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Copyright 2005
Smith Electric Motorworks

Rotary Converter

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Dynamic or Rotary/Capacitor
Phase Converters

 A more versatile phase converter actually generates the "third leg". It uses a large capacitor bank to energize a rotary phase converter motor (Figure 5).


 Figure 5 - A rotary generator distributes capacitor
current to multiple motors.


With this method the electrical charge from the capacitor bank is processed through the connected rotary motor (generator) into a useful AC sine wave form. This power may then be connected to several pieces of 3-phase equipment. The equipment may then be started and stopped in any combination up to the rotary motor's total load capacity. Any type of 3-phase load may be operated with capacitors as long as the rotary generator is at operating speed.

Some Rotary Converter Construction Details

A rotary converter motor has a lot of shared characteristics with the 3-phase motors it operates. There is a set of stationary field coils, or stator, that determines the magnetic poles in the electrical steel of the rotary. These coils and their poles have 120 spacing to produce a uniform 3-phase wave form. A squirrel-cage type rotor produces the poles of the rotating magnetic field. The rotor should have good bearing support in cast iron end-bells, and preferably the entire rotor will be of cast-iron construction. This insures that minimal distortion will occur and allow precise machining of the end-bell/rotor bearing relationship. The machining of these components is critical, since the rotor must run axially in magnetic dead center of the stator for optimum converter performance. The rotor-to-stator air gap is also critical, since a magnetic "flux" that produces 3-phase voltages must pass this air gap. A white gap, or misalignment of the rotor/stator relationship results in poorly defined 3-phase voltages.

Capacitors connected to the rotary unit then produce counter-EMF (electromotive force) which define the currents and voltages produced by your rotary converter. The phase-shifted current from the capacitors is absorbed by the electrical steel of the rotary, then distributed in a 3-phase waveform that is usable by any type of equipment.

What to Expect from a Rotary Phase Converter

 Compared to any static phase converter, a rotary converter is versatile and powerful. Compared your local power company, a rotary phase converter is a finite compromise -- but a viable option. This is stated to prevent unwarranted expectations of the equipment. Rotary phase converters have weaknesses and strengths which should be considered before a purchase is made.

On the positive side, phase converters are ecologically beneficial. No landscape or wildlife is disturbed for a converter, as it is when more poles and wires are set in place and the environment is trampled it is with a utility-supplied conversion from single to 3-phase wiring. There is less energy expended to build the converter as compared to the extra transformers and wire required for 3-phase line. Many people also fear the effects of the additional high-tension wires above their living space, and the greater EMF created by the fields in additional transformers. To this writer's knowledge, no one has yet satisfactorily disapproved the validity of these fears, and a rotary converter operating on the low-voltage secondary side of the grid is not in any way comparable to the utility's primary power voltage or the 3-phase transformers.

As mentioned earlier, rotary phase converters also have a minimal purchase cost -- about $50 per HP on multi-motor installations, far lower than the dollar cost to bring 3-phase even one mile via power lines. Operating cost is very low on rotary converters -- often less than five percent of the electrical cost of the operated load, that is, five dollars for every $100 worth of energy purchased. Customers who have replaced single phase motors with 3-phase equipment and a rotary converter have told me that their energy cost was unchanged, possibly because the higher efficiency of the 3-phase motors offset the operating cost of the rotary converter.

On the downside: A rotary converter in a standard, multi--motor installation will never balance each line's power (current), as well as a utility-supplied, 3-wire, 3-transformer 3-phase system. The rotary converter is as good as an "open delta" 3-phase (2 transformer) system, however, and may in fact even be better under certain conditions.

Rotary converters will not deliver the starting torque of a solid 3-phase line, and the rotary unit often must be greatly oversized relative to the largest motor operated to produce high starting torque. Since a converter must maintain its rpm and start the largest motor in your system, providing 2 of the 3-phase voltages, it is not a good idea to try to use a rotary converter that has an actual rotary HP that is no greater than the largest motor you wish to operate. More on this later.

Rotary converters will not maintain close voltage balance over a wide range of operation, and line-to-line 3-phase voltages will drop precipitously when heavy loads are introduced. If you have voltage-sensitive equipment such as computerized machine tools (CNC), best results can be obtained by using a separate converter for the CNC, and another one for general shop machines. A special CNC converter such as GWM's Digi-Series typically adds five to seven percent to the machine's cost. (See also, "Light Dimming and Phase Converters", GWM catalog.)

By analyzing the strengths and weaknesses of each option -- utility 3-phase or a converter -- you minimize your disappointments with either. Utility supplied 3-phase often brings higher electrical costs than single-to 3-phase converter power -- after all, someone has to pay the purchase and maintenance cost of the extra lines and transformers sitting out on a pole in the weather, don't they? The converter is owned by you: when you don't need 3-phase, you turn it off and you're not paying anything. Its 100 percent tax deductible for your business, and you can take it to a new location. But if it gets whacked by lightning, you you can't just call the power company to fix it. So these are all matters which bear consideration in your search for a 3-phase power solution. Most applications are routine. But the difficult ones crop up also.

 

Applying Motors to Ensure Good Performance on a Converter

This is not a fairy tale. Once upon a time there was a manufacturer. He designed three machines, and called for his engineers to tell him how large (in HP) a motor he needed to operate each machine. The engineers engineered vigorously and returned, saying, "Machine A needs a five HP motor 90 percent of the time, but during the other 10 percent of the time certain circumstances will occur that requires a motor torque equal to that of an 8 HP motor. At that point, however, the lower motor rpm and reduced machine speed is okay. We'll use the 5 HP motor and overload it part of the time to 160 percent of its full-load torque rating."

"Machine B needs 17.2 HP at a standard rpm -- 1700 is okay. We can't buy a motor that HP at a very good price; the EPA might squawk if we use a 20 HP motor, since it will waste energy. Let's buy a 15 HP motor with a 1.25 service factor, and run it overloaded to do the work. It may not make 17.2 HP, but we can run it at the increased load anyway, it'll just run a little slower."

"Machine C needs a motor that runs at 1770 rpm for maximum production. Standard motors are 1725 or 1730 rpm full-load. We'll use a new "brand Z." EPact rated motor -- they have a double squirrel-cage rotor and run faster to meet the federal Energy-Saving Mandates, but it will require a lot of inrush (starting) current. Good thing it's only 10 HP, or it would cause line disturbance."

Now, everyone of the above applications can be, (1) typical, and (2) a pain in the neck for a phase converter manufacturer. Here's why: Machine A is loaded up to 160 percent of full-load part of the time. It will take at least a 10 HP rotary to operate. If the motor is cheaply made, it will require a 15 HP converter, since cheap motors often create a more intense magnetic field with less electrical steel. This places greater demands on the phase converter. The rotary should then contain even more electrical steel to compensate for the motor's shortfall.

Machine B is even worse. Operated on a phase converter, a well-designed motor in this application can require a converter four times the HP of the load motor. One converter application of this type comes to mind: A 20 HP pump motor was running at a 1.23 service factor (123 percent of full-load current, or 23 percent overloaded) and required a huge phase converter. But when the same pump was operated on a 25 HP motor, the converter requirement was reduced from a 70 HP rotary to a 30 HP unit! So, on some applications, money can be saved by installing a larger motor.

Machine C will place very high starting and running demands on a rotary converter. If it were powered by 10 HP non-EPact motor (EPact motors are universal in the U.S. after November 1, 1997 except for special-purpose applications) fully loaded, within normal starting torque requirement, it would require a model 256 (20 HP) frame rotary unit. With the EPact motor it may require a 286 (30 HP) rotary. Why? Because in a phase-converter-generated power system, balanced currents and voltages are a function of rotor slip in the largest load motor, and a low-slip motor -- that is, a motor that wants to run very near its synchronous rpm (1800 in this case) at the full-load -- will place stresses on the converter that can only be overcome by a greater rotating magnetic field. If the larger rotary is not available, the motor does not attain its desired rpm and currents are severely unbalanced. Additional capacitors will not help, either.

The point at issue is this: for a converter to work properly on a heavily-loaded application using modern motors -- especially "energy efficient motors" -- mandated by government, you may find that the best way to lower overall costs on the installation is to change the motor on the machine to a special-purpose (non-EPact) or higher HP motor. This will not lower the machine's electrical efficiency, since the capacitors in a rotary converter provide increased overall system efficiency. While machine and woodworking tools generally are not a problem on phase converter installations, large pumps, grain dryers, regenerative blowers (fans), rotary screw compressors and other such intense, continuous-duty loads should be approached with caution.


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