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Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
An *ideal* transformer acts as a simple scaling, all inputs and outputs are in phase, the only thing that happens is that there is a ratio between the input and output voltage, and the inverse ratio between the input and output current.
If you are looking for phase shift other than 180 degrees (phasing) due to it being an ideal transformer, then there isn't any, period.
Now for the bad news - no transformer is ideal.
The good news - fortunately, for most purposes the non-idealities can be modelled by simple external components. And for good cores and use in the middle of their frequency range, those effects can be very small indeed. An evenly wound toroidal core being used in the right frequency range for the material will be very close to ideal.
Generally the low frequency limit of a transformer is dictated by the primary inductance, and the high frequency limit by the leakage inductance and the load, or its stray capacitance, whichever bites first.
If you do have a significantly non-ideal transformer, then take account of the effects like this.
Finite primary inductance - model with an inductor in shunt with the input Finite leakage inductance - model with an inductor in series with the output intra or inter-winding capacitance - model with a capacitor in shunt or between the windings respectively Copper loss - model with resistors in series with the windings
Obviously with all of those stray inductors and capacitances around, there's plenty of scope for phase shift.
Registered Member #2478
Joined: Mon Nov 23 2009, 03:24AM
Location: Texas A&M University
Posts: 47
If there is a large phase difference between voltage and current on the primary, the transformer is either driving a reactive load, or is severely under or overloaded. In an underloaded transformer, the magnetizing inductance will swamp a resistive load on the secondary. In an overloaded transformer, the leakage inductance will swamp the load.
Under normal operating conditions, primary current should slightly lag primary voltage.
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
Ok, if I had to vote, I'd go with Dr Slack on this one!
For an ideal transformer...
Input voltage and output voltage are "in-phase" (but possibly inverted depending on how you connect to the windings!)
...and...
Input current and output current are also in-phase (but possibly inverted depending on how you connect it.)
The phases of the currents relative to the voltages depends on what load is connected to the output of the transformer. You can make all the currents lead the voltages by 90 degrees using a capacitor, make them in phase with a resistor, or the current lag by 90 with an inductor as the load. Or you can get anywhere in between with any combination of R, L and C.
So far an ideal transformer primary and secondary V's are in phase, and I's are in phase. V relative to I depends on the load.
As Dr Slack already said, if you decide to run your transformer near the limits of frequency for which it was designed, then you need to add in the effects of those undesirable inductors and capacitors. They will introduce their own currents into the nodes, then you need to be very careful about exactly which two voltages or currents you want to measure the phase between!
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Transformer circuit
