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Registered Member #1749
Joined: Fri Oct 10 2008, 02:04AM
Location: Claremont New Hampshire
Posts: 497
Is it safe to parallel two identical transformers for more power like you would with battery's I only ask because I have two identical transformers that putt out 24.7V at 1.4A\8.6V at 0.66A and after the rectifier and capacitor combo the 8.6V secondary puts out 12VDC which is the voltage I am building my SR motor around. Hope this is the right topic for this if not I apologize.
Registered Member #2463
Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546
Parallel connections of small iron core transformers is not a good idea because they may not be exactly alike. Currents circulating from transformer to transformer on the secondary side effectively defeat primary protection systems, thermal fuse, or intrinsic impedance, in the case of shorted windings. This means one may set the other one on fire, even after its fuse has opened the primary.
It might be better to parallel the DC side, after the bridge, feeding each through a single isolating diode, if you can afford the drop.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
I have done this previously, I think if the voltages are within ~0.5% tolerance, the balancing current will be small because the transformers also have some output impedance.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
With small transformers there is no problem because their regulation is so poor (high winding resistance) .. I'd say +/- 5% matching of no-load voltage is ok.
At full load there will be no problem, with no load the transformers will draw current on the secondary side proportional to the mis-match of voltages, it's nothing to worry about though - quite a small current compared to full load.
Your 6 VA winding probably has 15% to 20% regulation (15% to 20% of the no-load voltage gets 'dropped' by the resistance of the windings..for a resistive load).
With a simple rectifier/capacitor arrangement the regulation will be even worse due to current being taken in short pulses only.
When you have large transformers their regulation is 1% to 5% so closer matching would be required.
Registered Member #15
Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068
radiotech wrote ...
Parallel connections of small iron core transformers is not a good idea because they may not be exactly alike. Currents circulating from transformer to transformer on the secondary side effectively defeat primary protection systems, thermal fuse, or intrinsic impedance, in the case of shorted windings. This means one may set the other one on fire, even after its fuse has opened the primary.
It might be better to parallel the DC side, after the bridge, feeding each through a single isolating diode, if you can afford the drop.
The paralleling of identical transformers is absolutely okay, and is permissible from small transformers or very large utility transformers. Its done all the time. Circulating currents only occur when voltages (turns ratios) are different, which is not the case in this example.
Also, you can even parallel transformers with different power (VA) ratings so long as the impedances are the same. In this instance, power is divided proportionally to the VA rating of the transformer.
So with the original posters question whether he can parallel (2) identical transformers. The answer is absolutely - 100%.
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Paralleling transformers
