Efficiency can be an important factor to consider
when choosing a phase converter.
Most three-phase loads are relatively
large consumers of electrical power since they are used in
commercial and industrial applications such as metal fabrication and
pumping, so the operating efficiency of the system can have a
significant economic impact over the life of the equipment.
There are two ways
to measure efficiency of a phase converter – the efficiency of the
phase converter itself and the efficiency of the operated load.
The quality of the three-phase power, primarily voltage balance,
supplied by the phase converter can impact the efficiency of
three-phase motors and rectifiers.
converters do not consume or waste much power in themselves.
They are very simple devices with no moving parts. However,
the operated motor will lose efficiency because it is being supplied
with very poorly balanced power. For all intents and purposes,
only two of the three windings of the motor are being supplied with
Because of this the
motor must be severely de-rated. If asked to perform more work
than about half its rated capacity, the motor will begin to overheat
and may suffer permanent damage.
converters do a better job of supplying balanced voltages to a load
but because their voltage balance is difficult to predict and
impossible to control, the load may or may not be supplied with
balanced three-phase power. If the load is not receiving
balanced power, motor efficiency is lost and it must be de-rated to
avoid damage. The rotary converter is also a consumer of power
itself. It is usually built with a motor frame larger than the
motor frame of the load it operates. And in order to improve
the voltage balance and the ability of the converter to deliver
starting currents, an even larger motor frame is used. It is
common for the converter to be 2-3 times the size of the operated
load for demanding applications. This contributes to losses in
efficiency, especially when the converter is running with no load on
poor power factor
of rotary converters also decreases their efficiency.
Oversized rotary converters may waste as much as 30% of the power
digital phase converters
are solid state devices with no moving parts that are very
efficient. And because they supply the load with perfectly
balanced power, the load operates at high efficiency. A
digital phase converter is 95-98% efficient, slightly less efficient
than a VFD. This is because the digital phase converter has
filtering systems for both the input current and output voltage to
remove harmonic distortion that can affect the load and the utility
transformer. Removing these harmonics results in heat loss
within the converter. A VFD generates less heat internally, so
in itself, it is more efficient than a digital phase converter.
However, the harmonics generated on the input and output of a VFD
cause efficiency lost through generation of heat in the load and in
the utility transformer. The efficiency loss is caused by the
VFD, but occurs at other points in the system. In fact, the
overall efficiency of a system powered by a digital phase converter
is probably more efficient than one powered by a VFD because a
digital phase converter only creates one voltage, while a VFD
creates all three, leading to more heat losses in the motor