T1

Question 1

five main components of PLC

Answer 1

• input interface
• memory
• micro processor
• output interface
  -power supply

Question 2

explain the purpose of a opto-coupler

Answer 2

the purpose of an opto-coupler to provide electrical isolation between the input and output circuits. the operation of the circuit is as follows: an electronic signal is input through the photo diode generating a proportional light output. the phototransistor is exposed to the light and generates a proportional current output. in this way, there is no electrical connection between the input and output, and these circuits are thereby islolated from one and another.

Question 3

describe to bus wiring (topology) system can provide distributed control in a large automation system and list the main benefits of a distributed control system

Answer 3

bus: a bus topology consists of one continuous cable, commonly called the backbone cable, subsystems each containing its own microprocessor can be connected along the cable, and the information travelling along the entire cable can be shared amongst the subsystems. subsystems and devices can be isolated and removed from the bus without disturbing the entire network

benefits:
- performance is faster
-control functions are much closer to the plant being controlled

Question 4

describe the procedures required to connect a large three phase synchronous motor to the mains electrical supply

Answer 4

1. the synchronous motor must be run up to synchronous speed.
2. then the field of the synchronous motor should be excited to produce the voltage as the mains electrical supply.
3. the machine may now be connected to the mains electrical supply synchronised by closing the motor contactor when the motor emf is in phase with mains supply emf

Question 5

explain how arcing horns protect the windings of a power tranformer

Answer 5

arcing horns are connected between the incoming line and the earthed tank of the transformer. if a high voltage surge comes down the line, the high voltage will cause the surge to jump the air-gap between the horns and discharge to earth, the surge is thus prevented from entering tranformers and damaging the insulation of the windings

Question 6

explain the principle of operation of a triac and state typically applications

Answer 6

a triac is a single device containing two thyristors connected in parallel but in opposition and controlled by the same gate. it is two directional thyristors which is triggered on both halves of each cycle of a a.c. input by either positive or negative gate signals. the power available to the load can therefore be varied between zero and full load. the triac connections are called main terminals 1 and 2 are applied between G and MT1

the dimming of electric lamps and the speed control of electric motors

Question 7

list the types of input devices that may be found connected to a home automation system and describe how their applications can improve the energy efficiency of the building?

Answer 7

occupancy sensors: can detect presence within a room and automatically control the artificial lighting accordingly through the home automation system

Question 8

list the types of input devices that may be found connected to a home automation system and describe how their applications can improve the energy efficiency of the building?

Answer 8

temperature sensors: can provide feedback to the home automation system to prevent the room temperature rising above the set point

Question 9

list the types of input devices that may be found connected to a home automation system and describe how their applications can improve the energy efficiency of the building?

Answer 9

day/lux level sensors: can provide daylight linking feedback to the home automation system, when daylight levels are highly within a room the home automation controller can dim the artificial lighting

Question 10

list the types of input devices that may be found connected to a home automation system and describe how their applications can improve the energy efficiency of the building?

Answer 10

status or pulse sensors: provide simple binary on/off feedback to the automation system.

Question 11

in relation to home automation systems (smart homes), list four devices which may be found connected to the system for the purpose of controlling the lighting with the home

Answer 11

dimming
time clock
daylight sensor
scene controller

Question 12

state the main advantage of using a synchronous motor in a large commercial/ industrial premises and describe how a buildings electrical system can benefit from the installation of a synchronous motor

Answer 12

the main advantage of a synchronous motor is its ability to operate at a leading power factor. this means that in a large commercial/industrial installation requiring power factor correction a synchronous motor can be used instead of static capacitors for the power factor correction with the added bonus of the mechanical output from the motor. the leading power factor is achieved by over exciting the rotor

Question 13

describe the function of the two safety features installed within the bucholtz chamber

Answer 13

1: the bucholtz relay consitis of a gas trap chamber with two float switches on switch is in the upper chamber and is activated by an accumulation of gas caused by a small breakdown of insulation. the upper switch only activates an alarm, which indicates that the oil requires replacement or filtration
2: the second is in the flow line and responds to absence of oil due to a leak or a large internal breakdown which causes a sudden large build up of gas. the presence of large bubbles operate the lower float switch which disconnects the transformer

Question 14

relays and transistors are two separate types of internal programmable logic control switching modules used individually to turn on the electrical supply to connected output devices. list one advantage and disadvantage for each type of switching module.

Answer 14

RELAY OUTPUT MODULE:
Advantage: suitable for switching a.c. and d.c. switching.
provides real separation when when the contact is open.
can withstand high surge currents
Disadvantage:
subject to mechanical failure and contact corrosion.

Question 15

relays and transistors are two separate types of internal programmable logic control switching modules used individually to turn on the electrical supply to connected output devices. list one advantage and disadvantage for each type of switching module.

Answer 15

TRANSISTOR OUTPUT MODULE:
advantage:
silent, no moving parts
does not suffer from contact water
capable of very high speed switching.
disadvantage:
strictly for d.c. switching
easily destroyed by overcurrent and high reverse voltages.

Question 16

describe the function of an opto-coupler protecting an output module within a programmable logic controller

Answer 16

opto-coupler provided electrical isolation and limit the effects of interference. each outgoing is routed through an opto-coupler for isolation

Question 17

state the one main advantage and disadvantage of the operation of a synchronous motor.

Answer 17

ADVANATAGE:
an overexcited synchronous motor can lead power factor correction
DISADVANTAGE:
the synchronous motor is not self starting.

Question 18

outline the main reason why automatic tap changes are used in the windings of electrical power distribution transformers.

Answer 18

increased load demand on the supply network can cause increased volt drops on electrical power transmission & distribution lines which can lead to a wide range of problem inside a consumers installation. to overcome these problems the electrical supply authorities, incorporates an automatic a tap changer in the windings of a transformer, to change the turns ratio as load fluctuates.