Unit 17 Two – Wire Controls Objectives
Unit 17 Two-Wire Controls Objectives
Unit 17 Two-Wire Controls Objectives After studying this unit , the student should be able to • List the advantages of two-wire controls . • Connect two-wire devices to motor starters . • Read and draw simple diagrams for two-wire controls . A two-wire control may be a toggle switch , pressure switch , float switch , limit switch , thermostat , or any other type of switch having definite on and off positions . As indicated in Unit 16 , devices of this type are generally designed to handle small currents . Two-wire control devices will not carry sufficient current to operate large motors . In addition , 230-volt motors and three-phase motors require more contacts than the one contact usually provided on two-wire devices . Two-wire controls may be connected to operating coils of magnetic switches , as shown in Figure 17–1 . When the switch is closed , the control circuit is completed through the coil (M) . When the coil is energized , it closes the contacts at M and runs the motor . When the switch is opened , the coil is de-energized and the contacts open to stop the motor . In the case of an overload , the thermal heaters open the overload contacts in the control circuit and de-energize the coil , thus stopping the motor . Two-wire control provides no-voltage (or low-voltage) release . When the starter is wired , as shown in Figure 17–1 , it will operate automatically in response to the control device . A human operator is not required . The control maintaining contact 2-3 (shown in the wiring diagram) is furnished with the starter . However , this contact is not used in two-wire control . For simplicity , this contact is omitted from the two-wire elementary diagram . The motor starter in Figure 17–1(A) is a line voltage , or across-the-line , starter (described in Unit 3) . The circuit shown in Figure 17–1 employs the use of line voltage controls . This simply means that the control components must be rated to operate on the voltage of the line supplying power to the motor . If the power line is 480 volts , the contactor coil must be rated at 480 volts and the contacts of the two-wire control device and the overload relay must be capable of interrupting this voltage . Two-wire controls often use a control transformer to reduce the control voltage to a lower value , Figure 17–2 . Typical control voltages are 120 and 24 volts . Control systems that operate on 24 volts are often used in hazardous areas . The NEC permits the use of intrinsically safe systems in areas that contain hazardous vapors . Intrinsically safe systems cannot provide enough energy to ignite the surrounding atmosphere . These systems are much more cost effective than installing explosion proof fixtures .