10.5Review of fundamental principles

Shown here is a partial listing of principles applied in the subject matter of this chapter, given for the purpose of expanding the reader’s view of this chapter’s concepts and of their general interrelationships with concepts elsewhere in the book. Your abilities as a problem-solver and as a life-long learner will be greatly enhanced by mastering the applications of these principles to a wide variety of topics, the more varied the better.

•“Normal” switch status: the “normal” status of a switch contact as defined by the manufacturer is its resting condition (minimum stimulus).

•“Seal-in” circuit: when an electrical relay uses one of its own switch contacts to continue its own coil energization after the initial triggering event has passed. Relevant to motor starter circuits.

•Lenz’s Law: any magnetic field arising from electromagnetic induction opposes the inducing field. Relevant to the operation of induction AC motors.

•Rotating magnetic field: this is necessary to cause an AC induction motor to spin in a particular direction, and is generated by polyphase field poles (i.e. multiple magnetic fields that are out-of-phase with each other). All AC induction motors require such a polyphase magnetic field to start up in a particular direction, although a single-phase magnetic field is su cient to maintain rotation once started.

References

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Croft, Terrell and Summers, Wilford I., American Electrician’s Handbook, Eleventh Edition, McGraw-Hill Book Company, New York, NY, 1987.

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Robichaud, S.N., “Plumb Your Plant Air System with Plastic Pipe”, Hydraulics & Pneumatics, June 1, 1999.