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Xiao C has been repairing motors for many years. One time, he received a customer who came to him because of his reputation. Of course, it was because the motors Xiao C repaired were good. He said that the motor burned out after a short period of use. He found someone to repair it locally, but it burned out again not long after. Later, he heard from his friends that the motors repaired by Xiao C would not break, so he came. After opening the motor and checking it, the winding showed that it was overloaded and burned. The customer was unhappy when he heard it: the voltage in our place is only about 340V, and a 380V motor would not burn; Xiao C patiently explained and made a more appropriate analogy: People carrying sacks on an empty stomach will definitely be exhausted to death! We can repair the motor, but if the voltage is not adjusted, it will soon break down. After the motor was repaired, the customer solved the voltage problem and praised the motor repaired by Xiao C.About rated voltage and frequency
Any kind of electrical appliance will have a rated voltage requirement. If it operates within the normal fluctuation range during actual operation, the appliance will work normally, otherwise it will cause failures. The power supply frequency adopted in my country is 50Hz, and the frequency deviation range is generally stipulated as ±0.5Hz. For example, if the voltage of a thermal radiation light source is too high, its service life will be greatly shortened; if the voltage is too low, the illumination of the light source will be significantly dimmed, seriously affecting work efficiency and human vision health. It can be seen that grid voltage fluctuations will affect the normal operation and service life of electrical equipment. Therefore, voltage, frequency and continuous and reliable power supply are basic indicators to characterize power quality.
All electrical equipment in the power system has a rated operating voltage and frequency. When electrical equipment works at its rated voltage and frequency, its comprehensive economic effect is the best. For example, if the voltage of an induction motor is too high, although the torque increases, the current also increases, and the temperature increases, which will seriously damage the insulation of the motor and shorten its service life.If the voltage is too low, the torque will decrease according to the square of the voltage. When the load torque requirement is constant, the winding current will inevitably increase, and the insulation of the motor will be damaged, shortening its service life. If the power supply frequency is too high or too low, it will also seriously affect the torque and service life of the motor.
Consequences of voltage not meeting requirementsThe change of power supply voltage being too high or too low will have adverse effects on the motor. When the voltage is too high (more than 10%), the magnetic circuit of the core is saturated (because when designing and manufacturing the motor, the magnetic circuit at rated voltage is generally near the saturation point). Therefore, the increase of main magnetic flux causes the excitation current to increase sharply (several times the value of the applied voltage increase), thereby increasing the stator current and overheating the motor, so that the temperature rise exceeds the allowable value and burns out.
When the voltage is too low (less than 5%), the motor torque decreases in the square of the voltage. First, the starting torque decreases a lot, making it difficult to start the motor (slow to start or even unable to start); second, if the load of the running motor remains unchanged (heavy load or rated load operation), the rotor must maintain the necessary electromagnetic torque to balance the load resistance torque. This forces the rotor current to increase, which in turn causes the stator current to increase, causing the motor to overheat, increase temperature rise, shorten the motor life, or even burn out the motor.There are many cases where the motor starts and runs at low voltage. At this time, the load should be reduced as much as possible or used intermittently. Generally, the voltage of the motor is allowed to fluctuate within the range of +10%~-5%. For example, if the power supply voltage is 380 volts, the motor is allowed to run for a long time between 418-361 volts.
XINDA