Common misconceptions
4 min read

Alternating Current

- What you are getting from a statement might not be necessarily true. Let us burst some of the common misconceptions
1
The reactive power is the unwanted power.
MYTH: In the famous beer analogy, it is assumed that the reactive power is as useless as the foam in the beer
FACT: The reactive power has its multiple uses and it is also productive in many ways.
The beer analogy says that the beer present in a glass is active power and foam above the beer is reactive power. When foam becomes more in a mug, the quantity of drinkable beer becomes low. It is correct up to this point but in few cases, students assume that the reactive power is totally useless as the foam in the glass. If we take an induction motor, it requires both active and reactive power to run. Without reactive power, no magnetic field will be generated in the motor and it can't be started. The reactive power can be easily distributed with help of capacitor banks without any distribution losses.
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2
How will the current flow when we consider that in AC the electrons flow forward and then reverse their direction for the next half-cycle?
MYTH: Do the electrons drift in one direction only in current flow?
FACT: The electrons only vibrate at their position and let the energy of this vibration move forward as a flow of current.
The flow of current does not mean that there is some current that will flows throughout the circuit. It only depicts that there are the charge carriers that will be flowing or moving the circuit. In the case of an alternating current, where the charged particles oscillate, they transfer charge from one carrier to other and maintain the flow of current.
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3
The students are mistaken in the concept of lead and lag of voltage and current.
MYTH: Looking at the phasor diagram, the students assume the one which looks being ahead as the leading one.
FACT: The leading and lagging ones are decided based on the value of waveforms at a particular instant of time.
The students on observing the shown graph of phasor and generally assume that the graph of seems to be the winning one as it is running ahead of . They consider to be the leading one. But actually, the leading phasor is decided by the value of phasors at a particular instant. The particular value of has occurred sometime earlier as compared to the same value on . Therefore, is the leading one here.
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4
The use of a capacitor in a fan that is operated on a single-phase supply.
MYTH: The capacitor is used in a fan in order to store the charge and start the rotation of the motor.
FACT: The capacitor actually provides a magnetic torque to the motor.
The capacitor present in a fan is usually mistaken to be used to store the amount of charge which in turn supports the motor to initiate its rotation. The capacitor is used in a single-phase driven fan to produce a phase difference which in turn leads to the magnetic torque in the motor of the fan and supports the rotation.