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Copyright 2005
Smith Electric Motorworks

Balanced Voltages

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Balanced Voltages
Interpreting Current Balance (Amp Readings) on Phase Converters

Balancing a 3-phase load can be likened to 3 circus tight-wire walkers carrying a grand piano. If one of the three walk-wires is not drawn up as tightly as the other two, two guys are going to carry most of the piano -- the load. Voltage is the "tension" on the circuit, while the current, or amps is the portion of the piano each line carries. Consequently, unbalanced power will harm a motor if the imbalance is so great that part of the motor "pulls a muscle" electrically. Achieving equal current balance on motors operated on any phase converter can be difficult. Motors operated on a capacitor static-type converter will only achieve reasonable current balance when the motor is operated at 60 to 65 percent of its actual HP rating. After that the L1 (A) current begins to climb dramatically. Rotary converters operated on a multi-motor system are considerably better -- but not perfect.


The chart in Figure 6 gives an approximation of how currents will lay in motors operated on a standard, symmetrically-wound rotary phase converter under varying loads. Assuming that voltages are reasonably balanced throughout the operating range, the reason for some of the line-to-line imbalance at moderate load lies in a phenomenon called power factor. Power factor as a percentage measurement is the ratio of apparent power (line current) to actual power required to move the load, including any frictional and windage losses in the motor. Without getting overly technical, power factor relates to the power required for actual work being done versus the amount of power required to magnetize the iron in a motor, or "fill it up" with electrical current so that it can do some work. This magnetizing current (called VARS) can be likened to the bottom 4 feet of water in a 10 ft. deep swimming pool -- it doesn't normally have much use, but it is necessary. One way of supplying these VARS of current is through capacitors.

Because capacitors store electrical current, the capacitors on a rotary converter actually store and supply the VARS -- or magnetizing current -- for motors operated on the converter. The result is a very high electrical efficiency, especially at light to moderate loads. Because of this, a motor operated at light load on the converter will have a higher than normal current on lines B and C -- these are the rotary lines which have their capacitor connection, and these sit like little suitcases full of power that jump in and out of the motor as magnetizing current. The lines begin to balance more closely as motor load increases. At full load the motor's amps may be above nameplate rating on phase A (L1) but lower on C (T3, the manufactured phase). For decades the phase converter industry has stated that this is acceptable, since one "hot" phase is adjoined in the motor by one or two "cool" ones with lower current. Personally, I would advocate the use of a larger or at least more well-designed motor if the one or more line currents is higher than the service factor amps -- usually 115 percent of full load amps -- of the motor in question, if oversizing the rotary or adding capacitors fails to correct the problem.

Suffice it to say, under the present circumstances, a converter that provides reasonably close current balance will allow most motors to operate well overall. But heavily-loaded motors on a converter installation will require special accommodation for trouble-free operation to result. And motors and machines which are very sensitive to voltage or current balance -- such as deep-well submersible pump motors or CNC machine tools -- require a specialized rotary converter dedicated to one motor or machine.

The Importance of Good Voltage Balance

Usually if the line to line voltages on a 3-phase converter system are balanced within five to 10 percent of each other, the currents on each motor will also be close. We discussed earlier that electrical current may be viewed as fuel. A rotary converter is a fuel distribution system for 3-phase equipment. Remember that voltage is the pressure at which the electrical fuel is delivered, while the current is the rate of flow of the fuel.

 Figure 7 - 3-phase power is viewed as a triangle. Each side represents a line-to-line voltage. Ideally, all sides are equal (10%). The phase angles will measure 120 each on an oscilloscope when voltages are balanced.

 Figure 8 - Power triangle on an overloaded rotary phase converter. Severe voltage imbalance results in improper phase angle relationship. Motors will operate at reduced power and at higher temperatures.

In Figure 7, three equally-spaced voltages appear as points in a triangle. Figure 8 shows what happens to the same voltages when the rotary phase converter system becomes heavily loaded: the power triangle has collapsed, leaving the phase voltages severely unbalanced. Two possible reasons for this operating condition follow.

Possibility 1: The rotary is large enough relative to the load, but the capacitor bank is too small. In this scenario, an insufficient bank of capacitors -- which actually force current, or fuel, into the system -- have left the fuel supply spread too thin over the operated load. Capacitors may be added to one or more large motors in the system to restore balance and prevent load distress (Figure 9).

 Possibility 2: The rotary is slowing under load and has synchronized its RPM with large or heavily loaded motors in the system. Adding capacitors in this situation will not solve the problem, since the timing of the 3-phase voltage is off. The 3-phase "sine" wave from the rotary must "lead" (by higher rotor speed) the operated motors, in order to create a magnetic field which is ahead of the load motor's rotor RPM. The rotor in the load motor must "chase" these magnetic fields, or else full motor torque is not attained with reasonable balance. In this case, late timing of the magnetic fields will throw most of the motor's load on the single-phase line (which is not affected by converter slowing). The net effect can be likened to a truck engine in which the ignition timing is late, and most of the fuel is burned after the work has taken place. The engine then has to work much harder to move the load, with resultant overheating and higher fuel bills.

Figure 9 - Additional capacitor panel installed on large motor to provide balanced power under load. The capacitor panel is connected at the load side of the motor starter so that excess capacitor current does not enter the system when the motor is not operating.

The solution to this problem is to increase the rotary size or parallel a second rotary in the system to increase power to operate loads. Cramming more current into the motor with extra capacitors will only make matters worse.


Figure 10 - A second or third rotary may be parallel connected to an underpowered installation. Connection is simple and the rotarys are self-synchronizing to the power grid.
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