1 School of Electrical Systems Engineering ABD RAHIM 2008 EET421 Power Electronic Drives - AC voltage controller Abdul Rahim Abdul Razak

2 School of Electrical Systems Engineering ABD RAHIM 2008 If a thyristor is connected between ac supply and load, the power flow can be controlled by varying the rms value of ac voltage applied to the load.  AC voltage controller Types of control : 1) On-Off control 2) Phase-angle control Common application : industrial heating, on-load transformer tap changing, light control, induction motor speed control etc.

3 School of Electrical Systems Engineering ABD RAHIM 2008 If a thyristor is connected between ac supply and load, the power flow can be controlled by varying the rms value of ac voltage applied to the load.  AC voltage controller Thyristors are line commutated switch  automatically Off during voltage commutation. 1-phase 3-phase Bidirectional / full-wave control Unidirectional / half-wave control

4 School of Electrical Systems Engineering ABD RAHIM 2008 Generic power switch selection :

5 School of Electrical Systems Engineering ABD RAHIM 2008 Principle of On-Off control -the switch is turned On for n- duration cycle then Off for m- duration cycle. -Only applicable for a high mechanical inertia or high thermal time constant loads  industrial heating and speed control of motors. -Harmonic found to be reduced due to zero-voltage and zero-current switching -Duty cycle, k = n/(m +n)

6 School of Electrical Systems Engineering ABD RAHIM 2008 Thus, the output voltage Vo : For a sinusoidal input voltage,

7 School of Electrical Systems Engineering ABD RAHIM The Power factor and output voltage vary with -If T is total On-Off control period. (m+n)T should be less than the mechanical or thermal time cosntant of the load, (usually << 1s). Sum of m and n is generally around k or T cannot be considered within a longer period of more than hours or days. It will give an erroneous results.

8 School of Electrical Systems Engineering ABD RAHIM 2008 Principle of Phase control -the switch is turned On only after a delaying firing angle, α. -The output voltage and input current are asymmetrical  contains DC components, thus may cause saturation on input transformer.

9 School of Electrical Systems Engineering ABD RAHIM 2008 Principle of Phase control -Thus this half-wave controller is suitable for low power resistive load only  heating and lightning -Also known as unidirectional controller. -The output voltage, Vo:

10 School of Electrical Systems Engineering ABD RAHIM 2008 The average value of output voltage, Vdc:

11 School of Electrical Systems Engineering ABD RAHIM 2008 Bidirectional 1-phase controller – resistive loads -Full wave controller are used to encountered the DC input current problems. - T1 is controlling the +ve half-cycle while T2 is to control the -ve half –cycle. - the firing pulses of T1 and T2 are kept 180 degrees apart.

12 School of Electrical Systems Engineering ABD RAHIM 2008 Bidirectional 1-phase controller – resistive loads -Full wave controller are used to encountered the DC input current problems. - T1 is controlling the +ve half-cycle while T2 is to control the -ve half –cycle. - the firing pulses of T1 and T2 are kept 180 degrees apart. - if the delay angles of T1 and T2 are the equal (α1=α2=α), and input voltage is - The output voltage, Vo:

13 School of Electrical Systems Engineering ABD RAHIM 2008 Possible arrangement of full-wave AC controller circuit :

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15 School of Electrical Systems Engineering ABD RAHIM 2008 Bidirectional 1-phase controller – inductive loads - in practical most loads are inductives. - assumes T1 is fired during the +ve half-cycle and carrying load current. Due to inductance in the circuit, the current in thyristor T1 would not fall to zero at ωt= π, when voltage starts to be -ve. - instead of π, T1 will cont to conduct untill its current falls zero at ωt=β - the conduction angle of T1, δ = β – α and depends on the delay angle α and power factor angle of load θ.

16 School of Electrical Systems Engineering ABD RAHIM 2008 Bidirectional 1-phase controller – inductive loads

17 School of Electrical Systems Engineering ABD RAHIM 2008 Bidirectional 1-phase controller – inductive loads

18 School of Electrical Systems Engineering ABD RAHIM 2008 Bidirectional 1-phase controller – inductive loads

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22 School of Electrical Systems Engineering ABD RAHIM phase transformer tap changers - One of the famous thyristor AC voltage control applications - advantage – very fast switching action, smooth transitions. - with the input voltage, Vp:

23 School of Electrical Systems Engineering ABD RAHIM phase transformer tap changers

24 School of Electrical Systems Engineering ABD RAHIM phase transformer tap changers

25 School of Electrical Systems Engineering ABD RAHIM phase transformer tap changers

26 School of Electrical Systems Engineering ABD RAHIM phase transformer tap changers

27 School of Electrical Systems Engineering ABD RAHIM phase transformer tap changers The output voltage, Vo:

28 School of Electrical Systems Engineering ABD RAHIM phase transformer tap changers The output voltage, Vo:

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30 -is a direct frequency changer that converts AC power at one frequency to AC power at another frequency by ac-ac conversion, without an intermediate conversion link - anyway, the maximum output frequency is limited to fraction of the source frequency only. -Major applications  low speed AC motor drives. School of Electrical Systems Engineering ABD RAHIM 2008 Cycloconverters

31 -During the first half period To/2 : operate converter P as a normal controlled rectifier with αp = α by gating T1 and T2 at α and gating T3 and T4 at π+α. -During the second half period To/2 : operate converter N as a normal controlled rectifier with αN = π-α by gating T1’ and T2’ at π-α and gating T3’ and T4’ at 2π+α. - The instantaneous values of the two output voltages may not be equal, so it is possible for large harmonic current to circulate wihin the converters. School of Electrical Systems Engineering ABD RAHIM 2008

32 -The circulating current can be eliminated by suppressing the gate pulses on converter which not delivering load current. -An intergroup reactor (centre tap transformer) will maintain a continous current flow and also limits the circulating current. School of Electrical Systems Engineering ABD RAHIM 2008

33 Circuit Cycloconverters – 3-phase School of Electrical Systems Engineering ABD RAHIM 2008

34 Gating Sequence 1.During the first half period To/2 : operate converter P as a normal three phase controlled rectifier with a delay angle α P = α 2. During the second half period To/2 : operate converter N as a normal three phase controlled rectifier with a delay angle αN = π-α School of Electrical Systems Engineering ABD RAHIM 2008

35 School of Electrical Systems Engineering ABD RAHIM Controlling an AC motor required 3-phase voltage variable frequency. Each phase needs 6 thyristor thus 3- phase would need 18 thyristors. But for full-wave control, 36 thyristors are needed.