Transcript Principle of On-Off control

EET421
Power Electronic Drives
- AC voltage controller
Abdul Rahim Abdul Razak
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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.
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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
1-phase
3-phase
Unidirectional / half-wave
control
Bidirectional / full-wave control
Thyristors are line commutated switch automatically Off during voltage
commutation.
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Generic power switch
selection :
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Principle of On-Off control
-the switch is turned On for nduration cycle then Off for mduration 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)
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Thus, the output voltage Vo :
For a sinusoidal input voltage,
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- The Power factor and output voltage vary with
k
-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 100.
-k or T cannot be considered within a longer
period of more than hours or days. It will give
an erroneous results.
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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.
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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:
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The average value of output voltage, Vdc:
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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.
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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:
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Possible arrangement of full-wave AC controller circuit :
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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.
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Bidirectional 1-phase controller – inductive loads
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Bidirectional 1-phase controller – inductive loads
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Bidirectional 1-phase controller – inductive loads
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1-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:
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1-phase transformer tap changers
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1-phase transformer tap changers
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1-phase transformer tap changers
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1-phase transformer tap changers
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1-phase transformer tap changers
The output voltage, Vo:
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1-phase transformer tap changers
The output voltage, Vo:
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Cycloconverters
-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.
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-
-
-
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.31
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-
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.
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Cycloconverters – 3-phase
Circuit
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Gating Sequence
1. During the first half period To/2 : operate converter P as a normal three
phase controlled rectifier with a delay angle
αP = 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
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3. Controlling an AC
motor required 3-phase
voltage variable
frequency. Each phase
needs 6 thyristor thus 3phase would need 18
thyristors. But for full-wave
control, 36 thyristors are
needed.
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