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THYRISTOR SWITCHES

Other than conventional contactors, our semiconductor-based switches allow for exact determination of the switching moment
to the millisecond. This brings about two advantages:
1. Connection of the capacitor exactly at the moment of equal voltage between mains and capacitor element(s): this eliminates
inrush effects almost completely without the need for current-limiting resistors, substantially enhancing the operating con-
ditions for the capacitor and resulting in a longer life expectancy.
2. Instantaneous power factor correction for rapidly changing loads: certain reactive loads, e.g. spot welding equipment or
elevator systems, call for a high amount of power factor correction for short but very frequent periods. Thyristor switches are
THYRISTOR SWITCHES Discharge resistors and thyristor switches:
capable of multiple switching operations per minute. Since they always switch at zero-voltage-differential (see pt.1), the
capacitors do no longer need to be discharged before re-connection, and can sustain substantially higher numbers of connec-
tions as compared with conventional magnetic contactors.

Thyristor switched capacitors do not require discharge resistors for proper operation. For the purpose of maintenance safety,
however, all power capacitors are required to be equipped with bleeding resistors ensuring a discharge below 75V within no
more than 3 minutes after disconnection. When selecting the discharge resistors for thyristor-switched capacitors, please
consider the following important points:

1. Due to carrier storage effects during switch-off, thyristor switches may charge the capacitor with an overvoltage of
up to 1.43 × U AC (= ca. 2 × U rms) at the moment of disconnection. This is unproblematic for ELECTRONICON power
capacitors, however frequent switching may overheat and even destroy the bleeding resistors.
2. Certain thyristor switches keep disconnected capacitors constantly charged at maximum AC voltage in order to
provide a defined “point of re-entry”. This permanent voltage will then be up to 1.41 times above the rated voltage of
discharge resistors normally used for “standard” power capacitors!

The resistors integrated in our CAPAGRIP K and CAPAGRIP II terminals are usually rated for the nominal voltage of the con-
nected capacitor and a discharge below 50 V within less than 60 seconds; in the 525 V range, the resistors are dimensioned for
such discharge from 480 Vrms (= 680 Vac). They are not suitable for the operation with thyristor switches. You are therefore
required to use capacitors from our Heavy Duty ranges with CAPAGRIP L or M terminals instead and select suitable resistor
modules from our selection charts on pages 46ff.
»!« Use resistors with a rated voltage of at least 2.2 × U . As a rule, our 275.106 modules rated 1000 V will be sufficient for
equipment in 400 or 440 V mains. N

Remember that the discharge is relevant for safety aspects only. Select resistor values which serve for a discharge of the
capacitors below 75V within 3 or – even better – 5 minutes. This drastically reduces the power dissipation losses of the
resistors.

Five minutes discharge period is not specified by standard IEC EN 60831. Make sure to place an appropriate warning sticker
on capacitors or equipment if selecting resistors this way.

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