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Inrush current and primary fuse
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Inrush current
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The higher the induction in the transformer core, the smaller the remaining air gap. And the higher the copper losses, the higher the inrush current. Guideline values for 230 V primary voltage:
Inrush current approx. 3 up to 5 ms:
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Type size
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Toroidal transformer
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Bobbin transformer
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up to 50 VA
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6 A
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5 A
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up to 100 VA
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15 A
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7 A
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up to 200 VA
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30 A
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20 A
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up to 300 VA
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60 A
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30 A
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up to 500 VA
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130 A
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45 A
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up to 750 VA
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250 A
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55 A
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up to 1000 VA
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350 A
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70 A
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up to 1500 VA
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450 A
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Primary fuse
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Small transformer can be fused with laggy safety fuses (sign T). For big transformers and toroidal transformers we recommend super laggy safety fuses (TT) on the primary side.
Selection of safety fuses with rated values conform to DIN 41662 IEC 127-2:
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32 mA
40 mA
50 mA
63 mA
71 mA
80 mA
100 mA
125 mA
160 mA
200 mA
250 mA
315 mA
350 mA
400 mA
500 mA
630 mA
700 mA
800 mA
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1 A
1,25 A
1,4 A
1,5 A
1,6 A
2 A
2,5 A
3,15 A
3,5 A
4 A
5 A
6,3 A
8 A
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10 A
2,5 A
16 A
0 A
25 A
35 A
50 A
63 A
80 A
100 A
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NTC thermistors as current inrush limitators
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NTC-resistors or so called Surge-Guards can be used as well. The current is limited for 1 to 2 seconds. While in cold condition, means during the start-up of the transformer, NTC-thermistors have a way higher resistance than during the warm condition and thus they can limit the current. After a short heat-up due to the inrush current, the NTC becomes low-impedant and are able to deal with permanent currents up to 20 A. The selfwarmth reduces the resistance of the NTC to less than 10% of the cold resistance during the start-up. After a break of the power supply, the NTC-thermistor cools down to ambient temperature and increases the resistance again. A certain time of cooling is needed though until the NTC reaches his full safety function again. This can take up to two minutes, depending on disksize and ambient conditions. Before the end of the cooling time, NTCs are not able to redeem their full limiting functions. We choose the NTC depending on transformer characteristics. During the usual operation of the transformer, means the low-impedant condition of the NTC, it becomes very hot. Temperatures up to 200°C and more are possible. Here is a small selection:
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Type
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Current
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max. temp
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R cold
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R warm
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Diameter
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Thickness
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9930031
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1,3 A
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220°C
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200 Ohm
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1,90 Ohm
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15mm
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8mm
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9930032
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2,0 A
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220°C
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40 Ohm
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0,60 Ohm
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15mm
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6mm
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9930033
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3,0 A
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220°C
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120 Ohm
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0,90 Ohm
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20mm
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6mm
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9930034
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4,0 A
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270°C
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12 Ohm
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0,22 Ohm
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12mm
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8mm
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9930035
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6,0 A
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270°C
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10 Ohm
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0,15 Ohm
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15mm
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6mm
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TSR device
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TSR, transformer-switch-relay, (helps when current inrush avoiders reach their limits)
The TSR is an electronical inrush current limiter, which does not only limit the inrush current of transformers but avoids it completely. This makes it possible, to quickly fuse transformers on their primary side with the rated current. A latency between the switchings is not needed at all. A transformer or a parallel- or series circuit of transformers is premagnetised for 0,2 seconds by the TSR and then started without any inrush current. The TSR is short circuit proof, if safeguarded properly with a 16 A B fuse. A control input allows the switching via a SPS etc. Switching losses are avoided completely and also frequent switching is no problem. Even fast clocking with versions is possible. One or three-phase TSRs are available. Please send us an inquiry.
PDF-Datasheet
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Alle table values are examples.
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Toroidal transformer and fast fusing
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 The picture shows a 1 KVA, 230 V toroidal transformer and a C 4 A circuit breaker.
The toroidal transformer is constructed for low losses and has a rated current of 4,3 A and an inrush current of about 56 times the rated current = 240 A.
The C 4 A circuit breaker reacts at 6 times the rated current = 24 A rather fast. The inrush current of the transformer is 10 times too high. Like this the transformer can’t be fused.
It’s too b ad, that those two don’t fit together without any problems, because each of them has a lot of advantages. The low loss transforemr is stress stiff, remains almost cold and has a no-lod current, which can be ignored.
More to be found here
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