Failure analysis of mediate frequency furnace - Part 1

The working principle of mediate frequency power is: use the 3-phase bridge type full control rectification circuit to rectify AC current as DC current, after smooth filtering by reactor, the DC current turn to constant DC power source, then via single phase inverter bridge, the DC current has been inverted as (200-8000Hz) single mediate frequency power, load was assembled by induction coil and compensation capacitor group, connected as parallel-resonance circuit.

In general case, the mediate frequency power failure can be divided into two kinds: 1. Unable to start ; 2.Unable to work after start.

Our normal practice: when failure happened, user should check whole system under power shut off condition, check points:

Ⅰ. Power: use multimeter to check main circuit switch (contactor) and control fuse ware back has power or not, this way can remove failure possibility of these components.

Ⅱ. Rectifier: the rectifier use 3-phase full bridge type rectification circuit, it has 6 fast fuses, 6 SCR, 6 pulse transformers and a fly-wheel diode. A red indicator on fast fuse, it shrink inside of shell, when fast fuse melt, it jump out, some fast fuse has tight indicator, so it has not jump out when fast fuse melt, so user should use multimeter to test fast fuse, judge it burn out or not.

The thyristor inspection has a simple method, that is use multimeter barrier(200Ωbarrier) to test its cathode-anode, gate-cathode resistance, do not take off the thyristor during testing. In normal condition, the resistance between anode- cathode is infinity, gate-cathode resistance should keep in 10—50Ω range, too big or too small means the thyristor is fault, cannot be triggering and conducting.

The pulse transformer rim edge connect with thyristor, original edge connect with main control board, use multimeter test original edge resistance about 50Ω. The fly-wheel diode is not easy broken part, so user can use multimeter barrier test two sides of diode.

Ⅲ. Inverter: the inverter includes 4 fast speed thyristors and 4 pulse transformers, can use above method do inspection.

Ⅳ. Transformer: every coil of each transformer should be connected.

Ⅴ. Capacitor: maybe had been breakdown, capacitors were installed on rack, user should figure out which group is breakdown one first. Cut off group bus bar with main bus bar, test resistance between two groups bus bar, normal value is infinity. When breakdown group confirmed, user need to check each capacitor and find out breakdown capacitors.

Ⅵ. Water cold cable: cable for power supply and induction coil connection, some cable has bend or crack after long term working. The mediate frequency furnace cannot start under cable broken condition, if user try to start furnace many times, that would burn the mediate frequency voltage inductor.

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