Electronics > Beginners

ASK: Determining transformer's windings phase

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Gyro:

--- Quote from: BravoV on December 12, 2018, 11:45:00 am ---
--- Quote from: Gyro on December 12, 2018, 08:42:00 am ---A big NOTE though... At switch-on the current will peak very high - worst case, the transformer will get switched on at a voltage zero crossing and the core may well saturate in the first half cycle, peak current then will be AMPS. With separate A and mA jacks on the meter, that's going to be impossible to handle.

Difficult to arrange, but if you really want to measure quiescent current, you need to bypass the meter with a switch or something during power-up. Difficult to arrange safely - especially when trying to tutor a friend at a distance! You may well be advised to stick with the dim bulb tester - you can switch a low wattage mains bulb.
--- End quote ---

Well, my friend works as technician, developed a technique at the field, that he used for years on skipping the initial big inrush using his meter, he works mainly on AC motors.

What he told and taught me with this technique, when he forgot to bring his AC clamp meter, is to use an ordinary DMM, and connect it like this illustration below, using a sacrificial wire and a pair of safe isolated wire cutter.



His DMM has those expensive Hirschmann cable piercing clips and bite into the sacrificial wire touching the copper, and on the initial power on at AC motor, especially big one will hum and slowly starting to rotate, he will wait until the motor's spin stabilized, and then cut the wire in the middle to measure the current.

Of course this is not totally fool proof, but he claimed he never blown any fuse, as he knows & aware of the load. He used this method only occasionally and with great care as he aware of the dangerous.

He will be using this technique.  :P

--- End quote ---

Haha, I was holding back on suggesting something like that!  He'd better make sure those clips are really high integrity, otherwise he's going to be searching his remote location for a mA fuse or Aluminium foil / copper nail  ;)

P.S. A really clean turn-off would be advisable under those circumstances too.

spec:
Hi BravoV

[1] Connect the primary windings any way that you like.

[2] Connect an incandescent bulb (40W to 100W) between the pair of primary windings and connect the mains across the bulb and the primary windings in series.

[3] Note the brightness of the bulb

[4] Reverse the connection of the primary windings and repeat [2] above.

[5] Note the brightness of the bulb.

The primary connections that gives the dimmest bulb are the correct connections for 230V.

ArthurDent:
Even if you think you're sure your connections to a transformer are correct you should always use an incandescent lamp in series for safety while testing.

An easy way to check to see if you have the two equal 110 VAC primaries connected in parallel correctly that only requires a DMM is to try the wiring I show in the modified drawing below. Wire the first of the two primaries through the bulb to 110 VAC with one wire of the second identical primary connected to one of the wires that is powering the first primary. Measure the AC voltage from the unconnected wire on the second primary to the single wire on the first primary. If the voltage is zero, the primaries are wired in phase which is correct  but if the voltage is 220 VAC then the primaries are out of phase and you need to revere the second primary wires and try again and you will now get zero volts. 

T3sl4co1l:

--- Quote from: basinstreetdesign on December 12, 2018, 09:07:03 am ---Has anybody ever had this situation with the dual primaries wired in series but out of phase?  I have and the result is non-climactic.  With all secondaries open, if the primaries are wired out of phase their magnetic fields are in opposition and cancel within the core.  All secondaries then show next to zero voltage.  Except for magnetizing current, the turn-on transient is minimal.  Swapping the leads on one primary brings it right.  Testing this requires nothing more than a meter.
--- End quote ---

You've contradicted yourself there -- you're applying flux (voltage at frequency) to the sum primary windings; if the field in the core is forced to zero, it can only be that all windings will appear short circuited under that condition!

Since zero minus zero equals zero, the anti-series primaries will see no back EMF, and full mains current will flow (likely limited by the resistance and leakage of that winding, more than the mains supply itself).

(As it happens, this condition does not force the field to zero; secondary voltage is independent of primary connection when the primary is anti-series, and vice versa.  Simply, the amp-turns cancel out, making that winding be a lump of wire wrapped around the core, but otherwise an open circuit to the field.)

If you did in fact have this and nothing happened, then something else was wrong, like the primary was wired correctly but the secondary was opposite (+Vsec - Vsec = 0), or the fuse opened, or the circuit wasn't closed after all.  FYI. :)



--- Quote ---Carrying this further, I have used some of the low voltage secondaries wired in series with, and, in or out of phase with the primary(s) to adjust the voltage on other secondaries as needed.
The only time there may be smoke is if there is a shorted turn or three.

--- End quote ---

Also known as a buck or boost transformer.  It's a normal application, and has the advantage of the autoformer: you only need as many VA as you're adding, not the whole thing. :)

Tim

ArthurDent:
T3sl4co1l - "Also known as a buck or boost transformer.  It's a normal application, and has the advantage of the autoformer: you only need as many VA as you're adding, not the whole thing."

That is the way the TrippLite automatic  power line conditioners work. They have a 20A transformer with a tapped secondary that is switched by relays in the older units to add or subtract two 12 volt steps to give 2400 watts corrected line voltage in a small package (LC2400 is 7"x6"x9.5"). If the 123 VAC input drops to 101 then the sensing circuit adds 12 volts to change the output back to 123. If the input drops one more step to 89, then it adds 24 to bring it back to 123. It just reverses the secondary to subtract 12 or 24 to correct for overvoltage of 135 and 147.

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