Page 289 - Electronic Mechanic - TP - Volume - 2

ELECTRONICS MECHANIC - CITS

In 3-phase power rectifiers, conduction always occurs in the most positive diode and the corresponding most
negative diode. Thus as the three phases rotate across the rectifier terminals, conduction is passed from diode
to diode.
Each diode conducts for 120° (one-third) in each supply cycle but as it takes two diodes to conduct in pairs,
each pair of diodes will conduct for only 60° (one-sixth) of a cycle at any one time as shown above.
We can correctly say that for a 3-phase rectifier being fed by “3” transformer secondary, each phase will be
separated by 360°/3 thus requiring 2 x 3 diodes.
Time period from 0° to 30°
V C > V > V ⇒ Diodes D and D are ON
5
4
A
B
Diode D is OFF (Anode voltage V is lower than cathode voltage V )
1
C
A
Diode D is OFF (Anode voltage V is lower than cathode voltage V )
A
2
B
Diode D is OFF (Anode voltage V is lower than cathode voltage V )
3 B C
Diode D is OFF (Anode voltage V is lower than cathode voltage V )
6 B C
So, the load voltage is equal to: V load = V - V B
C
Time period from 30° to 90°
V A > V > V ⇒ Diodes D and D are ON
1
4
B
C
Diode D is OFF (Anode voltage V is lower than cathode voltage V )
A
2
B
Diode D is OFF (Anode voltage V is lower than cathode voltage V )
3 B A
Diode D is OFF (Anode voltage V is lower than cathode voltage V )
5 C A
Diode D is OFF (Anode voltage V is lower than cathode voltage V )
6
B
C
So, the load voltage is equal to: V load = V - V B
A
Related Formula:
For half wave rectifier:
The peak input voltage vp, the average value of output waveform is:
Vdc = 3 x 1.732/2 x 3.14 x Vp = 0.827 x Vp
Vp = 1.414 x Vrms
Vrms = Vp/1.414
Where Vrms = rms value of input,
Vdc = average value of input
Vm=Peak value of input.
The average DC value of the output voltage waveform is:
Vdc = 3 x 1.732/3.14 x Vs = 1.65 x Vs
Where Vs is equal to VL /1.732
Ripple factor = Vrm fl/Vfl x 100%
For bridge rectifier:
Vdc = 3 x 1.732/3.14 x Vs=1.65 x Vs
Where Vs is equal to VL peak/1.732
VL is the maximum line to line voltage (VL*1.414)
Ripple factor =0.48
5 Repeat the above steps for bridge rectifier.
Result:

Hence, the construction and testing of three phase uncontrolled rectifier using diodes was verified and waveforms
were drawn.

272
CITS : E & H - Electronics Mechanic - Exercise 199