Transformer equivalent circuit
An ideal transformer with one primary winding and two
secondary windings can be represented as shown below
Such a transformer has the following characteristics:
• No losses
• Perfect coupling between all windings
• Infinite open circuit impedance (i.e., no input current when secondaries
• Infinite insulation between windings
In reality, practical transformers show characteristics
that differ from those of an ideal transformer. Many of these
characteristics can be represented by a transformer equivalent circuit:
• R1, R2, R3 represent the resistance of
the winding wire.
• C1, C2, C3 represent
the capacitance between the windings.
• Rp represents the losses which are due to the eddy
current and hysteresis losses. These are the real power losses, sometimes
called the core loss, that may be measured by performing an open-circuit
power measurement. Because there is no load current, there is very little
I2R copper loss in the energized winding, and the watts
measured at no load are nearly all due to the core.
• Lp represents the impedance due to the magnetizing
current. This is the current that generates the magnetizing force, H, used
in the B-H loop diagrams.
Note that this current may not be a simple sine wave, but
can have a distorted, peaked shape, if the transformer is operated in the
non-linear region of the B-H curve. This is usually the case for
line-frequency, laminate type transformers.
• L1 L2 L3 represents the leakage inductance of all the
(This is discussed in detail in Voltech Note 104-105,