This article was created from a post by Bruce Norton (norton_bruce) on Apr 08, 2013. Bruce has given me permission to post it here.
Methods of Phase Conversion
There are several methods to derive 3 phase power from single phase power.
The following is a list of the methods I am aware of and I am sure there
are some I do not know about. This summary starts with the simplest and
progresses to the more complicated.
Most Basic Method
Take a 3 phase motor and a 3 phase disconnect, hook single phase power to
two poles of the disconnect switch and wire all three of the disconnect
terminals (T1,T2,T3) to the idler motor. Then wire from these same 3
terminals to the motor starter. To use, spin idler motor with foot (not
recommended) or with a starter rope (also not recommended), then close the
disconnect switch to connect power to the idler. You will now have crude 3
phase with poor voltage balance and poor ability to start loaded motors.
It is cheap and simple.
This is a really a bad name for these as they are only a method to get a
three phase load motor started on single phase, then allow it to single
phase. Load motors will produce no more than 59% of nameplate HP with this
method. The start circuit consists of either a potential or timing relay
and some start capacitors.
Balanced Static Converter
This is a little better than the simple Static Converter as it has some
run capacitors to improve performance of the load motor. Some claims are
for 80% power from the load motor. These are still good only for lightly
loaded motors as one will encounter problems with starting heavily loaded
Unbalanced Rotary Converter
This is simply an idler motor (3 phase motor of proper voltage) which is
started with a static converter. It provides 3 phase power but with some
voltage imbalance. It is better than static converters but the
manufactured phase will experience substantial voltage drop with moderate
loads. It is common to use an idler twice the size of the largest load
motor that will be connected when using this design. This helps with the
voltage drop problem. These are not as noisy as the balanced RPCs.
One balanced phase Rotary Converter
Add run capacitors to one phase of the unbalanced rotary converter and one
has this design. These will start good size loads but suffer from
substantial voltage drop on the unbalanced phase. Not a bad design and
they will sometimes start without a starting circuit. This was my first
RPC build and it worked but I did not like the voltage imbalance nor did I
like the noise.
Two balanced phases Rotary Converter
Add run capacitors to both manufactured phases and one has this design. It
is good for starting loaded motors and can give good voltage balance and
will hold the voltage balance better than the other static or rotary
converter designs. It will need a start circuit to get it started and is
the most costly of the rotary converters, but is well worth the money for
it's reliable service and good performance. I have been building and
operating these for a number of years.
There are several methods of starting a rotary converter
Start with foot, start with a rope starter, start with a pony motor or
start with capacitors. All these methods work. I never use the foot
starter nor do I use the rope starter. These are just too dangerous for
me. I use the capacitor start method for RPCs through 15 hp, then look
seriously at using pony motor starters for 20 hp and larger RPCs.
There are various methods for controlling the starting capacitors with two
common methods being a potential relay or a time delay relay. I prefer the
potential relay method for RPCs of 15 hp and below. I use a time delay
relay for pony motor start RPCs. I have seen a centrifugal switch used
with pony motor RPCs but have not built an RPC with this starting method.
Electronic Phase Converters
These are excellent performers but costly. They will run multiple motors
up to their horsepower rating. Maybe I will own one someday. Phase Perfect
is one brand I have experience with.
Variable Frequency Drives
These are electronic drives designed to operate 3 phase motors at variable
speeds by varying the frequency. They will also convert single phase power
to 3 phase. What a nice side benefit!
These are intended for one VFD to operate one load motor. High end
industrial drives may be able to handle more than one motor, however the
ones hobbyists can afford are best dedicated to one motor.
VFDs are available as single phase input x 3 phase output and as 3 phase
input x 3 phase output. Many of the 3 phase input drives will also run
from single phase input, however the drive output must be de-rated to
about 50% of nameplate. I first tried this in 1989 and was surprised at
how well they work. I now own several. VFDs are now cost competitive with
Static Converters and do a MUCH better job. The VFD will give 3 phase
output and full power to the load motor plus variable frequency is
available if one wants variable speed also. These are great for drill
press and wood lathes, not to mention belt sanders and other common shop
tools. It is also possible to get higher voltages such as 460 volts with a
VFD simply by using a single phase transformer to step up 240 to 480 and
feeding this to a 460 volt VFD that will run on single phase. I have two
of these operating in my shop now.
This type provides the missing phases with an autotransformer. I know of
this method and know it works well for one motor. I do not know how to
build these but they are worth researching if one is working at 240 volts
and wants the converter to be dedicated to one load motor.
This discussion is not intended to be used for construction of a phase
converter. It is simply a brief description of several methods to achieve
3 phase from a single phase power.