CHAPTER ONE
1.1 Introduction
The development of power electronic switches and low cost computational hardware makes AC induction motor drives now compare favorably to DC motors on considerations such as power to weight ratio, acceleration, performance, maintenance, operating environment, and higher operating speed, cost and robustness of the machine, and perhaps control flexibility are often reasons for choosing induction machine drivers in small to medium power range applications [1, 2, 3]. The need for drive system arises due to the constant motion nature of an induction motor. Variable speed drive primarily controls the frequency, voltage, torque and current of a motor, it also controls motor behavior in a unique way. Below is a typical example of variable speed drive system.
Figure 1.1: Block diagram of simple drives system.
Due to the improvement of fast-switching power semiconductor devices and machine control algorithm, more precise variable speed drive method finds particularly growing interest in industrial machines [3].
AC motor drives are widely used to control the speed of conveyor systems, blower speeds, pump speeds, machine tool speeds, and other applications that require variable speed with variable torque [4]. As seen in Figure 1.2 below; the complete system consists of an ac voltage input that is put through a diode bridge rectifier to produce a DC output which across a shunt capacitor will feed the PWM inverter. The PWM inverter is controlled to produce a desired sinusoidal voltage at a particular frequency, which is fed through to the squirrel cage induction motor.
A large variety of methods for Pulse Width Modulation (PWM) exists which are efficient in motor drive. For the AC machine drive application, full utilization of the DC bus voltage is important in order to achieve the maximum output torque under all operating conditions. In this aspect PWM method for the voltage source inverter is implored to feed the induction motor. Moreover sine triangle PWM method produces little current ripple in steady state operation but can be minimized [5].
Figure 1.2: Variable voltage/frequency ac drive system.
The speed or torque of an induction machine can be controlled by various control strategies, in this research volt/hertz control of Induction Motor (IM) for both open loop and closed loop systems using a common and efficient modulation strategy known as Sinusoidal Pulse Width Modulation (SPWM) technique, is simulated (using MatLab/Simulink) and compared in both open and closed loop volt/hertz control, and volt/hertz slip difference control showing improved performance of induction motor characteristics. This research work shows the improved volt/hertz method by using constant flux control.
Open-loop and closed loop Variable Speed Drives (VSD) are used in a wide-spread manner in numerous industrial and commercial applications. The control of most of these drives is based on the conventional scalar constant volts-per-hertz (V/f) control, [6]. In spite of the simplicity of this approach and its associated reduced processing requirement, it can provide a smooth speed-torque control over a wide dynamic range. Although this control approach cannot provide precise speed-torque control such as in the vector-controlled closed-loop systems, the scalar (Volt/hertz) control method is adequate for most of real-world drive applications e.g. fan, conveyor belt, pumping machine. This approach is in essence the workhorse of the AC motor-drive industry, [4, 5, 7]. Nevertheless, we need to understand the primary concept of how the induction motor operates thus;
When the rated AC supply is applied to the stator windings, it generates a magnetic flux of constant magnitude, rotating at synchronous speed. The flux passes through the air gap, sweeps past the rotor surface and through the stationary rotor conductors. An electromotive force (EMF) is induced in the rotor conductors due to the relative speed difference between the rotating flux and stationary conductors.
The frequency of the induced EMF is the same as the supply frequency. Its magnitude is proportional to the relative velocity between the flux and the conductors. Since the rotor bars are shorted at the ends, the EMF induced produces a current in the rotor conductors. The direction of the rotor current opposes the relative velocity between rotating flux produced by stator and stationary rotor conductors [8].
To reduce the relative speed, the rotor starts rotating in the same direction as that of flux and tries to catch up with the rotating flux. But in practice, the rotor never succeeds in catching up to the stator field. So, the rotor runs slower than the speed of the stator field. This difference in speed is called slip speed. This slip speed depends upon the mechanical load on the motor shaft. The frequency and speed of the motor, with respect to the input supply, is called the synchronous frequency and synchronous speed.
Synchronous speed is directly proportional to the ratio of supply frequency and number of poles in the motor. The slip of a machine is generally denoted as;
Slip, S= 1.1
Where Synchronous speed
N= Relative speed between magnetic field and the winding.
Synchronous speed of an induction motor is shown in the equation (1.1)
Synchronous Speed, Ns = 1.2
Where f = rated frequency of the motor
P = number of poles in the motor
Synchronous speed is the speed at which the stator flux rotates. Rotor flux rotates slower than synchronous speed by the slip speed; this speed is called the base speed [8, 9]. The speed listed on the motor nameplate is the base speed. Some manufactures also provide the slip as a percentage of synchronous speed.
1.3 Problem Description
Most industrial machines especially induction machines are so difficult to control to desired output, sometimes the machines are just operated in rated values as specified by the manufacturers (constant speed application), but with the advent of variable speed drive (VSD) power electronics have taken machine operation to a whole new level.
A Variable speed drive (VSD) plays an important role in most industrial applications, they are commonly used for energy saving and in some applications an increased productivity is achieved. Machine industries have begun to demand higher power equipment and for this reason the Variable speed drive (VSD) technology has experienced rapid changes towards improvement. The use of scalar method has emerged as the essential need for economical solution for control of small and medium industrial machines and an attractive solution to control AC drives in most industrial layout. Solid state drives have become the new trend of the drives technologies in power and industrial sector because of the importance of industrial machines to have reliable systems.
Secondly; during start of most IM great vibrations, noise and jerky nature of the machine are experienced , thereby increasing the transient period of time, and these disturbances are caused by machine inertia, noise from PWM, high stator resistance and inductance (rotor and stator). It is of my interest to study the performance of variable voltage/frequency control methods for Variable speed drives (VSD) base on speed and torque control response by the use of a simulation tool called MatLab/Simulink.
1.4 Research Objectives
The objective of this work is to design a drive system that will allow the operation and control of both open loop and closed loop techniques of variable voltage/frequency method including slip difference approach and constant flux method which also works with a three phase Induction motor whose equations will be derived. The motivation behind this research is to gain experience and understanding of the problems associated with an implementation of a variable speed PWM induction motor drive system.
Some research questions to be answered during this research are:
- What kind of induction motor is used?
- What control method is suitable and why?
- How efficient this method can be economically as compare to others?
- What kind of inverter will drive the system efficiently?
- Why we used DC voltage boost system against low voltage at low speed drop?
- Why one has to maintain constant (flux) voltage/frequency ratio?
- Showing the periods when actual values is compared to desired values were captured.
The target now is to ensure that the speed drive system is maintained at constant voltage/frequency ratio which allows the flow of constant flux from zero speed to rated values and even beyond.
Another objective of this work is to integrate the use of voltage sources inverter (VSI) being operated by low cost conserving and simple technique like the pulse width modulator (PWM) this method can be very economical for small and medium scale industrial layouts.
We will model a three phase squirrel cage induction motor, showing its DQ model in both steady state and transient analysis displaying also their characteristics performance in torque, speed, current and output voltage generated.
Finally the use of MatLab/Simulink toolbox to design and simulate this variable speed drive is an important testing ground to observe the system design operation and check for possible errors. Compare to other mathematical analyses program the MatLab/Simulink is well equipped to simulate most industrial machines process made in recent times. It posses the ability to showcase the analysis, runtime and with its powerful graphic tool (scope) show the oscillogram of various wave generated.
MatLab/Simulink will be used to achieve this stage of the research after parameters of the induction motor have been given.
1.5 Limitation of work.
This research work is limit to the use of open loop and closed loop motor control of using pulse width modulated voltage source inverter. This research does not cover the use of a more efficient techniques like space vector PWM, vector control method or direct torque control method.
The quadrature rotor voltage (Vqr) and direct rotor voltages (Vdr) are considered zero voltages because the rotor bars of the induction machine are shorted.
1.6 Scope/Outline of work.
This research outline the basic steps in producing a simple volt/hertz control on a three phase squirrel cage induction motor.
The voltage source inverter was used so as to operate easily with pulse width modulation (sinusoidal) techniques. The inverter is a two-level three phase inverter with six pulses, three phase voltage and current measurement were also taken during the course of the experiment.
A three phase fed squirrel cage induction machine was used the rotor speed, torque, stator current, rotor current, three phase voltage and current were recorded.
The machine speed and torque were altered in other to show the various characteristics of the squirrel cage motor.
Finally a very powerful mathematical simulation software (MatLab/Simulink) was used to analyzed and simulate the drive system.
In this research a major limitation was confronted during simulation of both open loop and closed loop of the machine. Even though the system is closed loop noise and vibration can still be seen in the simulation of the both the torque and the rotor speed wave form, this is due to either the techniques adopted (pulse width modulation) or the machine inertia.
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