Capacitance in an AC circuit

1. Capacitance in an AC circuit

Level 3 Diploma in Electrical Installations (Buildings and Structures)
PowerPoint
presentation
Unit 302: Principles of electrical science
Capacitance in an AC circuit
© 2018 City and Guilds of London Institute. All rights reserved.
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2.

Level 3 Diploma in Electrical Installations (Buildings and Structures)
AC supply
Q = V.C
Where:
Q = Charge in coulombs
V = Voltage in volts
C = Capacitance in farads
© 2018 City and Guilds of London Institute. All rights reserved.
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3.

Level 3 Diploma in Electrical Installations (Buildings and Structures)
In a purely capacitive circuit, the current must flow before the pd can be
established across the capacitor and therefore the current leads the
voltage by 90°
© 2018 City and Guilds of London Institute. All rights reserved.
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4.

Level 3 Diploma in Electrical Installations (Buildings and Structures)
In a purely capacitive circuit, the current must flow before the pd can be
established across the capacitor and therefore the current leads the
voltage by 90°
V
XC =
I
1
XC =
2pfC
1
XC =
wC
Where:
XC
V
I
f
C
ω
=
=
=
=
=
=
Reactance of the capacitor in ohms
Voltage across the capacitor
Resulting current flow
Supply frequency in Hz
Capacitance in Farads
2πf
© 2018 City and Guilds of London Institute. All rights reserved.
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5.

Level 3 Diploma in Electrical Installations (Buildings and Structures)
The graph represents a
15µF capacitor on a 230
volt AC supply
The graph represents a
15µF capacitor on a 230
volt AC supply
The current flowing through a capacitor is proportional to:
size of the capacitor in microfarads ( F)
the voltage applied to the capacitor (VC)
the frequency (f).
© 2018 City and Guilds of London Institute. All rights reserved.
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6.

Level 3 Diploma in Electrical Installations (Buildings and Structures)
The graph represents a
15µF capacitor on a 230
volt AC supply
W =
The graph represents a
15µF capacitor on a 230
volt AC supply
1
CV 2 (joules)
2
© 2018 City and Guilds of London Institute. All rights reserved.
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7.

Level 3 Diploma in Electrical Installations (Buildings and Structures)
EXAMPLE – Calculate the capacitive reactance and current that will flow to
a 2 F capacitor, when connected to a 230 volt supply at:
a) 50Hz.
b) 200Hz.
a)
1