testing random transfos for 230v

[p.larner]

i have random old transfos unknown pri or sec,whats a safe way to test if ts safe to feed them with 230v if any?

[Andy Chee]

First, use a multimeter in resistance or continuity mode, to determine the separate windings.

Second, use a low voltage AC source (say, 10VAC from an AC adapter, NOT DC) and connect it to any winding. Use the multimeter in AC voltage mode to measure the other winding.

Then do some math to work out the voltage ratio between the windings.

Another consideration is insulation leakage, but you’ll need an insulation tester for that.

[p.larner]

does the resistance need to be a minimum value so the primary wont burn?.

[andrewtaylor]

does the resistance need to be a minimum value .

depends on transformer rating.

A 2VA has approx 2kR at 230V primary,
a 25VA  can be between 40..100 R,
a 3kVA less than 1R.

partly shorted prray might give confusing results.

as And Che posted: low voltage for testing, and once you identified the primary: place a light bulb (realy the heat bulb type, no LED/energy saver) in series with the primary when you fire it up on 230V.
A variac (buy or borrow) will ease your tests.

your mileage may vary .-)

[garageanalyze]

Testing unknown transformers to determine if they can safely handle 230V can be risky, as it involves exposing yourself to potentially hazardous electrical conditions. However, if you choose to proceed, here are some steps you can take to perform a basic safety test:
Visual Inspection: Inspect the transformer for any signs of damage, such as cracks, burns, or melted insulation. Ensure that the wires are securely attached and there are no exposed conductors.
Check Ratings: Look for any markings or labels on the transformer indicating its voltage and power ratings. These ratings should specify the input voltage (primary) and output voltage (secondary) that the transformer is designed to handle. If the ratings are unknown or unclear, it's best to err on the side of caution.
Perform a Continuity Test: Use a multimeter set to the continuity or resistance mode to check for continuity between the primary and secondary windings of the transformer. There should be no continuity between the primary and secondary windings, as they should be electrically isolated from each other.
Measure Resistance: Measure the resistance of both the primary and secondary windings using the multimeter. Compare these measurements to the expected values for a transformer of similar size and type. Abnormally low or high resistance readings may indicate a fault or damage.

[srb1954]

does the resistance need to be a minimum value so the primary wont burn?.

More important is the inductance of the primary winding. In all but the smallest power transformers the inductive reactance dominates the winding resistance as the current limiting factor under no load conditions. If you have an LCR meter you can measure the winding inductances and from those figures you can estimate the turns ratio and the expected no load primary current, which is known as the "magnetising current".

The turns ratio is calculated as n_pri/n_sec = sqrt(L_pri/L_sec).

 N.B. this turns ratio enables the calculation of the no load secondary voltage but the actual secondary voltage under load  (usually specified as the rated voltage) will be somewhat lower due to transformer losses.

The magnetising current can be calculated as I_mag = Vpri/(2*PI*f*L_pri).

If the transformer has any shorted turns the inductance will be lower than expected and the magnetising current will consequently be higher.

[CaptDon]

Many smaller transformers operate at 10 to 25% below saturation based on input voltage. If you have a Variac and an A.C. current meter you could raise the input voltage in 5 volt or 10 volt steps looking for the suddenly non-linear current increase which indicates you have entered into the saturation area. You may see this effect at around 135-140 volts for a 120 rated transformer and at 260-280 volts for a 240 rated transformer. You will have to figure out the most likely wires or terminals for the primary or primaries. Sometimes just a pair of blacks for a single primary. Sometimes a black and black with white tracer + a black and a black with a red tracer for a dual primary. Be aware this is a huge generalization and some transformers will already be in partial saturation even at the rated voltage. Microwave ovens run nearly in hard saturation and may draw 5 amps or more with no load!! Be careful to not get shocked from any floating leads like the 480 output from a control transformer or the high voltage windings of a transformer used in oscilloscopes or other test gear with C.R.T.'s inside.