7/27/2019 ee366-chap-3-3

1/30

Mode 2

At the time of turn-on, none of the

devices is conducting.

Just after turn-on, two diodes will

conduct in addition to the SCRSubsequently one diode will turn off

Followed by the SCR and the otherdiode.

The expressions for the instantaneous

out ut volta e are as follows:

322

7/27/2019 ee366-chap-3-3

2/30

(a) 65 (Dand)T2t0For :conductingD,T 65 0van

(b) 621 (Dand)D(),Tt2For :conductingNone 0van

(c) 6D32tFor :conductingD,D,T 621 tV

m sinvan

(d) 21 D(and)T67t32For :conductingD,T 21

6

sin2

3van

tV

m

7/27/2019 ee366-chap-3-3

3/30

7/27/2019 ee366-chap-3-3

4/30

(e) 243 D(and)D(),T32t67For

:conductingNone 0van

(f) 2D34t32For :conductingD,T,D 432 tV

m sinvan

(g)

43

D(and)T611t34For

:conductingD,T 43

6

sin2

3van

tV

m

(h) 465

(Dand)D(,)T34t611For :conductingNone 0van

(i) :D2t34For 4 :conductingD,TD 654,

tVm sinvan

7/27/2019 ee366-chap-3-3

5/30

7/27/2019 ee366-chap-3-3

6/30

Mode 3At the time of turn-on, none of the

devices is conducting.

Just after turn-on, only one SCR and

one diode will conduct.

At zero power is delivered to the

load.The expressions for the instantaneous

output voltage are as follows:

6732

67

7/27/2019 ee366-chap-3-3

7/30

(a) 65 (Dand)T3/2t0For

:conductingNone 0van

(b) 65 (Dand)T2

t3/2For

:conductingD,T 65 0van

(c) 21 (Dand)Tt2For :conductingNone 0van

(d) 21

(Dand)T67tFor

:conductingD,T 21

6

sin2

3van

tV

m

7/27/2019 ee366-chap-3-3

8/30

(e) 43 (Dand)T32t67For :conductingNone 0van

(f) 43 (Dand)T611t32For :conductingD,T 43

6

sin2

3van

tV

m

(g) 2t611For :conductingisNone 0van

T1, D2, T3, D4, T5, D6, T1, D2, T3,.....

7/27/2019 ee366-chap-3-3

9/30

7/27/2019 ee366-chap-3-3

10/30

The rms output voltage can be obtained asfollows

Mode 1 20

7/27/2019 ee366-chap-3-3

11/30

Mode 2 322

7/27/2019 ee366-chap-3-3

12/30

Mode 3 6732

7/27/2019 ee366-chap-3-3

13/30

Example 3The 3-phase half-controlled regulator

supplies a star connected resistive load of

R= 10 per phase and the line-to-line

voltage is 208 V, 60 Hz. = 60o. Find

(a) The rms output phase voltage

(b) The input power factor and

(c) Expressions for the instantaneous

output voltage of phase a

7/27/2019 ee366-chap-3-3

14/30

Solution(a)

(b) RMS phase currentOutput power

Input volt-amperes

The power factor PF

A094.111094.110 oI W3692.311011.09433 22 RI

o

VA3996.80094.1120833 oLIV

lagging0.923996.89

3692.31

VA

P

o

8

2sin

43

13

21

VV

o

21

8

32sin

123

1208

V110.94

7/27/2019 ee366-chap-3-3

15/30

The instantaneous output phase voltage,which depends on the number of conducting

devices are as follows:

anv

7/27/2019 ee366-chap-3-3

16/30

3.3 Other Regulator CircuitsDelta-connected ac regulator

It is used for delta-connected loads

where each end of each phase is

accessible.

It consists of three single-phase acregulators operating independently of

each other as shown below.

7/27/2019 ee366-chap-3-3

17/30

Delta-connected ac regulator contd

7/27/2019 ee366-chap-3-3

18/30

Delta-connected ac regulator contdThe SCRs should be capable of

carrying the phase currents and

withstand the line voltages.

The firing angle control range is 0 to180ofor resistance load.

The firing angle is measured from the

zero crossing of the line-to-line voltage.

This can be employed to reduce SCR

current ratings.

7/27/2019 ee366-chap-3-3

19/30

Open-Star 3-Phase AC Regulator withSix SCRs

They are used for star-connected load

where neutral exists and can be

accessed and opened.

In this regulator, shown on the next

slide, current can flow between twolines even if one SCR is conducting, so

each SCR requires one firing pulse per

cycle.

7/27/2019 ee366-chap-3-3

20/30

Ia

Ib

Ic

ZL

ZL

ZL

T2

T3

T6

T4

T1

T5

vc

vb

va

a

b

c

7/27/2019 ee366-chap-3-3

21/30

Open-Star 3-Phase AC Regulator withSix SCRs Contd

The voltage and current ratings of the

SCRs are nearly the same as those of

the delta-connected ac regulator.

7/27/2019 ee366-chap-3-3

22/30

Open-Star 3-Phase AC Regulator withThree SCRs

It is possible to operate the open-star 3-

phase ac regulator with three SCRs as

shown on the next slide.

Each SCR is provided with gate pulses

in each cycle spaced 120o

apart.Though, the number of devices is

fewer, their current ratings must be

higher.

7/27/2019 ee366-chap-3-3

23/30

7/27/2019 ee366-chap-3-3

24/30

Integral Cycle ControlIn SCR heating applications, load

harmonics are unimportant and integral

cycle control, or burst firing, can be

employed.

Basic triac burst-firing controller circuitis shhown on the next slide.

7/27/2019 ee366-chap-3-3

25/30

I l C l C l C d

7/27/2019 ee366-chap-3-3

26/30

Integral Cycle Control ContdThe output voltage waveforms:

7/27/2019 ee366-chap-3-3

27/30

Integral Cycle Control ContdCommercially produced gate drivers may be

used to trigger the Triacs.

The lowest order harmonics in the load is 1/Tp.

The rms output voltage isThe output power is

The supply displacement factor is unity

The input power factor where

n = the number of oncycles and

N = number of cycles in the period Tp

21

NnVVo

NnRVP 2

NnPF

7/27/2019 ee366-chap-3-3

28/30

Example 4A burst-firing ac voltage regulator

supplies power to a 100-ohm resistance

from a 250 V, 50 Hz supply. Determine

the values of (a) load voltage (b) load

Current (c) output power and (d) input

power when n=2andN = 8.

7/27/2019 ee366-chap-3-3

29/30

Solution(a)

(b)

(c) Output power

(d)Inputand hence input

V12582250 21

21

NnVVo

A25.1100125 RVIoo

W3.15610025.122 RIP

o

VA5.31225.1250 oVIS

50.05.3123.156 SPPF

A 3 h h lf l f d b d li

7/27/2019 ee366-chap-3-3

30/30

A 3-phase half-wave ac regulator fed by a star-connected source supplies power to a star-

connected resistive load ofR = 10 per phase. The phase voltages are va= 170 sint, vb= 170

sin(t120o) and vc= 170 sin(t + 120

o). The circuit operates with a delay angle =135

o.

(a)Draw the circuit diagram of the regulator with T1andD4connected anti-parallel in phasea, T3andD6anti-parallel in phase b and T5andD2anti-parallel in phase c. [4marks]

(b)Given that for a delay angle =135o, all switches are off before an SCR is fired and an SCRwhen fired switches on with only one diode, determine the following:

(i) The instant in degrees (= t) at which T1is fired, the diode that switches on with T1andthe instant (= t) at which T1 turns off. [3 marks]

(ii) The instant in degrees (= t) at which T3is fired, the diode that switches on with T3andthe instant (= t) when T3turns off. [3 marks]

(iii)The instant in degrees (= t) at which T5is fired, the diode that switches on with T5andthe instant (= t) at which T5turns off. [3 marks]

(c)Obtain expressions for the instantaneous output voltage of phase a over a period. Givenon zeroexpressions only. [4

marks]

(d)Using the expressions obtained in part (c), calculate the rms output phase voltage. [4 marks](e)Calculate the input power factor. [4 marks]