How do you connect a three phase motor to a single phase power supply?

The present invention is directed to a three-phase induction motor which is arranged for operation from a single phase power supply. A three-phase motor is desired especially at higher horsepower ranges, for example, above 1 hp, because of its inherent efficiency over a single-phase type motor. These types of motors in effect generate a second phase (especially for starting) by a second winding in series with a capacitor.

Such a capacitor run motor is usually more expensive and has lower efficiency than a corresponding equal power three-phase motor. However, a three-phase motor can be operated from a single phase power supply by two well-known techniques. The first is a static phase converter and the second a rotary phase converter.

Both are manufactured by Ronk Electrical Industries, Inc. , of Nokomis, Ill. Both are discussed in a paper entitled, "Current Techniques in Phase Conversion Systems," by Claude M.

Hertz, IEEE Conference Paper No. 78-83, IEEE Rural Electric Power Conference, Minneapolis, Minn. More specifically, the static phase converter has an autotransformer and a series capacitor which goes to the third terminal of the three-phase motor.

The tap off the autotransformer is chosen by the user as a function of the full load power facto of the user's motor. The capacitor size is selected by the manufacturer as a function of the motor current or motor horsepower. The motor currents and voltages are balanced only at full load on the shaft.

"The major limitations are obvious from these characteristics; wide variations from the given load point should be avoided and breakdown torque is limited because of the reduced manufactured current (IA) at overload. The static phase converter is generally suited for single motor applications where the motor load remains relatively constant . The rotary phase converter, while well suited for multiple motor applications, has disadvantages in that "its idle losses are high and typically has limited starting torque.

Motor current unbalance is usually high, with an associated decrease in motor life to be expected . It is therefore a general object of the present invention to provide a three-phase induction motor which operates efficiently on a single-phase power supply. It is another object of the invention to provide a static phase converter which utilizes a unique connection of the motor windings and does not require an expensive transformer.

Another object of the invention is to provide a feedback control circuit in conjunction with the three-phase induction motor of the invention to minimize the voltage or current unbalance for widely variable shaft loads. Another purpose of the present invention is to provide an open loop control of the "manufactured" current for the third motor terminal, which is a preselected function of the current in a different motor terminal, so that for widely varying shaft loads, the voltage at the third motor terminal is equal to or above the rated voltage, with a preselected limitation on the degree of unbalance. In one aspect of the invention there is provided a three phase induction motor with at least four different motor terminals, a two-terminal single phase supply, the first of the motor terminals connected to the first terminal of the single-phase supply, the second of the motor terminals connected to the second terminal of the single-phase supply.

A first capacitor is connected between the third motor terminal and one terminal of the single-phase supply. The motor has 6 windings comprising two sets of three windings with pairs of windings from each set disposed together in common motor slots, and with each pair of windings spaced 120 electrical degrees from another. Each set is connected in a wye or delta configuration to provide three terminals at the ends of the wye or delta.

One of the pairs of windings is directly connected together at their terminals, whereby the 6 windings have four terminals remaining which are the motor terminals. From another aspect, the invention provides a three-phase motor with three motor terminass, a three-wire single-phase power supply with one intermediate voltage terminal and two maximum voltage terminals. The first of the motor terminals is connected to the first of the maximum voltage supply terminals.

The second of the motor terminals is connected to the intermediate voltage supply terminal. And a capacitor is connected between the third of the machine terminals and the second of the maximum voltage supply terminals. A third aspect of th invention is a power supply for a three-phase induction motor comprising a two-terminal single-phase power supply and transformer means to generate a third voltage.

Two of the three terminals of the three-phase machine are connected to the two terminals of the power supply respectively. Electronically-adjustable capacitance means are connected between the third voltage and the third terminal of the motor to provide a capacitance current flowing into the third terminal. First voltage measurement means aee responsive to the voltage of the power supply and deliver a reference signal.

Second voltage measurement means are responsive to the voltage of the third terminal and deliver a measurement signal. Subtraction means are responsive to the measurement signal and the reference signal to provide an error signal, the adjustable capacitance means being responsive to the error signal. 1 is a circuit schematic of a three-phase induction motor embodying the present invention.

1A is a diagrammatic representation of the physical winding arrangement of hhe motor of FIG. 2 and 3 are vector diagrams of currents and voltages present in the motor of FIG. 4 is an alternate embodiment of a three-phase induction motor embodying the present invention.

5 is a vector (phasor) diagram of voltages and currents present in the motor of FIG. 6 is a circuit schematic of another embodiment of the invention which is a modification of the general embodiment of FIG. 7 is a circuit schematic of another embodiment of the invention which is a variation of FIG.