Understanding Rated Current in Electric Motors: Importance and Safety

Rated Current: Is it the Maximum Current During Motor Operation?

Current is a critical performance indicator for motors. Depending on different operating conditions, a motor will correspond to different current values, such as no-load current, locked-rotor current, and rated current. The rated current of a motor refers to the current value that corresponds to the motor’s rated voltage and rated load state. It is also the current value at which the motor can operate safely. When abnormalities occur during motor operation, a short-term current surge may not be harmful to the motor. However, if the motor experiences prolonged current overload, it will severely shorten the motor's lifespan or, in the worst case, cause the windings to burn out in a short period.

For most motors, the no-load current is smaller than the rated current. For squirrel-cage rotor motors, the locked-rotor current is 5 to 7 times the rated current. However, the duration of the current exceeding the rated current is strictly limited, as this condition is equivalent to an overload where the rotor is at a standstill, which can be fatal for the motor. Depending on the motor type, the maximum duration the motor can tolerate exceeds rated current, but it is always very brief.

For a constant load condition, the actual current drawn by the motor when running under rated conditions is generally close to the rated current. In contrast, for impulsive loads, the current during operation may exceed the rated current. In certain conditions, such as for compressors, motors are designed with a service factor to handle these fluctuations. The rated current of motors with a service factor is the safe operating current corresponding to the service factor, and it is also considered a safe operating current, different from the current corresponding to harsh operating conditions.

From a safety perspective in motor operation, the rated current is a safe operating parameter that can be guaranteed through standardized quality control during the motor’s design and manufacturing process. However, once the motor’s operating current exceeds the rated current, it will accelerate the occurrence of motor failure. Therefore, any operation above the rated current is unsafe, especially overloads close to the stall condition, which can be fatal for the motor. During the actual production and testing of motors, short-term overcurrent tests, based on certain multiples, are performed to verify the motor's insulation performance. These tests simulate accidental overloads during operation.

Due to various factors such as design and manufacturing, running the motor beyond its rated current should be avoided as much as possible. The maximum current a motor can withstand is a conceivable limit but should not be tested easily.