Re: Using SSR to switch transformer


I'm considering using an SSR to switch the primary side of a transformer. The transformer is a toroid, 230V -> 27V. The secondary side is fixed to a 1 Ohm nichrome wire heating element.

That 1R is reflected into the primary circuit as 1/(27/230)^2=72R making the maximum possible inrush current due to the secondary loading 1.41*230/72=4.5 Amps peak. This 4.5 Amps then adds in quadrature to the transformer magnetizing current, both currents passing through the series ohmic resistance of the primary winding.


I've read some articles that say switching on at zero crossings causes a high inrush current, and that it's best to switch on at peak voltage.

The big unknown is the residual magnetization of the core at turn on, but the worst case is usually zero flux because the initial permeablity, and therefore the inductance of the core, is smallest under that condition.


If I follow this advice, are there any other issues I need to look out for ?

What kind of (surge) current capabilities does the SSR need to have for reliable operation ? (DC resistance of primary side is about 1 Ohm)

My original idea was to switch the secondary side using a pair of SCRs, but at that current it would require a fairly decent heat sink. If I can switch the primary side, the dissipation would be reduced considerably.

As you can see from the calculations, the 1R in the secondary contributes little to the inrush compared to the core initialization or possible supersaturation effects, both of which may last for many line cycles before reaching steady state. There are lots of rules of thumb on overrating the SSR, bit if you /have/ to get it right the first time, then you need a soft start module:
http://www.crydom.com//userResources/productFamilies/54/crydom_sst.pdf

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