Understanding Rated Voltage and Frequency: Key to Optimizing Electric Motor Performance and Lifespan

About Rated Voltage and Frequency

Every electrical device has specific requirements for rated voltage. During actual operation, as long as the device operates within the normal fluctuation range, it will function properly. However, deviations outside this range may lead to malfunctions. In China, the standard power supply frequency is 50Hz, with an allowable deviation range of ±0.5Hz. For instance, in the case of thermal radiation light sources, excessive voltage can significantly reduce their lifespan, while insufficient voltage will noticeably dim the light, negatively impacting work efficiency and eye health. Therefore, fluctuations in grid voltage affect the normal operation and service life of electrical equipment. Voltage, frequency, and reliable continuous power supply are fundamental indicators of power quality.

All electrical equipment in the power system is designed to operate at specific rated voltage and frequency. Equipment achieves optimal economic performance when working within its rated parameters. For example, with an induction motor, if the voltage is too high, although the torque increases, the current also rises, resulting in higher temperatures that severely damage the motor's insulation and shorten its lifespan. Conversely, if the voltage is too low, the torque decreases proportionally to the square of the voltage. In cases where the load torque is constant, the winding current will increase, leading to insulation damage and reduced motor lifespan. Similarly, deviations in frequency, either too high or too low, can severely impact the motor's torque and lifespan.

Consequences of Voltage Deviation

Changes in supply voltage, whether too high or too low, adversely affect motor performance. If the voltage exceeds 10% of the rated value, the magnetic core's circuit becomes saturated. When designing motors, the magnetic circuit is typically near saturation at the rated voltage. Therefore, an increase in the main magnetic flux due to excessive voltage causes a sharp rise in the magnetizing current (several times the voltage increment), leading to higher stator current, overheating, and ultimately the motor burning out as the temperature rise exceeds the allowable limit.

When the voltage drops below 5% of the rated value, the motor torque decreases proportionally to the square of the voltage. This leads to two problems:

The starting torque reduces significantly, making it difficult for the motor to start (slow start or failure to start).

For a running motor with unchanged load (heavy or rated load), the rotor must maintain the necessary electromagnetic torque to balance the load's resistance torque. This forces the rotor current to increase, which in turn raises the stator current, causing the motor to overheat, increasing temperature rise, and shortening its lifespan. In severe cases, the motor may burn out.

Low-voltage starting and operation are common. In such cases, the load should be reduced or operated intermittently. Generally, motors allow voltage fluctuations within a range of +10% to -5%. For example, if the supply voltage is 380V, the motor can operate continuously within the range of 418V to 361V.