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Old 16th September 2010, 04:32 PM   #1
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Default Question Regarding Soft-Start

For a relatively large transformer, must the soft-start circuitry be on the primary side? Is the current in-rush required to charge the magnetic field of the transformer the primary issue, or is it the load seen on the secondary side, as presented by a bank of uncharged capacitors? Or is it that both are of equal concern?

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Old 16th September 2010, 04:34 PM   #2
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As I understand, the soft start should be on the primary of the transformer.
That way, it will handle both the primary surge and the charging of the capacitors on the secondary.
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Old 16th September 2010, 04:39 PM   #3
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Inrush current limiters are firstly put on the primary side.
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Old 16th September 2010, 04:40 PM   #4
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Hi Frank,

Thank you for your reply. Yeah, I think you are right. I've been considering a few soft-start methodologies for an upcoming project. It would be nice to have it on the secondary side - that way I could have the secondaries go straight from the transformer into a PCB containing the soft-start, and there's no messing with mains voltages. But I suppose that would be neglecting the in-rush current of the transformer.

I've been toying with the idea of making a time-delay relay, using a thermistor. I'll post up a schematic or two when I have something that might work :P

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Old 16th September 2010, 07:09 PM   #5
DF96 is offline DF96  England
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When you connect a transformer primary to the mains it may draw a DC current of up to the value of the initial AC current - it depends on where in the AC cycle the switch closes. This DC current then decays, according to the L/R time constant of the transformer. It can cause transformer saturation, which can often be heard.

Except for small items, I now routinely include an NTC inrush limiter in the mains circuit. This handles both the capacitor inrush and the DC, by temporarily increasing the resistance. Inrush limiting on the secondary side will help by limiting the initial AC current, but it won't quicken the decay of the primary DC.
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Old 16th September 2010, 07:38 PM   #6
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Old 18th September 2010, 07:51 AM   #7
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I always put the soft start on the primary side as it both covers inrush and filter capacitor charging. For a system with filter capacitors use a relay or contactor with a coil voltage equal to any convenient winding on the transformer usually the primary. wire the coil straight across that winding and use the contacts to short out a heavy duty resistor in series with the primary, Avoid the white ceramic block type wirewound resistors they blow operating into a heavy load. I have used this technique on power supplies up to 40 Kw NTC devices are fine for small power supplies I have found they give trouble once you get in the Kw range and restart after momentary power loss can be quite a problem.
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Old 18th September 2010, 02:18 PM   #8
DF96 is offline DF96  England
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Yes, I should have said that NTC is OK for 'domestic' type operation provided that you don't keep switching - the thermistor needs a few (tens of?) seconds to cool down before it will limit inrush current again.
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Old 18th September 2010, 03:46 PM   #9
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NTC is ok for powers up to hundreds of watts (the input current that passes through it is low enough - a couple of amps - to prevent the ntc from overheating). After that you need a softstart circuit.
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Old 18th September 2010, 04:23 PM   #10
AndrewT is offline AndrewT  Scotland
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NTCs are suitable for transformer soft start upto thousands of Watts (or VA).
Just choose the appropriate NTC or ask the manufacturer's sales rep.

40kW on the other hand is quite something else.
regards Andrew T.
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