Phase 4 Flashcards
In a capacitor start, single phase induction motor, the capacitor is used only during the starting operation. Explain the purpose of the capacitor.
The purpose of a capacitor in a capacitor start single phase motor is to give approx 90 degree phase displacement between the currents in the windings. Disconnected once 75% of synchronous speed reached.
List the various types of split phase induction motors, explain how they operate (3)
The three types of split phase induction motors are :
- Reactor start split phase induction motor, uses a choke in series with the running winding and is shorted out once 75% synchronous speed reached.
- Capacitor start split phase induction motor, uses a capacitor in series with the start winding to give 90 degree phase displacement between the currents.
- Capacitor start, Capacitor run, similar to capacitor start , but one capacitor is left in circuit and one is switched out once the motor runs to speed.
Fort a split phase induction motor, list the various methods of switching the start winding, provide an explanation for each switching method.
- Centrifugal switch, has 2 loaded weights, attached to springs designed to swing outward under centrifugal force.
- Current operated relay, has low impedance, when current energizes the relay, the N/O contact closes.
- Thermal type relay, uses a bi-metal strip which accentuates a N/C contact.
- Potential type relay, has high impedance, N/C contact opens when P.D is detected at 75% synchronous speed.
- Solis state relay, the thermistor gets current from the starting winding.
How would you identify a split phase induction motor?
The two windings are displaced physically by 90 degrees.
Start winding wound in lighter gauge.
Induction motor rotor speeds
Formulas used ?
Ns = f x 60 / P - synchronous speed
Nr = Ns (1-s) - Rotor speed
Briefly explain how the single phase repulsion motor works?
Single phase repulsion motors have 2 carbon brushes which are short circuited. When connected to A/C supply, an EMF is induced. A force of repulsion is set up between the rotor and stator, which creates torque. The direction of the motor depends on the position of the brushes.
Describe the design feature built into a single phase universal motor to make it operate more efficiently?
The universal series motor has its stator laminated to reduce eddy currents. Shellac is used.
Describe the essential difference between the repulsion motor and the universal series motor?
The main difference between both motors is the starting torque.
Repulsion motor 250% full load current.
Universal motor 450% full load current.

Universal can work with AC and DC current
Explain why the design of the shaded pole induction motor does not allow for the reversal of the rotor direction.
Reversing a shaded pole motor isn’t possible because of the shading rings on the poles. Therefore if 2 phases were changed, the shading rings wouldn’t allow EMF to travel, which in turn, the motor wouldn’t start.
Explain how a rotating torque is established in the rotor of a shaded pole motor. Explain the reason for this?
By producing an EMF when current increases, the EMF will oppose that of the shading poles and push the EMF in the opposite direction, creating torque.
The single phase universal motor can operate with A/C and D/C supply, explain the reason for this?
Because the armature is connected in series with the field windings.
Name the various types of couplers ?
Flexible
Solid
Diiscuss the purpose of a flexible coupler ?
Has rubber / leather bushings, gives flexibility to the shaft, reducing strain
The shaft of a pump has to be connected to a motor, discuss the process of aligning the shaft of both machines.
Level baseplate
lock out/ tag out
remove guard
confirm alignment (.15mm allowance)
use shims if needed
examine rubber on coupling
use feeler gauge for final alignment
Briefly describe the procedure for removing a worn bearing and replacing it with a new one.
To remove a worn bearing, a bearing puller may be used. Heating the bearing may help it fit into the shaft. lubricant is also used. The bearing in then pushed in while hot.
Where three phase motors are operating in hazardous (potentially explosive) areas, state one special precaution that must e taken with the power circuit.
To prevent single phasing , use a three phase overload or MCB
In relation to lighting, explain the term ‘‘luminous efficacy’’.
The number of lumens per watt of power.
efficacy = lumens / watts
Provide an approximate value for the luminous efficacy for an LED if it was to replace a 100w incandescent lamp.
16-20watt LED
LEDS last longer, allowing users to save money.
List one location where the low pressure sodium vapour lamp (SOX) is suitable and one it is not?
Suitable - General Street Lighting
Un-Suitable - Spray painting Booth
Briefly describe the operation of the high pressure mercury vapor (MBF) lamp from instant of switch on, to full illumination.
Discharge takes place in discharge tube between main electrode and secondary electrode through the argon gas. The discharge vaporizes the mercury allowing the charge to travel through the tube and across the two main electrodes increasing internal pressure, the mercury continues to vaporize until full illumination which takes around five minutes, the current is limited through the choke.
Give two reasons why mercury halide lamps are used in high bay industrial installations, high bay lighting and sports stadia.
- glow greenish white has good colour rendering
- good efficacy 44-83lm/w
- Intermediate colour appearance
The type of reflector or diffuser and the location chosen for a luminaire is important when designing a lighting installation. What factors are important to minimize negative impacts on the buildings occupants.
Glare caused by significant amount of luminance between the task at hand and the glare source. Angle between task and glare and eye adaption have significant impact on glare.
Calculate the total luminous flux ( F ) produced by a light source using the formula provided, given the average illuminance is 300 lux for a room measuring 20m X 10m.
Formula : E = F/A
There are 2 types of automatic lighting control systems available for luminaries, Standalone and addressable.
Describe one advantage and one disadvantage of each.
Stand Alone
Advantage : Easy to install
Disadvantage : each luminaire has to be configured separately.
Addressable
Advantage : usually supplies feedback to a central control system ( BMS/DALI)
Disadvantage : more expensive to install, need mains and comms cable.
You are required to design a lighting control strategy for a commercial building containing offices. State four considerations that should be taken into account in the design.
On/Off or dimmable luminaires
Time controlled switches
Remote controlled switches
Automatic switching (PIR)
Describe briefly the operation of the low pressure sodium vapour (SOX) lamp from the start of switch on to full illumination?
At switch on, a discharge takes place through the neon argon gas.
Has a reddish glow
Discharge vaporizes the sodium which begins to conduct
Gradually changes from red to yellow light
takes about 10 mins for full illumination
ballast limits current
180 lumens per watt
Compare low pressure sodium vapor lamps (SOX), and High pressure mercury vapor lamps (MBF), under the following headings:
colour rendering, efficacy and run up time.
colour rendering
MBF - fair
SOX - poor
Efficacy
MBF - 40 lm/w
SOX -180 lm/w
Run time
MBF - 5 mins
SOX - 10 mins
List four functions on an emergency lighting circuit that a test facility provides.
- simulates power failure, with minimal disruption to normal lighting.
- Simulation of failure for set periods (1,2,3 hr tests)
- Provides method to check battery is connected
- battery is sufficient to last for 1-3 hours
In terms of emergency lighting what is meant by standby lighting?
Part of emergency lighting provided to enable normal activities to continue unchanged in the event normal power supply fails.
Provide an example where standby lighting may be used?
Standby lighting is usually used where a process needs to be shut down safely while keeping light or where a hazardous process is carried out. (Hospital theater room)
With reference to the national standard IS10101.
when wiring systems are selected for an installation where the presence of water or high humidity can occur, what are the three requirements the installer must consider.
522.3
In relation to a self contained emergency lighting luminaire. State 4 main components that make up this type?
Battery
Charger
LED
Control unit
Clearly define what is meant by an emergency lighting luminaire?
An emergency lighting luminaire is a luminaire which may or may not be provided with its own power supply for safety services and which is used for safety or emergency lighting.
Define what is meant by a combined (sustained) emergency luminaire?
An emergency luminaire containing at least 2 lamps, one is energized by normal lighting supply, one is from another emergency supply (Battery)
In emergency lighting schemes, list three factors that must exist for escape lighting to comply with the Irish national standard code of practice for emergency lighting.
That lighting be provided at,
change of direction
change of level
escape routes
illuminate escape routes
When testing lead acid batteries used for a large diesel generator, describe the method to test the condition of the batteries. Include in your answer the required equipment to carry out the test, the procedure and the protective clothing required to perform the test safely?
Ensure batteries are fully charged
Ensure all PPE is being worn ( gloves, glasses)
Disconnect batteries, move to well ventilated area
Ensure no open flames
Visually inspect for cracks or breaks
Connect voltmeter and record reading
Place load tester across terminals for 15 seconds, not less than 9.5v DC.
Rest for 30 seconds, then repeat test for 15 seconds
Voltmeter reading still no less than 9.5v
Temp can effect batteries.
Multi compartment trunking is a popular means of cable installation in commercial buildings.
give 2 reasons for its popularity.
Reduce eddy currents
Segregation of band 1 and band 2
List one dis-advantage that an overhead busbar trunking system has over fixed wiring systems in an industrial application
More expensive
In long runs of busbar trunking, ‘expansion joints’ are fitted at intervals along the busbar. Explain the reason for fitting these and one fault that could occur if they are not fitted.
Expansion joints are fitted every 30m to allow expansion from heat caused by current. If not fitted the busbar can warp and there could be excessive movement which can loosen joints.
Describe 6 advantages when rising main busbar systems are installed in a multi story building compared to using electrical cables.
- greater mechanical strenght
- replaces multiple runs of cable
- easier to install
- minimum volt drop
- better resistance to spread of fire
- takes up less space
Describe 6 advantages when overhead busbar systems are installed in a manufacturing facility compared to using electrical cables.
simple to install
allows lighting and power in same housing
re arrange power connections whenever you need them
less termination space required
replace multiple runs of cable
plug in units can be plugged or unplugged without de-energizing the busbar.
A depletion layer is formed in PN Junction diodes, during their manufacturing, explain what causes this to exist.
The depletion layer along the PN Junction is caused by the recombination of ‘holes’ and electrons at the junction resulting in the junction region being depleted of such charge carriers.
What is ripple voltage?
Ripple voltage is the amount of A/C voltage mixed with a rectifiers D/C output. (extra ac voltage)
With regard to magnetism, define what is meant by magnetic flux density ?
Magnetic flux density is the magnetic flux per m2 at right angles to the magnetic field.
With regard to magnetism, what is meant by Reluctance?
Reluctance is the ratio off MMF to magnetic flux.
What is magnetic hysteresis ?
The lagging of the flux density (B), compared to the magnetizing force (H), when a ferromagnetic material is magnetised.
Explain the following when applied to a three phase motor:
Synchronous speed
Rotor speed
Slip speed
:The speed of which the rotating mag flux revolves
: the speed the rotor spins depends on synchronous speed+slip
: The difference in speed between rotor and stator shown as %
What determines the speed of the rotational magnetic field?
the number of poles and the frequency of the supply determines the speed of magnetic field
Explain the function of thermal cutout device and the effect of operation on the storage heater?
The thermal cutout acts as protective device and protects against overheating and fire, heater will cease to function until thermal switch is reset.
Electric storage heaters installed in a domestic premises, the energy consumed is billed using what type of meter?
An analogue Day + night rate kWh meter is used.
the upper dial displays the day reading
the lower dial displays the night reading
State three precautions to take when installing electric storage heaters?
- The heater must be securely fixed to a wall
- Transport fire bricks safely using hand truck or correct manual handling.
- The fire bricks can crack or break easily, care must be taken when installing the fire bricks in to the storage heater.
Transistors are sometimes mounted on heat sink, explain the purpose of this ?
Heat sink is used to dissipate heat from the transistor.
Show with the aid of a diagram how the direction of the rotor in a single phase capacitor start induction motor can be reversed
Draw a diagram of a shaded Pole motor ensure to label each part of the motor in the diagram
Draw a diagram of a single phase capacitor start induction motor identify all components
With a circuit diagram describe how to reverse the direction a series universal motor rotates
Sketch a graph of the torque or speed characteristics of a universal motor comparing the operation of the motor with AC and DC current
Describe the age of sketches two types of misalignment problems that can occur and what can be done to minimise the misalignment problems
Draw a delta connection and label
Draw a star connection and label
Draw a HBC fuse and label
Draw a neozed fuse and label
Draw an MCB and label
Draw an RCD and label
Draw and Mercury high-pressure Mbi lamp and label
Draw a PN junction diode curve
Draw a single phase full wave rectifier
Draw a stabilised voltage supply
Draw a three phase 4 wire rectification circuit
Draw a tree phase 3 wire rectification Circuit
With regard to magnetism, hysteresis loss occurs when a coil wound on a magnetic circuit carries an alternating-current , due to reversing magnetic flux set up by the reversing current, using clearly labelled diagrams sketch the hysteresis loop for the following: High loss and low loss
With reference to NSAI10101 use a labelled diagram to show the minimum and maximum mounting height for wall switches socket outlets and distribution boards
List 4 advantages of XLPE over PVC cables.
- XLPE insulation tends to last longer than PVC
- XLPE doesn’t contain chloride, its more environmentally friendly
- It can withstand higher temperatures
- Has more moisture resistance
State precautions that should be taken when connecting aluminium cables onto copper busbar?
Use bi-metal or aluminium lugs
add anti corrosion compound
Describe the construction characteristics of a UTp cable used in cat5e structured cabling systems.
Cat5e contains four twisted pairs
Used in ethernet based LANS
Compare aluminium to copper under the following headings:
Conductivity
Current carrying capacity
Mechanical strength
Cost
Conductivity - Aluminium cable is only 61% as conductive as copper
Current carrying capacity - Aluminium has lower current carrying capacity than copper, but higher resistance.
Mechanical strength - Copper has 40% more mechanical strength than copper
Cost - Aluminium is cheaper than copper
List four locations a DB must not be installed ?
Under timber staircase
Where it may be covered
In a bathroom
Over a cooking appliance
Draw a diagram of a storage heater
Draw a diagram of a fly back diode
Draw a linear power supply
And give 4 advantages
Draw a switch mode power supply
Give 4 advantages
The fault current will be the lowest at point C.
This is because the further we move out along the circuit from the supply transformer the higher will be the impedance of the circuit.
What’s is the difference between a primary and secondary cell ?
A primary cell is disposable and has no electrical input. ( AA battery )
A secondary cell is are-chargeable, and will repeatedly release energy as long as it receives charge
What is polarisation?
The layers of bubbles that collect on the copper plate, causing a reduction in output voltage.
What is local action?
When the plates is energised in the electrolyte , impurities form tiny cells with the zinc, the zinc is then eroded away.
What is the initial EMF of a healthy lead acid battery ?
2.2volts
What is a nickel cadmium battery used for ?
It is used in portable hand tools
Explain the term, battery capacity ?
Battery capacity is the total amount of energy a battery or cell will hold is measured in terms of the current which is supplied
State one advantage of double caged induction motor has over a single cage induction motor
It has a higher starting torque
it has very good running torque
How does the universal Motor operate ?
When supply is witched on, the stator and the rotor set up magnetic fields, which oppose each other causing the rotor to turn
Draw a diagram of a D.O.L starting
What are the 3 types of batteries ?
Lead acid
Nickel cadmium
Ni-mH
Provide one suitable application for nickel-meta hydride (Ni-mH) batteries.
Mobile phones
Medical equipment
What is the most common method of charging for a Ni-mH battery ?
Trickle charging
