|
Image may be copyright |
CHOPPER:
Chopper is basically static power electronics device which converts fixed dc voltage/power to variable dc voltage or power. It is nothing but a high-speed switch which connects and disconnects the load from the source at a high rate to get variable or chopped voltage at the output.
Chopper Principle:
The basic principle of the chopper is to connect or disconnect the load from the constant input supply by using 1) Power semiconductor device acting as an on/off switch. 2) Control circuit to control the on-off time of the switch.
The operation of chopper can be controlled in the following two ways:
1) Time Ratio Control (TRC):
In this method, the chopper can be operated both as a constant frequency or variable frequency.
- Constant Frequency Operation: In this technique, the chopper frequency is kept constant. Hence, the total period T remains constant. Ton and Toff both are varied to vary the duty cycle. The advantage of this strategy is that the filter components are easy to design.
- Variable frequency operation: In this technique, the frequency of the chopper varies when the duty cycle is to be varied. When Ton is varied, Toff is kept constant and vice versa. The filter design is difficult for this strategy.
2) Current Limit Control (CLC):
In this method, the output current is sensed. When it exceeds i
o(max), the chopper is turned off. When it reduces below i
o(min), the chopper is turned on.
Four Quadrant Chopper:
A class E chopper is also known as a four-quadrant chopper. It operates in the four quadrants of vo-io. The output current, as well as voltage both, can take positive as well as negative values. The four-quadrant drive operation can be explained in four modes of operation. They are:
- First quadrant: Forward motoring operation
- Second quadrant: Forward braking operation
- Third quadrant: Reverse motoring operation
- Fourth quadrant: Reverse breaking operation
|
Image may be copyright |
The following figure shows a four-quadrant chopper with transistor switches. As it has the capability of operating in all four quadrants, it is used in reversible dc drives. The braking is regenerative. Hence, they are highly efficient. Their dynamic response is also fast.
Application:
- Switched mode power supplies, including DC to DC converters.
- Speed controllers for DC motors.
- Class D electronics amplifiers.
- Switched capacitor filters.
- Variable-frequency drive.
- DC voltage boosting.
- Battery operated electrical cars.
Comments
Post a Comment