Distinguish between the normally open and normally closed contacts of a contactor

 Methods for Distinguishing between Normally Open and Normally Closed Contacts of a Contactor

 I. Basic Concepts

1. **Normally Open Contact (Normally Open, NO)**

   - **In the unpowered state**: The contact is in an open state (open circuit).

   - **After being powered on**: The electromagnetic coil is attracted, and the contact closes (closed circuit).

   - **Symbol**: It is usually represented by "NO" or "⏹️".

2. **Normally Closed Contact (Normally Closed, NC)**

   - **In the unpowered state**: The contact is in a closed state (closed circuit).

   - **After being powered on**: The electromagnetic coil is attracted, and the contact opens (open circuit).

   - **Symbol**: It is usually represented by "NC" or "⏺️".

 II. Distinguishing Methods

1. **Identification and Recognition**

   - **Shell Marking**: "NO" or "NC" is marked beside the contactor body or the terminal block.

   - **Contact Structure**:

     - Normally open contact: Generally located on the top or side of the contactor, with a relatively small number (such as 1-2 pairs).

     - Normally closed contact: Usually close to the coil, and the number may be relatively large (such as 2-4 pairs).

2. **Measurement with a Multimeter**

   - **In the powered-off state**:

     - Normally open contact: The resistance is infinite (the multimeter shows "OL").

     - Normally closed contact: The resistance is close to 0Ω (conducting state).

   - **Power-on Test**:

     - After applying the rated voltage, the resistance of the normally open contact becomes 0Ω, and the resistance of the normally closed contact becomes infinite.

3. **Observation of Actions**

   - **Manually pressing the armature**: When simulating the attraction of the coil, the normally open contact closes, and the normally closed contact opens.

   - **In the natural state**: After releasing the armature, the contact returns to its initial state.

 III. Application Scenarios

1. **Normally Open Contacts**

   - **Starting Control**: Used in self-locking circuits (such as the motor start button).

   - **Signal Transmission**: Used as a switch signal output in the control logic.

2. **Normally Closed Contacts**

   - **Interlock Protection**: To prevent multiple contactors in the circuit from operating simultaneously (such as forward and reverse rotation control).

   - **Fault Alarm**: To open the circuit when the equipment is abnormal (such as overload protection).

 IV. Precautions

1. **Safe Operation**: Be sure to cut off the power before operation to avoid electric shock or misoperation.

2. **Contact Maintenance**: If the contacts are oxidized or ablated, they need to be polished with fine sandpaper or the contact assembly needs to be replaced.

3. **Model Selection and Matching**: Select the contact type according to the circuit requirements (such as the number of auxiliary contacts and current capacity).

If you need further assistance, you can provide the specific model of the contactor or the circuit schematic diagram, and I will provide you with a targeted analysis.