parallel transformers

[GlennH]

We often see small transformers doubled up (i.e. 40va and 40va) to power up zone valves for heating systems.

Someone asked today what would happen if two different size transformers were paralleled? like a 40va and a 75va.

would the load be proportional beween the two or would the load be equal between them?

just curious

 

[Speedskater]

Most likely they would fight each other to the death. When paralleling transformer secondaries, you have to be careful that both transformers are identical and have the same model and part number. Even the smallest difference in voltage will cause a high internal current as each transformer tries to set the output voltage level and the transformers overheat.

With two very different transformers, even if the voltages are exactly the same at one current load as soon as the load changes the fight begins.

 

[skeshesh]

Kevin covered the basics with his post. Basically the easiest way to go about is to use two identical transformer. The most important characteristic in paralleling is having a matching impedance so that the current division does not overload one transformer. Here's a nice article from SqD I read a while ago, and while it's describing sizable transformers way upstream in the distribution, you should get a good idea of the principle involved:

http://ecatalog.squared.com/pubs/Electrical Distribution/Medium Voltage Transformers/7400DB0701.pdf

 

[G._S._Ohm]

Someone asked today what would happen if two different size transformers were paralleled? like a 40va and a 75va.

would the load be proportional beween the two or would the load be equal between them?

40 VA @ 24 vac = 1.7 A
75 VA = 3.1 A
Let's say unloaded they both go to 27 vac, so the first one has an internal impedance of (27-24)/1.7 = 1.8 ohms and the second has 1 ohm.

But, it's likely that the open circuit voltage of the 75 VA unit won't go as high as for the 40 VA so the 40 VA would be pushing some current into the 75 VA unit when there's no load on the network.
IMO, no problem.

Anyway, when they're paralleled you get a 27 vac source with an internal impedance of 0.64 ohms, so at 24 vac this network puts out ~4.7 A for a rating of ~115 VA.

Problem is, as this network is loaded down, they don't share current equally.
To fix this you'd need to put a resistor in series with one of them to ensure each one only sources its rated current.
Then you'd get less than 115 VA.

 

[hurk27]

Most likely they would fight each other to the death. When paralleling transformer secondaries, you have to be careful that both transformers are identical and have the same model and part number. Even the smallest difference in voltage will cause a high internal current as each transformer tries to set the output voltage level and the transformers overheat.

With two very different transformers, even if the voltages are exactly the same at one current load as soon as the load changes the fight begins.

Kevin
What you are describing is what happens when two DC power supply's with voltage regulation are paralleled, while it is still done with bias resistors all the time, this does not happen with just transformers, as a matter of fact many transformers are intentionally paralleled when wired for lower voltage, the only thing that matters is the phasing, the winding impedance will balance the loading proportionately as to the size of the transformer, higher va transformer will have a lower impedance then a lower va transformer.

 

[ggunn]

Aren't the secondaries of two transformers even with their primaries on the same feed "separately derived" and therefore necessarily kept isolated?