Exam 1 Electrical priciples

What happens to resistance of a conducting material if temp decreases

The resistance decreases, as shown on tables in as3008

If a voltmeter was connected between two points of equal potential, what result would be displayed on the meter?

The result would be 0v as the voltmeter measures the potential difference between the two points

When measuring across an open switch in a 230V circuit with the supply connected, what result would be displayed on the meter?

The result would be 230v

Describe an open circuit in an electrical circuit and what would happen to the current flow?

An open circuit is a circuit that is not complete, preventing current flow. In this case, the current flow would be zero because the circuit is interrupted.

Describe a short circuit in an electrical circuit?

A short circuit is when active bypasses the load by connecting to another point in the circuit, lowering resistance and spiking the current

Write down Ohm’s Law, and transpose for the three different values

V= IxR I=V/R R=V/I

Write down the single-phase power formula (for resistive only circuits), and transpose for the three different values

P= VxI V=P/I I=P/V

Write down the series circuit laws for: Total Voltage: Total Current: Total Resistance: Total Power:

VT=V1+V2

IT=I1=I2

RT=R1+R2

P=P1+P2

If five lights are connected in series and one of the lights becomes open circuit, describe what will happen to the other lights in the circuit?

All lights will turn off, current will no longer flow through the circuit

Give an example of where you would typically see a series circuit in your workplace

Earth fault loop impedance, each cable of fault loop is in series

Write down the parallel circuit laws for: Total Voltage: Total Current: Total Resistance: Total Power:

VT=V1=V2

IT=I1=I2

1/RT=1/R1+1/R2

PT=P1+P2

If five lights are connected in parallel and one of the lights becomes open circuit, describe what will happen to the other lights in the circuit?

Still work correctly

Give an example of where you would typically see a parallel circuit in your workplace?

Light or socket outlet circuits

If the voltage is increased in an electrical circuit, what will happen to the following: Current will: Increase/Decrease/Stay the same Resistance will: Increase/Decrease/Stay the same Power will: Increase/Decrease/Stay the same

Current will increase

Resistence will stay the same

Power will increase

How many different phase-to-neutral voltages does the alternator generate?

3 phases

How are the three different phases typically labelled?

A,B,C L1,L2,L3, Red, White, Blue

How many degrees apart are the phases generated

120 degrees

What piece of equipment will show you the phase rotation/sequence of the supply

Phase rotation meter

What piece of equipment will measure the power supplied by the alternator to an electrical installation

Watt meter

What piece of equipment will measure the energy supplied by the alternator to an electrical installation

KW per hr meter or smart meter

What is the Australian Standard RMS phase-to-neutral voltage output by the alternator

230v +10% -6%

What is the Australian Standard RMS phase-to-phase voltage output by the alternator

400v +10% -6%

Calculate RMS phase to neutral

Vpeak x 0.707

What is the term used to describe the number of cycles of AC completed in one second

Frequency

What is the term for the time it takes for an AC waveform to complete one full cycle (360º)?

periodic time, PT = 1/frequency

Sinusoidal _________ and _________ waves are produced using the principle of Faraday’s electromagnetic induction with a rotating coil in a magnetic field

Voltage and current

Electroplating is a method of transferring a thin layer of metal to a surface using ________________.

current flow

Explain basically how an electric motor rotates?

The electromagnetic effect of the current as it passes through the stator windings.

What type of enclosure would suit a motor that may have splashes of water dropped on it and requires cooling?

Totally enclosed, fan cooled (TEFC)

What type of enclosure would suit a motor to be installed in a hazardous area?

Explosion proof (XP)

What factors must be considered when selecting a motor starter?

To comply with the requirements of the local supply authority, and to achieve the required starting torque and current.

different methods of overload protection for electric motors?

thermistors, thermal protection and Circuit breakers

What is the point of a star delta starter and how does it work

Starts in star then switches to delta, used to limit starting current of a big motor

How is a normal light switch different to a relay/ contactor

A switch is manually operated whereas a contactor is a result of switching (automatic)

Squirrel cage motors have a rated start up current of how many amps

4-7 x the amount

How are voltmeters, amp meters and ohmmeters connected?

Voltmeter = In parallel to load A-N

Amp meter = In series to load

Ohmmeter = in parallel with power off

What is a variable speed drive

Electric motor control device that controls the frequency and voltage supplied to the motor inturn controlling the speed of the motor

What is a Direct on line starter

Motor is directly connected to supply at full line voltage when started, resulting in full current and torque

When testing a balanced 3 phase load which method is correctly used to determine total true power supplied to a three-phase circuit

Phase voltage x phase current x power factor

A three-phase motor operates at a constant level for 7 hrs. Which method is used to determine the electrical energy used by the three-phase circuit

Power (KW) x time of operation (Hrs)

What is the definition of phase sequence in a three phase supply and what is the standard sequence

Phase sequence refers to the order of conductor colours used in a three-phase cables, The standard sequence is red, white, blue

What is an auto transformer

An autotransformer is an electrical machine that contains only one winding and is used to transform or change the level of voltage or current in an electrical system or circuit.

It is named an autotransformer because it consists of only one winding acting as the primary and secondary windings.

What is an isolation transformer

An isolation transformer provides physically and electrically isolation between two circuits. It isolates as well as protects the electrical & electronic circuits.

A transformer is a static device that is used to either step up or step down a particular voltage level from a different voltage level which it does with constant frequency and works on the principle of magnetic induction it has different voltage levels on either side and can function as both steps up or step down transformer.