Rotary phase converters produce three-phase power
from a single-phase source and are relatively versatile. They are
capable of powering resistive, capacitive and inductive loads and
can power multiple loads. They have been in use since the 1960’s
and are especially popular with home machinists that operate a
variety of small three-phase machine tools. Their simple design
makes them reliable and relatively inexpensive. They are
manufactured by hundreds of companies large and small, and in fact
can be built by almost anyone with components that are cheap and
easy to find. Their main shortcomings are in the quality of the
three-phase power they produce and in efficiency.
Voltage balance can be very
important to the safe and efficient operation of three-phase motors
and other three-phase equipment.
A rotary phase converter consists
of a three-phase motor (usually without external shafts) and a bank
of capacitors wired together to act as a single large capacitor.
Two of the leads to the motor are connected to the single-phase
power source and the third lead to the motor is connected in series
with the capacitor bank to either one of the single-phase inputs.
The output leads from the phase converter are connected across the
three motor terminals. Typically the motor used in the phase
converter is larger than the loads it is supplying. For example, a
rotary converter designed for a 7.5 Hp load might use a 10 Hp motor
frame. The electrical interaction between the capacitor bank and
the free-running phase converter motor generates a voltage on the
third motor terminal which approximates the voltage needed for a
balanced three-phase system.
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The voltage on the
third leg generated by the converter is affected by the incoming
voltage on the single-phase line, by the amount of capacitance wired
in series with the motor winding, by the motor frame size, and by
the amperage draw on the load side. By knowing something of
the load demand and relying on past experience, the right
combination of motor frame and capacitors approximately produces a
voltage equal to the other two legs. However, it usually isn't a
very good approximation. For example, measurements on a 7.5 Hp
rotary converter in an actual machine shop installation resulted in
line-to-line voltages of 252 V, 244.2 V and 280.5 V, which is about
a 12% imbalance in the voltages. With this much voltage
imbalance, a motor should not be loaded anywhere near its rated
capacity or it will suffer damage. In fact, the lead with the
lowest voltage could be completely disconnected and it would not
significantly change the performance of the motor.
Theoretically you
could install a rotary converter and operate the load while
measuring the output voltages phase to phase. The voltages
could be balanced by adding and subtracting capacitance to the phase
converter winding This is not very practical for the average
end-user and the system remains balanced only if nothing changes.
Unfortunately in the real world, utility voltages fluctuate and
amperages demanded by the load change, all of which will upset the
voltage balance of the converter.
For loads that
require good voltage balance it is necessary to increase the size of
the converter motor frame. It is also advisable to restrict
the converter to operation of one machine so that the range of
amperage the converter has to supply is not as wide.
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A phase converter
must also be able to supply the current needed for
across-the-line-starting
of motors. These currents are 5-6 times the full load running
currents of the motor, and even greater for high efficiency motors.
The voltage on the generated phase of a rotary drops as the current
demanded by the load increases, so it is easy to see how starting
currents can render that phase useless in accelerating the motor up
to speed unless the motor frame of the converter is large enough.
Motors that start under load require a larger rotary converter than
those that are lightly loaded.
While increasing the
size of the rotary converter can improve its voltage balance and
ability to start motors, that strategy has some drawbacks. The
overall efficiency of the system will decrease as the size of the
converter increases, with some systems having efficiency as poor as
70%. Care must also be used when operating small motors alone
on a large rotary converter. Because the amperage demand of
the small load does not reduce the voltage of the generated leg like
a full load on the converter would, that voltage might remain high
enough to damage the small motor.
Rotary phase
converters offer an affordable, versatile solution for many users,
especially those with lightly loaded, simple motor loads. For
larger loads and demanding applications like
CNC machines,
converters with higher efficiency and better power quality may be a
better choice.
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