CONTROL CIRCUITS

393. Electric control circuits constitute a broad subject, and the reader is urged to refer to special books devoted to this important phase of motor circuitry. Two such books are Control of Electric Motors, by Paisley B. Harwood, published by John Wiley & Sons, Inc., and Modern Electric Controls, by J. F. McPartland, published by Electrical Construction and Maintenance of McGraw-Hill, Inc. These textbooks provide an excellent insight on how to understand, select, and design control circuits.

394. Arrangement of control wiring in grounded circuits. It is a requirement of the National Electrical Code that if one side of a control circuit is intentionally grounded, the control circuit shall be so arranged that an accidental ground in the control circuit remote from the motor controller will not start the motor nor bypass manually operated shutdown devices or automatic safety shutdown devices. Figure 7.192 shows two methods that satisfy this code rule. The elementary wiring diagram at the top of Fig. 7.192 shows the arrangement of a START-STOP pushbutton connected to the holding coil of a magnetic starter. Note that all control devices are connected to the L1 side of coil C and that the other side is connected to the grounded circuit conductor L2.

FIGURE 7.192: Arrangement of control wiring in grounded circuits. [ Electrical Construction and Maintenance]

The L2 conductor could be the grounded conductor of an end-grounded three-phase delta supply or the grounded...