Power Electronics Power Electronics Power Electronics Power Electronics
Teacher : Sir Mohsin ShehzadTeacher : Sir Mohsin Shehzad
By Spo9-Bee-8ABy Spo9-Bee-8A
Need Of Commutation and Difference between Natural and
forced commutation
Commutation In engineering “The term commutation basically
means transfer of current from one path to other.”
•
In power electronics
• It refers to the process of turning off semiconductor switch
• A static power converters ,use power semiconductor that function as a switch.
• These switches are turned off and ON .
• Repetitively or as required to implement desired function.
• So when ever switch turned
off ,then path of current changes, i.e commutation is there.
• In the terminology of power electronics turn off switching of the device is commutation
Example
• A conducted diode automatically commutated when reverse bias voltage is applied across it .
• A conducting thyristor turned off in a similar fashion because gate is inefficient to achieve this.
Commutating voltage
• The reverse voltage that serves to turned off the thyristor or diode is called commutating voltage…..
Switching Activity
• Due to need of switching activity inside converters ,we need commutation, (ON and OFF )
Thyristor• Thyristor being a switching
device ,so commutation is there
• The commutation circuit may need additional components to achieve goals
Properties of commutation • Hold negative voltage across
device during turn off time.
• During conduction (SCR) is flooded with carriers
• Ia (anode current) should flows through device until all carrier are removed
Continue • Then J1 and J2 achieve forward
blocking state.
• Trr (reverse recovery time)
• Commutation current has to hold negative voltage until Tgr
• So that excesses carriers near J2 recombined
• If negative voltage removed before Tgr ,then device may ON due to Carriers near J2.
• Because they act as a gate derive to SCR
• Negative voltage can be removed at the end of tgr
• Tq= turn off time of SCR
• Tq=Trr+Tgr
Turn off time • The time required to achieve
forward blocking capability after commutation initiated .
• Tq varies between 5 to 200 u sec
Tc
• Tc is the turn off time of commutation and it should be greater then Tq
• Tc>Tq
• SCR having Tq<25 u sec are called
Inverter grade SCR and used in Inverters and Choppers
• SCR having Tq>25 u sec are used in controlled rectifiers ,AC voltage controllers
Commutation Applications (where used and why)
• Rectification• AC to DC converters• DC to DC converters• DC to AC converters• AC to AC converters (1) Ac voltage regulators (2) Cyclo converters
Rectification (why commutation
required)• Switching activity involve • I/p Ac
• O/p DC
• It may be 1 phase or 3 phase (AC)
Application(rectification)
• Electric traction, battery charger,
Electroplating, Electrochemical processing, Power supplies ,
Welding and in UPS
Ac to DC converters(phase controlled rectifiers)
• I/p constant AC Voltage O/p Variable Dc Voltage
• These are also called Line Commutated
• Because they use line or source voltage for commutation
Applications
• In Dc drives ,
• Metallurgical and chemical industry
Excitation system for synchronous motors
DC to Dc converters
• I/p Fixed Dc O/p Variable DC
• Forced commutation is used
• Thyristor are used in high power
• In low power transistors and MOSFET
Application • Choppers are widely used in
• Dc drives, Subway cars,
• trolley trucks,
• Battery driven vehicles
Dc to AC Converter
• I/p fixed Dc O/p AC voltage
• Forced commutation as well as line is used
• When forced then used in Induction motor derives, synchronous motors derives, Induction heating , UPS
Continue
• In HVDC Inverters are line commutated
Ac to AC converters
• I/p Fixed AC O/p Variable AC
• Further two types
• (1)Ac voltage controllers (convert at same frequency)
• Line commutation is used
• O/p is controlled by firing angle
Used in
• To control heating and lighting
• Speed control of Fans and pumps
• (2) Cyclo converter
Frequency is different
Line commutation
Forced commutation also common
Used in
• Low speed large Ac MotorsThat derive loads like rotatory kilns
•
Natural (Line commutation)
• Doesn’t required any external component
• Use supply main voltage
• So called line commutation
• In Ac voltage is used to affect the current transfer.
Natural commutation
~
T
+
v ov s R
Continue
• A current always passes through a zero crossing .
• At each cycle
• Most widely used in Line commutated converters, voltage
regulator and cyclo converter
Vab ,vbn and vcn are 120 degree phase voltages
Ta conducts when (a) phase is at positive
when (b) is positive then Tb conducts and Ta OFF and so on
Continue
• By sequentially firing thyristor when the line voltage have correct polarity, we can successfully commutate each thyristor.
Two aspects of Line commutation
• Possible only in AC voltage bus
• Line commutation is not possible at any instant ,(possible only during 180 degree interval)
Forced commutation
• When supply voltage is Dc
• Artificial means are used to turn off the SCR
• Apply a reverse bias or pass a impulse of negative current.
Continue
• Normally LC circuit are used
• They store energy during SCR ON and use this energy for turning Off
• LC circuit impose a reverse voltage across SCR
Continue
• L And C are called commutating components
• These brings immediately the forward current to zero
Continue
• Then SCR Ia falls below holding current
• Forced commutation are of different types
Types of forced commutation
• (1)self commutation by resonating load or L C Ckt
• (2)Axuillary voltage commutation (impulse commutation)
• (3) Auxiliary current commutation resonant commutation)
• (4) Complementary commutation
• (5) External pulse commutation
Thank you