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Converter Voltages and Small-Motor Operation

3-phase Voltages from Converters

All phase converters produce a "delta voltage" (figure 2). The third leg voltage to the single-phase neutral will be at least 1.73 times (square root of 3) higher than the 110 or 120 V. from your power company -- about 200 to 230 volts. This results because of the 3rd-voltage-to-neutral is a greater distance through the rotary and transformer windings. It is perfectly normal, and will not affect 3-phase equipment, since most 3-phase machines do not recognize a line-to-neutral voltage. If you happen to have the rare machine that does, this will require a wye -- as opposed to the delta -- voltage. (The junction in the wye -- Y -- is the neutral point.) This cannot be produced by a wye-winding converter, but only by a transformer connected between the converter and the load, which has a secondary wye winding.


Do not let anyone mislead you that something is inherently wrong or inferior about converter voltage because it has a "wild leg" to neutral or that this will prevent operation of your equipment. Many utilities 3-phase supplies produce a delta voltage also. Just make sure that any non-CNC machine you operate has its controls isolated from the generated leg (T3), and you will get good results. The third leg voltage can fluctuate somewhat during load initiation and will cause starter chatter if connected to the control circuit.


3-phase Voltages and the "Minimum HP Load" Rating on Standard Rotary Converters


Each model of standard rotary phase converter has a "minimum load operated" and "largest single motor" rating, as well as a total operated HP load. The minimum load applies because a standard rotary's 3-phase voltages are dictated by its large capacitor bank. Two of the voltages may be quite high under light-load or unloaded conditions. As loads on the converter increase and the capacitor current is spread to the system, voltages decrease. On large multi-motor loads this is necessary so that good voltage under full-load is available. But high no-load voltages may "saturate" the magnetic fields of a small motor that does not require enough current by itself to bring the voltages down to reasonable level. The little motor may then overheat after 15 minutes or so when operated alone. To avoid this, you may either, (A) disconnect several motor run capacitors from the rotary's capacitor bank if loads are not too great, or (B) idle enough 3-phase equipment to exceed the minimum when a small machine is operated. Electrical cost will not increase enough to notice. As a third option, a small-motor operation switch may be installed on the rotary converter to manually switch off some of the rotary's capacitor bank under light loads. Yet a fourth option follows in a moment.


Some manufacturers of phase converters offer capacitor switching in an "automatic" mode or offer an "automatic voltage regulation" device (AVR) on certain models. Our knowledge and experience in this field mitigates vigorously against such a "solution" to high-voltage problems. More on this later. (See "Problematic Voltage Regulation Attempts," in the next section. See also "Harmful Effects of Capacitor Switching.")


Small Motor Voltage Solutions: Shifting Capacitors to the Load


 A rotary phase converter normally has one group of capacitors connected to its rotary motor at the rate of 20 to 23 units of capacitor power (measured in microfarads) per rotary HP for normal running.


In many farm applications we find one or two very large motors and several very tiny motors operated on the same large converter. As in the example above, as long as all the motors are running there is reasonable voltage balance to the entire load. However a farmer with an irrigation pump of 30 HP and eight or 10-1 HP motors driving a sprinkler will have a problem in the spring or autumn when he wants to move the sprinkler for planting or harvest without the pump motor running. The sprinkler may take 12 hours to move, and the motors--operated at saturation level voltages with the larger motor turned off--will overheat soon.

Figure 11 - Rotary capacitors may be relocated to a large load motor when the large converter may operate small motors alone.

The way to solve this is by removing some of the motor-run capacitors from the rotary converter (to lower the converters light-load voltage) and placing them in a panel connected to the 30 HP motor--along with some additional capacitors it needs to perform well--but at the load-side of its motor starter. Now the voltages from the converter will remain low at light-load while achieving normal levels at full load (Figure 11).