Portfolio Stufy Flashcards
Draw a power triangle clearly labelling reactive power, apparent power and true power include units
True power watts, apparent power VA, reactive power VAR
State 3 effects of operating an installation with low or poor power factor and explain how to improve it
Higher cost, low efficiency, wasted power
Connect capacitors in parallel
The power that actually does work in a circuit is called
True power
The power that is a product of line voltage and current is called
Apparent power
The power stored and returned to a circuit is called
Reactive power
The ratio between true power and apparent power is called the
Cos - power factor
The common name for either the angle of leaf or lag is called
Phase angle
Name the instruments used to measure
Single and tree phase power
Energy consumption
Maximum demand
Power factor
Wattmeter
Kilowatt meter
Maximum demand meter
Power factor meter
Does operating a circuit with a low power factor significantly increase costs associated with running installation
Yes
State two methods to improve power factor
Operate motors at full load
Connecting capacitors
Name an instrument to measure resistance
Ohmeter
Name three measuring instruments required to enable the calculation of the power factor of a single phase circuit
Voltmeter, ammeter, wattmeter
Where on the manufacturers piece of equipment would you find the power factor requirements
Name plate
Do question 13
Check
Name one purpose for providing a neutral in an unbalanced three phase, four wire system
A return path for unbalanced current
State one effect of an open circuit neutral on an unbalanced three phase four wire MEN installation under load
It will creat unbalanced load voltages across the 3 phases
Harmonics can be defined as
Multiples of the fundamental frequency
State one effects of harmonics
Distortion of electricity supply voltage
Name an instrument used to test harmonics
Harmonic analyser
State the dangers of operating a series circuit at resonance
XL and XC cancel each other out allowing high levels of current and voltage to flow across reactive components
Name two common sources of harmonics
Electronic lighting equipment, computers
Under what conditions will resonance occur in a series or parallel circuit
If inductive reactance and capacitive reactance are the same in a ac circuit
State the dangers of operating a parallel circuit at resonance
Current will increase extremely high
Name method used to reduce the problems caused by harmonics
Harmonic filters
A delta connected 3 phase motor is supplied from a star connected alternator with a phase voltage of 240v. What is the line voltage of the delta motor
415v
26
Use as 3000
Define earth fault loop impedance
The total opposition to current flow
Give one reason why it is important to measure energy, power, voltage, current and power factor of an installation or electrical load before alterations are made
So you can calculate max demand and make sure alterations will work
29 in book
Check book
The peak value per phase of a 3 phase alternator is 320v. Calculate the rms voltage per phase
0.707 x 320 = 226.24v
The two advantages of a 3 phase distribution system compared to a single phase
more efficient
Gives 2 voltages
State one effect of a three phase motor connected with one phase windings reversed
current would increase as Motors work against each other
Do question 33
In book
Give a brief explanation of how a 3 phase supply is generated inside a single alternator
3 coils are rotated at a constant speed tho a two pole magnetic field creating an EMF
Do question 35
Book
Explain the terms balanced and unbalanced loads give examples of each
Balanced - 3 phase motor
Unbalanced - network supply transformer
37
Check book
38
Book
39
As/3000 nameplate
40
Clause 3.5.2
41
No, clause 2.3.2.1 (b)(I)
42
415 x 2% = 8.3v
43
You cannot exceed maximum current without overheating
Starting motors
44
The supply system shall be no less than 0.8 lagging
45
The device will overheat
46
If one phase is reversed then it will likely burn out
47
Balanced - 3 phase hot water system
Unbalanced - network supply transformer
