AC 2 Flashcards
POWER IN AC CIRCUIT: THE POWER FACTOR
P=VI cos p
vi - virtual factor
cos phi - power factor
power in each case
i) purely resistive circuit
ii) purely inductivecapacitive circuit
iii) LCR circuit
iv) lcr at resonance
Case (i) Case (i) Case (i) Resistive circuit: If the circuit contains only pure R, it is called
resistive. In that case φ = 0, cos φ =1.Thereis maximum power dissipation.
Case (ii) Case (ii) Case (ii) Purely inductive or capacitive circuit: If the circuit contains
only an inductor or capacitor, we know that the phase difference between
voltage and current is π/2. Therefore, cos φ = 0, and no power is dissipated
even though a current is flowing in the circuit. This current is sometimes
referred to as wattless current.
Case (iii) Case (iii) Case (iii) LCR series circuit: In an LCR series circuit, power dissipated is
given by Eq. (7.30) where φ = tan–1 (Xc– XL
)/ R. So, φ may be non-zero in
a RL or RC or RCL circuit. Even in such cases, power is dissipated only in
the resistor.
Case (iv) Case (iv)Case (iv) Power dissipated at resonance in LCR circuit: At resonance
Xc
– XL
= 0, and φ = 0. Therefore, cosφ = 1 and P = I
2Z = I
2 R. That is,
maximum power is dissipated in a circuit (through R) at resonance.
) For circuits used for transporting electric power, a
low power factor implies large power loss in transmission. Explain.
We know that P = I V cosφ where cosφ is the power factor.
To supply a given power at a given voltage, if cosφ is small, we have to
increase current accordingly. But this will lead to large power loss
(I
2R) in transmission.
At an airport, a person is made to walk through the
doorway of a metal detector, for security reasons. If she/he is carrying
anything made of metal, the metal detector emits a sound. On what
principle does this detector work?
The metal detector works on the principle of resonance in
ac circuits. When you walk through a metal detector, you are,
in fact, walking through a coil of many turns. The coil is connected to
a capacitor tuned so that the circuit is in resonance. When
you walk through with metal in your pocket, the impedance of the
circuit changes – resulting in significant change in current in the
circuit. This change in current is detected and the electronic circuitry
causes a sound to be emitted as an alarm.
what are transformers
it is an electrical device used for converting ac of low voltage to high voltage and vice versa.
There are two types of transofmers:
i) step up transformer: output> input
ii)step down transformer: output<input
principle of transformer
it works on the principle of mutual induction.
When a changing current is passed through on ofe the coils, an emf is induced in the other coil.
construction
It consists of a soft iron core made of laminated sheets, well insulated from one another.
The 2 coils (primary and secondary) are wound on the same core.
They are well insulated from each other and from the core as well.
The source of alternating emf is connected to the primary coil while the load is connected to the secondary coil
what type of winding to use for best efficiency
if the primary and secondary coil are wound on the same limb,one over the other, the flux leakage is minimum. SO efficiency is more
