Electronics > Beginners
ASK: Determining transformer's windings phase
BravoV:
Scope of my question is on common mains step-down transformer which has 2 windings on the primary side.
The case here is, at the primary mains side it has 2 x 110 V windings, and to use it at 220 V mains, both windings must be connected in series with the right phase connection (the left illustration below, instead of the right one), cmiiw.
Questions :
1. How to determine the right phase connections on both windings "easily/practically", if the transformer doesn't have the proper labeling, say like the label is torn off or lost ? (only for the primary side, assuming the secondary side winding already properly identified).
2. What will happen if the primary side is connected like the one at the right at above picture ? Magic smoke ?
TIA
Messtechniker:
Simply apply some low level 50 or 60 Hz AC from a signal generator and measure or scope the output.
The connection with the higher output on the secondary side is the proper series connection.
To double check:
When reversing, the output on the secondary will be lower (half). i.e. no scope needed.
Ian.M:
The incorrectly phased primaries on the right will act pretty much like a dead short (limited only by the lowish winding resistance), as their magnetic fields cancel out. Result: one dead smoking transformer.
For first powerup of a mains transformer, if you aren't 100% certain of *all* winding phasing, its a good idea to have a mains voltage incandescent* bulb (ideally of a wattage between 5% and 50% of the transformer's VA rating) in series with the supply, and to *not* connect the secondaries to any load till you've checked voltages and phasing. If there is a fault, the bulb will limit the current so its low enough not to burn out the primary. If the bulb lights, you've got a problem . . .
* It must be a real tungsten filament bulb. CCFL and LED bulbs are totally unsuitable, because you need the non-linear resistance characteristic of a heated metal filament.
mvs:
--- Quote from: BravoV on December 11, 2018, 06:41:23 am ---1. How to determine the right phase connections on both windings "easily/practically", if the transformer doesn't have the proper labeling, say like the label is torn off or lost ? (only for the primary side, assuming the secondary side winding already properly identified).
--- End quote ---
Connect both primary windings in series, apply some small AC voltage (you may use signal generator or another transformer) to one primary winding only and then measure voltage over both.
If the connection was correct, you will get voltage doubled.
sorin:
You can use a LCR meter (use 100 Hz test frequency).
The correct winding's should have higher Inductance.
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