Page 214 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

In Fig 11, the action of Q1 - Q2 is to provide adequate gate current to trigger the SCR into conduction.
Q1 - Q2 and their associated resistors acts as a voltage sensitive switch. In each half cycle, C2 is able to
charge via R1. As soon as voltage across C2 rises to suitable value. Q1 and Q2 both switch- on and partially
discharge C2 into the gate of the SCR, thus delivering a pulse of high current to the SCR gate, independent
of any current drive limitations of RV1. The Q1 - Q2 and C2 network thus enables virtually any SCR to be used
in the circuit almost irrespective of its sensitivity characteristics.
The universal motor speed control circuit is in Fig 11 enables the motor speed to be smoothly varied from zero
to 75% of maximum via a single control. It also incorporates built - in feedback compensation to maintain the
motor speed virtually constant at any given speed setting, regard-less of load changes.

Voltage stabilizer and UPS

At the end of this exercise you shall be able to
• state the basic concept of stabilizer
• draw the block diagram and explain the function of each blocks
• state the working various types of voltage stabilizers
• state the basics of UPS system
• explain the block diagram of OFF line UPS and its various controls and functions
• explain the block diagram ON line UPS and advantages and disadvantages.

Voltage stabilizer

It is an electrical supply device controlled by electronic circuit which gives the constant output voltage irrespective
of the variation in the high input supply voltage or disconnect the output circuit if the input voltage is very low or
very high.

Every electrical device is designed to operate at a certain rated voltage for optimum efficiency and maximum
length of service. Power supply voltages should not drop or rise by more than 5% of rated voltage as per IS.
The effect of voltage variations in commonly used electrical appliances are given below.

Sl.No. Name of the equipment Low voltage High voltage
1 Incandescent lamp Lamp efficiency decreases Life of the lamp decreases or the
if the voltage is decreased. lamp fuses in extreme cases.
2 Fluorescent lamp If voltage is too low, lamp Life of the tube/choke
will not light up. decreases.
3 Electric stove, electric Increases the heating time Shortens the life of heating
iron, water heaters, as heat produced is low. elements or heating elements
toasters etc. burnt out.
4 Fans, vacuum cleaners Efficiency decreases. Life of the equipment is
decreased
5 Washing machines, Motor of the machine will draw The motor insulation may fail and
refrigerators and more current from the line that draw excess current which can
air-conditioners results in overheating of the lead to burn out.
motor which may lead to burn out.
6 Radios and television sets Poor quality of reception, Life of the equipment is decreased
picture will not be clear in the
television sets.

Some of the electronic equipment such as colour television sets are designed by the manufacturers
with built in electronic stabilizers like Switch Mode Power Supplies (SMPS). Hence there is no need
to provide an additional external stabilizers for these equipments.

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