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LE 06 - Using a contactor to turn on an AC bulb light

 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 contactor. In this experiment, you will rework previous installation but introducing a contactor as if you were to power up an electric motor.

Objective

Learn how does the contactor works and to install it along with the emergency stop button and its signaling pilot light.

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

  • Bulb light. AC

What to do

For this exercise a contactor is to be introduced. The role of a contactor is to power equipments demanding high voltage and that could represent a danger for operators. The contactor is similar to an encapsulated relay: it has a coil too. A contactor also have two type of contacts: auxiliary and power contacts. The auxiliary contancts are to be used for low voltage and the power contacts for hugh voltage.


In this exercise an AC light bulb is to be used as a high voltage equipment.


  • Follow the same steps as for LE 05

  • 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 contactor installation



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 05 is to be used. Details shall be omitted but refer to LE 05 for more information.


Step 1. Identification of contactor

There are several models and here three of the most common brands are presented.

Example contactors from three different manufacturers.Notice that all of these have NO contacts.

These components have two type of contacts: ones called power contacts used for high voltage connections required by electric motors for example, and auxiliary contacts used for low voltage connections such as pilot lights or other type of alarms.

Parts of a Siemens contactor. This component has a 24V DC coil, one NO and one NC contact. The power contacts are indicated as L1, L2, L3 and T1, T2, T3.



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 contactor

In this case the AC bulb light is to be turn on through the contactor. Also the bulb only needs one contact of the three available.

In other words, the AC bulb wont be related to the contacts of the encapsulated relay as previously done. As the contactor coil is energized the AC bulb is also powered so that a dangerous situation of high voltage is created and as a consequence a visual indicator should also be triggered: the red pilot light.

The red pilot light is to be link to the NO contact of the contactor.

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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