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LE 09 - Level control with 1 float switch and a HOA button

 WARNING

Make sure the whole control station is not energized before touching or starting your exercise!


If you have been performing previous experiments this could be the right moment to introduce another component of control cabinets: the float level switch. In this experiment, you will rework a previous installation for a contactor but introducing a float switch to control the high level of a tank.

An I&C diagram for the control of level using one switch.


Objective

Learn how to control the level of a tank by using a float switch to turn off a pump and button of three positions: manual, off and automatic.

Material

  • Multimeter

  • AC/DC power supply. Input 120V AC and output 24V DC, 1A (at least)

  • Push button (green )

  • Push button (red)

  • Pilot light (green). 24V DC

  • Pilot light (red). 24V DC

  • Pilot light (yellow) or buzzer. 24V DC

  • Encapsulated relay 24V DC with contacts for 8A 250V

  • Contactor. 25V DC coil with power contacts for up to 30 A

  • Emergency stop button

  • Centrifugal pump. 110 V AC

  • Float switch

  • HOA button

What to do

For this exercise a float switch is to be introduced. The role of a float switch is to open or close a circuit so that large equipments demanding high voltage that could represent a danger for operators are safely turn on or off. The float switch can be NO or NC and is to be installed in the tank directly so that level of liquid inside it activates the switch.


In this exercise an AC centrifugal pump is to be used as a high voltage equipment.


  • Follow the same steps as for LE 06

  • Connect the emergency stop button on the (+) DC wire (before any of the rungs)

  • Set on the power supply

  • Use the following ladder diagram:

Ladder diagram for level control with 1 high level float switch and a HOA button


In the above diagram ES stands for emergency stop.

  • The whole circuit should turn off when the emergency button is depressed

  • Check for continuity if needed

How to do it

For this exercise the same setup developed for LE 06 is to be used. Details shall be omitted but refer to LE 06 for more information.


Step 1. Identification of the float switch

There are several models and here one of the most common is presented.

An example of a float switch. Notice that it can be NO or NC by just turning it round. This model is to be attached to the tank wall.


The two wires coming out of it correspond to the contact in the inside.


Step 2. Emergency pilot light (yellow) connection

As the emergency stop button is depressed a yellow pilot light should turn on to visually indicate that an emergency has occurred and that an action has to be taken. This is a visual alarm. This pilot light is to be in series with the emergency stop button while the NO contact is in parallel with it.

Yellow pilot light used for emergency alarm

Step 3. Connecting the switch

In this case the AC pump is to be turn on through the contactor and the float switch should open the circuit once the liquid level passes it.

The red pilot light is to be link to the NO contact of the contactor.
Hand-Off-Automatic button


Step 4. Energizing the loop

In order to power the loop you should activate the circuit breaker to the ON position with the lever upl and the green indicator being shown.

Thermal circuit breaker

Step 4. Using the buttons

If your connection was right then as the loop was powered, nothing should happen and only the green pilot light should turn on, indicating that everything is OK. However, when you push the start button the following should occur (in this order):

  • the encapsulated relay coil is energized
  • the encapsulated relay NO contacts close and the NC contacts open
  • the green pilot light turns off
  • the contactor coil is energized
  • the contactor NO contacts close and the NC contacts open
  • the red pilot light is turned on
  • the AC bulb light also turns on

If you had a pump connected instead of the AC bulb light it should start pumping liquid.


Now, supposedly, an emergency is occurring and you need to power off of the whole system (equipment, for example). Then, what you need to do is to press the emergency stop button. The bulb should turn off and the yellow pilot light should turn on (to indicate that an emergency has occurred). Also, the red pílot light turns off.
Only the yellow pilot light should be on.

Once you push the emergency stop button it automatically latches itself so that power to the system remains interrupted so that the causes of the emergency can be attended

If you need to restore power to the system the emergency stop button needs to be unlatched first. Depressed down and swirl it to the right to release the button to its unlatched state. The system could be started again if the start button were depressed and the green pilot litght is turn on again indicating safety.

Step 5. Continuity and DC voltage test

Check for continuity at every contact and for 24V DC using the multimeter. Since you are using a power source for 24V DC you should get a reading not far from this value.



Any question? Write in the comments and I shall try to help.

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