Application of fuses in electric motors

Fuses play a crucial role in motor protection systems, providing short-circuit protection and auxiliary overload protection. They are particularly suitable for small and medium-power motors. Their application must be considered in conjunction with the motor's operating characteristics (such as starting current, rated current, and load type) and protection requirements. Fuses should be used in conjunction with other protection components (such as thermal relays and circuit breakers) to form a reliable, comprehensive protection solution.

I. Core Protection Function and Compatibility

The main electrical faults encountered during motor operation include short circuits, overloads, and phase failures. Fuses focus on short-circuit protection and provide limited auxiliary protection against overloads.

II. Selection Principles for Motor Protection Fuses

Selection is crucial to ensuring fuses perform their protective function. Focus on the following 5 core parameters to avoid "false tripping" (blowing during startup) or "failure to trip" (not blowing during faults).

1. Rated Voltage

It must match the voltage level of the circuit where the motor is located (e.g., 220V and 380V are commonly used for low-voltage motors). This ensures the fuse can reliably extinguish the arc after blowing, preventing insulation breakdown.

2. Rated Current (Fuse Element Current)

This is the core of selection and needs to be calculated based on the motor's rated current and starting characteristics:

For direct-start motors: The starting current is large and the duration is short (usually 1-3 seconds). The rated current of the fuse element should be 1.5-2.5 times the motor's rated current (e.g., a 10kW motor has a rated current of approximately 20A, so the fuse element current can be 30-50A).

For reduced-voltage start motors (e.g., star-delta starting): The starting current is smaller (about 1/3 of direct starting). The rated current of the fuse element can be 1.2-1.5 times the motor's rated current.

For frequently starting motors: The fuse element current should be appropriately increased (2.0-3.0 times the motor's rated current) to avoid fatigue blowing caused by frequent starting.

3. Breaking Capacity

It must be greater than the maximum short-circuit current that may occur in the motor circuit (obtainable through system short-circuit current calculation or equipment manual lookup). If the breaking capacity is insufficient, the fuse may fail to extinguish the arc when blowing, causing explosions or equipment damage. Fuses commonly used for low-voltage motor protection typically have a breaking capacity of 50kA-100kA.

4. Ampere-Second Characteristic

A fuse with an "inverse time characteristic" matching the motor protection requirements should be selected:

During short circuits (high current): Blowing time < 0.1 seconds to quickly cut off the fault;

During startup (medium overcurrent): Blowing time > startup duration to avoid false tripping;

During mild overloads (low current): Blowing time is longer, serving as a supplement to thermal relays.

5. Type Compatibility

Select the appropriate fuse type based on motor power and application scenarios:

Low-voltage small motors (≤5kW): Porcelain plug fuses (RC series) or screw-type fuses (RL series) can be used, featuring simple structure and low cost.

Medium-voltage motors or important motors: Fast current-limiting fuses (RS series) are required, which can quickly limit the peak short-circuit current during short circuits and reduce impact on the motor.

Control circuit protection: Motor control circuits (e.g., contactor coils, relay circuits) have small currents, so miniature glass tube fuses (5×20mm) or chip fuses can be used.

III. Typical Application Scenarios and Coordination Methods

Fuses are rarely used alone for motor protection and are usually coordinated with other components to form an integrated protection system covering "short circuit + overload + phase failure". Common coordination methods are as follows:

1. Fuse + Thermal Relay (Classic Low-Voltage Motor Protection Scheme)

This is the most commonly used scheme for small and medium-power low-voltage motors (≤30kW), with complementary advantages:

Fuse: Responsible for short-circuit protection (quickly cutting off high-current faults);

Thermal relay: Responsible for overload and phase-failure protection (acting through the thermal bending of bimetallic strips, with high delay accuracy, capable of handling mild overloads and phase failures).

Connection method: The fuse is connected in series in the main circuit (near the power supply side), the thermal relay is connected in series in the motor stator circuit, and the contactor is responsible for on-off control.

2. Fuse + Motor Protector (Intelligent Protection Scheme)

For medium and high-power motors or important occasions (e.g., motors in chemical and metallurgical industries), the "fuse + intelligent motor protector" scheme can be adopted:

Fuse: Serves as the "last line of defense" for short-circuit protection;

Intelligent motor protector: Monitors motor current in real-time through current sensors, enabling precise protection against multiple faults such as overload, phase failure, locked rotor, overvoltage, and undervoltage. After actuation, it cuts off the contactor coil power supply through a relay.

3. Simple Protection for Small Motors (Fuse Alone or with Circuit Breaker)

For micro-motors (≤2.2kW, such as fan and water pump motors), if the protection accuracy requirement is not high, a fuse can be used alone (focusing on short-circuit protection) or coordinated with a miniature circuit breaker (MCB):The fuse is responsible for short-circuit protection, and the miniature circuit breaker is responsible for overload protection (with a built-in thermal release). This scheme features simple structure and low cost, and is commonly used in household or small equipment.

Summary

Fuses are the "core component" for motor short-circuit protection but are not "universal protectors". In practical applications, precise selection based on the motor's power, starting method, and load characteristics is required, and coordination with components such as thermal relays and intelligent protectors is necessary to achieve comprehensive protection effects of "quick short-circuit cutoff, precise overload delay, and reliable phase-failure protection", thereby extending the motor's service life and ensuring the safe operation of the power system.