Hand-Off-Automatic Controls

GOALS:

• Describe the operation of a hand-off-automatic control switch.
• Connect a hand-off-automatic control circuit.
• 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 Ill. 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 Ill. 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's 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's energized.

Ill. 1 Hand-off-automatic switch provides manual control of a motor or control can be provided by a float switch.

Ill. 3 Push-button control station with hand-off-auto witch and pilot light.

Ill. 2 A water-cooled motor must have a flow of water before it's permitted to run.

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 Ill. 3.

The flow switch shown in Ill. 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's possible to use a single-pole double-throw switch with a common terminal, a normally open terminal, and a normally closed terminal in this circuit. Notice that one terminal of both the normally open switch and normally closed switch are 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, Ill. 4. Although the circuit shown in Ill. 4 looks different than the circuit in Ill. 2, electrically they are the same and will operate in the same manner.

Ill. 4 A flow with two separate switches has been replaced with a single-pole double-throw switch.

QUIZ:

1. Refer to the circuit shown in Ill. 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 Ill. 1? Explain your answer.

2. Refer to the circuit shown in Ill. 2. Would the temperature have to increase or decrease to permit the fan to turn on?

3. Refer to the circuit shown in Ill. 2. Which starter controls the holding contacts connected in parallel with the start push button?

4. Refer to the circuit shown in Ill. 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.

5. Refer to the circuit shown in Ill. 2. Assume that the HOA switch is in the Hand position and that the motor is running. Now assume that OL2 opens its contacts. Would this stop the operation of the fan motor? Explain your answer.

6. Refer to the circuit shown in Ill. 4. Assume that the HOA switch is in the Auto position. Also assume the fan is running. Now assume that the fan stops running and the red warning light turns on.

Which of the following could cause this condition?

a. OL1 contacts are open

b. Temp 1 switch is open

c. OL2 contacts are open

d. The HOA switch has been moved to the Off position

7. Refer to the circuit shown in Ill. 2. Assume that the HOA switch is set in the Hand position.

When the Start button is pressed, the red warning lamp lights. When the Start button is released, the light turns off but the fan motor does not start.

Each time the Start button is pressed, the warning lamp lights for as long as the Start button is held down, but it goes out each time the Start button is released, and the fan does not start. Which of the following could cause this condition?

a. Temp 1 switch is open

b. M2 starter coil is open

c. M1 starter coil is open

d. The Stop push button is open