Transformer Surge current?

[m_c]

I'm looking for a guide as to what surge current to allow for during power-up of a transformer.

I'm thinking about using an SSR to control the primary on a 625VA Toroidal, but have so far drawn a blank as to a guideline figure for surge current, so I can size something appropriate.

Any suggestions?

[Al_The_Man]

There are usually two aspects to consider for inrush current, the first stage is the transformer itself which offers a high load initially for approx 1/2 to 1 cycle, where the current is limited by the resistance of the winding, before the inductive effect takes over and offers a high impedance to the supply, but if the transformer has a rectifier and capacitive component on the secondary, then this will offer a load as the capacitors charge, these also will offer a very high load initially, which will be reflected back to the primary.
Generally, the transformer inrush does not present a problem, due to the short duration, if the transformer is otherwise offloaded.
It is usually the capacitive charge that creates the longer current duration, the maximum current would be limited by the secondary impedance of the winding and the bridge rectifier, plus any plate resistance of the capacitors.
Probabally the best method to measure Imax would be to put a peak registering Clamp-on meter on the primary on switch on.
Al.

[m_c]

Just been and done some measuring and some rough calculations, and I'm going to fry the bridge rectifier before the SSR!

The SSR I've looked at, has a peak rating of 250A for 16.6ms.

Now given a a primary resistance of 1.6 ohm, initial power up is going to see around 150A flow if switch on is at peak voltage (240V), which works out around 37.5kW, but is within spec of the SSR, provided the load settles within the time frame.

Now if the primary remains at 150A for the 16.6ms, then the bridge rectifier will be seeing over 400A for that same time, and it's only rated for a peak current of 300A (or fuses at 375A).
If the BR dies, then I'll look into some sort of resistor/time delay relay set-up to reduce the start up surge.

[Al_The_Man]

The bridge will not 'see' any of the primary current.
The bridge current should only consist of capacitor charge , in the absence of any other load.
Al.

[irving2008]

The bridge will see the initial capacitor charge current which will be roughly given by I = CV/.01 (from Q=It and Q = CV and t = .01 sec for a 50Hz supply), so for a 50v DC supply and a 22000uF cap (as mine is), I = 50 * .022/.01 or about 110A peak. That's worst case assuming the capacitor charges to the full output voltage in the first 1/2 cycle, which it wont due to the secondary resistance (about 0.3ohm on my transformer) which will mean it actually takes about 10 half cycles (0.1sec) with the peak current at 112A in the first cycle, then 55A, 38A, 24A, 17A, 13A, 10A, 8A, 7A, 5A and the capacitor is fully charged in the first 0.15sec. For a 35A bridge rectifier it need to stand the surge current for about 7mS in each of the first 3 half cycles after which it is back inside spec.