rectifier circuits. Controlled rectifier circuits; half-wave
and full wave. Function of choppers; commutator principles,
operation of chopper circuits, mark space ratio or time ratio
control, step-up and the step-down chopper.
RECTIFIER AND CHOPPER
General objective : To understand the concept of a rectifier.
Specific objectives : At the end of the unit you should be able to:
- Identify the power of an uncontrolled rectifier, semi-controlled rectifier, controlled rectifier and chopper.
- Identify the uncontrolled rectifier and chopper circuit.
8.1 INTRODUCTION OF RECTIFIER
8.2 PRINCIPLE OF RECTIFIER
8.3 SEMI-CONTROLLED RECTIFIER
i. Diode rectifiers
ii. AC - DC converters (controlled rectifiers)
8.4 HALF-WAVE RECTIFICATION
8.5 FULL-WAVE RECTIFICATION
Figure 8.5.1(a) shows the direction and path of current flow for the ½ cycle when the polarity of the transformer is as marked. Notice that only D1 is conducting and that only the top half of the transformer is providing power. This is because D2 is reverse-biased.
Figure 8.5(b) shows a full-wave, bridge rectifier circuit. Notice that this circuit provides twice as much dc voltage as does the previous full-wave circuit when both circuits use the same transformer. The bridge rectifier circuit does not use the center tap of the transformer and it requires four diodes.
During ½ cycle, two of the diodes in Fig. 8.5(c ) conduct and allow the full secondary voltage to force current through load resistor R1. the remaining two diodes are reverse-biased and thus prevent the diode bridge from short-circuiting the transformer secondary.
8.6 INTRODUCTION OF CHOPPER.
8.7 PRINCIPLE OF CHOPPER.
A chopper can operate on either fixed frequency chopper or variable frequency. A variable-frequency chopper generates harmonics of variable frequencies and a filter design. A fixed – frequency chopper is normally used. A chopper circuit uses a fast turn off as a switch and requires commutation circuitry to turn it off. The circuits are the outcome of meeting certain criteria: (1) reduction of minimum on-time limit, (2) high frequency of operation, and (3) reliable operation.
8.8 TYPE AND BASIC OPERATION OF CHOPPER FUNCTION CIRCUIT
8.8.1 IMPLUSE-COMMUTATED CHOPPERS
8.8.2. IMPULSE-COMMUTATED THREE-THYRISTOR CHOPPERS
8.8.2. RESONANT PULSE CHOPPERS
8.9 SKETCHING CURRENT AND VOLTAGE WAVE
There are no fixed rules for designing or sketching of copper circuit and the design varies with the types of circuit used. The designer has a wide range of choice and values of LmC components are influenced by the designer’s choice of peak resonant reversal current, and peak allowable voltage of the circuit. The voltage and current ratings LmC components and devices is left to the designer based on the considerations of price, availability, and safety margin. In general, the following steps are involved in the design:
a. Identify the modes of operations for the copper circuit.
b. Determine the equivalent circuits for the various modes.
c. Determine the currents and voltages for modes and their waveforms.
d. Evaluate the values of commutation components LmC that would satisfy the devices.
A chopper with a highly inductive load is shown in Fig. 8.9(a). The load current ripple is negligible (DI=0). If the average load current is Ia, the peak load current is Im=Ia + DI= Ia The input current, which is of pulsed shape as shown in Fig 8.9(b).
The wave forms for currents and voltages are shown in figure 8.9(b). In the following analysis, we shall redefine the time origin t=0 at the beginning of each mode.
Fig 8.9(c): Equivalent circuit for modes.
Mode 1 begins when main thyristor T1 is fired and the supply is connected to the load. This mode is valid for t = kT.
Mode 3 begins when T2 is self-commutated and the capacitor discharges due to resonant oscillation through diode D1 and T1. Assuming that the capacitor current rises linearly from 0 to Im and the current of thyristor T1 falls from Im to 0 in time tx.
Mode 4 begins when current through T1 falls to zero. The capacitor continues to discharge through the load at a rate determined by the peak load current.
Mode 5 begins when the freewheeling diode Dm starts conducting and the load current decays through Dm. the energy stored in commutation inductance Lm and source inductance Ls is transferred to capacitor C.
Mode 6 begins when the overcharging is complete and diode D1 turns off. The load current continues to decay until the main thyristor is refired in the next cycle. In the stedy-state condition Vc = Vx.
8.10 DEFINITION OF MARK SPACE RATIO (TIME RATIO CONTROL)
The sequence of events within the frequency counter is controlled by the time ratio base, which must provide the timing for the following events: resetting the counter , opening the count gate, closing the count gate, and storing the counted frequency in the latch. The resetting of the counter and storing the count are not critical events as long as they occur before and after the gate period, respectively. The opening and closing of the count gate, on the other hand, determine the accuracy of the frequency counter and are very critical in its timing.
8.11 COMPARING STEP-UP AND STEP-DOWN CHOPPER DOWN.
8.11.1 PRINCIPLE OF STEP-DOWN OPERATION
The principle of operation can be explained by Fig. 8.11(d). When switch SW is closed for time t1 , the output voltage Vs appears across the load. If the switch remains off for a time t2 , the voltage across the load is zero. The waveforms for the output voltage and load current are also shown in Fig. 8.11(e). The chopper switch can be implemented by using a (1) power BJT, (2) power MOSFET, (3) GTO, or (4) forced-commutated thyristor. The practical devices have a finite voltage drop ranging from 0.5 to 5 V, and for the sake of simplicity we shall neglect the voltage drops of these power semiconductor devices.