In this post we are going to discuss about the working of 25KVA Automatic Servo control voltage stabilizer made by Fuji Electric. If you want to know about its technical specifications, you can go through our previous post which based upon specifications of stabilizer whose link given below:

Working principle of 25KVA ASC Voltage Stabilizer made by Fuji Electric

Transformer is most important Electrical machine. Its primary function is; deliver the power from one circuit to another while doing so it can produce voltage difference at output side at same frequency. Transformer can step up & step down the voltage easily. It is the efficient Electrical machine without any moving part. That's the reason transformer is most widely used around the world.

You may think off why I'm talking about transformer in this post but Transformer is the main component behind the working of servo voltage stabilizer which we are going to discuss next. A 'Buck boost transformer' is behind the working servo voltage stabilizer.

Three phase input supply divided into 3 individual phases, Each single phase of three phases stabilize individually that's why for all the three phases there is three buck-boost transformers.

Each phase passes through HV or secondary side of transformer in a series If we produce Voltage variation on LV side; which is primary; it produces proportional variation on HV side.

{{ NOTE:

In a transformer, a voltage variation on primary side produces similar variation on secondary side. If voltage on primary side increases, voltage on secondary side increases proportionally. Similarly, the case when voltage decreases on primary, voltage decreases on secondary proportionally.

For an Example:

Let suppose we have a 500KVA transformer with 11kV/415V Voltage ratio.

HV/primary voltage is 11KV

LV/secondary voltage is 415V

Voltage ration becomes 11÷415=26.5

When voltage on primary side decreases to 10kV than voltage on secondary becomes 10kV÷26.5= 377.3V
When voltage on primary side increases to 11.5 KV than voltage on secondary becomes 11.5KV÷26.5=433.9V }}

25KVA automatic servo voltage stabilizer by Fuji

Application of voltage variation of transformer is used in this servo voltage stabilizer.

Full working is explained next in this post.

Parts of the Stabilizer

In this machine like other Electrical machines, There is two type of wirings:

Power wiring
Control wiring

We are going to explain power wiring & its components first; after that we will explain control wiring.

Power Wiring & it's components

Power wiring as name implies heavy load current flows through this wiring. Since heavy current flows through this wiring that's why heavy wires & heavy components are used in this wiring. We will explain the wiring step by step, starting from the input.

Input/Output terminals

Input/Output terminals are provided at the back down side of stabilizer. Earth+3phase+neutral inputs/outputs are connected at these terminals.

25KVA automatic servo voltage stabilizer by Fuji

INPUT/OUTPUT Terminals

MCB

Input supply then connected to the 3pole 50Amps MCB. MCB provides protection in short circuit conditions.

MCB & BYPASS SWITCH

Buck boost Transformers

From the output of MCB, all the 3phases gets divided into 3individual single phases. Voltage rectification is done on all the 3phases individually that's why 3 Buck boost transformers are used. Each Phase is connected to the secondary of buck boost transformer in a series manner.

Transformers secondary conductor is thicker in size as compare to primary conductor because load current flows at secondary & control current flows at primary.

Buck-Boost Transformer

Contactor

Three phases from all the three buck boost transformer are connected to the 3pole contactor. Contactor works when control card gives signal to the contactor. When everything is in permissible limit, control card energies the contactor & supply becomes available at the output of contactor.

Contactor MI50 by L&T

Changeover Switch

Changeover has 3 points at which 3phase rectified output supply from contactor, 3phase direct input supply & output terminals are connected.

Changeover Switch

We can connect the output terminals with servo stabilized voltage by selecting changeover at SCVS or we can connect the output directly with input by selecting the changeover switch at BYPASS or we can disconnect the output terminals at 0 point of changeover.

Control Wiring & it's Components

HMI

HMI with small display for checking of voltage on both Input & Output sides, load current & frequency is being provided.

HMI ASC Voltage stabilizer

We can ON & OFF the stabilizer output with the HMI. It also displays the error logs on display.

Parameters & setting changes, all done with the help of HMI.

Control Card

It is Brain of the stabilizer. It continuously monitors voltages on both the Input & Output sides.

It takes feedback from output voltage & produces variation to meet the pre-set value.

Control Card

Phase reversal protection, protection from single phasing, neutral failure protection, Earth-Neutral voltage cut of - all the protection commands are produced by the control card.

Variac & it's Components

Principle of operation of stabilizer; explained above. It's an action of buck boost transformer on which load current passes through the secondary of the transformer in a series configuration. Changes on secondary occurs when changes occurs on primary side.

Primary of transformer gets control supply through a Variac such that a carbon brush arrangement moves on variac with the help of single phase rugged AC synchronous motor along with gear system.

This servo motor is slow speed high torque motor, It start & stop instantly. It is 230Volts,27Watt motor. It runs at a speed of 60 rpm. An RC (Resistance & capacitor) circuit for motor functioning has been provided with the motor.

Variac

Servo Motor

This motor is capable of rotation in both forward & reverse directions. Command of movement 'HOW MUCH & IN WHICH DIRECTION' to servo motor is given by triac PCB. Triac printed circuit board is actually work as a relay card upon which 6 switch components are present.

Variac with Servo motor

Each switch components operates the motor in one direction but we need to rotate the motor in both the directions that's why we need 2 switch components for single motor. These components are control switches which switches the servo motor according to the command of main control card.

To Limit the movement of carbon brushes on variac, micro limit switches have been installed. These micro switches are installed on both the ends of variac. Command wire to servo motor is actually in series with the NC point of the limit switch.