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SOLID-STATE SOFT-START CONTROLLER BASED RECTIFIER

IP.com Disclosure Number: IPCOM000173523D
Publication Date: 2008-Aug-11
Document File: 5 page(s) / 169K

Publishing Venue

The IP.com Prior Art Database

Abstract

A plasma cutting system was re-configured with a soft-start controller in an effort to replace the contactor based rectifier circuit currently used in plasma cutting systems. Preliminary cost analysis shows that the proposed method is roughly 20% cheaper than the conventional contactor based method. This method should improve system reliability and be more attractive for systems with a higher power rating.

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SOLID-STATE SOFT-START CONTROLLER BASED RECTIFIER

    A plasma cutting system was re-configured with a soft-start controller in an effort to replace the contactor based rectifier circuit currently used in plasma cutting systems. The system was then subjected to a reliability stress test that lasted for about 50-60 hours.

    Preliminary cost analysis shows that the proposed method is roughly 20% cheaper than the conventional contactor based method This method should improve system reliability and be more attractive for systems with a higher power rating.

A schematic diagram of the contactor based rectifier circuit is shown below:

In a contactor based rectifier circuit, the contactor is used to:
1) Provide isolation and user safety by disconnecting the power supply from the 3- phase power input during process idling periods and during servicing/repair or changing of consumables.

2) Connect power to the power supply and charge the dc bus as and when the process demands it.

Drawbacks of the contactor based rectifier circuit:
1) During contact closure, the contactor contacts are subject to an extremely high surge current. This is due to:
a. An extremely large transformer magnetizing inrush current that is typically about 500-1000A and lasts for about 100-200ms.

b. High DC bus charging current that is typically of the order of about 1000A and lasts for about 2-3ms. This parameter has little effect on the contact

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life but could cause degradation of the rectifier and dc capacitors over a period of time due to repeated charging of the dc bus.
2) Contactors are known to have a short field life while operating under adverse environmental conditions. It is observed that the field life of a contactor is typically 12-18 months resulting in expensive replacements.

    The purpose of this concept and its use of a soft-start controller based rectifier circuit is to eliminate the contactor, while substantially performing its functions in a reliable, safe and cost-effective manner.

    A schematic diagram of the soft-start controller based rectifier circuit is shown in Fig. 2 below:

    Here, the main transformer is connected to the line source through the disconnect switch. The diode bridge in Fig. 1 is replaced by a half-controlled bridge and a soft-start controller board. The half-controlled bridge consists of 3 SCRs and 3 diodes as shown.

The soft-start controller performs the following functions:
1) Gradually charge up (soft-start) the dc bus when there is a request for the power supply to be used. This reduces current stress in the dc capacitors and the rectifier bridge improving device reliability.

2) On completion of the charging period, turn-on all the SCRs in complete conduction mode. Here, the half-controlled bridge operates like a diode bridge.

3) Switch-off the rectif...