Transformer wiring in reverse - Page 1
Forum section: electronics / beginners

author: kelemvor
author_rank: Regular Contributor
timestamp: September 30, 2019, 07:00:47 pm
content: Is there any reason not to wire a transformer in reverse? Meaning, this transformer:https://www.mouser.com/datasheet/2/410/media-845803.pdfWired from a 28vac source (home made pure sine inverter) to produce 110/220 on the secondary. Clearly the datasheet indicates it's meant to be used as a step-down from 120vac to 28vac (for example) but I want to use it as a step-up.As an aside, it's only 1kva rated. I need 4kva so I guess I'll need four of them at $183USD each. Pricey. Is this the wrong solution to step-up inverter voltage?

author: MosherIV
author_rank: Super Contributor
timestamp: September 30, 2019, 07:14:11 pm
content: Depends on what the input and output voltages are.The output (secondary) was not designed for mains voltages. The insulation will not withstand mains voltages. The impedance of the secondary will be way too low to directly connect across mains, it may trip breakers.The input (primary) was design to take mains voltages, not high voltages.You can try it. It may work, for some time but the insulation may break down at some point.

author: kelemvor
author_rank: Regular Contributor
timestamp: September 30, 2019, 07:19:01 pm
content: Quote from: blueskull on September 30, 2019, 07:16:31 pmCorrect.Not sure why I'm struggling to find 28v-120v transformers. With the popularity of 28v solar battery systems it seems like those would be all over the place.

author: capt bullshot
author_rank: Super Contributor
timestamp: September 30, 2019, 07:26:04 pm
content: Inverters (to create mains voltage from battery voltage) often use a switching mode step up converter (often including the isolation) and an H-Bridge PWM circuit to generate the output voltage from an intermediate DC rail (the output of the step up converter). This saves the bulky and heavy line frequency transformers.Otherwise, products exists that use a line frequency transformer and an H-Bridge operated at the battery voltage. But these transformers aren't ordinary mains transformers, they have a nominal line voltage secondary and a lower than the minimum battery voltage primary to have some headroom for the PWM circuit to create a stable output voltage from the varying battery input. These transformers usually are custom made and not available as off-the-shelf components. And don't forget the sqrt(2) factor from DC voltage to RMS AC (as blueskull said)

author: floobydust
author_rank: Super Contributor
timestamp: September 30, 2019, 08:39:46 pm
content: As mentioned, power transformers are slightly overwound, to correct for sag due to losses in the windings and core.120:28VAC looks like 4.3:1 turns ratio but could be closer to 3.7:1 (assuming 15% regulation) so running the transformer backwards can give lower than expected results.That would give, with 28VAC in only 104VAC out. It depends on the transformer's quality.edit: large 1kVA transformer, assuming better 5% regulation would give, with 28VAC in only 114VAC out. (assuming 5% regulation)For your power levels, running transformers at mains frequency means they are bulky and huge.Modern inverters use high frequency SMPS to step-up to HV DC and then commutate that to a pseudo-sine wave.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 30, 2019, 08:59:34 pm
content: At the 1 KVAR level the efficiency is usually rather high and thus the tuns ratio is pretty close to the nominal voltage ratio. So the output voltage would be a little smaller than expected from the pure voltages, but not that much. I would expect only some 5% less.Usually larger transformers will be even more efficient and often smaller than 4 small ones in parallel.Due to the weight and shipping costs I would no necessary see Mouser as a good source for a large transformer.With an inverter one has to be careful with 2 effects: one is a possible DC part that can saturate the transformer, that might cause hum and peak current problems. The other are spikes that could exceed the design voltage and stress the insulation.

author: Zero999
author_rank: Super Contributor
timestamp: September 30, 2019, 10:02:56 pm
content: When you mean 28VAC is that the peak voltage, i.e. the battery voltage? If so, then transformers are specified in RMS voltage, so you need a 28/root(2) = 28/1.414 = 19.8V primary, probably a little lower, to make up for the regulation so that's 18V.

author: kelemvor
author_rank: Regular Contributor
timestamp: September 30, 2019, 11:06:02 pm
content: Quote from: blueskull on September 30, 2019, 07:21:50 pmYou're right, I hadn't considered it. When I connected the inverter circuit to my scope and meter, I'm actually getting 20v rms out of it (unloaded) so.. there ya go.

author: L_Euler
author_rank: Regular Contributor
timestamp: October 01, 2019, 01:32:09 am
content: I’ve been running a 480-120 3KVA transformer backward for more than 20 years with no problems at all. I use it to single phase source 2 VFDs, one on my lathe and one on my drill press. Go for it.

author: kelemvor
author_rank: Regular Contributor
timestamp: October 01, 2019, 01:38:33 am
content: Quote from: Kleinstein on September 30, 2019, 08:59:34 pmI'd love to hear suggestions for other sources, if you'd care to make any.