circuiting end-rings [2]. This design is referred to as a cage rotor because the conductors, if examined on their own, would look like one of the exercise wheels that squirrels or hamsters run on. It is worthy of note that the rotor bars are permanently short-circuited on themselves, making it impossible to add any external resistance in series with the rotor circuit for starting purposes.  The  rotor  slots  are  also  usually  not  quite  parallel  to  the  shaft  but  are deliberately given a slight skew which serves the purposes of making the motor run

quietly by reducing the magnetic hum and reducing the locking tendency of the rotor [5]. This skewing may however yield results which may or may not be desirable such as increase in the effective ratio of transformation between stator and rotor, increased rotor resistance due to increased length of rotor bars, increased impedance of the machines at a given slip and increased slip for a given torque [5].

The other type of rotor as earlier stated is a wound rotor. A wound rotor has a complete set of three-phase windings that are mirror images of the windings on the stator. The three phases of the rotor windings are usually starred internally, and the ends of the three rotor wires are tied to slip rings on the rotor’s shaft. The rotor windings are shorted through brushes riding on the slip rings. Wound-rotor induction motors therefore have their rotor currents accessible at the stator brushes, where they can be examined and where  extra  resistance  can  be  inserted  into  the  rotor  circuit.  It  is  possible  to  take advantage of this feature to modify the torque- speed characteristic of the motor [2].

Almost 90 per cent of induction motors are squirrel cage type, because this type of rotor has the simplest and most rugged construction imaginable and is almost indestructible [5]. Wound-rotor induction motors on the other hand are more expensive than cage induction  motors,  and  they  require  much  more  maintenance  because  of  the  wear associated with their brushes and slip rings. As a result, wound-rotor induction motors are rarely used [2].

Poly-phase induction motors are the most extensively used for various kinds of industrial drives and according to [5] has the following advantages:

i.        They have very simple and extremely rugged, almost unbreakable construction

(especially squirrel cage type).

ii.       They are low cost and very reliable.

iii.       They  have  sufficiently  high  efficiency.  In  normal  running  condition,  no brushes are needed, hence frictional losses are reduced. They have a reasonable good power factor.

iv.      They require minimum maintenance.

v.         They start up from rest, need no extra starting motor and have no need to be synchronized. The starting arrangement is simple especially for squirrel-cage type motor.

Some of the disadvantages are as follows:

i.        Their speed cannot be varied without sacrificing some efficiency.

ii.       Just like a dc shunt motor, their speed decrease with increase in load.

iii.      Their starting torques are somewhat inferior to that of a dc shunt motor.

Over the years, there have been attempts to further improve the efficiency and reliability of  induction  motors  using  various  winding  configurations.  In  the  early  1980’s  the Wanlass motor winding design claimed higher efficiency than the conventional winding motor [6]. The configuration uses an induction motor that has dual three phase armature windings. According  to  [6],  this  is  quite  conventional and  is  in  fact  the  technique employed in dual-voltage machines; in these machines the windings are connected in parallel for the low voltage connection and in series for the high voltage connection. In

the Wanlass motor winding design, one winding termed the control winding, is connected directly to the three-phase source, and the other winding known as the main winding, is connected in series with a capacitor that has reversed leads and a shift by one phase from the control winding [6, 7]. Another of such motors is the Roberts configuration which also has two stator windings but only one so-called power winding is connected to the source. The other winding termed the floating winding is floating with integral capacitors connected across its terminals [7]. Another distinction from the Wanlass design is that the floating winding has no reversed leads and is in phase with the power winding.

In the analysis of induction machines, [8] points out that all rotating electrical machines work on the same basic principles and that the various types differ from each other majorly in their winding arrangements and in the method of exciting these windings. The attempts to unify the various treatments of rotating electrical machines have led to the generalized theory of electrical machines.

1.2      Statement of the Problem

Contemporary induction motors such as the squirrel cage type have a number of drawbacks. For instance, when heavily loaded, they draw excessive current as the rotor slows  down,  which  can  result  in  motor  burn  out.  Such  motors  must  have  a  high breakaway torque to running torque ratio to prevent motor damage in the event of motor overload, and as a result the flux density must be maintained at considerably less than saturation levels [9]. Other problems encountered in conventional induction motors are the high starting current inherent in their operation [10] and poor starting torque

Consequently, this study focuses on a dual stator winding induction motor with integral capacitors on one of the windings. The auxiliary winding is not connected to source but is magnetically coupled  to  the  main  winding  which  is  connected to  the  source.  Both windings are identical and wound for the same number of poles and the rotor is a squirrel cage rotor. This arrangement in principle will reduce the foregoing disadvantages of conventional induction motors by providing a system in which the magnetic flux density in the stator is maintained at a maximum level. In addition, the system permits the current in the rotor also to be maintained at a large magnitude relative to those permitted in conventional electric motors of the induction type.

The steady state model of this dual stator winding motor is mathematically derived and then, analysed in the MATLAB Simulink environment to ascertain the level of improvement, if any, over the conventional winding configuration.

1.3      Aims and Objectives of the Study

The aim of the project is to develop the steady state model of the dual stator winding induction motor with integral capacitors connected to the auxiliary winding and simulate its performance using MATLAB with a view to determining whether the configuration has any advantages over the conventional winding arrangement. The specific objectives are to:

i.          Mathematically derive the steady state equations of the dual stator winding induction motor with integral capacitors connected to the auxiliary winding.

ii.       Develop the steady state model of the motor winding configuration in the

MATLAB environment and study its performance at different slips.

iii.       Compare simulation results of the dual winding configuration with capacitors with the conventional single winding arrangement of equivalent parameters.

1.4      Scope of the Study

This work covers:

i.          Modelling  and  simulation  of  the  steady  state  performance  of  dual  stator winding induction motor with integral capacitors connected to the auxiliary winding.

ii.         Comparison of the simulation results of this motor configuration with that of conventional single winding arrangement.

1.5      Significance of the Study

The ever increasing cost of electrical energy has resulted in an ardent exploration of ways in which energy losses in distribution systems can be greatly reduced. Accordingly, electric motors were prime targets for improvement in efficiency since they consume nearly two-thirds of all electric power produced [7]. The dual winding induction motor under study is one of such attempts at efficiency improvement, the success of which marks a major milestone for energy distribution systems.

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