Aim of the Experiment: o To study the working principles of an AC-DC Converter with Ohmic inductive load and Ohmic-Battery load. o To study the effect of no-linear characteristics of AC-DC converter on the power factor and harmonic distortion of AC input source.
Introduction
PART I: Ohmic-Inductive load
A single-phase one-pulse diode rectifier feeding Ohmic-inductive RL load is shown in Fig.1 (a). Current i o
continues to flow-even after source voltage Vs has become negative; this is because of the presence of inductance L in the load circuit.
After positive half cycle of source voltage, diode remains on, so the negative half cycle of source voltage appears across load until load current i o
decays to zero at ω t = β . Voltage V
R
=i o
R has the same waveshape as that of i o
.
Inductor voltage V
L
=V s
- v
R
is also shown. The current i o
flows till the two areas A and B are equal. Area A (where V s
> V
R
) represents the energy stored by L and area B (where Vs < V
R
) the energy released by L. It must be noted that average value of voltage V
L
across inductor L is zero.
When i o
=0 at ω t = β ; V
L
=0, V
R
=0 and voltage Vs appears as reverse bias across diode D as shown. At β , voltage v
D
across diode jumps from zero to V m
sin β where β > π:. Here, β = ϒ is also the conduction angle of the diode.

Average value of output voltage:
Average value of load or output current
The solution of equation below can give the value of extinction angle β .
Performance of single-phase one-pulse diode rectifier with RL load can be improved by connecting a freewheeling diode FD across the load as shown in
Fig. 2 (a).

Output voltage is V o
=V s
for 0 ≤ ω t ≤ π. At ω t = π, source voltage Vs is zero, but output current i o
is not zero because of L in the load circuit. Just after ω t = π, as Vs tends to reverse, negative polarity of Vs reaches cathode of FD through conducting diode D, whereas positive polarity of Vs reaches anode of FD direct. Freewheeling (or flywheel) diode FD, therefore, gets forward-biased. As a result, load current i o
is immediately transferred from D to FD as Vs -tends to reverse. After ω t = π, diode, or source, current is =
0 and diode D is subjected to reverse voltage with PIV equal to V m
at ω t =
3 π/2, 7 π/2 etc.
After ω t = π, current freewheels through circuit R, L and FD. The energy stored in L is now dissipated in R. When energy stored in L =energy dissipated in R, current falls to zero at ω t = β < 2π. Depending upon the value of R and
L, the current may not fall to zero even when ω t =2π, this is called continuous conduction. But in Fig. 2 (b) , load current decays to zero before
ω t =2π; load current is therefore discontinuous .
The effects of using freewheeling diode are as under:
► It prevents the output (or load) voltage from becoming negative.
► As the energy stored in L is transferred to load R through FD, the system efficiency is improved.

► The load current waveform is smoother, the load performance, therefore, gets better.
It is seen from Fig. 2 (b) that, average output voltage and current are,
PIV equal to V m
PART II: Ohmic-Battery load
Practically single-phase half wave AC-DC converter is not efficient due to the high output ripple and low output DC voltage. Hence, a single-phase full-bridge rectifier is preferred upon half-wave one.
The full wave single phase rectifier offers as good performance as possible from a single phase rectifier in terms of the output voltage form factor and ripple factor. They have a few disadvantages however. These are
 They require a split power supply which is not always available.
 Each half of the split power supply carries current for only one half cycle.
Hence they are underutilized.
 The ratio of the required diode PIV to the average output voltage is rather high.
These problems can be mitigated by using a single phase full bridge rectifier charging a battery as shown in Fig.(a). This is one of the most popular rectifier configurations and is used widely for applications requiring dc. Power output from a few hundred watts to several kilo watts. Fig.3(a) shows the rectifier supplying an R-L-E type load which represent a storage battery. These rectifiers are also very widely used with capacitive loads particularly as the front end of a variable frequency voltage source inverter. However, in this experiment analysis of this rectifier supplying an R-L-E load will be presented.

When the switch S is turned on at the positive going zero crossing of vi no current flows in the circuit till vi crosses E at point A. Beyond this point, D1&
D2 are forward biased by vi and current starts increasing through them till the point B. After point B, vi falls below E and i o
starts decreasing. Now depending on the values of R, L & E one of the following situations may arise.
• i o
may become zero before the negative going zero crossing of vi at point C.
• i o
may continue to flow beyond C and become zero before the point D.
• i o
may still be non zero at point D.

It should be noted that if i o
>0 either D1D2 or D3D4 must conduct. Fig.3 (b) shows the waveforms for the third situation.
If i o
>0 at point C the negative going input voltage reverse biases D1 & D2.
Current io commutates to D3 and D4 as shown in the associated “conduction
Diagram” in Fig .3 (b). It shows pictorially the conduction interval of different devices. The current io continues to decrease up to the point D beyond which it again increases. It should be noted that in this mode of conduction io always remain greater than zero. Consequently, this is called the continuous conduction mode of operation of the rectifier. In the other two situations the mode of operation will be discontinuous.
The steady state waveforms of the rectifier under continuous conduction mode is shown to the right of the point ω t = 0 in Fig.3 (b).
If the parameters of the load (i.e, R, L &E) are such that the conduction of the rectifier becomes discontinuous i.e, the load current becomes zero for a part of the input cycle. Fig.4 (b) shows the waveforms of different variables under discontinuous conduction mode of operation.
In this mode of operation D1 D2 are not forward biased till vi exceed E at ω t =
θ . Consequently, no current flows into the load till this time. After ω t= θ , the load is connected to the input source through D1D2and io starts building up.

Beyond ω t = π - θ , iostarts decreasing and becomes zero at ω t = β < π. D1D2 are reverse biased at this point. D3D4 are forward biased at ω t = π + θ when io starts increasing again. Thus none of the diodes conduct during the interval β
< ω + ≤ π + θ and io remains zero during this period.

The operation of the bridge rectifier can be reduced the input power factor of the utility and distort the input current waveforms which has drawbacks to another devices connect on the same AC utility. Figure below show the effect of the rectifier on the input current for two different L values.
1
20V
2
400mA
0V 0A
-20V -400mA
1 V(D7:1,V5:-)
400mA
V(R3:1,D8:1) 2 -I(V5)
0A
-400mA
0s
-I(V5)
1
20V
2
400mA
10ms
0V 0A
20ms 30ms 40ms 50ms
Time
60ms 70ms 80ms 90ms 100ms
-20V -400mA
400mA
1 V(D7:1,V5:-) V(R3:1,D8:1) 2 -I(V5)
0A
-400mA
0s
-I(V5)
10ms 20ms 30ms 40ms 50ms
Time
60ms 70ms 80ms 90ms 100ms
This distortion and low input power factor can be increased further as the DC voltage of the battery reach close to the maximum voltage of the supply, as shown below.

1
20V
2
400uA
0V 0A
-20V -400uA
400uA
1 V(D7:1,V5:-) V(R3:1,D8:1) 2 -I(V5)
0A
-400uA
0s
-I(V5)
10ms 20ms 30ms 40ms 60ms 70ms 80ms 90ms 50ms
Time
Part I: Ohmic-Inductive load
1Assemble the single-phase half-wave rectifier circuit as show below:
100ms
Part II: Ohmic-Inductive-Battery load
1Assemble the single-phase full-wave rectifier circuit as show below: