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RPC Construction Methods

Modified on 2013/04/25 16:23 by Frank Categorized as Uncategorized
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.

Static Converter

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 motors.

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.

Autotransformer Method

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.

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