2.6 QL - Q factor
When a transformer is energized the changing magnetic field in the core
causes losses in the core.
Two types of losses occur in the core: - Hysteresis losses and Eddy
These losses are described in section 1.3 and 1.4
The total of these losses can be represented on the transformer
equivalent circuit by a resistance associated with the inductance of the
This resistance may be shown either in series with an inductance or in
parallel with an inductance, as shown in the following diagrams:
Either the parallel or series circuit can be used, with equal validity,
in the transformer equivalent circuit where: -
RP = (RS2 + ω2LS2)/RS
LP = (RS2 + ω2LS2)/ω2Ls
It is clear from this equation that series and parallel inductance do not
necessarily have the same value, so when a value for inductance is
specified, it must be specified as series or parallel equivalent circuit.
For a series circuit, the 'quality factor' Q is defined as:-
For a given inductance, the lower the value of the equivalent series
resistance, the higher is the value of Q, i.e. the 'better' the coil.
Typical value of Q range from about 2 to several hundred
The Q factor measurement would normally follow a measurement of the
inductance of the primary winding in the test program.
As with an inductance measurement, the Q factor test would normally be
used for signal, pulse and switched mode power transformers, where the
normal operating conditions require only small excursions of the B-H
curve, never extending beyond the linear regions.
A Q factor test is one way of highlighting shorted turns within the
To measure Q factor, the tester performs exactly the same steps that
would be used to measure inductance.
The only difference is in the calculation at the end of the test: the
measured voltage is divided by the current to obtain a complex impedance
from which the Q factor is calculated.
The test signal can have a frequency in the range 20Hz to 3 MHz, and an
amplitude from 1 mV to 5 V.
Normally when following an inductance test, you would choose the same
test conditions for the QL test. If the QL test does not have an
associated inductance test, then choose the test conditions as detailed in
the Table on page 23, based on the value of the inductance of the winding