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13. Notes on the Use of Transformer Load
Purpose of Using Transformer
1) In case heater voltage is different from line voltage, the voltages are matched.
2) When the heater circuit needs to be insulated from the power source.
3) In case insulation to the earth lowers as experienced in vacuum devices, withstand voltage to the earth should be raised by using a compound
transformer.
13-1. Transformer Flux Density
If a magnetic circuit is saturated while a transformer is being used, excessive current flows (applying a load only to transformer winding resistance)
may occur and destroy the thyristor.
As thyristor control requires switching (ON-OFF) every half a cycle, a slight imbalance of output waveforms tends to cause saturation when the
load increases.
Therefore, the flux density should be designed to be lower than that of an ordinary transformer.
Example: The flux density of an ordinary transformer is about 10,000 to 13,000 Gs.
It should be designed to be less than 8,000 Gs when a transformer is used together with a thyristor.
In case an ordinary transformer is used, no problem will be caused when the load rate is held below 70% of the rating for the transformer.
13-2. When Electromagnetic Switch (Contactor) Is Used
If an electromagnetic switch (contactor) is used in a circuit connected with a transformer (inductive load), a bounce of the contact may cause an
erroneous movement.
In this case, use the designated noise filter or connect a capacitor (0.1–0.5μF) across the power side terminals R and T to absorb the noise.
13-3. Use of Rapid Fuse
In order to protect the thyristor elements from excessive current which is generated by high frequency noise or load trouble while a
transformer is being used, we recommend that you use a Rapid fuse.
13-4. Prohibition of Open Load
In case a load is unable to be connected, for example, in a trial run, disconnect wiring for the transformer and connect a dummy load such
as an electric heater or a light bulb. Do not put the transformer in operation when its load remains open.
You should not switch the load while the transformer is energized. If you do this, the soft start function will not work, resulting in
generation of excessive current or activation of the protection circuit of the PAC27.
