What Are NO and NC Contacts in Thermal Relays?
In the electronic components industry, thermal overload relays are commonly used as overload protection devices in motor control circuits. They are primarily designed to prevent electric motors from overheating and being damaged due to prolonged overloading. Thermal overload relays typically feature normally open and normally closed contacts, which differ in their states under normal operating conditions and serve different functions in control circuits. Understanding how these contacts work and how they are used helps engineers design electrical control circuits correctly and improve equipment reliability.
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I. What Are Normally Open and Normally Closed Contacts in a Thermal Overload Relay?
II. Applications of Normally Open Contacts in Thermal Overload Relays
III. Applications of Normally Closed Contacts in Thermal Overload Relays
I. What Are Normally Open and Normally Closed Contacts in a Thermal Overload Relay?
A thermal overload relay detects changes in motor current and uses the thermal effect of electric current to determine whether an overload condition has occurred. When the current remains above the set range and reaches the operating threshold, the relay changes the states of its auxiliary contacts to send signals to the control circuit.
A normally open (NO) contact remains open when the thermal overload relay is in its normal, untripped state. When the relay trips due to an overload, the normally open contact typically closes, allowing it to trigger a fault alarm or send a corresponding control signal.
A normally closed (NC) contact remains closed when the thermal overload relay is in its normal, untripped state. When the relay trips due to an overload, the normally closed contact opens, interrupting the contactor coil control circuit and causing the motor to stop.
It is important to note that the terms normally open and normally closed refer to the contact states when the relay is in its normal, untripped condition, not to whether the motor is running or stopped. The specific contact terminals and operating characteristics should be verified in the manufacturer's product documentation.
II. Applications of Normally Open Contacts in Thermal Overload Relays
Normally open contacts in thermal overload relays are primarily used for fault alarms, status feedback, and auxiliary control. When the relay detects an overload condition that meets its tripping criteria, the normally open contact closes, allowing external circuits to identify the fault.
In a motor overload alarm circuit, the normally open contact can be connected to an indicator light, an alarm device, or a PLC digital input. When the relay trips, the contact closes and sends a signal to alert maintenance personnel, prompting them to inspect the motor load, power supply, and mechanical transmission system. In automated equipment controlled by PLCs, normally open contacts can also be used to record fault conditions, making troubleshooting and maintenance more efficient.
Normally open contacts can also support auxiliary interlocking and fault status feedback, depending on the control logic. However, they generally do not directly interrupt the motor's main power circuit to provide overload protection. Instead, they transmit signals or work with other control components to initiate the required actions.
III. Applications of Normally Closed Contacts in Thermal Overload Relays
Normally closed contacts are primarily used for motor overload protection and are among the most common components in contactor control circuits. In a typical motor start-stop circuit, the normally closed contact of the thermal overload relay is connected in series with the contactor coil control circuit. When the motor experiences an overload that reaches the relay's tripping threshold, the normally closed contact opens, de-energizing the contactor coil. The contactor's main contacts then open, disconnecting the motor from the power supply.
This arrangement allows the motor to stop when the thermal overload relay trips, reducing the risk of overheating caused by prolonged excessive current. Normally closed contacts can also be used for fault interlocking in automated equipment. When the relay trips, the control circuit is interrupted, allowing the equipment to stop the relevant operations and helping prevent the fault from escalating.
It is important to note that thermal overload relays are primarily designed for overload protection and cannot replace short-circuit protection devices. Standard thermal overload relays also cannot independently provide reliable protection against electrical leakage, overvoltage, or phase reversal. Appropriate protective devices must be selected according to the specific requirements of the application.
IV. Conclusion
The main differences between normally open and normally closed contacts in thermal overload relays lie in their contact states under normal conditions and their functions in control circuits. Normally open contacts are typically used for overload alarms, fault feedback, and auxiliary control, while normally closed contacts are primarily connected in series with the contactor coil control circuit to interrupt the control circuit and stop the motor when the relay trips.
In practical electrical system design, engineers should select the appropriate contacts based on the control logic, protection requirements, and relay specifications, while ensuring that the overload current setting is correct. Proper use of normally open and normally closed contacts helps improve the safety and operational reliability of motor control systems.
