Circuits with half-bridge thyristor inverters are well known and used in different secondary power supplies with the transformer load power to 3 - 4 kilowatts .
Range extension of powers to hundreds of kilowatts is urgent proposition. It enables to use such inverters to supply of energy-intensive customers. For example, such inverters are used to supply of electric-arc furnaces, plasmatrons, high-power welding sets and other devices demanding a stabilization of load power.
Half-bridge thyristor inverters are preferable used at high power loads. Such inverters must have two condensers in-series and two thyristors in-series. Running ends of those thyristors are must be connected to the dc source moreover thyristors are must be switched to a current conducting direct.
A transformer type loads are usually switched to the diagonal of alternate current . The deficiency of such inverters is impossibility theirs commutation over the small current range, that is at the quiescent condition and light loads. When load is equal the half rated load and higher the thyristor current will break after total condenser discharge of "working" arm takes place.
The complementary commutating choke 1 works in the inverter circuit (fig. 1 ). The mid point of it is connected to the load contact 2 and running ends of it are connected between the cathode of first thyristor 3 and the anode of second thyristor 4 of half-bridge arm. That permitted to increase the range of inverter loads that a thyristor commutating is possible.
Though this variant have sequent grave disadvantages:
- The coil (half-winding) of choke 1 must be meant for the root-mean-square of current "half-wave", i.e. for the 70 percent of load ampere rating;
- The choke must be unsaturable;
- The thyristor commutating is possible over the small current range subject to condenser voltage of the turned on arm is more then condenser voltage of the turned off arm. It is impossible over the light load and quiescent condition.
Thus disadvantages of the circuit (fig. 1) are great mass and overall characteristics of the choke and this device is efficient over the bounded range of load.
Two complementary commutating condensers work in the inverter circuit (fig. 2 ). That permitted to make the commutating choke is quickly saturable and designed only for switching currents, i.e. to decrease steeply copper and iron bulks and as a whole to decrease choke gabarits. Besides that permitted to ensure an operability of a half-bridge thyristor inverter all over the load range that is from the quiescent condition to the maximum load.
The circuit (fig. 2) works in the following way.
Commutating condensers charged to half of source voltage U when source voltage turn on. Let enable pulse arrive at the thyristor 3 from the control system that is standard and don´t shown at fig.2 for simplicity. Condenser 1 was running down to load 5 and commutating condenser 7 was running down simultaneously through the unblanking thyristor 3 and the half-winding 9 of the commutating choke 6 when thyristor 3 open. Capacity of commutating condensers 7, 8 is selected such that blanking time of thyristors 3, 4 was assured. These capacities are considerably smaller then capacities of condensers 1, 2 therefore their discharges were rapidly occurred. After the commutating condenser 7 had uncharged the commutating condenser 8 should charge to the supply voltage U. If load current is small condenser 1 uncharged slowly and voltage of it don´t have time to decrease to zero. The commutating condenser 8 was charging through half-winding 10 of the commutating choke 6 and the thyristor 4 when the thyristor 4 open. Here voltage equal to voltage of half-winding 10, i.e. voltage U, is induced in the half-winding 9 of the commutating choke 6. This voltage has plus on the cathode of the thyristor 3 therefore thyristor 3 was blocking up. Then the commutating process will repeat similarly.
Condenser 1 or 2 was uncharged total during the half-period of inverter voltage and currents of thyristors 3, 4 decreased to zero when load current increase to an appointed value that is near (0.4 - 0.5) of the load ampere rating according to inverter parameters. Commutating elements (commutating choke 6 and commutating condensers 7, 8) don´t influence to commutating process at this operation.
Since half-windings 9, 10 of the commutating choke 6 transmit only short-time current impulse, overall sizes of the commutating choke 6 are small in comparison with overall sizes of the choke 1 (fig. 1).
The positive circuit singularity attached to half-bridge inverter is a coercive switching is necessary over the small current range therefore blanking times of thyristors 3, 4 and gabarits of commutating component part would decrease.
Thus the half-bridge thyristor inverter can be used at wide power range particularly if supply is the three-phase rectifier.
- Pryashnikov V.A. Electronics. S-Petersburg, 1998.
- Rudenko V.S., Senko V.I., Chizhenko I.I. Converter technique. Kiev, High school, 1978, 42 p., fig. 5.19.
- Half-bridge thyristor inverter. Patent RU 2312440 10.12.2007, priority 5.06.2008. Authors: Magazinnik L.T., Magazinnik A.G.
The work was submitted to III international scientific conference «Actual problems of science and education», Cuba, March, 19-29, 2008, came to the editorial office 21.04.2008.