Chopper | Power Electronics - Electrical Engineering (EE) PDF Download

CHOPPERS

  • Chopper circuit provides adjustable DC from fixed DC. If is analogous’ to the transformer of AC.

It is mainly categorized into two types: 

  • AC link chopper 
  • DC chopper

Principle of Chopper Operation

  • It is high speed ON/OFF semiconductor switch (e.g. SCR). 
  • Inductor L and FWD are used to make load current to be continuous. 
  • From figure, it can be seen that average

  Chopper | Power Electronics - Electrical Engineering (EE)

where, T = TON + TOFF

f = [1/T] is called as chopping frequency 

 δ = [TON/T] = duty cycle

Chopper | Power Electronics - Electrical Engineering (EE)

  •  Form equation it is clear that VO can be controlled either by controlling TON or by controlling f.

 Types of Chopper Circuits Classification Of Choppers

Choppers are classified as per the direction voltage and current in five categories,
(a) Class A choppers
(b) Class B chopper
(c) Class C chopper
(d) Class D chopper
(e) Class E chopper. 

Chopper | Power Electronics - Electrical Engineering (EE)    Chopper | Power Electronics - Electrical Engineering (EE)Chopper | Power Electronics - Electrical Engineering (EE)                   Chopper | Power Electronics - Electrical Engineering (EE)
Chopper | Power Electronics - Electrical Engineering (EE)

Class A Chopper The step down chopper described in sections 5.2, 5.3, 5.4 and 5.5 is class A chopper. The load current II can flow out of the source and into the load, both VI and II. are positive and chopper operates in first quadrant only. Figure (a) shown its quadrant of operation.

Class B chopper It is also a one quadrant chopper but operates in the second quadrant. The load voltage is positive but the load current is negative. Thus the current flows out of load and into the source. Figure (b) shows the quadrant of ‘is operation.

Figure (a) shows the circuit of class B chopper.
Thyristor is shown by switch S. When thyristor is conducting S is closed. When thyristor is off, S is open. 

Chopper | Power Electronics - Electrical Engineering (EE)

When switch is closed, the battery e.m.f. E sends a current through the inductance and the voltage VI. is zero. KVL equation in this mode is

  Chopper | Power Electronics - Electrical Engineering (EE)
Let the current at t = 0 be donated by Imin. Solution of equation with current equal to Imin at t =0 is

  Chopper | Power Electronics - Electrical Engineering (EE)

This mnode exists for 0 ≤ t ≤ Ton.
Ton can also be written as αT where α is the duty cycle.
When switch S is open, energy stored in inductance is returned to supply through diode D. The current decrease during this mode. If time during this mode is written as t’, KVL equation for this mode is

 Chopper | Power Electronics - Electrical Engineering (EE)

Let current at t' = 0 (or t = αT) be denoted by Imax. Solution of equation is

 Chopper | Power Electronics - Electrical Engineering (EE)

This mode exists for 0 ≤ (T–Ton) or for aT ≤ t ≤ T.

Class C chopper

Figure below shows the circuit of class C chopper.
It uses two thyristors shown by switches S1 and S2 and two diodes D1 ,D2. It is also known as two quadrant chopper. The load voltage is always positive.
The current can be either positive or negative. It is a combination of class A and B choppers. ,S1 and Dwork as class A chopper while S2 and D1 work as class B chopper. When S1 or D2 are on, the load current is positive On the other hand when S2 or D1 are on, load current is negative. Power car. flow both from source to ioad and from load to source. It is used in operation and regenerative braking of DC motors.
When motor is driving the load, power into the source.
It should be ensured that both thyrisotrs, S1 and S2 should not be turned on together because this would cause a Short-circuit across the source. 

    Chopper | Power Electronics - Electrical Engineering (EE)

    Chopper | Power Electronics - Electrical Engineering (EE)

Class D chopper Figure shows a class D chopper. The load current is always positive but the load voltage can be positive or negative. This it can operate in first or fourth quadrant. When both S1 and S2 are on, load voltage is positive. If S1 and S2 are off but D1 , D2 conduct, load voltage is negative. In both cases the direction of IL, is the same. When Ton is less than Toff, load voltage is negative. Since VL can be positive or negative, power can flow in both directions i.e. from source to load or from load to source.

Class E chopper A class E chopper can operate in all the four quadrants. Both VL and IL can be positive or negative.
Figure (a) shows the circuit. Two class C choppers when combined give a class E chopper. The operation in the four quadrants is as under 

Chopper | Power Electronics - Electrical Engineering (EE)

 

                      Chopper | Power Electronics - Electrical Engineering (EE)Chopper | Power Electronics - Electrical Engineering (EE)

Quadrant I: For this operation S4 is on, S2 and Sare off and S1 is operated. When both SS4 conduct, iL flows from source to load. When .S1 is off, iL freewheels through S4 and D2. Thus both v1 and iL are positive and wc get first quadrant operation.
 

Quadrant II: In this operation S2 is operated while S1 ,S3 and S4 are off. When S2 is on, negative current Hows through L, S2, D4, E. Inductance L stores energy during this current flow. When S2, is off, current is led back to source through D1 and D4. For this feedback of energy, Chopper | Power Electronics - Electrical Engineering (EE) should be more than V..

Quadrant III: F or this operation S1 and S4 are off, S2 is on and S3 is operated. When S3 is on, the load is connected to the source through S2, S3 and both vI, and i1, are negative. When S3 is turned off, the negative current freewheels through S2 and D4.

Quadrant IV: S1, S2, S3 are off and S4 is operated. The polarity of e.m.f E has to be reversed for this operation. When is S4, positive current flows through E, S4, D2 L. Energy is stored in inductance L. When .S’4 is turned off, current is fed-back to source through D3 and D2. The load current is positive but load voltage is negative giving operation in fourth quadrant.
The components which conduct are shown in Figure (b).

Thyrsitor Chopper Circuits

The process of opening or turning-off, a conducting thyristor is called commutation. lis dc choppers, ii is essential to provide a separate commutation circuitry to commutate the main power SCR. It may be recalled that a conducting thyristor can be turned, off by reducing its anode current below holding current value and then applying a reverse voltage across the device to enable it to regain its forward blocking capability. T here are several ways of turning-off a thyristor. All these methods differ from

one another in the manner in which commutation is achieved. In DC choppers commutation circuitry has passed through numerous innovations. All these commutation circuits can, however be broadly classified into two groups as under:

1. Forced Commutation: In forced commutation, external elements L and C which do not carry the load current continuously, are used to turn- off a conducting thyristor. Forced commutation can be achieved in the following two ways:

a. Voltage Com m utation : In this scheme, a conducting thyristor is commutated by the application of a pulse of large reverse voltage.
This reverse voltage is usually applied by switching a previously charged capacitor. The sudden application of reverse voltage across the conducting thyristor reduces the anode current to zero rapidly. Then the presence of reverse voltage across the SCPv aids in the completion of its turn-off process (i.e. aids in gaining the forward blocking capability of SCR).

b. Current com m utation: In this scheme, an external pulse of current greater than the load current is passed in the reversed direction through the conducting SCR. When the current pulse attains a value equal to the load current, net pulse current through thyristor becomes zero and the device is turned, off. The current pulses is usually generated by an initially charged capacitor.
An important feature of current commutation is the connection of a diode in anti-parallel with the main thyristor so that voltage drop across the diode reverse biases the main SCR. Since this voltage drop is of the order of 1 volt, the commutation time in current commutation is more as compared to that in voltage commutation.
In both voltage and current commutation schemes, commutation is initiated by getting an auxiliary SCR.

2. Load commutation: In load commutation, a conducting thyristor is turned off when load current flowing through a thyristor either.

a. becomes zero due to the nature of load circuit parameters or

b. is transferred to another device from the conducting thyristor

The document Chopper | Power Electronics - Electrical Engineering (EE) is a part of the Electrical Engineering (EE) Course Power Electronics.
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FAQs on Chopper - Power Electronics - Electrical Engineering (EE)

1. What is a chopper in electronics?
Ans. In electronics, a chopper refers to a device that is used to convert a continuous input voltage into a pulsed output voltage. It is commonly used in power electronics applications to control the power flow and voltage levels.
2. How does a chopper work?
Ans. A chopper works by rapidly turning on and off a switch in order to control the output voltage. When the switch is turned on, the input voltage is connected to the load, and when it is turned off, the connection is broken. By adjusting the switching frequency and duty cycle, the chopper can regulate the average output voltage.
3. What are the advantages of using a chopper in power electronics?
Ans. Using a chopper in power electronics offers several advantages, including: - Voltage level control: A chopper allows for precise control of the output voltage, enabling voltage level adjustments as per the requirements. - Power flow control: By controlling the switching frequency and duty cycle, a chopper can regulate the power flow to the load. - Energy-efficient: Choppers can improve energy efficiency by reducing power losses and minimizing energy dissipation in the system. - Compact size: Choppers are typically smaller in size compared to other power electronic devices, making them suitable for space-constrained applications. - Compatibility with different power sources: Choppers can be used with various power sources, including batteries, generators, and renewable energy systems.
4. What are the applications of choppers in power electronics?
Ans. Choppers find application in a wide range of power electronic systems, including: - Electric vehicle systems: Choppers are used in electric vehicles to control the power flow from the battery to the motor, enabling efficient speed and torque control. - Renewable energy systems: Choppers are employed in renewable energy systems, such as solar and wind power, to regulate the power flow and ensure compatibility with the grid. - Uninterruptible power supplies (UPS): Choppers are utilized in UPS systems to convert the DC power from the battery into AC power for supplying critical loads during power outages. - Adjustable speed drives: Choppers play a crucial role in adjustable speed drives, allowing for precise control of motor speed and torque. - Battery charging systems: Choppers are used in battery charging systems to regulate the charging voltage and current, ensuring safe and efficient charging.
5. What are the different types of choppers used in power electronics?
Ans. There are several types of choppers used in power electronics, including: - Step-down chopper: Also known as a buck converter, it converts a high input voltage into a lower output voltage. - Step-up chopper: Also known as a boost converter, it increases a low input voltage to a higher output voltage. - Step-up/down chopper: Also known as a buck-boost converter, it can step up or step down the input voltage based on the requirements. - Forward chopper: It is used for high-frequency applications and operates by using a transformer. - Flyback chopper: It operates by storing energy in an inductor and releasing it to the load when the switch is turned off. - Half-bridge and full-bridge chopper: These configurations are used for high-power applications and provide more flexibility in controlling the power flow.
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