Electricity Basics - Part Three


Q1: What is the voltage levels used for single phase circuits? 
Voltage Levels for single phase circuits are: 120V – 220V -230V – 277V – 347V.

Q2: What is the voltage levels used for three phase circuits? 
Voltage Levels for three phase circuits are: 120/208V – 220/380V – 220/400V – 230/415V – 277/480V – 347/600V.

Q3: what are the electrical systems Configurations for a single phase system? he electrical systems Configurations for a single phase system

  • 220V or 230V or 240V, Two wire – single phase system (Not as per NEC code) (fig.1)
  • 120/240 V, Three wire – single phase system (as per NEC code) (fig.2)
fig.1

fig.2

Q4: what are the electrical systems Configurations for a three phase system?
  • 120/208V or 220/380V or 220/400V or 230/415V or 277/480V or 347/600V, Four Wire-three phase system Wye.(fig.3)
  • 120/240V, Four Wire-three phase system Delta (high Leg).(fig.4)
  • 220V or 240V, Three Wire-three phase system Delta (corner ground).(fig.5)


fig.3
fig.4
fig.5

Q5: What is the difference between a series circuit and a parallel circuit?
 A series circuit has one path; the parallel circuit splits into branches

Q6: what is the definition of Apparent power (S)?
Apparent power is the product of the current and voltage of the circuit. Due to energy stored in the load and returned to the source, or due to a non-linear load that distorts the wave shape of the current drawn from the source.

Q7: what is the definition of Real power (P)?
Real power is the capacity of the circuit for performing work in a particular time

Q8: what is the difference between the Real power (P) and Reactive power (Q)?
Reactive power does not transfer energy, so it is represented as the imaginary axis of the vector diagram. Real power moves energy, so it is the real axis.

Engineers care about apparent power, because even though the current associated with reactive power does no work at the load, it heats the wires, wasting energy.

Q9: what are the unit measurements for Apparent power(S), Real power (P) and Reactive power (Q)?
Unit for Apparent power(S) is VA (volt ampere)

Unit for Real power (P) is W (watt)

Unit for Reactive power (Q) is VAR (volt ampere reactive)

Q10: what is the phase difference Angle?
Phase difference angle (φ), the angle of difference (in degrees) between voltage and current; Current lagging Voltage (Quadrant I Vector), Current leading voltage (Quadrant IV Vector)


Q11: What is the power factor?
The power factor of an AC electric power system is defined as the ratio of the real power flowing to the load to the apparent power in the circuit

Q12: what is the value range of the power factor?
The power factor is a dimensionless number ranging between 0 and 1

It is zero when the current leads or lags the voltage by 90 degrees.

Q13: Can we add the real power (P) of different loads to get the total load for them?
No, we can’t .we must be sure that the phase difference is the same for all these loads

Q14: Can we add the Apparent power (S) of different loads to get the total load for them?
Yes, we can sum the complex values of the Apparent power for these Loads.

Q15: when the current lead the voltage?
Purely capacitive circuits cause reactive power with the current waveform leading the voltage wave by 90 degrees

Q16: when the current lag the voltage?
while purely inductive circuits cause reactive power with the current waveform lagging the voltage waveform by 90 degrees.

Q17: why the inductors cause the lagging of current?
When a voltage is initially placed across the coil a magnetic field builds up, and it takes a period of time for the current to reach full value. This causes the current to lag the voltage in phase, and hence these devices are said to absorb reactive power.

Q18: why the capacitors cause the lagging of voltage?
When current is driven through the capacitor, it takes a period of time for charge to build up to produce the full voltage difference.

Q19: what is the relation between for Apparent power(S), Real power (P) and Reactive power (Q)?



S = P + jQ







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