enchancing the performance of speed control of three phase induction motor using voltage/frequency speed control technique

Description

CHAPTER ONE

1.1 BACKGROUND OF STUDY

Three-phase induction motor drives are employed in several industrial areas with a good power, ranging from few 100W to many MW. In industrial-oriented countries, more than half the total electrical energy used is converted to mechanical energy through AC induction motors. Induction motors have industrial and household applications and expend over 50% of the total generated electrical energy. Single phase induction motors are widely utilized in home appliances and industrial control. During the last few years, speed and torque control principle are asynchronous with motor drives which gained significant popularity. It is possible to combine the induction-motor structural robustness with the control simplicity and efficiency of a direct current motor. This evolution resulted to the replacement of the dc machines by induction motors in many applications in the last few years. Earlier only dc motors were employed for drives requiring variable speeds due to facilities of their speed control methods ( Zubek, 2010). The conventional methods of speed control in an induction motor are very expensive or too inefficient thus restricting their level of application to only constant speed drives. Examples include to drive pumps, fans, compressors, mixers, agitators, mills, conveyors, crushers, machine tools and cranes. They are very simple, reliable, low maintenance and low cost. Today, with advancements in power electronics, microcontrollers, and digital signal processors (DSPs), electric drive systems have improved drastically. Initially the principle of speed control was based on steady state consideration of the induction motor. Voltage/frequency control was suitable for the open-loop speed control of drives with low dynamic requirements.

There are different methods of controlling induction motor for industrial application. Voltage/frequency ratio method offers an easy way to regulate both the frequency and magnitude of the voltage applied to a motor. However, better efficiency can be obtained by these motor drives with less noise. The most rampant technique is the constant Voltage/frequency principle which requires that frequency and the magnitude of the voltage applied to the stator of a motor maintain a constant ratio. So, by this, the

magnetic field in the stator is kept almost constant for all operating points. Thus, constant torque is maintained.

Figure.1.1: Block diagram of closed loop Voltage/frequency control of induction motor

2          STATEMENT OF PROBLEM

Controlling the speed of a motor using traditional methods involve running the motor at full speed and then using mechanical means such as gears, hydraulic couplings or pulleys. This is not only expensive, but also consumes tons of energy. There are many means of controlling induction motor speed, but the most popular problem discovered in them is that motor used to experience speed instability and the efficiency of the motor drops when the motor speed is varied.

4. SIGNIFICANCE OF THE STUDY

This research work will throw more light on the best techniques for controlling the speed of electric motor. This study will also be designed to be of immense benefit to all the users of electric motor most especially in industries.

It will also serve as a guide to whoever that wants to purchase speed controller for induction motor.

Finally, it will also serve as a useful piece of information for both producers and users of electric motor speed controller.

OBJECTIVES

iii.        To model and simulate Voltage/Frequency speed control technique in  MATLAB environment.

1. To compare the perform simulation of an induction motor speed control using the model and V/F control technique.
2. REVIEW OF RELATED STUDIES

In modern countries, more than half the electricity used is converted to energy through induction motors. Induction motors are extensively utilized in industrial and household appliances and consume more than 50% of the entire generated electricity. Single-phase induction motors are widely utilized in home appliances and industrial control. During the previous couple of years, the concept of speed and torque control of asynchronous motor drives has gained significant popularity. This way, it’s been possible to mix the induction-motor structural robustness with the control simplicity and efficiency of an immediate current motor. This evolution resulted to the replacement of the direct current machines with induction motors in many applications within the previous couple of years. Earlier only dc motors were employed for drives requiring variable speeds due to facility of their speed control methods.

Speed control of an induction motor were either too extravagant thus limiting their application to only constant speed drives. They are used to drive pumps, fans, compressors, mixers, agitators, mills, conveyors, crushers, machine tools, cranes, etc. sort of electrical motor is so popular thanks to its simplicity, reliability, less maintenance and low cost. Today, with advancements in power electronics, microcontrollers, and digital signal processors (DSPs), electric drive systems have improved drastically. Consideration of the induction motor, V/f control was the commonly used method for the open-loop speed control of drives with low dynamic requirements which is the focus of this study. Below are the related works on this same study.

Deraz1, et al. (2004), has been presented a replacement current- limiting soft-starter for a three-phase induction motor drive system using pulse width modulation (PWM) chopper unique in configuration with three-phase. A good AC-chopper using only four insulated gate bipolar transistors (IGBTs) is additionally proposed. It requires just one current sensor. The duty ratio of the chopper IGBTs from the closed loop current control so as to limit the motor starting current at a preset value. Only two complementary gate pulses are obtained from the negative feedback circuit to regulate the four IGBT switches.

Pratibha Naganathanl et al,(2017),posit that phased two-level inverters voltage space vector for the three-level inverter-controlled induction motor with cascades, this study proposes a five-level torque controller (FLTC)-based torque control technique to boost steady-state motor torque performance and retaining the high dynamic performance.

1. Prathap Reddy et al,( 2003), has suggested two cascaded two-level inverters can synthesize three-level voltage space vector for three-level inverter-controlled induction motor with cascade , this study proposes a five-level torque controller (FLTC)-based direct torque control (DTC) method especially for improve steady-state motor torque performance and retaining the high dynamic performance.

Gonzalez-Prieto et al, (2005), has been presented the foremost serious and up to date competitor to the quality field oriented control (FOC) for induction motors (IM) is that the finite control set model predictive control (FCS-MPC), has discussed Direct torque control (DTC) has been widely used as an alternate to traditional field-oriented control (FOC) methods for three-phase drives. They also suggested that Direct torque control (DTC) has been recently used for the event of high efficiency in five-phase induction motor (IM) drives. This work analyzes the fault-tolerant capability of six-phase drives with parallel converter supply. Scenarios of  up to 3 faults for single and two neutral configurations are examined, optimizing off-line the post- fault currents and modifying accordingly the control strategies. Has evaluated a model to control scheme for multiphase induction machines with multi three-phase. Complete details about the predictive control scheme and adopted flux observer are included, has been suggested Direct torque control (DTC) has been recently used for the event of high performance five-phase induction motor (IM) drives in normal operation of the system and therefore the ability of DTC to manage things has been analyzed as compared with different rotor field-oriented control (RFOC) strategies.

Mario Bermudez et al,(2006), discussed Three-phase machines are the industry standard for electrical drives, but the inherent fault tolerance of multiphase machines makes them a beautiful alternative in applications requiring high reliability. This novel strategy is then combined with minimum losses and maximum torque criteria to urge a variable current injection method that minimizes the drive derating, reduces the copper losses and improves the braking transients. It was observed that the six-phase induction motor drive can perform successfully within the different zones.

Toliyat et al, (2003), in their study reviewed the high order and nonlinearity of the dynamics of an induction motor, estimation of the angle speed and rotor position without the measurement of mechanical variables becomes a challenging problem. The benefits of position and more so speed of a sensorless induction motor drives are to reduced hardware complexity and lower cost, reduce size of drive machine, eliminate of sensor cable, better noise immunity, increasing reliability and fewer maintenance requirements.

Benchaib et al (1999), presented a sliding mode controller with rotor flux estimation for induction motor drives. Rotor flux was also estimated using a sliding mode observer. Most methods are basically supported the Model Reference Adaptive System schemes (MRAS) (Cirrincione and Pucci, 2005).

Bilal et al, (2004),confirmed reactive-power-based-reference model derived in both motoring and generation modes but one among the disadvantages of this algorithm is its sensitivity to detuning within the stator and rotor inductances. An MRAS pattern is so simple but its greatest drawback is that the sensitivity to uncertainties within the motor parameters.

Ouhrouche, (2002), proposed another method based on the Extended Kalman Filter (EKF) algorithm. The EKF may be a stochastic state observer where nonlinear equations are linearized in every sampling period. An interesting feature of the EKF is its ability to estimate simultaneously the states and therefore the parameters of a dynamic process. This is generally useful for both the control and therefore the diagnosis of the method.

Negm, (2000), in his findings coined the advances in microprocessor and power electronics which provides permission to implement modern techniques for induction machines like field-oriented control also referred to as vector control. This provides higher efficiency; lower operating costs and reduces the value of drive components. In sensor-less field with oriented control, the speed or position cannot be estimated, their values are estimated using other parameters like phase voltages and current, that are directly measured. Sensorless drives are getting more and more important as they will eliminate speed sensors maintaining accurate response. While observing only the stator current and voltages, it is possible to estimate the necessary control variables.

Kyo and Frede, (2006), used the EKF pattern to simultaneously estimate variables and parameters of the IM in healthy case and under different Induction Motor faults. An extended Kalman filter was also used by Kim et al (1994) for speed estimation of vector-controlled induction motor drive. Unfortunately, Cheng and Hai, (2002) stated that this approach contains some inherent disadvantages such as its heavy computational requirements and difficult design and tuning procedure. Luenberger Observer for state estimation of Induction motor was used in. The Extended Luenberger Observer (ELO) may be a deterministic observer which also linearizes the equations in every sampling period. There is other sort of methods for state estimation that’s supported the intelligent techniques is employed within the recent years by many authors (Sbita and Ben, 2007).

Ignacio González-Prieto1, et al. (2014) reviewed a direct torque-flux control is described for a six-phase asymmetrical speed and voltage sensor less induction machine (IM) drive, based on non-linear back stepping control approach. First, the decoupled torque and flux controllers are developed supported Lyapunov theory, using the machine two axis equations within the stationary coordinate system.

Seyed Mohd. J. R. Fatemi et al, (2014) gave an overview on distributed generation (DG) into low voltage (LV) systems demands that the generation system remain grid connected during voltage sags to ensure the operational stability. A variable frequency drive (VFD) having a 440-V front-end current source rectifier (CSR) interface to a voltage source inverter (VSI) feeding a Permanent-Magnet Axial-flux Air Core motor combination is a solution for low-horsepower pump and fan control that is both power dense and compatible with a shipboard environment. Power density and efficiency comparisons are made between equivalent CSR/VSI- and voltage source- conversion-based VFDs to demonstrate that the CSR/VSI-based VFD is more power dense, has presented the implementation of the controller is based on the machine air gap flux which is measured by detecting the third harmonic component of the stator phase voltages. This new controller doesn’t require any sensors within the air gap of the machine nor does it require complex computations. Only access to the stator neutral connection is necessary to measure the air gap flux has been suggested Comprehensive analysis of the starting period of inverter-fed large induction motors proved that these motors are subjected to pulsating torsional torque. These torque pulsations may coincide with the natural torsional frequency of the massive motor system and produce hazardous shaft torque oscillations. To alleviate the torsional toque problem and limit the motor starting current, a continuing air-gap flux using slip frequency control scheme is proposed to work the motor inverter.

Yifan Tang et al, (2001), has discussed Variable-speed constant- frequency generating systems are used in wind power, hydro power, aerospace, and naval power generations to enhance efficiency and reduce friction. In these applications, the slip power recovery system comprising of doubly excited induction machine or doubly excited brushless reluctance machine and PWM converters with a dc link.

1. Rahman et al. (1995), has discussed steady state and transient operation of thyristor and diode controllers for variable voltage control of three-wire three-phase induction motors is considered.

Takayoshi Matsuo et al, (1977), has been presented the standard method of induction motor torque control uses the indirect field orientation principle during which the rotor speed is sensed and slip frequency is added to form the stator impressed frequency. For the sake of this study,two new field-oriented control schemes are presented which employ rotor end ring current detection and thereby remove the dependence of the controller accuracy on temperature so that the controller is entirely independent of rotor time constant variations. The field orientation schemes don’t require an incremental encoder for rotor position sensing. The motor torque is often accurately controlled even right down to zero speed operation. has been suggested Adjustable-speed operations of induction motors are required to maintain their maximum efficiency levels which is achievable through constant slip operation of induction motors. In applications like submersible motor pumps, variable-speed operation is additionally needed to get maximum efficiency in the least loads. To maintain a continuing slip operation of induction motors, it’s necessary to watch the motor’s speed from its shaft. Conventional methods use speed sensors attached to the shaft.

Thus from the working rule of three phase induction motor, it’s going to be observed that the rotor speed shouldn’t reach the synchronous speed produced by the stator. If the speeds become equal, there would be no such relative speed, so no emf induced within the rotor, and no current would be flowing, and thus no torque would be generated. Consequently, the rotor cannot reach the synchronous speed. The difference between the stator (synchronous speed) and rotor speeds is named the slip. The rotation of the magnetic flux in an induction motor has the advantage that no electrical connections got to be made to the rotor.

6. REVIEW OF VARIOUS METHODS OF CONTROLLING THE SPEED OF        INDUCTION MOTOR

a. CONTROL FROM STATOR SIDE

This method involves changing the supply frequency, by changing number of stator poles, by changing the supply voltage.

i. Changing the applied voltage.

From the torque equation of induction motor,

2.2

Rotor resistance R₂ is constant and if slip s is small then (sX₂)² is so small that it can be neglected. Therefore, T ∝ sE₂² where E₂ is rotor induced emf and E₂∝V

Thus, T ∝ sV², which means, if supplied voltage is decreased, the developed torque decreases. Hence, for providing the same load torque, the slip increases with decrease in voltage, and consequently, the speed decreases. This method is the easiest and cheapest, but rarely used,due to the following factors;

1. large change in supply voltage is required for relatively small change in speed.
2. large change in supply voltage will result in a large change in flux density, hence, this will disturb the magnetic conditions of the motor

b. CHANGING THE APPLIED FREQUENCY

Synchronous speed of the rotating magnetic field of an induction motor is given by,

= rev/min                                                                                               2.3

where, f = frequency of the availability and P = number of stator poles.

Hence, the synchronous speed changes with change in supply frequency. Actual speed of an induction motor is given as

N = Ns (1 – s).                                                                                                2.4

Where N = Actual speed,  = Synchronous speed, S = Slip

However, this method is not widely used. It may be used where, the induction motor is supplied by a generator (so that frequency are often easily varied by changing the speed of prime mover). Also, at lower frequency, the motor current may become too high thanks to decreased reactance. And if the frequency is increased beyond the rated value, the utmost torque developed falls while the speed rises.

1. Constant Voltage/Frequency control of induction motor

This is the foremost popular method for controlling the speed of an induction motor. As in above method, if the supply frequency is reduced keeping the rated supply voltage, the air gap flux will tend to saturate. This will cause excessive stator current and distortion of the stator flux wave. Therefore, the stator voltage should even be reduced in proportional to the frequency so on maintain the air-gap flux constant. The magnitude of the stator flux is proportional to the ratio of the stator voltage and therefore the frequency. Hence, if the ratio of voltage to frequency is kept constant, the flux remains constant. Also, by keeping Voltage /Frequency constant, the developed torque remains approximately constant. This method gives higher run-time efficiency. Therefore, majority of AC speed drives employ constant Voltage/Frequency method (or variable voltage, variable frequency method) for the speed control. Along with wide selection of speed control, this method also offers ‘soft start’ capability.

1. CHANGING THE NUMBER OF STATOR POLES

From the above equation of synchronous speed, it can be seen that synchronous speed  can be changed by changing the number of stator poles. This method is usually used for cage induction motors, as cage rotor adapts itself for any number of stator poles. Change in stator poles is achieved by two or more independent stator windings wound for various number of poles in same slots.

For example, a stator is wound with two 3phase windings, one for 4 poles and other for 6 poles for supply frequency of 50 Hz

1. Synchronous speed when 4 pole winding is connected, Ns = 120*50/4 = 1500 RPM
2. Synchronous speed when 6 pole winding is connected, Ns = 120*50/6 = 1000 RPM
3. CONTROL FROM ROTOR SIDE

This method involves three different ways:

1. Rotor rheostat control

This method is analogous thereto of armature rheostat control of DC shunt motor. But this method is merely applicable to slip ring motors, as addition of external resistance within the rotor of cage motors isn’t possible.

1. Cascade operation

In this method of speed control, two motors are used. Both are mounted on a same shaft so that both run at same speed. One motor is fed from a 3phase supply and therefore the other motor is fed from the induced emf in first motor via slip-rings.

The arrangement is as shown in following figure.

Figure. 2.4:Motor Arrangement

Motor A is called the main motor and motor B is called the auxiliary motor.
Let, N_s1 = frequency of motor A

N_s2 = frequency of motor B

P₁ = number of poles stator of motor A

P₂ = number of stator poles of motor B

N = speed of the set and same for both motors

f = frequency of the supply

Now, slip of motor A, S₁ = (N_s1 – N) / N_s1.

Frequency of the rotor induced emf in motor A,   f₁ = S₁f

Now, auxiliary motor B is supplied with the rotor induce emf

Therefore, N_s2=(120f₁)/P₂ = (120S₁f)/P₂.
Now putting the value of  S₁ = (N_s1 – N) / N_s1

2.5

At no load, speed of the auxiliary rotor is almost same as its synchronous speed.
i.e. N = N_s2.

from the above equations, it can be obtained that

2.6

With this method, four different speeds can be obtained

1. When only motor A works, corresponding speed = .Ns1 = 120f / P₁
2. When only motor B works, corresponding speed = Ns2 = 120f / P₂
3. If commutative cascading is done, speed of the set = N = 120f / (P₁ + P₂)
4. If differential cascading is done, speed of the set = N = 120f (P₁ – P₂)

iii. By injecting EMF in rotor circuit

In this method, speed of an induction motor is controlled by injecting a voltage in rotor circuit. It is necessary that voltage (emf) being injected must have same frequency as of the slip frequency. However, there’s no restriction to the phase of injected emf. If we inject emf which is in opposite phase with the rotor induced emf, rotor resistance will be increased. If we inject emf which is in phase with the rotor induced emf, rotor resistance will decrease. Thus, by changing the phase of injected emf, speed are often controlled. The main advantage of this method may be a wide range of speed control (above normal also as below normal) is often achieved. The emf are often injected by various methods like Kramer system, Scherbius system etc.

7. REVERSING THREE PHASE INDUCTION MOTORS

Three phase induction Motor rotation can be reversed by changing any two of the three lines feeding power to the motor. It is common standard practice to modify line 1 and line 3. When the motor needs to be run in a clockwise and a counterclockwise rotation, a reverse starter is needed

Reverse starters are two-three pole contactors during which one among the contactors contains one set of overloads.

Both contactors must contain a set of normally closed auxiliary contacts along with the normally open seal-in contacts. The normally closed contacts are going to be wont to provide a way of interlocking. The auxiliary interlocking is wired serial with the other coil. This method of wiring the coils through the opposite auxiliary normally closed contacts prevents the coils of the starters from being energized at the same time, which could be very dangerous even if it is wired incorrectly.

Most reversing starters also contain a mechanical interlocking device that also is a way to exclude the coils from being pressed in at an equivalent time. Often technicians will use screwdrivers to manually engage the coils of the starter. This method of troubleshooting is prohibited shortcutting and really dangerous.

Pushbutton Interlocking

Another method of interlocking is that the use of pushbutton interlocking. Pushbutton or button interlocking is a method of control wired done by the control technician. The station consists of three pushbuttons one pushbutton is a normally closed stop pushbutton and the other two are make and break pushbuttons containing both normally open and normally closed contacts.

The pushbuttons are wired in such how that permits the motor to be started in either “forward or reversing” rotation. If the load is required to spin within the other way while one among the contactors is energized, pressing the other rotation pushbutton will cause the energized coil to lose power to its seal-in contact. At this moment both contactors are now de-energized allowing the pushbutton to interact the non-running coil.

The built-in interruption of the negative feedback circuit allows time for both coils to be de-energized before the other coil are often started. To ensure the utmost safety for personnel and equipment, all three methods of interlocking should be implemented.

Reversing Starter Operation

Figure. 2.6: Configuration 1: Reversing Three Phase Induction Motor

The Reversing Starter in Configuration 1 operates as follows:

• The contactors controlling the forward or the reverse rotation of the motor can be started by pressing the forward pushbutton or the reverse pushbutton.
• If the forward pushbutton is pressed power will be sent to the coil through the reverse auxiliary contact.
• The coil will be energized through the forward normally open seal-in contact.
• At this time the reverse pushbutton has been isolated out of the circuit because the forward auxiliary contact is open so the reverse contactor cannot be energized            simultaneously or at an equivalent time the forward contactor is running.
• In order for the reverse pushbutton to be pressed and turn the motor in the reverse direction, the stop pushbutton must be pressed releasing the power from the normally open seal-in contact of the forward motor starter.

8. Elements of Software Development

Developing useful computer program and application require a substantial amount of detailed planning. The importance of program design cannot be overemphasized. It is easier, faster, and cheaper to design a good program than to try to fix and maintain a poorly designed done. Application and program development is a continuous process which is called the ‘software development lifecycle.

A brief description of the steps is given below:

Step one: Column definition

Computer programs are written because there is a problem to be solved or a need to be satisfied. Problem definition is perhaps the most crucial stage in the development of a program.

Step two: Analysis and design

The analysis step takes the problem definition and produces all necessary documents and performs all necessary activities that are required before the computer program can be written.

Step three: Language Selection

Different programming languages possess different characteristics. In selecting a particular programming language, these characteristics must be weighed against the nature of the problem to be solved.

Step four: Program coding

This is the process of writing the necessary instructions in the language selected in step three to solve the problem defined in step one. The computer programmer follows the plane and documents in the previous steps. This ensures that the software actually accomplishes the desired result.

Step five: Testing and Debugging

Testing and debugging are vital steps in the development in a computer program .In general, testing is the process of making sure that the program performs as intended, while debugging is the process of locating and eliminating errors.

Step six : Documentation

The next step is to document the program. For most applications, two different types of documentations are required, technical documentation and user’s documentation.

Analyst and programmers use the technical documentations in case there are any problems or if the program needs modification. The user documentation is developed for individuals who use the program.

Step seven: Implementation

The last step in developing a new computer program is implementation. Implementation is the process of taking the program and taking it into operation

Step eight: Maintenance

The useful life of a program depends on the initial design and the extent to which the hardware changes over time. The major cost of program maintenance is due to users request normally for program enhancement.

9. SIMULATION OF THE MODELS

The equation (3.38) developed for the three-phase induction motors were modeled and simulated using Matlab. Simulations were done to determine the effects of changing parameters on the torque and speed performance characteristic of the three-phase induction motors and the Matlab script and model is presented in the appendix below.

9.1 MATLAB software

The acronym MATLAB stands for Matrix Laboratory and it is developed by Mathworks. MATLAB is a high-performance language for technical computing. It integrates computation, visualization, and programming in an easy-to-use environment where problems and solutions are expressed in familiar mathematical notation. Typical uses include: Math and computation, Algorithm development, Modeling, simulation, and prototyping, Data analysis, exploration, and visualization, Scientific and engineering graphics, Application development, including Graphical User Interface building. MATLAB is an interactive system whose basic data element is an array that does not require dimensioning. This allows users to solve many technical computing problems, especially those with matrix and vector formulations, in a fraction of the time it would take to write a program in a scalar non-interactive language such as C or Fortran.

The sequence to the simulation of the developed models in Matlab is given in the flow sequence below.

Figure 3.3:  Project Flow Sequence

9.2 Data Collection

The data used for this work comprise machine parametres gotten from published literatures online. The following data are used for this work

Table 3.1: SIMULATION PARAMETER

Parameter	Value
	Base	Sensitivity
Line to Line supply voltage (RMS value)	400 V	400V, 440V, 480V, 520V, 560V
Stator Resistance	0.1 ohm	0.1ohm, 0.15ohm, 0.2ohm, 0.25ohm, 0.3ohm
Rotor Resistance	0.15 ohm	0.15ohm, 0.2ohm, 0.25ohm, 0.3ohm, 0.35ohm
Number of poles	4	4, 6, 8, 10, 12
Frequency	50 Hertz	40 Hertz, 50 Hertz, 60 Hertz, 70 Hertz, 80 Hertz
Stator leakage resistance @ 50 Hz	0.75 ohm	0.55ohm, 0.65ohm, 0.75ohm, 0.85ohm, 0.95ohm
Rotor leakage resistance @ 50 Hz	0.75	0.55ohm, 0.65ohm, 0.75ohm, 0.85ohm, 0.95ohm
V/F ratio	8 V/Hz	6V/Hz, 7V/Hz, 8V/Hz, 9V/Hz, 10V/Hz
Power Rating	1.8 kw

 9.3 Simulation

The equation (3.38) developed above were modeled in Matlab. Matlab scripts were written for the equations developed and the result are seen in figure 4.1 to figure 4.9 below. Nine script files were written to accommodate the nine sensitivity analyses performed on the investigation of the effect of changing parameters on the three-phase induction motors. The Matlab codes are presented in the appendix.

10. PRESENTATION OF MATLAB SIMULATION RESULTS

The results for the Matlab simulations of equation (3.38) are presented in this section. Simulation was done on the torque Vs speed for the three-phase induction motor. Various sensitivities were performed to investigate the effect of the changing parameters on the torque-speed value for the three-phase induction motors. It is pertinent to note that the target of the simulation is to determine factors that will lead to enhancement in the speed control of the three-phase induction motor. Thus, the evaluations made herein are geared towards improvements in characteristics of the voltage frequency method proposed in this study. Graphs are presented in this section to represent the steady state behaviour of the three-phase induction motor.

10.1 Variable Stator resistance

The base stator resistance used was 0.1 ohm. Sensitivities were performed on the following values of stator resistances: 0.1 ohm, 0.15ohm, 0.2ohm, 0.25ohm, 0.3ohm

Figure 4.1: Torque -speed at varied stator resistance.

The simulation ran from the Matlab script written for variable stator resistance method is represented by Figure 4.1. From Figure 4.1, it can be observed that the lower the value of the stator resistance, the higher the torque but the speed remains constant. The highest torque was observed for 0.1ohm (blue line) with a value of 35 Nm. Conversely, the lowest torque was observed for 0.3ohm (the green line) with a value of 30.5 Nm. The difference in torque from a stator resistance of 0.1ohm to 0.35ohm (0.25-ohm difference) is 4.5 Nm. Furthermore, from figure 4.1, it can be observed that all the stator resistances considered all peaked at the same value of rotor speed. They all peaked at 1350 rpm. It is evident from the graph that stator resistance is inversely proportional to the electromagnetic torque at constant rotor speed. Higher torque is produced at smaller values of stator resistance and thus the degree of speed control is minimal. This maximum torque could be applied during starting. Once the motor is started, external resistance could be reduced to obtain high starting torque necessary for the range of acceleration. If this is achieved, then the heat loss due to temperature rise would be reduced because the heat loss is directly proportional to the resistance of the system.

10.2 Variable rotor resistance.

The effect of varying rotor resistance on the torque and speed of the three-phase induction motor was evaluated using the following rotor resistances: 0.15ohm, 0.2ohm, 0.25ohm, 0.3ohm, 0.35ohm.

Figure 4.2: Torque -speed at varied rotor resistance.

The result of the simulation of the Matlab script written for the Torque-speed for varied rotor resistance is illustrated in Figure 4.2. From Figure 4.2, it can be observed that higher rotor resistance yields higher torque. From Figure 4.2, the maximum torque produced is independent of the rotor resistance as all the rotor resistances peaked at 35 Nm torque value. Nevertheless, the rotor resistances did not achieve their peak torque at the same rotor speed. The higher the rotor resistance the higher the speed at which the torque peaks. For instance, for rotor resistance value of 0.35 ohm, the torque peaks at 35 Nm and the corresponding value of the speed is 1156 rpm, while a rotor resistance of 0.1 ohm achieved the peak torque at 35 NM but the corresponding rotor speed is 1350 rpm. Thus, the higher the rotor resistance, the lower the rotor speed.

10.3 Varying number of poles

To see the effect of varying number of poles on the torque-speed performance, the following poles were analyzed: 4, 6, 8, 10, 12 poles

Figure 4.3: Torque -speed at varied poles.

The simulation of the Matlab script written for the effect of number of poles on the torque-speed characteristics of the three-phase induction motor is represented by figure 4.3. From Figure 4.3 it can be observed that as the number of poles increases the torque increases and the speed decreases. Thus increasing the number of poles increases the torque value and reduces the rotor speed. For 4 poles, the torque value peaked at 35 Nm and the corresponding rotor speed is 1350 rpm, but for 12 poles, the torque peaked at 105 Nm and the corresponding rotor speed is 444 rpm. Thus as the pole increases, the speed decreases (slip increases) and the electromagnetic torque increases.

10.4 Varying the supply voltage

The supply voltage was varied and the effect was investigated in the torque-speed characteristics performance of the three-phase induction motor. The following line-to-line RMS voltage were investigated: (400v, 440v, 480v, 520v, 560v).

Figure 4.4 Torque -speed at varied supply voltage

 

 

The simulation of the Matlab script written for the effect of varying supply voltage on the torque-speed characteristics of the three-phase induction motor is represented by Figure 4.4. From figure 4.4, it can be observed that as the supply voltage increases, the torque increases and the speed remains constant. To understand this, for a voltage of 400v, the torque peaked at 35 Nm corresponding to 1350 rpm. While for 560v, (green line) the torque peaked at 68.2 Nm corresponding to 1350 rpm. Thus as the voltage increases, the torque increases while the rotor speed remains constant (i.e. at the same slip).

10.5 Varying the line frequency

The line frequency was varied and the effect was investigated in the torque-speed characteristics performance of the three-phase induction motor. The following line frequencies were investigated: 40Hz, 50Hz, 60Hz, 70Hz, 80Hz.

Figure 4.5: Torque -speed at varied line frequency

The simulation of the Matlab script written for the effect of varying line frequency on the torque-speed characteristics of the three-phase induction motor is represented by Figure 4.5. From Figure 4.5, it can be observed that as the line frequency increases, the torque decreases and the speed increases (or slip decreases). For a line frequency of 40Hz, the torque observed from figure 4.5 is 43.6 Nm while the rotor speed is 1076 rpm. But for a line frequency of 80Hz, the peak torque was observed at 22 Nm while the corresponding rotor speed was 2152 rpm.

10.6 Varying the stator leakage reactance at 50 Hz

The stator leakage reactance was varied and the effect was investigated in the torque-speed characteristics performance of the three-phase induction motor. The following stator leakage reactance were investigated: 0.55ohm, 0.65ohm, 0.75ohm, 0.85ohm, 0.95ohm.

Figure 4.6: Torque -speed at varied stator leakage reactance at 50 Hz

The simulation of the Matlab script written for the effect of varying stator leakage reactance on the torque-speed characteristics of the three-phase induction motor is represented by Figure 4.6. From Figure 4.6, it can be observed that as the stator leakage resistance increases, the torque increases and the speed remains constant. For a stator leakage reactance of 0.55ohm (the blue line), the torque peaked at 40Nm corresponding to 1350 rpm. While for 0.95ohm, (green line) the torque peaked at 32 Nm corresponding to 1350 rpm. Thus as the voltage increases, the torque increases while the rotor speed remains constant (i.e. at the same slip).

10.7 Varying the rotor leakage reactance at 50 Hz

The rotor leakage reactance was varied and the effect was investigated in the torque-speed characteristics performance of the three-phase induction motor. The following rotor leakage reactance were investigated: 0.60ohm, 0.64ohm, 0.68ohm, 0.72ohm, 0.76ohm.

Figure 4.7: Torque -speed at varied rotor leakage reactance at 50 Hz

The simulation of the Matlab script written for the effect of varying rotor leakage reactance on the torque-speed characteristics of the three-phase induction motor is represented by Figure 4.7. From Figure 4.7, it can be observed that as the rotor leakage reactance increases, the torque increases and the speed remains constant. For a rotor leakage reactance of 0.6ohm (the blue line), the torque peaked at 36.8 Nm corresponding to 1350 rpm. While for 0.95ohm, (green line) the torque peaked at 28.3 Nm corresponding to 1350 rpm. Thus as the voltage increases, the torque increases while the rotor speed remains constant (i.e. at the same slip).

10.8 Varying frequency at constantV/F ratio

The line frequency was varied but the V/F ratio was kept constant at and the effect was investigated in the torque-speed characteristics performance of the three-phase induction motor. The following line frequency were investigated: 40Hz, 50Hz, 60Hz, 70Hz, 80Hz.But the V/F ratio was maintained at 8 V/Hz

Figure 4.8: Torque -speed at varied frequency and constant V/F ratio

The simulation of the Matlab script written for the effect of varying line frequency on the torque-speed characteristics of the three-phase induction motor is represented by Figure 4.8. From Figure 4.8, it can be observed that as the line frequency increases at constant V/F ratio, the torque increases and the speed also increases (or slip decreases). For a line frequency of 40Hz, the torque observed from figure 4.8 is 27.9 Nm while the rotor speed is 1080 rpm. But for a line frequency of 60Hz, the peak torque was observed at 42 Nm while the corresponding rotor speed was 1614 rpm. Thus increasing the line frequency at constant V/F ration increases the electromagnetic torque and also increases the rotor speed (decreased slip).

10.9 Varying frequency and Varied V/F ratio

In this part, the line frequency was varied and the V/F ratio was also varied and the effect was investigated in the torque-speed characteristics performance of the three-phase induction motor. The following V/F ratio were investigated: 6V/Hz, 7V/Hz, 8V/Hz, 9V/Hz, 10V/Hz,

Figure 4.9: Torque -speed at varied V/F ratio

The simulation of the Matlab script written for the effect of varying V/F ratios on the torque-speed characteristics of the three-phase induction motor is represented by Figure 4.9. From Figure 4.9, it can be observed that as the V/F ratio increases, the torque increases and the speed remains constant. For V/F ratio of 6V/Hz (the blue line), the torque peaked at 19.8 Nm corresponding to 1350 rpm. While for 10V/Hz, (green line) the torque peaked at 54.8 Nm corresponding to 1350 rpm. Thus as the voltage increases, the torque increases while the rotor speed remains constant (i.e. at the same slip).

11. RESULTS FOR HARDWARE DESIGN IMPLEMENTATION

The results for the hardware design and implementation are given and discussed in this section

11.1     Transformer Rectification Process

Figure 4.10: Transformer Rectification

TR1 is as step down transformer from 315v to 12v, br1 is bridge rectifier which rectifiers the out coming voltage from tr1, c1 filter the rectified voltage, R1 and R2 forms a voltage divider, this sub circuit create a power supply to the micro-controller which is used to control  F/v of the induction motor.

11.2 Micro-controller Sub-Process

Figure 4.11 Micro-controller Sub-Process

U2 is a controller which control the Frequency/Voltage of an induction motor, U2 is the heart bit of the circuit, U1 and U2 forms the control card of the entre circuit and some biasing component.

11.3     Driver Sub-Process

Figure 4.12: Driver sub-process

U3 to U5 are drivers from the controller which drives the triacs in three phase mode which control the F/V induction motor, R7 to R9 limit the current from U3 to U5

4.2.4 Circuit Diagram and Principle of Operation

Figure 4.13: Circuit Diagram and Analysis

12. PRINCIPLE OF OPERATION

The main of 220v a.c passes through bridge rectifier BR₂ which rectifies the incoming a.c into a pulsating dc., C₂ filters the rectified voltage form BR₂. TR₁ is a step-down transformer, which step down the main of 220v to 230v into 12v aC, BR₁ rectifies the 12v ac form TR₁ into pulsating d.c C₁ filters BR₁. U₁ is 7805 regulator which regulator which regulate the input Dc volt 12v to 5v dc, C₄ filters the regulated voltage to U₂. U₂ is the heart beat of the control circuit which control the voltage and frequency and also generate pulses to fire U₃, U₄ and U₅. U₂ pin 15 and pin 16 is connected to a crystal oscillator which oscillates at 20MH_Z driving U₂ into oscillation. U₂  pin 2, pin 3, pin 8 and pin 9 are used for controlling the frequency  and voltage. U₃ , U₄,  and  U₅ are opta isolated drivers. R₇, R₈ and R₉ are current limiter which protects U₃ to U₄ from excess current. V₆, V₇ and V₈ are power amplifiers which amplifies the signal form U₃, U₄ and U₅ to the induction motor.

13. CONCLUSION

Induction motors are the most widely used electrical motors due to their reliability, low cost and robustness. However, induction motors do not inherently have the capability of variable speed operation. Due to this reason, earlier dc motors were applied in most of the electrical drives. But the recent developments in speed control methods of the induction motor have led to their large scale use in almost all electrical drives.

Several techniques of induction motor speed control have been developed; these include: pole changing, frequency variation, variable rotor resistance, variable stator voltage, constant V/f control, slip recovery method etc. The closed loop constant V/f speed control method is most widely used; in this method, the V/f ratio is kept constant which in turn maintains the magnetizing flux content so that the maximum torque remains unchanged. Thus, the motor is completely utilized in this method.

During starting of an induction motor, the stator resistance and the motor induction (both rotor and stator) must be kept low to reduce the steady state time and also to reduce the jerks during starting.