The following points highlight the three important types of static voltage regulators. The types are: 1. Servo Type Voltage Regulators 2. Magnetic Amplifier Regulator 3. Electronic or Solid State Automatic Voltage Regulator.
Static voltage regulators are those which have no moving parts, the circuits comprising of static apparatus like transformers, capacitors, resistors, transistors, magnetic amplifiers etc. Static voltage regulators are superior to electromechanical regulators in respect of accuracy of control, response, reliability and maintenance.
The important types of static voltage regulators are described below:
Type # 1. Servo Type Voltage Regulators:
The main feature of a servo type voltage regulator using an amplidyne is depicted in Fig. 17.8. The system comprises of a constant voltage main exciter driven from the alternator shaft and an auxiliary exciter whose field is controlled by amplidyne. Both the auxiliary exciter and amplidyne are driven by a dc motor coupled to both the machines.
The dc motor is supplied from the main exciter. The main exciter has a saturated magnetic circuit and hence has a roughly constant output voltage. The armatures of main and auxiliary exciters are connected in series and this series combination excites the field winding of the alternator. Thus, the auxiliary exciter controls the excitation current of the alternator by a buck-boost action.
The potential transformer (PT) provides a signal, proportional to the alternator output voltage, to a voltage sensitive bridge one arm of which consists of saturated diode. Any deviation in the output voltage of the alternator produces a corresponding change in the output of an electronic amplifier. The amplifier output feeds the amplidyne control field. The amplidyne output alters the auxiliary exciter field.
Thus, the auxiliary and the main exciter in series adjust the excitation current of the alternator, according to the load needs, to maintain its output voltage constant within the specified limits. The several time lags in the circuit needs negative feedback for stabilization of the system response. Feedback is also given from the amplifier circuits to the amplidyne as well as the auxiliary exciter.
Type # 2. Magnetic Amplifier Regulator:
In a magnetic amplifier, the basic component is a steel-cored coil with an additional winding energized by direct current. It is intended for controlling alternating current of relatively large power by means of a low power dc.
The simplest practical magnetic amplifier has two identical steel cores with identical ac windings, called the load winding, on each. In addition they have a common dc winding called the control winding. The ac windings are connected together in series or in parallel, and in series with them there is a load.
The series connection is used when a short-time response and a high voltage are required, usually in controlling loads requiring low power. The parallel connection, on the other hand, has a slow response of approximate 1 to 3 seconds, but permits control of large current. The control winding is energized by dc.
With no current in control winding, the ac winding has a large inductance, and a high reactance is offered to an ac potential. As a result alternating current to the load is limited to a low value by the inductive reactance, and the load voltage is small. When a dc voltage is applied to control winding the dc magnetic flux passes through the core and the core approaches magnetic saturation, even when a small value of dc voltage is applied.
This causes the effective inductance of the ac winding to decrease, so that the ac winding impedance drops. Consequently the alternating current flowing through the ac winding (and the load voltage) increases as the flow of direct current through the control winding increases. Thus, varying the magnitude of control current over a small range causes the load voltage to vary over a wide range.
Magnetic Amplifier (or Transconductor) Voltage Regulator:
Figure 17.10 depicts an excitation circuit using magnetic amplifier for regulating the alternator voltage. It is a simplified circuit that makes use of a magnetic amplifier (transconductor) for voltage control of an alternator. The field winding of the shunt wound exciter is connected in series with the resistor R and in parallel with this resistor is connected a single-phase full-wave rectifier I.
Single-phase full wave rectifier I is supplied from the alternator through the transconductor ac winding. The two main dc windings of the transconductors arranged to provide opposite magnetic effects are supplied from the alternator through the rectifiers II and III. For a particular value of the output voltage of the alternator, the ampere-turns of the reference and feedback control windings are equal and opposite and, therefore, their resultant is zero.
Stabilizing circuit transformer A connected across the exciter to feed an additional control winding on the transconductor is provided so as to prevent hunting. Exciter reversal is avoided by means of a small transformer II supplied from the alternator output which feeds the exciter field to assist self-excitation.
Assume that the terminal voltage of the alternator exceeds the preset value, may be due to decreased load. An increased voltage will appear across bridge rectifier I resulting in increased voltage drop across resistor R in opposition to that fed by the exciter armature. This reduces the field current of the exciter which in turn reduces the alternator excitation to reduce the output voltage of the alternator. Thus, the alternator output voltage is restored to the present value.
Type # 3. Electronic or Solid State Automatic Voltage Regulator:
Figure 17.11 depicts the block diagram of an electronic (solid state) AVR.
The output of the alternator obtained through a potential transformer (PT) is rectified, compared with reference voltage VREF and then amplified with the help of error amplifier and power amplifier stages. The amplified error is supplied into the exciter field which provides the excitation for alternator such that the terminal voltage of the alternator remains constant.
The stabilising transformer gives feedback of exciter voltage for improving dynamic response. Earlier the electronic regulators used to employ vacuum and gas tubes but the advent of semiconductors made the valve type regulators obsolete. The present day electronic regulators invariably use ICs.
Electronic voltage regulators provide the benefits of accuracy of control, high speed response, high reliability and minimum maintenance.