Presentation on theme: "Instructor: Eng.Moayed N. EL Mobaied"— Presentation transcript:
1Instructor: Eng.Moayed N. EL Mobaied بسم الله الرحمن الرحيمThe Islamic University of GazaFaculty of EngineeringElectrical Engineering DepartmentPOWER ELECTRONICSEELE 5450 — FallInstructor: Eng.Moayed N. EL MobaiedLecture 24
2AUXILIARY IMPULSE COMMUTATED INVERTER When thyristors are used as the power electronic switches, a separate turn-off circuit is required in the situation where there is no natural supply voltage zero,i.e. where the supply is a steady D.c. Examples of forced commutation in inverter circuits.
3AUXILIARY IMPULSE COMMUTATED INVERTER T1 and T2 are the main power controlling thyristors, and T1A and T2A are the auxiliary commutating thyristors. D1 and D2 are the feedback diodes for inductive loads.
4AUXILIARY IMPULSE COMMUTATED INVERTER T2A is switched on to pre-charge the commutating capacitor. When the charging current ceases, the capacitor is charged to the full battery voltage Vb, with the polarity shown. The thyristor turns off naturally at current zero, when the capacitor is fully charged.
5AUXILIARY IMPULSE COMMUTATED INVERTER T1 is turned on to start the load cycle, and conduction occurs left to right through the load with the left-hand terminal positive. At the end of the half-cycle period, T1A is turned on, T1 is reverse-biased by the capacitor and prepares to turn off.
6AUXILIARY IMPULSE COMMUTATED INVERTER The capacitor meanwhile recharges to opposite polarity, through the load first, and then through D1 when T1 turns off. When the capacitor current reaches zero, T1A turns off leaving the left-hand plate of the capacitor positive.
7AUXILIARY IMPULSE COMMUTATED INVERTER T2 is now switched on and conduction occurs right to left through the load with the right-hand terminal positive. At the end of the negative half-cycle period, T2A is switched on, T2 is reverse-biased and prepares to turn off, and thecapacitor recharges to the original polarity through the load, and then through D2 when T2 turns off. The cycle is then repeated.
8A single firing pulse is shown, but if the load is inductive a train of pulses would be used.
9T1 and T2 must never be on together because this will short-circuit the supply.
10If T2 is fired after T1 is off, but before T1A is off, the capacitor charge is topped up.
11Control of inverter frequency is by control of the periodic time of the gate firing pulses.
12Control of the mean voltage can be achieved by variation of the delay between gating pulses of main and auxiliary thyristors.
13A block schematic diagram of the gate pulse control is shown below. AUXILIARY IMPULSE COMMUTATED INVERTERA block schematic diagram of the gate pulse control is shown below.
14A typical commutation interval is shown in three stages AUXILIARY IMPULSE COMMUTATED INVERTERA typical commutation interval is shown in three stagesT1 is on and TIA is tumed on to begin the commutation of T1. The capacitor discharges through an oscillatory circuit, first of all through the load, and then through D1. When T1A is turned on the capacitor current, ic, rises and the current in thyristor T1 falls until it reaches zero, at which point T1 turns off. Capacitor current continues to rise, and the excess of ic over IL flOWS through diode D1. The discharge current passes through maximum, and begins to fall as the capacitor begins to charge in the reverse direction.
15Stage2 AUXILIARY IMPULSE COMMUTATED INVERTER ic falls below IL, the load inductance acts to keep IL constant, and in so doing it forward biases D2 and load current flows in this diode. When the capacitor is charged in the reverse direction, ic ceases and T 1A turns off, leaving the capacitor charged to rather less than Vb.
16End of Lecture Stage 3 AUXILIARY IMPULSE COMMUTATED INVERTER If T2 is turned on just before T1A goes off, a further charge will flow into C to recharge it to Vb. When T1A turns off, the capacitor is charged to a polarity such that T2 can be turned off by firing T2A in the negative half-cycleEnd of Lecture