Topic 5 capacitance in AC circuits topic a5

Bài giảng ngành Điện: Giải các bài toán trong mạch điện điện thế thấp một pha và ba pha (Phần A) gồm các chủ đề sau Solve problems in single and threephase low voltage circuits Part A Content Topic 1 Sinusoidal Alternating Voltage and Current_Topic_A1 Topic 2 Phasors_Topic_A2.ppt Topic 3 Resistance in AC Circuits_Topic_A3 Topic 4 Inductance in AC Circuits_Topic_A4 Topic 5 Capacitance in AC Circuits_Topic_A5 Topic 6 AC Circuit Analysis_Topic_A6 Topic 7 Resonance_Topic_A7 Topic 9 Harmonics_Topic_B9

Solve problems in single and

three-phase low voltage circuits

Topic 5: Capacitance in

AC Circuits Part A

Capacitance in AC Circuits

 Key Characteristic

– A capacitive component will ACT TO STORE

ELECTRICAL CHARGE THAT WILL OPPOSE THE APPLIED VOLTAGE (and therefore current)

– It achieves this by storing a negative charge on one plate and a positive charge on the other

– How strongly it does this is indicated by the

capacitor’s CAPACITANCE, symbol C, measured

in Farads (F)

C

Capacitance in AC Circuits

 Effect in an AC circuit

– Since the voltage in an AC circuit is continually changing (ie alternating between positive voltage and negative voltage), a capacitor in an AC circuit continually charges and discharges which then opposes the change in voltage (and current)

– As a result, the capacitor produces a constant

opposition to the flow of alternating current

– ƒ is the frequency in Hertz (Hz)

– C is the capacitance in Farads (F)

XC = 1

2π ƒ C

Capacitance in AC Circuits

 Important!

– CAPACITIVE REACTANCE is an opposition to current flow like RESISTANCE, but it is NOT the same as resistance, even though both are

measured in Ohms

– As a result, CAPACITIVE REACTANCE and

RESISTANCE can NOT be simply added to find the total opposition to current flow in a circuit

Capacitance in AC Circuits

 Key Advantage

– Ability to produce better operation for circuits

containing highly inductive loads

– Ability to limit AC current flow without consuming any power

Capacitance in AC Circuits

 Examples of Capacitive Components

– Capacitor

– Capacitor banks (for power factor correction)

– Filter or tuning circuits

Capacitance in AC Circuits: Ohm’s

Capacitive Reactance: Exercises

VS=230V ƒ=50Hz

IC=?

C=47µF

VS=230V ƒ=50Hz

IC=0.723A

C =?

Q1 Determine:

Capacitive reactance XC, and

Current though capacitor Ic

Q2 Determine:

Capacitive reactance XC, and Capacitance of capacitor C

Capacitive Reactance: Answers

Capacitive Reactance: Exercises

 A 230V, 50Hz AC supply is to be applied to a ceiling fan circuit using a capacitor speed

control switch If the current for the ceiling

fan motor needs to be reduced to 0.362

Amps for the “Low” setting, determine the

appropriate value of capacitance required by for this setting (assume the motor has no

opposition to current flow).

Capacitive Reactance: Answers

 XC = 635.36Ω

 C = 5.01μF

Capacitance in AC Circuits:

Capacitance in Series and Parallel

C1

VSƒ

VSƒ

Series Capacitive Circuit

C1

VSƒ

C2

Kirchoff’s Voltage Law

•The ‘sum’ of the voltage drops in the circuit will equal the supply voltage

Vs = VC1 + VC2+…

[Purely capacitive circuit only]

VC1 VC2

Parallel Capacitive Circuit

Kirchoff’s Current Law

•The ‘sum’ of the currents entering

a junction will be equal to the sum

of the currents exiting the junction.

Is = IC1 + IC2+…

[Purely capacitive circuit only]

VSƒ

Capacitance in AC Circuits

 Exercises on series / parallel combinations

Capacitance in AC Circuits:

Capacitance in Series and Parallel

C1 = 15μF

VS=230V ƒ=50Hz

VS = 32V ƒ=50Hz

 At this stage, you should have a clear understanding

of Capacitance in AC circuits, including:

– Understand the concept of capacitive reactance;

– Understand the application of Ohm’s Law to capacitive

Phase Relationship between

Voltage and Current

Resistive Circuit

Phase Relationship between Voltage

and Current: Resistive Circuit

‘V’ Waveform

‘I’ Waveform

V I

In-phase Phasor diagram

Simple Resistive

Circuit

V

VSƒ I

R

Phase Relationship between

Voltage and Current

Inductive Circuit

Phase Relationship between Voltage

and Current: Inductive Circuit

Simple Inductive Circuit

V

VSƒ

Phase Relationship between

Voltage and Current

Capacitive Circuit

Phase Relationship between Voltage

and Current: Capacitive Circuit

I C

‘V’ Waveform

‘I’ Waveform

V I

Out-of-phase LEAD by

90 0 E

Phasor diagram

Summary of Phase Relationships

 Purely Resistive component

– Current will be IN-PHASE with the Voltage

 Purely Inductive component

– Current will LAG the Voltage by 90 0 E

 Purely Capacitive component

– Current will LEAD the voltage by 90 0 E

NOTE: From this point on, you should always describe

what the current is doing with respect to the voltage

when describing a phase relationship

Remembering Phase Relationships