11-04-2012, 10:53 AM
CLOSED LOOP SPEED CONTROL OF DC MOTOR WITH PWM
CLOSED LOOP SPEED CONTROL OF DC MOTOR WITH PWM.doc (Size: 159 KB / Downloads: 121)
INTRODUCTION:
Today’s industries are increasingly demanding process automation in all sectors. Automation results into better quality, increased production and reduced costs. The variable speed drives, which can control the speed of A.C/D.C motors are indispensable controlling elements in automation systems. Depending on the application, some of them are fixed speed and some of the variable speed drives.
The variable speed drives, till a couple of decades back, had various limitations, such as poor efficiencies, larger space, lower speeds, etc., However, the advent of power electronic devices such as power MOFETs, IGBTs etc., and also with the introduction of micro-controllers with many features on the same silicon wafer, transformed the scene completely and today we have variable speed drive systems which are not only smaller in size but also very efficient, highly reliable and meeting all the stringent demands of the various industries of modern era.
Direct current (DC) motors have been used in variable speed drives for a long time. The versatile control characteristics of DC motors can provide high starting torques which are required for traction drives. Control over a wide speed range, both below and above the rated speed can be very easily achieved. The methods of speed control are simpler and less expensive than those of alternating current motors.
There are different techniques available for the speed control of DC motors. The phase control method is widely adopted, but has certain limitations mainly, it generates harmonics on the power line and it also has got poor p.f when operated at lower speeds. The second method is of PWM technique, which has got better advantages over the phase control.
In the proposed project, a 5HP DC motors circuitry is designed and developed using pulse with modulation (PWM). The pulse width modulation can be achieved in several ways. In the present project, the PWM generation is done using a micro-controller.
In order to have better speed regulation, it is required to have a feedback from the motor. The feedback can be taken either by using a tacho- generator or an optical encoder or the back EMF itself can be used. In the present project, we implemented the feedback by using the EMF of the armature as the feedback signal.
The project proposed is a real time working project, and this can be further improvised by using the other safety features, such as field current, air gap magnetic flux, armature current, etc.,
CHAPTER 2
DC motor: Basic Principles
Introduction: Controlled Dc drivers are extensively used as process prime- movers in all industrial process. With the increasing emphasis on automation, accuracy and higher process speeds are specifications for the drive units also are becoming more and more stringent. Dc motors with their excellent speed torque characteristics and ease of control, have been the natural choice in industrial controlled drives. In the earlier period the Dc drives had ‘rotary converters’ and ‘mercury arc converters’ as their power regulators. Thyristors converters have distinct advantages over these two power regulators, because of the efficiency and excellent control qualities. Thus, thyristors have become economically reliable power regulators for small as well as medium and high power drive systems. The present day integrated circuits further enhance their capabilities. Thyristor controlled Dc drives are being introduced increasingly into various industries. These thyristors based drives also had some problems, they are generation of harmonics on the power line, and also the poor P.F at higher firing angles. For this reason the other newer power electronic devices such as power MOSFET and IGBT based drives are seeing the light. The IGBT based PWM drive for a Dc motor has become the latest and had several advantages. Most of the today’s controlled drive requirements are met by the PWM drives, and are efficiently used in the industry.
In the present project, micro-controller based PWM generator is used to drive the IGBT, which in turn provides a variable Dc output to the armature as required, and the speed is also regulated by the feed back provided.
In the present chapter, the Dc motor fundamentals and the control methods are discussed here with.
2.1 Motor Principle
An electric motor is the one, which converts electrical energy into mechanical energy. The action is based on the principle that when a current carrying conductor is placed in a magnetic field, it experiences a mechanical force. The magnitude of the mechanical force is dependent on the magnetic field strength and current through the conductor. The direction of the mechanical force is also determined by “Flemings left hand rule”. Thus, when a current carrying conductor is placed in the magnetic field it experiences the mechanical force, and this force rotates the wire.
On the contrary, in accordance with the laws of electromagnetic induction, when a conductor moves in a magnetic field, a emf is induced in it. In the motor this emf is called back emf. The magnitude of this emf is obtained from faraday’s second law. Generally back emf is directly proportional to the flux and speed. The direction of induced emf is obtained from lenz’s law, which states that the direction of induced emf is always opposite to the cause to produce it.
In a Dc motor, the back emf depends on armature speed, governed as follows:
a) If the speed is high, back emf is high hence Ia decreases.
b) If the speed is low, more current flows which develops more torque resulting in acceleration.
Because of thsee two reasons, the speed of a Dc motor is regulated automatically.
2.2 Types of DC Motors:
In Dc motors, basically two windings are there. They are: field and armature windings. Based on how these windings are connected, the motors can be classified as:
a) Series motors
b) Shunt motors, and
c) Separately excited Dc motors
The series and shunt motors are not useful for speed control applications, because of their limitations.
Among all existing electric motors, the separately excited Dc machines has the best ability to fulfill the demands of adjustable drive systems, as its speed can be varied over a wide range through voltage and field flux control. The possibility of speed control through these parameters gives increased matching ability to drive requirements. If the field is excited from a source separate from that supply the armature, as is usually the case with speed control drives, two distinct ranges of operations are obtained as shown in f.g.2.1.
There are many methods to obtain variable voltage for armature from Ac supply. The following are few important methods used for speed control of Dc machines:
Line commutated thyristor converters.
Uncontrolled bridge rectifier followed by chopper.
2.3 Line commutated thyristor converters for Dc motor speed control:
The basic function of the phase-controlled converter is to convert an alternating input voltage to a controllable direct voltage. In the operation of the converter, each thyristor turns on and conducts for a certain fraction of the Ac cycle time. Then the current is transferred (or commutated) to the next thyristor in the sequence. The turn-on of the thyristor is achieved simply by providing a gate pulse when the respective thyristor is forward biased. The turn off of the thyristor is achieved by natural means ie., the commutation of the current from one thyristor to the next is made to occur at a print at which the incoming Ac voltage wave had a higher instantaneous potential than that of the outgoing wave. Thus, there is a natural tendency for the current to be commutated from the outgoing to the incoming thyristor, without the aid of any additional commutating circuitry. This commutation process is often referred to as natural commutation.
The converters, they themselves can be classified as:
Single phase converters, and
Three phase converters.
Single-phase converters are normally used for motors of a capacity less than 10 Hp, usually up to 5Hp. When the motor capacity is higher than 10 Hp, one selects the three-phase converters.
In the present project, the speed control application is designed for a 5 Hp motor, thus a single-phase converter is sufficient. When it is single-phase or three-phase, the operating principle is same, and is required to convert Ac voltage Dc voltage by triggering the appropriate thyristors, ie., when that thyristor is forward biased. By varying the trigger pulse position with respect to a reference point on the Ac waveform, the Dc output voltage can be varied. The single-phase inverters can be further classified as
Center-tapped transformer version
Bridge version.
2.3.1 Center-tapped transformer version:
The circuit diagram of the center-tapped transformer version single-phase converter is shown in fig. 2.2.
From the figure 2.2, it is clear that this particular operation cannot be practically implemented, as it requires a center-tapped transformer for its use. Even the center tapped transformers utilization factor is also very poor, apart from the magnetization current flows one half of winding for only one half cycle. The other half cycle the current does not flow in that particular half winding, which leads to core saturation, and ultimately leads to saturation of transformer. For these obvious reasons, the center tapped transformer full-wave converter is not practically used in industry.
2.3.2 Full-wave controlled bridge converters:-
The single-phase controlled bridge converters had basically two versions. They are:
Half-controlled converter, and
Fully controlled converter.
The half-controlled converter has three different types of circuits, the circuit diagrams are as shown in fig 2.3. All the circuits behave in the similar fashion as far as resistive load is concerned. The waveforms are as shown in fig2.3. In the waveforms itself, the elements that are conducting during that position are also indicated.
But of the different half controlled rectifiers shown in fig 2.3, (a) & (b) work in the same fashion even with R.L load. The version shown in fig 2.3(a) is preferred, as it requires less complicated circuit as far as triggering circuit is concerned.
In case with inductive loads, the inductive current forces the conducting thyristor and same branch diode to be ‘on’ and current flows through them, but no negative pulse appears across the load. But for proper commutation of thyristors. It is advised to provide a free-wheeling diode in parellel with the load. This free-wheeling diode forces the conducting thyristor to get commutated. This happens the same way with circuits shown in fig 2.3 (a) & (b).
In the circuit shown in fig 2.3 ©, the two diodes are in the same arm and it does not require extra free wheeling diode.
These half controlled converters can be used for Dc motor speed control applications, but this does not have braking action.
2.3.3 Fully controlled converter:-
In the fully controlled converter, instead of single thyristor, two thyristors are required to be triggered in each half cycle. In this case, the conducting thyristors are kept in that state until the inductive load current becomes zero or the other set of thyristors are turned on. This conduction of thyristors during the negative half cycle of the Ac supply forces a negative pulse to appear across the load. If the firing angle is between 00 and 900, the average output voltage is positive, and the motor receives power from the supply, and thus motoring action takes place. When firing angle is between 900 and 1800, the average output voltage is negative and the energy stored in the motor is taken away by the supply and the motor is stalled. This happens only when the free wheeling diode is not connected.
When the free wheeling diode is connected, the negative pulses won’t appear across the load, because the inductor current now flows through the free wheeling diode, thus commutating the conducting thyristors. This way breaking action cannot be derived.
Thus, a fully controlled converter without free wheeling diode can be used to have both motoring and breaking actions. Motoring action is possible when the firing angle is between 00 and 900 and breaking action is possible when the firing angle is between 900 and 1800.
The circuit diagram and the associated waveforms are as shown in fig 2.4.
Though the controlled bridge converters can be used for Dc motor speed control, there are certain problems associated with these circuits. They are:
The basic characteristics of a phase-controlled converter are that the load voltage and line current waves are rich in harmonics, and the line fundamental current lags the line voltage wave, which often creates problems in the reliable operation of sensitive equipment operating on the same power line bus. It also loads the line equipment, creates interference with the communication lines, and may create harmful resonance problems with the line parameters. The recent growth of this type of power electronic equipment on utility systems is creating power quality problems that are of serious concern. The harmonics in line current and load voltage can be improved by increasing the pulse number of the converter. Again this requires a complicated transformer at the input section of the converter passive filters and active power line conditioners can combat the harmonic distortion and lagging var problems.
These problems can also be solved by PWM type rectifiers or a diode rectifier cascaded with a chopper.
2.4 Speed control using PWM:-
PWM is critical to modern digital motor controls. By adjusting the pulse-width, the speed of a motor can be efficiently controlled without the problems of line harmonics and also with improved P.F.
The general circuit diagram of a PWM based Dc motor speed control is shown in fig: 2.5. in this the Ac supply is converted to Dc by using an uncontrolled diode rectifier. The rectifier output is filtered with a capacitor, so that a constant Dc is appearing at the input of the chopper.
This fixed Dc voltage can be converted to a variable average voltage on a load by placing a high-speed switch between the Dc source and the load. The switches may be a static one such as SCR or power transistor, power MOSFET or IGBT. Depending on the power rating a power transistor, SCR, power MOSFET or IGBT may be used as a switch. The L and C components form a filter and diode D acts as a free wheeling diode. The input and output wave forms of the chopper are shown in fig: 2.6
the average output voltage of the chopper is given by:
Average output voltage > V0 =ton / ton + toff . Vi
V0 = f ton - Vi
Where f is the switching frequency.
The switching frequency is a parameter which should be carefully selected at the time of design. For high frequencies SCR can not be used because of the forced commutation circuit. If the frequency is low, the switching losers are minimum, but the filter components are like. Thus, the switching frequency is a trade off between these two. For higher switching frequencies, as one cannot use SCR. Power MOSFETS or IGBTS can be thought off.
The design details and the problems associated with such circuits are discussed in the subsequent chapters.