Transformer questions - Page 1
Forum section: electronics / beginners

author: soligen
author_rank: Contributor
timestamp: April 02, 2016, 11:36:44 pm
content: I've been searching, but I have a couple questions I cant find answers to.First, looking to transformers for sale, mostly what I see have 115V primaries. I'm in the USA and we have 120V. The measurement with my multi meter is 122V. Is it OK to use a 115V transformer? I have read about a 5% tolerance, but this gap is more than 5%.Second, I have a salvaged transformer and the secondary measured 64 VAC with no load, which is too high to be useful to me. This transformer has alternate primary connections for 220V and 240V. Would there be any issue running 120V on the 240V primary so that I would have 32 VAC. This would be useful to me as it is center tapped. Will this damage anything or cause a safety issue?Thanks

author: Jeroen3
author_rank: Super Contributor
timestamp: April 03, 2016, 12:00:20 am
content: Transformers are very flexible.A 120/60 volt transformer will work fine with 100 or with 140 input voltage. Howewer, the output will also change by the same percentage. (Around nominal voltage)The current is more important, since this will determine the temperature, and the temperature will determine if the wire isolation stays above several mega ohms.The frequency is also important, if you deviate fron nominal frequency, derate properly. You can't run a transformer on 10 hz, since it will have too much resistance. You also cant run then on 1KHz, since it will have too much inductance. Impedance stuff.You can use transformers for measurement. They will work fine on lower or higher voltages. Within practical limits. You can't double the input voltage, maybe x1.2.Transformers also have a high voltage droop under load. Open terminal voltage is always higher than nominal, and nominal load is either a bit lower, or spot-on nominal voltage.Sometimes the open terminal voltage can be double the nominal voltage!You cannot magically increase the VA's.

author: Seekonk
author_rank: Super Contributor
timestamp: April 03, 2016, 12:13:25 am
content: A transformer ceases to be a transformer once the core magnetically saturates. Then it is just a resistor. It is all about cost. Small transformers are more likely to saturate at slightly elevated voltages than larger ones. The primaries have so much resistance that it isn't an issue. I've seen sine wave get pretty squashed on top. If you ever get a variac, monitoring primary current as an increase in voltage is an interesting experiment.

author: rstofer
author_rank: Super Contributor
timestamp: April 03, 2016, 03:32:03 am
content: If the primary is rated for 240V and the secondary for 64V then, yes, if you run the primary on 120V, you will get 32V on the secondary.110V vs 115V vs 117V vs 120V - all of these were standards at one time. Today, the number is 120V and the fact that you are a little high is no problem.

author: Seekonk
author_rank: Super Contributor
timestamp: April 03, 2016, 06:50:29 am
content: A friend has a HVAC business, I visit and scrap transformers out of units. Standard is 208/240/480 to 24V. Use them on 120V all the time. Good thing they are big because their power rating will drop about four times. Lower voltage won't fully magnetize the iron.

author: Audioguru
author_rank: Super Contributor
timestamp: April 03, 2016, 02:14:16 pm
content: Quote from: Seekonk on April 03, 2016, 06:50:29 amI have never seen "crossover distortion" on the output of a transformer. A mains or audio transformer has a sinewave AC input that has many alternating voltages from 0V to the positive peak then back to 0V then to the negative peak. The iron magnetises perfectly at every voltage.I think you mean that the power rating must be reduced to half when the input voltage is halved so that the maximum allowed current is not exceeded. Many power transformers have two primary windings. In series they use 230V and in parallel they use 115V. The same power for each way.

author: ghulands
author_rank: Contributor
timestamp: April 03, 2016, 04:44:32 pm
content: What brands of transformers are the more reputable ones?Which ones should i stay away from?

author: soligen
author_rank: Contributor
timestamp: April 05, 2016, 12:42:31 pm
content: Thanks everyone. This info really helps me.

author: alanb
author_rank: Frequent Contributor
timestamp: April 05, 2016, 01:34:55 pm
content: Quote from: Seekonk on April 03, 2016, 12:13:25 amI may be wrong but I recall that saturation is a function of current and turns rather than voltage.