bridge rectifier

Thyristor control of a resistive load 1

THYRISTOR CONTROL OF A RESISTIVE LOAD

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Abstract

This report is a study and design of single full wave controlled rectifiers using two

single phase half-wave thyristor converters designs with R load. Using an oscilloscope, the

output voltage waveforms were observed. At firing angles within the range of 0 °<α <90 °,

the converter was operating in the rectifying mode, hence power was flowing from the AC

source into the DC load while at firing angles within the range of 90 °<α <180 °, there were

small conducting angle for the output voltages. The converter was operating more in the

inversion mode hence little power was flowing into the DC load. It was found that at firing

angle, α , of 0° the thyristor exhibited the characteristics of a normal diode, while at firing

angle of 180° it did not conduct at all.


Introduction

Earlier, most of the DC power was achieved via AC power or motor generator

conversion into the DC power by using thyratrons or the mercury arc rectifiers. Nowadays,

thyristor are commonly used in conversion of the AC to DC power, this is possible by the

application of phase controlled AC to DC converters that change constant AC voltage into

controlled DC output voltage (Visintini, 2014).

The thyristors are part of the semiconductor control devices that fall under the Silicon

Controlled Rectifiers (SCR). They are applied mostly applied in industries since they more

efficient, reliable, rugged, and less expensive. They are mainly used in generator field

control, solid state relays, DC motor control, lighting systems and Variable Frequency Drive

(VFD) (Visintini, 2014).

Objective

To study a full wave half controlled bridge rectifier in common cathode configuration

with a resistive load.

Background

Single phase half-wave thyristor converters are not widely applied in power

electronics due to the fact that they low DC voltage, as well low DC power; hence full wave

rectifiers are suitable instead (Bird, 2010). These rectifiers, half wave controlled rectifiers,

employ only one SCR in their circuit, that is, between the load and the AC source. The single

thyristor device used in this rectifier gives output controls for each half of an input AC

supply. The characteristics or performance of a controlled rectifier is dependent on the

parameters and type of load or output circuit (Irwin, 2002).


The single full wave controlled rectifiers are a combination of two half wave rectifiers

in a single circuit. This implies that the input’s half cycles are utilized as well as converted

into a unidirectional output current via the load for the production of two pulse output

waveform. The full wave controlled can be Achieved either via a centre tapped transformer

or bridge. A single phase full wave bridge controlled rectifier is further divided into a half

(semi) and fully controlled bridge converter (Krein & Pilawa-Podgurski, 2016).

Phase controlled bridge rectifiers are used in the provision of unregulated DC voltage

that can be processed into either a regulate AC or DC output. A phase controlled rectifier is

made up of a thyristor, load, and AC supply as the main circuit components. These rectifiers

are divided into R load, RL load and RL load with freewheeling diode; their classification is

dependent on the load requirements.

Figure 1. Single phase half wave controlled bridge rectifier with a resistive load

From Figure 1 above, the input AC voltage to the thyristor converter from the

transformer is dependent on the required voltage output. vp and vs represent primary and

secondary AC supply of the transformer respectively. At positive half cycle of the input,

usually when the upper end of the secondary part of transformer has a positive potential when

compared to the lower end, the thyristor’s anode becomes positive as compared to its

cathode; hence the thyristor enters a forward biased state (Dollah, 2013).


When a suitable gate trigger pulse is applied to the thyristor’s gate lead, it implies that

there is a triggered delay angle expressed as ωt=α. An ideal thyristor is assumed to behave

like a closed switch when a suitable gate trigger pulse is applied. Therefore, when a thyristor

is triggered to delay at an angleα , the input supply voltage is equal to the voltage across the

load as the thyristor conducts from α to π radians. This implies that that the output voltage

vo=vs for an ideal thyristor conducting from α=ωt to π radians (Erney Fabian et al., 2016).

The load current or output, io , of purely resistive load flowing when the thyristor T 1 is

on is expressed as follows:

io=vo

RLwhenα ≤ωt ≤ π

At the thyristor’s conduction time α to π, its output load current and output voltage

waveforms are similar; that is, they are in phase. The load current increases in a direct

proportion to the input supply voltage; hence the maximum load current appears at ω=π2 as

the supply voltage is maximum at the input. The thyristor is turned off when the current

flowing through it is zero; whenωt=π .

The peal value or maximum value of the current is obtained as:

I m=io (max )=V m

RL

Where R=RL=¿Load Resistance; and I m=peak value of the load current when ω=π2

When the thyristor is on, the source current is expressed as:

is=iTi=io=vo

R=

V msin ωtR

;when α ≤ ωt ≤ π

Where iT 1=Thyristor current; io= Load current viaRL; is=Source current flowing via the

transformer’s secondary windings.


When the supply voltage reverses, in negative half cycle, and becomes negative at

ωt=π to 2π radians, the thyristor’s anode has a negative potential with respect to the cathode

(Hughes, 2008). This implies that the thyristor is reverse biased hence it is at cut-off or

reverse blocking mode. In this state, a thyristor cannot conduct and therefore an ideal

thyristor in this mode behaves like an open switch. At cut-off, the load current and voltage

are zero, atωt=π to 2 π . The maximum reverse voltage appearing Across the cathode and

anode terminal is given as V m.

The delay or trigger angle, α is obtained at beginning of each half cycle up to an

instant a gate trigger pulse is provided. Therefore, the thyristor’s conduction angle is

δ=( π−α ); in other terms it from π to α . The maximum conduction angle occurs at 180 ° (π

radians) when trigger angle is 0° (α=0°).

vs=V m sin ωt= AC supply voltage across the secondary side of the transformer.

V m=maximum value of the input AC supply voltage across the secondary side of the

transformer.

V s=V m

√2=RMS value of AC supply voltage across the secondary side of the transformer.

vo=vL=output voltage at the load

io=iL=output current

vo=vL=V m sin ωt at ωt=α to π the thyristor is switched on or Act as a closed switch.

io=iL=vo

R=Load current at ωt=α the thyristor is switched on.

Finding the average output voltage, V dc;

V o ( dc )=V dc=1

2π∫απ

vo .d (ωt )


V o ( dc )=V dc=1

2 π∫απ

V m sin ωt .d (ωt )

V o ( dc )=V m

2 π∫απ

sin ωt . d (ωt )

V o ( dc )=V m

2π[−cos ωt ]α

π

V o ( dc )=V m

2π[−cos π+cos α ] where −cos π = −1

V o ( dc )=V m

2π[ 1+cosα ] ; V m=√2V s

V o ( dc )=√2 V s

2π[1+cos α ]

V dc(max )=V dm=V m

π

The single phase half-converter circuits consist of two diodes and thyristors in order

to make a full wave half controlled bridge converter. Half controlled bridge converter is

divided into ssymmetrical and asymmetrical configurations (Robertson, 2008). Figure 2 is

symmetrical configuration, which is the commonly used, since a single trigger is used when

firing the two thyristors without any application of electrical isolation.

Figure 2. Single phase full wave converter

The two thyristors in the circuit above are controlled by applying suitable gating

signal or trigger pulse, hence they Act as power switches. On the other hand, the two diodes


are uncontrolled switches that turn-on or conduct when forward biased. During the positive

half cycle with respect to the input AC supply voltage, line ‘A’ is positive when compared

with line ‘B’; this implies that diode D1 and thyristor T 1 are forward biased. Thyristor T 1

becomes triggered at angle ωt=α by applying a suitable trigger signal to the gate. This

implies that current flows via line ‘A’ via T 1 then via the load in a downward direction and

eventually via D1 to line ‘B’. Diode D1 and T 1 conduct together as from ωt=α to π while

load is connected to an input AC supply.

Finding the average output voltage, V dc;

V dc=2

2 π∫απ

vo . d (ωt )

V dc=2

2 π∫απ

V m sin ωt . d (ωt )

V dc=2V m

2 π ∫α

π

sin ωt .d (ωt )

V dc=2V m

2 π[−cosωt ]α

π

V dc=V m

π[−cos π+cos α ] where −cos π = −1

V dc=V m

π[ 1+cosα ]

V dc varies from 2V m

π to 0 with the variation of α from 0 to π.

Maximum average voltage=V dc (max )=2V m

π=V dm


Figure 3. Waveforms of single phase full wave half-converter for R and RL loads

Materials and method

1 - PE481 power electronics control unit

1 - PE481B full-wave thyristor circuits module

1 - PE483 power electronics control unit with integral full-wave thyristor circuits

1 – Diginess hand-held multimeter

1 – Tektronix TDS1002 digital double beam oscilloscope

1 – Slide-wire rheostat

2 – Resistors, 1 MΩ and 10 kΩ, 0.5 W mounted on a component board

Inter-connectors.

Experimental procedure

The apparatus were set as shown in Figure 2 using wires. The 240 V, 13 A bench

supply was switched OFF and the PE482A Control “set value” maintained at zero when

connecting the apparatus. The 0 V from the +15-15 V output was connected to the earth. The

rheostat was connected as a fixed resistance using the bottom terminals, one at each end. An


earth from the rheostat’s stud was connected to an earth point. All meters connected were

eventually checked if they were at their zero mark.

The PE481 Base Unit was adjusted to the 30 V/1 A level using the “Max O/P”. The

“Meter 1” was set to “normal” while the 50 V range was adjusted to forward. The Base

Unit’s meter 2 was set to the 60 V range.

The PE483 Unit was set to meter at V/A, the 60 V range and in forward. The PE482A

unit was used in applying the “set value” that controlled the point in the cycle at which the

thyristors fired. The 1 MΩ and 10 kΩ resistors were used to act as potential divider, so as to

enable simultaneous display of the voltage as well as the trigger pulse waveforms. Initially

the thyristor firing circuit on the PE482A unit was set to multi-pulse but later changed to a

single-pulse at some point in order to observe the effect on the pulse wave form.

The Power electronics control unit was switched on. The delay angle range was

maintained for the resistive loads with “set value” requiring minor adjustment in order to

reach the maximum output voltage. The oscilloscope was switched on with the vertical

position controls used in setting the baselines or coupling ground of the Channel 1 and 2. The

observations for the variation of the load voltage with firing angle was achieved by cursor on

the oscilloscope as it can measure or indicate time, therefore, delay angle. Additionally, the

handheld multimeter was used in determining the voltage.

The maximum load voltage was set and the horizontal position control was used for

the arrangement of the voltage waveform into the 180° position; that is the ‘end’ of a half-

cycle. This step facilitated subsequent estimations of the delay angle. The handheld

multimeter was used to check the voltage in PE481.

Results

Input voltage= 42 V


Table 1. Input and output values of the rectifier

α (° ) V o(Measured) VV o ¿Calc)V o=

V m

π[1+cosα ] V

0 36.2 37.836 32.8 34.254 28.5 30.072 23.3 24.790 18.3 18.9108 12.1 13.1126 7.3 7.8144 3.3 3.6162 0.9 0.9180 0.0 0.0

0 20 40 60 80 100 120 140 160 180 2000.0

5.0

10.0

15.0

20.0

25.0

30.0

35.0

40.0

VO(dc) Vs. Trigger Angle α in Degrees

Trigger angle α in degrees

VO

(dc)

Figure 4. Vo (DC) versus the trigger angle (α )

From Figure 4 and Table 1, the voltage out is inversely proportional to the trigger angle.

Table 2. Error analysis

α (° ) V o(Measured) V V o ¿Calc) V Absolute Error, V Relative Error (%)


0 36.2 37.8 1.6 4.236 32.8 34.2 1.4 454 28.5 30.0 1.5 572 23.3 24.7 1.5 690 18.3 18.9 0.6 3108 12.1 13.1 0.9 7126 7.3 7.8 0.5 6.5144 3.3 3.6 0.3 8162 0.9 0.9 0.1 6180 0.0 0.0 0.0 0

Average =4.97%

η=95.03%From Table 2, η=95.03% which is Acceptable, since typical rectification ration, η, is about

81%. Additionally, the measured and calculated values have small deviations as seen in

Figure 5.

0 20 40 60 80 100 120 140 160 180 2000.0

5.0

10.0

15.0

20.0

25.0

30.0

35.0

40.0

Error Analysis of the VO(dc) Vs. Trigger Angle α in Degrees

Measured Calculated

Trigger angle α in degrees

VO

(dc)

Figure 5. Error analysis Vo (DC) versus the trigger angle (α )

The discrepancies in measured and calculated values are due to the sudden changes in

voltages at the thyristor’s firing point hence large current spikes are produced.


Figure 6. The output sketch on the oscilloscope


From Figure 6, the wave forms are less regular. The problem emanates from the

process of commutation. In the half controlled case, the thyristor and diode pair does not turn

off and on the same time. In this case, the diodes conduct up to the occurrence of

commutation at the firing point of the next diode or thyristor.

Discussions

The single-phase mains supply was stepped down prior to being fed to the control

circuit; the control circuit has components that work at low voltage and current. The PE481

power electronics control unit played a major role in the experiment because of its duty that

triggers the firing angle to the two thyristors. Therefore, the performance of this circuit

component is vital in determining the outcome or errors. The diodes were used in order to

avert the load voltage from becoming negative, moreover extend the conduction period at

reduced AC ripples (Mubeen, 2014).

The working of single phase full wave half-converter for R is studied along with the

waveforms. The full wave rectifier converted both polarities of AC into DC. The rectifier

exhibited less ripples when compared to the half wave rectifier. Additionally, the circuit

components shared the same main current supply for a synchronized signal that was used for

gating the SCR. If there were no synchronization, possibly the SCR firing signal could be

random in nature hence leading to an erratic system response. From the calculations, its

efficiency was over 81%, hence twice that of the half wave rectifier.

The SCR conducted after meeting the following conditions, the SCR was forward

biased and current had been applied to the gate. It was observed that, the SCR conducted after

being triggered despite the gate current going to zero. At firing angles within the range of

0 °<α <90 °, there were large conduction angle of the average output voltage. This implies

that the converter was operating in the rectifying mode, hence more power was flowing from

the AC source into the DC load. Moreover, at firing angles within the range of 90 °<α <180 °,


there were less conduction angles for the average output voltages (SCILLC, 2016). The

converter was operating more like in the inversion mode hence less power was flowing to the

DC load from the AC source as seen in Figure 6; it was not possible for the circuit to work in

the inversion because of the presence of the diodes. The diodes averted the average output

voltage from becoming negative. At firing angle α=0°, the circuit behaved liked the one

with diodes only; it had the characteristics of the uncontrolled rectifier. Additionally, the

source current for the circuit with resistive load was sinusoidal; hence it was in phase with the

average voltage at trigger angle, α=0 ° which further implies that the power factor was

approximately 1, p.f=1 (Hughes, 2008).

Since conduction is via diode, the output voltage wave in R load cannot extend in a

negative direction but in the case of an RL load, the circuit’s output voltage wave will extend

towards the negative portion. If there was an inductive load, there would be a tendency of

slowed change in the anode current with time, hence leading to an increased charge with the

value of inductance. According to ON Semiconductor (2016), long pulse width or DC current

triggering, there are little effects of inductive loads but the effects increase significantly at

short pulse widths. Increase in charge is as a result of short pulse widths hence the trigger

signals are decreased to negligible values prior to the anode current reaching a level sufficient

of turn−on sustenance.

Conclusion

At the firing angle, α , of 0° the thyristor has the characteristics of a normal diode,

while at firing angle of 180° it does not conduct at all. The circuit exhibited an experimental

error of 4.97%, which is negligible. These are random errors that affected the precision of

measurement due to the sum of component errors as they are not ideal; hence non-traceable.

Therefore, the study objectives of a full wave half controlled bridge rectifier in common


cathode configuration with a resistive load was successively carried out via design,

construction and implementation.

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