Within certain power limits, and depending upon the type of equipment you
intend to drive, it is possible to convert a single-phase supply into
three-phase and thus avoid the costs of bringing in a true three-phase
supply system. Geoff Brown describes three ways of doing it, and compares
their advantages and disadvantages
The industrial world runs on three-phase power. Three-phase motors are
more efficient, cost less, last longer and are more dependable than their
single-phase counterparts, which are after all basically three phase
motors with additional capacitors. Yet, often a three-phase supply is not
available, particularly in remote areas where it may not be economical to
run the necessary cabling or, for historic reasons, where alternative
distribution was used. If a single-phase supply is all that is available
to run your three-phase motors, then some form of phase conversion method
is needed. There are three main methods: static conversion, rotary
conversion and variable speed drives.
Static conversion
Static converters use capacitors to produce a three-phase supply. The two
single-phase conductors are connected to two of the inputs on a
three-phase motor and a capacitor is then connected to one of the
single-phase inputs and the third leg of the motor. A phase shift through
the capacitor shifts the voltage in time from its parent voltage,
producing a voltage distinct from the two single-phase lines.
A motor requires about six times as much current to start as it does to
run, so a static-capacitor phase converter must be able to switch a large
group of capacitors in and out during motor starting. Using this method,
a three-phase motor can be run at 70% of its nominal load all day or make
short bursts of power at around 90-95% load for 15 minutes at a time.
Some devices will start a three-phase motor on capacitors but will then
allow the motor to run on single-phase. This can dangerously overheat the
motor on loads above 45-50% of its real power rating. Stall occurs past
65-70%, since only one third - or one phase - of the three-phase motor is
actually receiving power.
The static method is thus suitable for moderately loaded motors, but a
motor improperly applied to a static converter may suffer from too little
power. On the other hand, if too large a static converter is used, the
capacitors may force too much current into the motor, resulting in
overheating and eventual failure. While some power tools and machines
equipped with flywheels work satisfactorily with static converters, there
is a large body of equipment that is entirely unsuited, including most
pump drives, large fans and CNC machine tools.
Three from two:
Northumbrian Water shows how
Northumbriam Water's Winston Village pumping station near Barnard Castle
is on a split-phase supply - a common problem for many sites in rural
areas, says project manager, Bob Dixon, who explains that this is
essentially a farm supply, which does not have the capacity to run the
larger pumps such as those at the Winston station. A standard UK
three-phase supply will typically consist of a star connected three-phase
arrangement, with 120 degrees between phases, and neutral connected to
the star point. This provides 415V between phases and 240V between a
phase and neutral. A split-phase arrangement, on the other hand, has two
phases 180 degrees apart, providing 480V between phases and 240V between
phase and neutral.
Winston Village pumping station has two 3kW duty standby submersible
pumps on a sewage pumping application. They are designed for three-phase
operation, but upgrading the supply from split-phase to true three phase
would have cost somewhere in the region of £50,000. The problem was
solved by Slater Drive Systems (an ABB Drives alliance partner), using an
ABB ACS 400 variable speed drive to create a synthetic three-phase
supply. This SDS-pione