Download How does inductive filter work

SINGLE PHASE (1Ø ) BRIDGE
RECTIFIER
WITH INDUCTIVE FILTER ON DC SIDE
By Zoran Bilic
November, 2011
What is a rectifier ?
It is an electrical device that converts
alternate voltage and current (AC) into
direct voltage and current (DC)
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Difference between AC and DC Voltage
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Basic Circuit Schematic
Major circuit components:
•AC Voltage Source
•Diodes D1, D2, D3, D4
•Inductive Filter Ls
•Load (consumer) R
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Major circuit parameters
•AC voltage E
• AC current I
•Unfiltered DC Voltage EKA
•Filtered DC Voltage Ed
•Filtered DC Current Id
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How does it work?
Diodes D1 to D4 are “semiconductor valves” that can conduct
electric current in one direction only while they block it in the
opposite direction
Diodes are connected in so called BRIDGE CIRCUIT
AC voltage E and rectified (unfiltered) voltage EKA waveforms
Voltage EKA is not smooth Dc voltage, it contains strong “ripple”
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DC voltage EKA has an average value that is related to AC input
voltage E
EKA (average) = 0.9 • E (Volts)
Also, peak value of EKA
EKA peak = √2 • E (Volts)
Example:
What would be average and peak value of EKA if AC input
voltage E=120V?
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Solution:
EKA (average) = 0.9 • E = 0.9 • 120 = 108Vdc
EKA peak = √2 • E = √2 • 120 = 170Vdc
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Since the ripple (fluctuation) of the EKA is too high for many
applications, there is a need for smooth ripple-less DC voltage
This can be achieved by use of inductive or capacitive filter
located between EKA and load.
Without a filter DC current Id would be rippled
Current Id
Voltage EKA
It also important to observe that AC current I at the input has
sinusoidal shape
AC Voltage E
AC Current I
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How does inductive filter work?
Inductor’s nature is to always oppose to any change in
electrical current , including ripples of DC current Id
Installing sufficiently large inductor Ls, current ripples
will disappear completely
Inductive filter LS installed on DC side of the rectifier
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Here we can observe waveforms for DC voltage EKA along
with smoothed DC current Id
DC voltage EKA
DC current Id
Consequently, output voltage at the load will be smooth,
ripple- less equal to the average value of DC voltage EKA
DC voltage EKA
DC voltage Ed
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As a consequence of use of inductive filter, AC input current I
will not be sinusiodail anymore.
AC Voltage E
AC Current I
AC input voltage E and Current I with L filter.avi
And that is unwanted consequence because creates another
problem.
It will be rectangular shaped curve.
This phenomenon is due to fact that smooth, flat DC current
flows in DC side, and AC input side must provide that.
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There is direct correlation between input AC current I and output DC
current Id
Id(DC) = I (AC)
However input current I is still ALTERNATING (AC) current.
Example:
What would be input ac current value if load on DC demands 10 Amps
of DC current?
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Solution:
I d(DC) = I (AC) = 10 Amps
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Summary
1. Rectifiers are electrical devices or circuits that convert
alternating (AC) currents/voltages into direct (DC)
currents/voltages
2. Diodes are semiconductor “valves” that provide rectification.
They conduct current in one direction and block current flow in
opposite direction
3. Rectified DC voltage and current are not smooth and without
ripple unless we apply filters
4. Filters can be capacitive or inductive. Inductive filter smoothes
rectified current ripple.
5. Consequence of inductive filter use is that input AC current is
not sinusoidal any more. It becomes rectangular.
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