Scientific journal
European Journal of Natural History
ISSN 2073-4972

THYRISTOR INVERTERS WITH AN IDLE LIMITER FOR TRANSFORMER LOADS

Magazinnik L.T.

Electroarc welding sets of an inverter type, i.e. welders in which a welding transformer is operated from the inverter with frequency till 100 kHz, are widely used [1]. Inverters in such welders may be realized as the single-ended circuit as well as the push-pull circuit. Single-ended inverters use the core of the welding transformer slightly worse than push-pull inverters. But push-pull bridge inverters form the supply voltage asymmetry of the welder transformer. When frequency increases this asymmetry increase too and result in the saturation of the transformer core that minimize advantages of bridge (push-pull circuit) inverters to single-ended inverter. The asymmetry is eliminated in push-pull circuits if condensers are used in the power circuit. Specifically, the asymmetry is lacked in the half-bridge inverter shown in [2]. The deficiency of such inverter is poor utilization of capacitor capacity. Utilization of the condenser is better fourfold, but a surge voltage is possible in the diagonal of the bridge load at the quiescent condition and light loads.

The general deficiency of half-bridge inverters as well as bridge inverters is the need to use gate-turn-off (GTO) keys, i.e. transistors or GTO thyristors. Transistors constraint a power range of welders, and GTO thyristors have the more complicated control system, and main, have large losses and lesser permissible switching frequency in comparison with ordinary (SCR) thyristors.

Inverters using ordinary (SCR) thyristors with coercive commutation by means of condensers ("C"-commutation) or of a combination of condensers and inductor ("L-C"-commutation) are known too. But coercive commutation complicates the power circuit of the inverter. A "classic" thyristor inverter using gate-turn-off thyristors or ordinary thyristors with coercive commutation is described in [3]. The circuit provides for the series condenser turn on with the primary winding of the transformer welder that may result in voltage surges in the condenser and the primary winding at the quiescent condition and light loads. Gate-turn-off thyristors are much expensive than ordinary thyristors and it losses are greater. The control system of it is more complex too. Frequency behaviors of gate-turn-off thyristors are worse than of ordinary thyristors too. Therefore such decision may used only for comparatively low frequencies that increase integrally the mass and gabarit characteristics of the device.

Decisions described in [4] permit to optimize the device. Specifically, gate-turn-off thyristors of the inverter are substitute for ordinary (SCR) thyristors; the inverse diode bridge is excluded from the thyristor inverter circuit, and a standard control circuit has in addition a current sensor, a resistance transducer of the "welding die - welding surface" gap, a delay cell, an AND element, a gate-tape diode and four keys. After primary winding current of the transformer welder was dropped the delay cell provided the interval was required to restore locking properties of the conducting current diagonal thyristor pair. If the welding electrode don´t contact with a welding surface at the same time then current was missed in the circuit of the resistance transducer and enable pulses were don´t given from the control system to thyristors driving points of the inverter. I.e. the quiescent condition is excluded. Only after the electrode touch with a welding surface enable pulses were begun to give to driving points of thyristors. Those decisions permit to eliminate voltage surges at the circuit of the inverter load diagonal, to increase the frequency, to decrease losses of the inverter, to substitute gate-turn-off thyristors for ordinary thyristors, to exclude the inverse diode bridge from the power circuit.

In conclusion should note that the device was considered with the load in the form of a transformer welder. Though the proposed device is available for other electro technical transformer loads in which the quiescent condition isn´t operating condition and may be excluded.

References:

  1. «Invertec» - V - 130-S-Lincoln. USA, 1998.
  2. V.A. Pryashnikov. Electronics. Saint-Petersburg, 1998, fig. 33.11.
  3. I.L. Kaganov. Industrial electronics. «High school».-M.-1968.
  4. L.T. Magazinnik, A.G. Magazinnik. Thyristor inverter. Patent RU 2314631 10.01.2008.

The work was submitted to international scientific conference «Priorities for Science, Technology and Innovation», Egypt (Sharm el-Sheikh), November, 20-27, 2008, came to the editorial office оn 28.05.2008.