It has been said that Washington, D.C., is the home of the largest invertebrate population in the U.S. This, of course, jokingly refers to the population of politicians and their lack of backbone or guts required to make difficult decisions. The same analogy could be applied to the single-phase motor, as it has only one-third the “guts” of its three-phase cousin. However, it can still perform well as long as expectations are reasonable.
Single-Phase Magnetic Field
Figure 1 (below) illustrates one cycle (Hertz) of a single-phase, AC sine wave. Unlike the three-phase curve discussed in Part Two of the series (Pumps & Systems, April 2011), this one peaks only twice per cycle, and the peaks occur 180 degrees apart. It was those peaks that occurred just 60 degrees apart that provided a true rotating magnetic field in the three-phase stator. Can the single-phase wave form produce a rotating field in a single-phase stator, and if so, what does it look like?
Figure 1: One cycle of a single-phase AC sine wave
Figure 2 (below) shows a single-phase, two-pole stator with the poles oriented in a north/south position. It looks similar to the three-phase version except that it is missing four poles. It is not as easy to visualize the field that is created between the two poles without animation, but let's give it a try.
Figure 2: A single-phase, two-pole stator with the poles oriented in a north/south position
At 90 degrees, the wave reaches its positive peak, and the field strength in the upper pole is at its maximum. At 270 degrees, the wave reaches its negative peak, and the magnetic field is transferred to the bottom pole. During this downward migration, the magnetic field travels toward the lower pole on both sides of the stator.
When the wave begins to rise again, the magnetic field begins to reverse direction and travels toward the upper pole, and again, it occurs on both sides of the stator. The result is a pulsating field that occurs twice each cycle. It is not a rotational field, and if single-phase power were applied to this stator, the rotor would not turn. If, however, you give it a twist with your hand (in either direction), it will begin to rotate and will remain in rotation as long as power is applied.
Twisting the shaft causes induction to occur, and the interaction between the induced fields and the stator fields plus the inertia of the rotor maintains the rotational motion. Unlike the three-phase motor, all single-phase motors require an auxiliary starting component to begin rotation.
Although a number of single-phase starting techniques are available, most use a second set of poles to create a “synthetic” second phase (split phase) that works in conjunction with the primary phase to produce a rotating magnetic field among the four poles.
A good comparison is the two-phase motor. Figure 3 (below) shows the pole positions of a two-phase, two-pole motor. A two-phase generator produces phases that are separated by 90 degrees, and the positive and negative peaks will create a rotating field in a two-phase stator. Single-phase motors mimic the two-phase motor by using the Phase Two poles as a start winding that can create a rotating field similar to that of a two-phase motor.
Figure 3: The pole positions of a two-phase, two-pole motor
Figure 4 (below) is a schematic of a split-phase, resistance-start, induction-run motor. In this arrangement, the auxiliary or start winding is wound with a smaller wire size than that of the main winding. This increases the resistance (and lowers inductance) in the winding and results in a smaller delay in current flow than does the main winding. An external resistance can also be used in conjunction with a larger wire size to obtain the same result.
Figure 4: A schematic of a split-phase, resistance-start, induction-run motor
The resulting phase shift between the two windings is about 30 degrees and is sufficient to produce a low-torque, rotating field. It is not as good as the 90-degree shift produced by the two-phase motor, but it works. A centrifugal or thermal switch removes the start winding from the circuit once the motor reaches a certain speed. Resistance-start motors are available up to ½ horsepower and are popular due to their low cost. Their disadvantages include low starting torque and high inrush current, which produces additional heat during starting.
The split-phase design shown in Figure 5 is called a capacitor-start, induction-run motor. In this design, the start winding is wound with a normal wire size and connected in series with a capacitor. AC power current leads voltage in a capacitive circuit, but in an inductive circuit, main winding current trails voltage. This results in a phase shift of nearly 90 degrees and gives rise to a rotating field that is almost identical to the one produced by a two-phase motor.