Fractional HP Motors for AC Supply

Posted By Tom Feltham on 10 May 2014

Posted in The Vintage Machinery Almanac

This article first appeared in Practical Engineering 1940 Vol1 No22. The piece relates to historical information as of 1940. This article describes new developments within Industry and Commerce at the time.

The increasing use of small electric motors for driving industrial, commercial, and scientific machines of all types has resulted in many developments in the design both of standard motors and special motors to suit particular applications.

New Range of Fractional HP Motors

An entirely new range of G.E.C. fractional-h.p. motors for single-phase and three-phase A.C. supplies is of interest. The range covers outputs from 1/8 h.p. to 7/8 h.p. Single-phase motors are available with "split phase start" or "capacitor - start" characteristics, while the already well-known range of "repulsion-start" motors is included.

Three-phase motors are of the squirrel-cage induction type. Repulsion-start induction motors are designed for applications where high-starting torque combined with minimum starting current and a high accelerating capacity are required. There are no separate condensers or switches.

Two Main Types

In the brush-lifting type, the motor starts as a high-torque repulsion machine with brushes in contact with a radial commutator. As it approaches full speed the brushes are automatically withdrawn from the commutator, which is at the same time short-circuited. This type is most suitable where silent running is important, owing to the absence of brush noise; for example, for certain domestic appliances.

In the case of the brush-trailing type the motor starts as a high-torque repulsion machine with brushes in contact with an axial commutator. As it approaches full speed the commutator is short circuited, the brushes remaining in contact. It has a higher starting torque and is more suitable for industrial and commercial applications, particularly for such loads as compressors (where the torque required at start and running-up fluctuates considerably) and for loads with a heavy flywheel effect.

Four leads are brought out to provide series-parallel connections, enabling two separate voltage ranges to be covered by one winding.

Refrigerator Motors

The refrigerator motors, 1/6h.p. and 184h.p., resiliently-mounted capacitor-start induction types, have been specially developed for the purpose and have the advantage of being silent. This is due to the absence of magnetic and mechanical noise, achieved by liberal rating, special bearings, resilient mounting and other features. There is overload protection by means of a thermal cut-out which automatically stops the motor should it be overloaded to a dangerous extent.