Page 216 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

Servo - voltage stabilizer
The servo voltage stabilizer employs a toroidal auto-transformer and a servo motor driven by a sensing circuit
which senses the voltage. The difference between the output and nominal voltage is sensed by a sensing circuit
which drives the servo motor. Any variations in mains cause the motor to move clockwise or anticlockwise thus
correcting the voltage.
A servo voltage stabilizer is provided with three transformers function along with control circuits and a servo motor
as in Fig 3. T1 is a continuously variable toroidal auto-transformer (variac) driven by a servo motor M.

Fig 3

The output from the variac, drives a series buck/boost transformer T2 so that boost takes place when the variable
tap arm moves down and bucks the voltage when the arm moves up. The transformer T3 provides the required
reference voltage and sensing voltage for the electronic circuit which drives the motor.
When the output voltage is less than the reference voltage, the electronic circuit senses the difference, drives the
motor in one direction which results in increase in the output voltage.
When the output voltage increases above the ratings, the motor is driven in the opposite direction so that the
output voltage increases. When the voltage difference in output and the reference are equal, the servo motor is
switched off by the circuit.

A servo stabilizer provides constant voltage to an accuracy around ±1% or ±0.5% and a correction range 10 to
30 volt/sec.
A servo stabilizer is more accurate and also costlier, and, therefore, used with costlier equipments such as computers,
xerox machines, medical electrical equipments etc.
Constant voltage transformer
A constant voltage transformer works on ferro-resonant principle. The variation in the primary flux with an unsaturated
iron core does not affect the secondary flux with saturated iron core. Thus, the secondary induced voltage remains
relatively independent of the voltage impressed upon the primary winding.
Basics of UPS systems : Most people take the mains AC supply for granted and use it almost casually without
giving the slightest thought to its inherent defects and the danger posed to sophisticated and sensitive electronic
instruments. For ordinary household appliances such as incandescent lamps, tubes, fans, TV and fridge, the
mains AC supply does not make much of a difference, but when used for computers, medical equipments and
telecommunication systems, a clean, stable, interruption-free power supply is of utmost importance.
UPS (Uninterrupted Power Supply) is the only solution available to an individual customer faced with the problem
of ensuring high quality of power for critical loads. All UPS designs contain a battery charger to keep the battery
fully charged by the power from mains. Small UPS normally comes with a sealed maintenance free (SMF) batteries
which can provide 10 to 15 minutes of power backup, the backup time increases with the capacity of the battery.
Tubular batteries or automotive batteries are used in medium and large capacity UPSs.

UPS classification : There are two broad categories of UPS topologies - OFF line, and ON line . These topologies
differ in the way they serve the load when the mains is present and is healthy. They vary in features & pricing.
Off-line and on-Line : Off-line UPS filters the mains and feeds it directly to the load for most of the time. When the
mains is unhealthy, perhaps due to a slight drop in voltage, the load is switched by a fast relay, in typically less than
half a cycle, to an inverter deriving its power from a battery. The inverter generates a square or stepped waveform to
emulate the mains-satisfactorily for most computers. This particular technique represents the lowest cost solution.

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CITS : Power - Electrician & Wireman - Lesson 93-100 CITS : Power - Electrician & Wireman - Lesson 93-100