Calculating transformer input impedance at different frequencies - Page 1
Forum section: electronics / beginners

author: mvno_subscriber
author_rank: Regular Contributor
timestamp: March 02, 2021, 09:51:12 am
content: I wanted to do a simple check of a transformer in order to see if it was bust or not, without turning it on. This transformer is meant for 117V, 60Hz on the primary. I attached my LCR meter, but it can go no lower than 100/120Hz. At 100Hz, impedance was about 90 ohms, at 120Hz it was about 110 ohms. At 1K: 1.17 Kohm. In other words, the ratio between frequency and impedance seemed to scale somewhat linearly. (At 10Khz it started behaving quite different, however).Can I, as a rule of thumb, assume that the impedance at 60Hz will be half that of 120Hz?Or would that vary a lot based on the build of the transformer?What about temperature?Also - the appliance says "70W", but 55ohm@117V would consume more like 250W? Is there something I'm not getting here?

author: Zero999
author_rank: Super Contributor
timestamp: March 02, 2021, 10:34:15 am
content: You're right that at low frequencies, close to those the transformer is designed for, the impedance will scale linearly with frequency. Assuming you measured the primary impedance, with the secondary open circuit, those numbers are much too low and are indicative of a shorted turn.Have you powered the transformer up? Did it get hot? Try connecting it in series with an old fashioned incandescent lamp. The tiny magnetising current shouldn't be enough to cause the lamp to glow and the transformer shoudn't get too hot to touch.

author: mvno_subscriber
author_rank: Regular Contributor
timestamp: March 02, 2021, 10:43:37 am
content: The secondary is connected. Disconnecting the secondaryis rather difficult, but I thought this would anyway be the configuration in which the transformer is used, so would anyway be a correct indication?There are too many components inside that need to be replaced before I dare power it up (e.g. old paper wax capacitors).

author: rstofer
author_rank: Super Contributor
timestamp: March 02, 2021, 04:24:40 pm
content: Wire the input voltage in series with a 100W light bulb. Worst case, a shorted primary, you just get full brightness.

author: Zero999
author_rank: Super Contributor
timestamp: March 02, 2021, 04:30:06 pm
content: Quote from: mvno_subscriber on March 02, 2021, 10:43:37 amThe problem is what's connected to the secondary, will affect the primary impedance reading.If the only thing between the secondary and the rest of the circuitary, is a bridge rectifier, choosing a test voltage, which will keep the peak secondary voltage below, the forward turn on voltage of a diode will effectively eliminate it. If you have a filament such as a heater, or incandescent lamp, you won't get valid readings, because the cold resistance will be much lower, than the hot resistance, so the impedance measured, will be lower than normal operation.

author: Wolfram
author_rank: Frequent Contributor
timestamp: March 02, 2021, 04:35:13 pm
content: Mains frequency transformer iron usually has a much lower permeability for small levels of excitation, compared to at full applied voltage. As you increase the applied voltage, magnetization impedance will first rise as the iron is pushed into the linear region, then start dropping as you approach saturation. To get an idea of the large-signal inductance, you can apply a significant fraction of the rated voltage and calculate the impedance from the magnetizing current.

author: mvno_subscriber
author_rank: Regular Contributor
timestamp: March 03, 2021, 11:08:00 am
content: Aha, I'm starting to understand.. Transformers (and the details inductance) is a difficult concept to master for a hobbyist - at least for meHighly interesting, though!I found a so-called "death cap" connected between the primary and chassis, removed it - now, the impedance @120Hz increased first to 123Ω, then it started climbing, up to 170Ω, then back and stabilized at around 160Ω.. it's obvious that there issomethinghappening on the secondary side, affecting the readings. I've pulled out all tubes (including rectifier), but there are capacitors and resistors there doing..stuff. Which gives me hope the transformer may not necessarily be bust after all!Quote from: Wolfram on March 02, 2021, 04:35:13 pmWhat constitutes a significant fraction? Are we talking 10%, or more?