Two New Methods of Accelerating Electric Motors Automatically

The first of the two methods discussed in this paper is particularly applicable to a-c slip-ring motors. Seriesresonance circuits responsive to the variation in rotor frequency with motor speed provide a means of accelerating the motor automatically, while a series-parallel resonance circuit similarly connected to the rotor can be used to protect the motor when ``plugged'' or suddenly reversed. The sensitive response of the latter circuit within a narrow frequency range makes possible its use as a ``zero-speed'' switch. The second method has a more general application in that it can be used with any type of motor. A neon tube timing circuit measures the time intervals between the closures of accelerating contactors. One ``timer'' can be used for all steps of acceleration, with or without separate time adjustment. For a-c motor control, the introduction of a current transformer and a rectifying circuit makes possible the obtaining of time intervals which are increased automatically with increase in motor load. Such a scheme is often referred to as ``time-current'' acceleration in order to distinguish it from ``constant-time'' acceleration where the time interval is independent of the load.