Motor overheating or burnout caused by adding a frequency converter is often attributed to a problem with the motor itself due to the lack of understanding of the cause. In fact, these seemingly motor malfunctions are not actually caused by the motor itself, but are mostly due to improper frequency converter debugging or using a non-frequency converter motor as a frequency converter motor. The main situations are as follows:
1. Using a regular motor as a variable frequency motor.
Because the cooling fan of a regular motor is connected to the shaft, the speed is unstable when the speed is adjusted by a frequency converter and cannot reach the rated speed of the motor. As a result, the cooling fan cannot function properly, leading to poor heat dissipation of the motor. In addition, regular motors are not designed according to frequency conversion requirements, which can cause the motor to overheat or burn out.
2. The variable frequency motor and the variable frequency drive are directly connected together for use without being debugged.
The two most common methods for controlling motors with frequency converters are vector control and V/F curve control. For each method, the following parameters must be set first: motor type (synchronous, asynchronous, with or without encoder), rated power, rated voltage, rated current, speed or number of poles, rated frequency, maximum operating frequency, acceleration and deceleration times for motor start-up and shutdown, the protection method and proportional coefficient of the frequency converter, and carrier frequency. All these parameters are indispensable. After setting these parameters, you can then choose between vector control and V/F control. When selecting vector control, the motor must undergo dynamic self-learning with the frequency converter under no-load conditions or static self-learning under load. Only after self-learning can the motor and frequency converter work together to achieve the accuracy of vector control. When selecting V/F control, self-learning is not required; after parameter adjustment, the motor can be directly powered on and run.
3. If the direction of operation of the variable frequency motor fan is inconsistent with the rotation direction marked on the fan, the fan cannot function properly, resulting in poor heat dissipation of the motor. The heat generated by the motor cannot be dissipated, causing the motor to overheat or burn out.
4. Of the three situations above, items 2 and 3 occur most frequently.
When the motor current setting in the frequency converter is too high, or the motor protection parameters are set too high, the frequency converter cannot protect the motor according to the actual motor current when the motor is overloaded, which will cause the motor to overheat or burn out due to overload.
When the motor speed is set incorrectly, if the set speed exceeds the rated speed of the motor, the motor will run at a higher speed in the constant power region at the rated frequency point. The higher the speed, the lower the output torque, which will cause the motor to overheat or burn out due to excessive current.
If the motor's acceleration and deceleration time is too short, the frequency converter will report an overcurrent fault and activate its protection.
The frequency converter uses vector control, but the motor and the frequency converter do not have a self-learning pairing, which causes the parameters controlled by the frequency converter to be inconsistent with the actual values of the motor, resulting in poor control accuracy, motor overheating or burnout;
When the carrier frequency is set relatively high, the switching speed of the inverter's switching transistors is relatively high, and the heat generation increases. At this time, the inverter's ability to resist changes in load current decreases. When the load current increases, the inverter may trip due to overcurrent, which manifests as a problem with the motor.
Without setting a motor protection current, most frequency converters default to a protection current of 150% of the converter's rated output current. The frequency converter will only protect itself when the motor current reaches this value. However, the output current of the frequency converter is greater than the rated current of the motor. By the time the motor current reaches the frequency converter's protection current, the motor is already overloaded by far more than 150%, causing it to overheat and burn out. What customers see is that the frequency converter is fine, but the motor is unreliable.
5. In view of the above, it is recommended that customers choose variable frequency motors when selecting variable frequency drives to control motors, and choose variable frequency drives from reputable manufacturers. Although the initial investment may be slightly higher, the quality is guaranteed, the trouble-free operation time is long, and it is less likely to cause production stoppages due to motor or variable frequency drive failures. In addition, high-quality variable frequency drives have guaranteed after-sales service and fast response time.
For more information, please visit the Medium and Low Voltage Frequency Inverter channel.
