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Appendix B:

Perfect 3-phase From a Single-Phase Line.

120° Phase Relationships

Occasionally confusion arises about the phase angles--or timing--of a 3-phase supply produced by adding a leg to single-phase.

The question goes something like this: Since single phase has a 180° relationship (Fig. 13), and 3-phase voltages are 120° apart (Fig. 14), how can you add a leg to the 180° source and get 120° phase angles on all 3 lines?

Let me first say that all our rotary converter models do produce true 120° 3-phase, and this we verify by an oscilloscope. However, this does not explain the apparent contradiction noted above.

There is a vector analysis involved that is too complicated to present here, and a separate publication dealing with that is available on request. But I believe that the attacks on our method are the result of a bad experience when CNC or other voltage-sensitive equipment was operated on a high-resistance rotary, or on a rotary that was much too small for the applied load.

The phase angles in the 3-phase power triangle represent 3 equal voltages. If it is a 230 volt system then each side of the triangle is 230 volts long. However, if a rotary is used that slows severely under load, the voltage on the generated phase drops, and 2 sides of the triangle become shorter (or disappear completely). The phase angles then cease to be 120°--in fact, under severe load the only voltage remaining may be the single-phase line.

Review again what I said about the High Resistance Rotary and the Importance of Good Voltage Balance. I am happy to report that the converters we build--with a low-resistance rotor--are much tougher under applied loads and that our power triangle will resist distortion.

One other important fact about the 3-phase distribution of phase converters:

If our converters did not produce properly-timed, 120° 3-phase, then you would not expect fully-loaded motors to perform well on our phase converters, or would expect them to overheat under full-load. If the 3-phase voltages were at 180°, 90° and 90° (see Figure 15) instead of 120° (see Figure 14) the power would not align itself with the magnetic poles in the motor being operated. The result would be the same as if you switched 4 spark-plug wires on a 6-cylinder car engine--overheating and extremely poor power.

In fact, the toughest motors in the world to operate are deep-well submersible pumps. The currents and voltages must balance within 5% on all 3 legs or the motor will destroy itself.

These pump motors have been known not to balance acceptably on "Open Delta" 3-phase supplied by the power company--but they will balance within 4% on our Rotary Converters, at full motor and pump load. All it takes is the right equipment.

For additional information on the products discussed in this website, or for hard copies of this publication, contact:


Smith Electric Motorworks

10288 Hwy ZZ

PO Box 282

Marceline MO 64658

FAX 660-376-9701

Toll-Free 800-437-4273

© 1992-2016 by Gary A. Werner, author. All rights reserved.

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Figure 13 - Single-phase voltage is a 180° relationship.

Figure 14 - 3-phase voltages are 120° apart. How can you

add a leg to single phase and get 3-120° voltages?

Figure 15 - Does this represent a phase converter's voltage output? Impossible!