Most of the fractional horsepower motors found in household appliances, ranging from about 1/8 to 1/2 H.P., are of the “split-phase” type. A variation is the “capacitor-start” type generally found in refrigerators and ironers, but the same physical construction is used as in the split-phase variety.
Inside the case of the motor are two interwoven “stator” coils of enameled wire of different diameters. The thinner wire is on the starting winding; the heavier on the running winding. When the motor is at rest, the windings are connected in parallel.
One end of one winding is connected permanently to one end of the other winding, and this junction represents one terminal of the motor. The other two ends are connected by a simple single-pole spring switch mounted in one of the end bells; this juncture is the other terminal. The rotor has no outside connections of any kind.
On the end of the shaft nearest the stator switch, however, is a pair of spring-loaded weights. These are collapsed when the motor is at rest. When the line switch is turned on, alternating current flows through both windings causing twisting magnetic fields to be set up around them and through the rotor which is in close proximity.
The rotor is dragged around by the magnetic fields and builds up speed rapidly, provided it is not overloaded by the mechanism it is supposed to spin. In a second or two the centrifugal force of the rotation pushes the weights outward against the fixed switch causing it to open and to remove the starting winding from the power line. The motor now operates only with the single running winding.