Electrical Equipment

Question 1

Disadvantages of relays

Answer 1

-Slow operating
-Noisy
-Can weld under fault conditions (short circuit)
-mechanical wear at the moving contact

Question 2

Advantages of relays

Answer 2

• Can switch both AC and DC loads
• Can switch high currents
• Physically breaks circuit when turned off
• Can withstand high surge currents

Question 3

Diagram of a three phase synchronous motor

Answer 3

• Circle (Frame)
• Inner circle with square teeth (Stator)
• Small circles in-between square teeth (Stator windings)
• Center of circle has tall rod with both ends shaped like the head of a round headed screw (Salient pole (N/S) rotor)
• DC supply wrapping around Salient pole rotor
• 3 phase supply exiting to the right of the frame

Question 4

Advantages of a synchronous motor

Answer 4

• Can operate at a leading power factor
• Highly efficient

Question 5

Disadvantages of a synchronous motor

Answer 5

• Non-self starting
• Expensive
• requires two supplies

Question 6

Input devices for home automation systems

Answer 6

• Occupancy sensors (detects presence in a room
• Temperature sensors (offers zone heating control for individual rooms)
• Daylight sensors (adjusts internal lighting according to natural Lux levels)
• Status/pulse sensors (on/off feedback from pumps/valves)
• actuator valves (motor controlled valve operator)

Question 7

Home automation devices to control lighting

Answer 7

• Occupancy sensors
• Time switch
• absence detectors
• daylight sensors
• dimmer switch
• Scene controller (ambiance)

Question 8

How to connect a Synchronous motor to a live supply

Answer 8

• motor voltage must be the same as the supply
• Phase rotation must be the same as supply (test with rotor-scope)
• frequency must be the same as the supply
• Generated voltage must be the same as the supply

Question 9

Physical difference between a D.C motor and 3-phase induction motor

Answer 9

• 3- phase induction motor has stator windings and a squirrel cage
• 3 phase induction has no electrical connection between the rotor and stator
• D.C motors need a commutator (connects to the armature via carbon brushes (negative temp coefficient) and commutator segments)

Question 10

Advantages of home automation

Answer 10

• remote control (phone)
• can still work on normal switches
• can operate on timers
• can incorporate motion switch’s
• can incorporate dimmer switch’s

Question 11

What happens when you the vary the resistance of the shunt field in a DC generator

Answer 11

Varying the shunt field resistance will vary the slope of the resistance line, the point of intersection and hence the output voltage of the generator on open circuit.

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Question 12

What is the point of intersection on a DC generator

Answer 12

The point of intersection establishes the output voltage of the generator on open circuit