Power Electronics – APSEEE

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What is VFD?

VFD stands for Variable Frequency Drive. VFD is used to speed control of AC induction motors and protect the motors. A VFD can also control the speed of motor during start and stop cycle, as well as throughout the run cycle. VFD is also known as VVVFD. The meaning of VVVFD is Variable Voltage Variable Frequency Drive. It means with change in frequency voltage can also be changed and vice versa. 

Working of VFD

The working of VFD is very simple. An AC supply given to the VFD which is first converted into DC supply then it is converted into AC supply again at desired frequency and voltage. Changed magnitude of frequency and voltage used to control of speed of AC induction motors. For better understanding, we write a relation between speed and frequency which are given below

N= (120×f)/P

NS = Synchronous speed in r.p.m

f = frequency of power supplied or input power or supply frequency

P = No. of Poles

In above relation we can see, with change of frequency of input power the speed of the induction motor is changed. Simply we can say that the speed of the induction motor is directly proportional to the supply frequency.  
variable frequency drive VFD

Rectifier Converts ac into dc. SCRs are used in rectifier. In three phase rectifier six SCRs are used. Control Unit control the supply frequency and voltage. inverter converts dc into ac.

Advantages of VFD

VFD have many advantages. some of advantages are given below:-

  • VF drive save energy.
  • It improves the power factor of the machines.
  • It gives smooth starting to the induction motors.
  • It reduces the power when not required.
  • We can change the direction of induction motor very easily.
  • It provides controlled starting and stopping. 
  • It provides protection to the induction motors against short circuit, overload, earth fault etc.

Disadvantages of VFD

  • The initial cost of VFD is very high
  • Skilled workers are required to operate it.

Applications of VFD

  • It is used in industrial application such for the control of speed of motors.




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Methods of Triggering of Thyristor

Methods of Triggering of Thyristor

The method by which a thyristor is turned on is called triggering of Thyristor. In this process the thyristor brings in conducting state from non-conducting state. A thyristor can be turned on by any one of following techniques. In this article, we will discuss about different methods of triggering of thyristor or SCR.

READ – Construction and working of Silicon Control Rectifier  or Thyristor

  • Forward voltage triggering
  • Gate triggering
  • Thermal triggering
  • dv/dt triggering
  • Light triggering
  1. Forward Voltage Triggering

When a thyristor is forward biased and the forward voltage applied across the anode and cathode, the width of the depletion layer, starts decreasing. At a breakover voltage the depletion layer vanishes and thyristor starts conducts. The triggering of the device is caused by the cathode, that is why it is called forward voltage triggering methods.

  1. Gate Triggering

In this method of triggering, source (say battery) connected across gate terminal and cathode. This junction makes forward biased with respect to cathode. Small gate signal applied to a thyristor turned on it. Gate triggering is most common method of triggering. The firing angle of device can be controlled by varying the gate signal.

methods of triggering of thyristor gate triggering

The Gate triggering methods can be classified as.

  • DC gate triggering
  • AC gate triggering
  • Pulse Gate triggering or UJT triggering

AC triggering may be Resistance r triggering or resistance capacitance rc triggering of scr or thyristor.

Why gate triggering is preferred?

 In gate triggering, we can control the output of an SCR or thyristor by gate signal which is applied to the junction J2 that helps to make junction J2 forward biased and small gate signal  can be triggered the SCR or Thyristor that is why gate triggering is preferred.

  1. Thermal Triggering

In this method, heat energy is applied to the thyristor to trigger it. We known semiconductor materials have negative temperature coefficient of resistance. As the external heat energy is applied to thyristor the resistance of the device is caused by using heat, hence the name is thermal triggering method.

  1. dv/dt Triggering

In this method, the triggering of thyristor is caused by using high rate of rise of voltage is called dv/dt triggering.

  1. Light Triggering

In this method, light energy fall on the thyristor, which results in electron hole pairs are generated in the device. This increase the flow of current with in the device which in turn causes triggering light activated silicon controlled switch are the examples of used light triggering method.


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What is Triac?

What is TRIAC?

Triac belongs to thyristor family. The triac is the most widely used device in an alternating current applications. Basically, it is a combination of two SCRs connected in anti-parallel manner. Triac can conduct in both directions positive as well as negative of an ac supply. A triac is widely used in industries for control purposes. The triac can conduct alternating current. So, it is used for ac current applications.

Construction of Triac

The triac is a three terminal, four layer bidirectional semiconductor device. The name of three terminals is MT1, MT2 and Gate(G). The same is used for the construction of triac that is used used for thyristor.


Basically, a triac consists of two SCRs connected in anti-parallel connection with a common gate.

antiparallel connected scr triac

Symbol of Triac

symbol of triac

I-V Characteristics of Triac

As we discuss above the triac has three terminals. MT1 MT2 and Gate (G). The triac can trigger by two methods.

Firstly when input voltage level reaches at breakover voltage or exceeds the breakover voltage.

Secondly, Gate signal is applied to the triac.

The first quadrant characteristics are just like an SCR but on the other hand, the third quadrant characteristics of a triac are identical to its first quadrant. Only polarities of the main terminals (such as MT1 and MT2) change.

vi characteristics of triac

The Triac can be operated in both the half-cycle of an ac supply. The output of the triac can be controlled by with the help of gate current.

Applications of Triac

  1. It is used to control power in steps.
  2. It is used as a fan regulator.
  3. It is used to control the speed of ac motors. Triac can control the speed by adjusting the gate current.

The triac are available in different sizes and ratings. They have the capacity to handle current from 0.5A to 100A and voltages upto 2000V. The size of the device depends upon its current carrying capacity. BT 151 is an example of triac.

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A diac belongs to thyristor family. Once it is triggered, it starts conducting in both directions. It is similar to diode but there is a main difference between diode and diac.  The diode conducts in one direction, when it is forward biased. On the other hand, diac conducts in both directions or we can say that diac conducts both half cycle of the alternating current. That is why the diac is also known as a bidirectional avalanche diode. It is known as bidirectional avalanche diode because it is conducts only when applied voltage becomes equal or more the forward breakover voltage.

Basically, diac is a two terminal bidirectional semiconductor device that can conduct alternating current.

Di means (two terminal device)

Ac means conduct alternating current.

Construction of Diac

The material used for manufacturing of diac is silicon like thyristor. Silicon materials have many advantages.

A diac is a four layer (PNPN), two junctions (J1, J2) and two terminal (MT1 and MT2) semiconductor device.

MT1 is known as Main Terminal One.

MT2 is known as Main Terminal Two.

There is no control terminal in this device.


Symbol of Diac

symbol of diac

Working of Diac

AC supply is given to the input of circuit. Diac is connected in series with load. The current does not flow through the load because the voltage applied to the diac is less than breakover voltage and at this stage  diac remains in non-conducting state.

When input voltage exceeds the breakover voltage, the diac starts conduct and current start flowing through the load.

working of diac

I-V Characteristics of Diac

The device can be triggered by either positive or negative half of an ac cycle. When the supply voltage is less than the breakover voltage, a very small amount of current called the leakage current flows through the device as shown in figure.

i v characteristics of diac

Leakage current is not sufficient to conduct the device. The device remains practically in non-conducting mode.

When the voltage level reaches the breakover voltage, the device start conducting. The current increased exponentially at this stage.

For positive half cycle the characteristics obtained in the first quadrant.

Negative half cycle the characteristics obtained in the third quadrant.

Diac does not give controlled output like Triac.

Applications of Diac

Diac are basically used to triggering of triac. Triac and diac pairs are available in market.


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Silicon Controlled Rectifier

Silicon Controlled Rectifier  (SCR)

Silicon controlled rectifier belongs to thyristor family. SCR is used in industrial application. It is used for controlling high voltage in the order of 10KV and current in order of 3000A. It plays an important role in industries. In this topic we will discuss about what is an SCR, construction of SCR and working of SCR.

What is an SCR?

SCR (is called silicon controlled rectifier) is a three terminals (namely anode, cathode and gate), three junction (J1, J2, J3) and four layer semiconductor device used for rectification, inversion etc.


SCR consists of four-layer P-N-P-N semiconductor materials. Silicon material uses as the semiconductor to which the proper impurity is added. The junction may be diffused or alloyed.

It has three terminals.

SCR and Working of SCR

  • The terminal taken from outer P-type material is called anode.
  • The terminal taken from outer N-type material is called cathode.
  • The terminal taken from inner P-type material is called Gate.

The SCR is a unidirectional device like power diode. The difference between power diode and SCR. There is an extra terminal is used in SCR.

Working of SCR

Action of Anode Voltage ( Gate Being Open)

In this mode, anode made positive with respect to cathode, with gate circuit open. In this case junctions J1 and J3 become forward biased whereas junction J2 become reverse biased under such conditions small current flows through the device. This current flows due to minority carries. The device remains in blocking state. If the voltage at anode is continuously increased, the reverse biased junction J2 will have an avalanche breakdown and SCR starts conduct.

silicon controlled rectifier

The forward anode voltage at which SCR starts conducts called breakover voltage.

Action of Cathode Voltage (Gate Being Open)

In this case, cathode made positive with respect to anode, the junction J1 and J3 become reverse biased. If the voltage increases gradually at one stage. It may results in breaking of depletion region at junctions J1 and J3 and the current through it suddenly increases at a very high value. This may damage the device. This is called reverse breakdown.

Action of Gate Voltage

When gate is connected to the junction J2 , it becomes forward bias. A small gate voltage is applied to the junction J2 to turn on the SCR.

Applications of SCR

  • It is used as a rectifier in industries to control speed of dc motors
  • It is used to control large power.
  • it is used as a switching device.
  • It is used for over voltage protection.

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