Ensuring Safe and Stable Operation of Industrial Motors

In the realm of industrial electrical systems, three-phase asynchronous motors represent the most prevalent category of power-consuming equipment. Their continuous, reliable performance is paramount to maintaining production efficiency. Thermal overload relays, installed in motor control circuits, are essential for safeguarding against overload conditions. The relay's thermal elements are connected in the main circuit, while its auxiliary contacts interface with the control circuit.

A recent incident at an industrial site revealed the consequences of improper component selection. A thermal relay malfunction led to the unintended shutdown of a motor, halting production. This underscores the importance of choosing the correct components in the motor’s main circuit. Any deviation—be it undersizing or oversensitivity—can result in either nuisance tripping or failure to respond during genuine faults.

Incident Overview: False Tripping of a 5.5kW Motor

In one workshop, a 5.5kW three-phase asynchronous motor experienced repeated unexpected shutdowns. During startup and normal operation, the thermal relay tripped prematurely, disconnecting the motor. Inspection revealed the motor’s rated current to be 10A. Surface temperatures were within safe limits, and no mechanical or electrical faults were found in the motor itself.

The culprit was an incorrectly set thermal overload relay. The model in use was a Chint NR2-25, with its trip current calibrated at 9A—below the motor's rated current. This misconfiguration triggered false trips under normal load conditions, interrupting production and reducing overall efficiency.

Thermal Overload Relay Selection Criteria

Thermal relays must be selected and set based on the motor’s nominal current rating. The proper selection principle is as follows:

• Relay rated current should slightly exceed the motor's rated current.

• Trip current adjustment range should be set between 0.95 to 1.05 times the motor's rated current under standard conditions.

• For motors exposed to high inrush or fluctuating loads, the range can be extended to 1.1 to 1.5 times the rated current.

Given the motor's 10A rating, the thermal relay's trip current should fall within 9.5A to 10.5A. The NR2-25 relay, with its adjustable current range, was appropriate in model but misconfigured. Re-adjusting the trip current to 10A resolved the false tripping issue, restoring operational stability.

Contactor Selection: The Heart of Motor Control

AC contactors serve as critical switching elements in motor control systems, capable of repeatedly connecting and disconnecting circuits carrying high currents. The proper selection depends on the following criteria:

• Rated voltage of main contacts must meet or exceed the control circuit voltage.

• Rated current of main contacts must be at least equal to the motor’s load current.

• For frequent start-stop operations, the rated current should be multiplied by 1.3 to 1.5 to ensure thermal and mechanical resilience.

For the 10A motor in question, a Chint CJX2-1210 contactor was selected. The “12” denotes a 12A current rating—sufficient for the motor’s needs under standard operating conditions. With a rated voltage of 380V, the device is suitable for general applications where starting frequency is moderate or low.

Molded Case Circuit Breaker (MCCB) Selection: Short-Circuit Protection

In addition to acting as a power switch, a molded case circuit breaker (MCCB) offers crucial protection against short circuits. Selection principles include:

• Rated voltage and current must match or exceed the circuit's operating voltage and expected load current.

• Thermal trip current setting should be within 1.05 to 1.1 times the motor's rated current.

• Instantaneous magnetic trip setting must exceed the motor’s peak inrush current during startup.

Given the motor’s 10A rating, the expected starting current (I_st) is approximately 7 times the rated current:
I_st = 7 × 10A = 70A

Therefore, the MCCB’s magnetic trip setting must be greater than or equal to 70A. The selected device was a Chint NM1-63H/33202:

• Rated current: 16A

• Thermal trip current: Set to 16A

• Magnetic trip current: 10× thermal setting = 160A

• Rated voltage: 380V

• Breaking capacity: 50kA

This breaker provides both the necessary overload protection and the robustness to withstand short-circuit currents during faults or startup surges.

Conclusion: The Art of Precision in Motor Circuit Design

This case study demonstrates the critical nature of selecting and configuring motor protection components. Through an analysis of the thermal relay, contactor, and MCCB selection for a 5.5kW motor, it becomes evident that even a minor misjudgment in settings can cascade into downtime and production losses.

By adhering to established selection principles and leveraging reliable product lines—such as Chint’s NR2, CJX2, and NM1 series—engineers can ensure the safe, stable, and efficient operation of industrial motors. In an era of increasing automation and electrification, attention to such details is not optional—it is foundational.