main downside is the practi-
cal lower limit on frequency
(core area required is inversely
proportional to frequency), so
these can’t really be used for
current feedback on a traction
inverter’s outputs (which de-
liver a very low AC frequency
at low RPM).
The other type of current
transformer is the Rogowski
coil, which is simply an
air-core solenoid bent into a
nearly-closed circle around the
conductor to be monitored.
The lack of a magnetic core
implies an ability to read very
high frequency and amplitude
currents, because air doesn’t
suffer from non-linearity or
saturation, unlike all ferro-
magnetic materials. However,
the output of a Rogowski coil
is proportional to the rate of
change in current (i.e. dI / dt),
rather than just the magnitude
of current itself, so its output
has to be integrated, first, and
the integrator inevitably limits
the frequency and amplitude
range that can be covered.
Rogowski coils are rarely used
as current sensors in equip-
ment below 1 kA or so, but
they do find widespread use
as current probes for meters
and oscilloscopes. The very
simplicity of their design -
and consequent cheapness to
manufacture - strongly sug-
gests they will be used more in
the future, especially in OEM
EVs that are already produced
in the kind of volumes that
most power supply and motor
drive manufacturers could
only dream about.
michael s
(Michael S)
#1