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