Automatic controls may be used to control a motor or an operation without requiring the Presence of an operator. For example, if the power fails, the motor will restart if the contacts of the two-wire control device are closed when the power is restored.

(A) Basic two-wire control circuit—elementary diagram. (B) Basic two-wire control circuit—wiring diagram.

Two wires are required for the device. One wire carries the control signal to the device, while the second wire provides the return path, allowing the control circuit to operate properly.

Intrinsically safe systems cannot supply enough energy to ignite the atmosphere in a hazardous location. The NEC  permits  the  use  of  intrinsically  safe  systems  in  areas  that  contain  hazardous  vapors. These  systems  are  much  more  cost  effective  than  installing  explosion  proof  fixtures.

24 volts is the most common control voltage used in intrinsically safe systems. This low voltage helps minimize the risk of sparks or excessive heat, making it suitable for hazardous locations where flammable gases or vapors may be present.

A selector switch and the automatic pilot device can control a motor directly if their contacts are large enough to pass safely the full-load current of the motor. However, the contact capacity of these devices is usually not large enough for large motors.

Selector switch with multiple contacts.

The direction of rotation of three-phase motors can be reversed by interchanging any two-motor leads to the line. If magnetic control devices are to be used, then reversing starters accomplish the reversal of the motor direction. Reversing starters wired to NEMA standards  interchange  lines L1 and L3.

Reversing rotation of an induction motor.

The purpose of interlocking is to prevent contactors from being energized simultaneously or closing together; in this way short circuits are avoided. There  are  three  basic  methods of interlocking.

• Mechanical Interlock
• Push-Button Interlock
• Auxiliary Contact Interlock

Interlocking is obtained by normally closed auxiliary contacts on the forward and reverse contactors of the starter. When  the  motor  is  running  forward,  a  normally closed contact (F) on the forward con-tactor opens and prevents the reverse contactor from being energized by mistake and by closing. The same operation occurs if the motor is running in reverse.

The “forward” indicating light should be connected in parallel with the “forward” coil (across the coil) to receive full voltage when the coil is energized. The “reverse” light should be connected in a similar manner to the reverse coil.

Sequence  control  is  the  method  by  which  starters  are  connected  so  that  one  cannot  be started until the other is energized. This type of  control  is  required  whenever  the  auxiliary  equipment  associated  with  a  machine,  such  as  high-pressure lubricating and hydraulic pumps, must  be  operating  before  the  machine  itself  can  be  operated  safely.  Another  application  of  sequence control is in main or subassembly line conveyors.

A symbol (NEMA) for a solid-state logic element “AND”

A stop button of a control circuit can be padlocked in the off position; thus, it is difficult for anyone to start a machine. This procedure is similar to pulling a safety disconnect switch and locking it in the off position. Lock stops open only the control circuits and provide safety to workers who may be injured while working on a machine if it is started suddenly.

Lock-stop push button in a jogging circuit.

A time-delay, low-voltage release relay assembly protects equipment and personnel during power interruptions.

• It prevents the motor from automatically restarting after a prolonged power failure when voltage is restored, reducing the risk of damage or unsafe conditions.
• For brief voltage dips, the relay allows the motor to reconnect to the power supply after a preset time delay, enabling normal operation without unnecessary shutdowns.