Saturday, October 29, 2016
What Is Called A DC Chopper ?
A d.c. chopper is a static device (switch) used to obtain variable d.c. voltage from a source of constant d.c. voltage.
In the right below figure you can see circuit diagram of a chopper. Therefore, chopper may be thought of as d.c equivalent of an ac. transformer since they behave in an identical manner. Besides, the saving in power, the dc.chopper offers greater efficiency, faster response, lower maintenance, small size, smooth control, and, for many applications, lower cost, than motor-generator sets or gas tubes approaches.
Solid-state choppers due to various advantages are widely used in trolley cars, battery-operated vehicles, traction-motor control, control of a large number of d.c. motors from a common d.c. bus with a considerable improvement of power factor, control of induction motors, marine hoists, forklift trucks and mine haulers. The objective of this chapter is to discuss the basic principles of chopper operation and more common types of chopper configuration circuits.
Classification Of Choppers or DC To DC Power Converters
DC choppers can be classified as:
(A) According to the Input/Output Voltage Levels
(i) Step-down chopper: The output voltage is less than the input
(ii) Step-up chopper: The output voltage is greater than the input
voltage.
(B) According to the Directions of Output Voltage and Current
(i) Class A (type A) chopper
(ii) Class B (type B) chopper
(iii) Class C (type C) chopper
(iv) Class D (type D) chopper
(v) Class E (type B) chopper
The voltage and current directions for above classes are shown in below figure.
(C) According to Circuit Operation
(i) First-quadrant chopper: The output voltage and both must be positive.(Type A).
(ii) Two-quadrant chopper: The output voltage is positive and current can be positive or negative (class-C) or the output current is positive and the voltage can be positive or negative (class-D).
(iii) Four-quadrant chopper: The output voltage and current both can be positive or negative (class-E).
(D) According to Commutation Method
(i) Voltage-commutated choppers
(ii) Current-cornmutated choppers
(iii) Load-commutated choppers
(iv) Impulse-commutated choppers