POWER ELECTRONICS

بسم الله الرحمن الرحيم The Islamic University of Gaza Faculty of Engineering Electrical Engineering Department POWER ELECTRONICS EELE 5450— Summer 2012 Instructor: Eng. Jalal Al Roumy Lecture 19

AUXILIARY 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 DC Examples of forced commutation in inverter circuits.

AUXILIARY 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.

AUXILIARY 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.

AUXILIARY 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.

AUXILIARY 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.

AUXILIARY 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 the capacitor recharges to the original polarity through the load, and then through D2 when T2 turns off. The cycle is then repeated.

A single firing pulse is shown, but if the load is inductive a train of pulses would be used.

T1 and T2 must never be on together because this will short-circuit the supply.

If T2 is fired after T1 is off, but before T1A is off, the capacitor charge is topped up.

Control of inverter frequency is by control of the periodic time of the gate firing pulses.

Control of the mean voltage can be achieved by variation of the delay between gating pulses of main and auxiliary thyristors.

AUXILIARY IMPULSE COMMUTATED INVERTER A block schematic diagram of the gate pulse control is shown below.

AUXILIARY IMPULSE COMMUTATED INVERTER A typical commutation interval is shown in three stages T1 is on and TIA is turned 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.

AUXILIARY IMPULSE COMMUTATED INVERTER Stage2 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.

AUXILIARY IMPULSE COMMUTATED INVERTER Stage 3 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-cycle End of Lecture

POWER ELECTRONICS