HAND-OFF-AUTOMATIC CONTROLS
Jun 05, 2015 Contact position on a selector switch can easily be illustrated using truth tables. Learn how this truth table illustrates the manual or automatic contact positions of two contact selector switch. Salzer is a well known name in the field of manufacturing electrical switches, cam rotary switches and modular switches in India. We exports electrical switches more than 50 countries with international affiliations. Electrical Symbol Category: Selector Switches. Free electrical, electronic, pneumatic and hydraulic symbols library with DXF, DWG and Visio formats, ordered by categories. Manual Switch. Designing circuits should be as easy as using Microsoft Office. Selector Switch Long Left Position 3D. Selector Switch Long Mid Position 3D.
Recognize hand-off-automatic switches on a schematic diagram.
Hand-off-automatic controls are used to permit an operator to select between automatic or manual operation of a motor. The circuit shown in Figure 27–1 permits a motor to be operated by a float switch or to be run manually. The switch is shown as a single-pole double- throw switch with a center Off position.
Another symbol for a hand-off-automatic switch is shown in Figure 27–2. This switch is shown to contain two separate sets of contacts. One set is labeled 1-2 and the other is labeled 3-4. The switch chart indicates that when the switch is in the Off position, there is no connection between any of the contacts. When set in the Hand position, connection is made between terminals 1 and 2. When set in the Automatic position, connection is made between terminals 3 and 4. This circuit is the control for a large fan that pulls air through a building. The motor in this example is water cooled: it requires a flow of cooling water when running. Starter M1 controls the fan motor, and starter M2 controls a pump used to circulate water through the motor. Flow switch FS1 detects the flow of water to insure that the fan motor cannot run if there is no flow of cooling water. A warning lamp indicates that there is no flow of water when the circuit is energized.
When the hand-off-automatic switch is set in the Hand position, connection is made between terminals 1 and 2. This permits the motor to be controlled by a start-stop push-button station. In this mode, the fan will run continuously until the Stop button is pressed. When the HOA switch is placed in the Auto position, a thermostat controls the action of the fan. A combination push-button station with a hand-off-automatic switch and pilot lamp is shown in Figure 27–3.
The flow switch shown in Figure 27–2 employs two separate contacts, one normally open and the other normally closed. The dashed line indicates that the two switches are mechanically connected. When one switch changes position the other switch changes position also. If a flow switch with two separate switches cannot be obtained, it is possible to use a single-pole
connected together. This forms a common point for both switches and could be used as the common terminal for a flow switch that contains a single-pole double-throw switch, Figure 27–4. Although the circuit shown in Figure 27–4 looks different than the circuit in Figure 27–2, electrically they are the same and will operate in the same manner.
Review Questions
1. Refer to the circuit shown in Figure 27–2. Would it be possible to replace the hand-off-auto switch in this circuit with a hand-off-auto switch that contained a common terminal and center Off position like the HOA switch shown in Figure 27–1? Explain your answer.
2. Refer to the circuit shown in Figure 27–2. Would the temperature have to increase or decrease to permit the fan to turn on?
3. Refer to the circuit shown in Figure 27–2. Which starter controls the holding contacts connected in parallel with the start push button?
Industrial Selector Switch
4. Refer to the circuit shown in Figure 27–4. Assume that the HOA switch is in the Hand position and that the motor is running. Now assume that OL1 opens its contacts. Would this cause the pump motor to stop operating? Explain your answer.
