Quality risk of power frequency motor used under variable frequency conditions
Ordinary asynchronous motors are designed according to the constant frequency and constant voltage operation scheme. The narrower area near the rated working point can be operated as expected according to the design. It is generally not suitable for wide-range variable frequency speed regulation. Some customers rely on cost considerations to directly use ordinary asynchronous motors as variable frequency motors, resulting in frequent motor failures or shortened service life. In fact, many motor companies have designed and developed wide-band motors, which are designed for wide-range variable frequency speed control applications. Then, the topic returns to the "normal asynchronous motor used as a variable frequency motor" proposition, what kind of potential harm is this simple and rude application? Is there a need to upgrade to a broadband motor? Today, we start with theoretical analysis, clarify the internal mechanism of frequent failures, and briefly explain the problem.
Efficiency cooling
All frequency converters generate different levels of harmonic voltage and current during operation, allowing the motor to operate under non-sinusoidal voltages and currents. The higher harmonics of the inverter cause an increase in the stator copper loss, rotor copper loss, iron loss and additional loss of the motor, most notably the rotor copper loss. These losses will cause the motor to generate extra heat, reduce the efficiency, and reduce the output power. For example, if the ordinary three-phase asynchronous motor is used under the condition of the inverter, the most direct increase is the temperature rise, especially for the IP23 series motor.
Cooling effect deteriorates at low speed
When the power supply frequency is low, the loss caused by higher harmonics in the power supply is large. Secondly, when the speed of the ordinary asynchronous motor is reduced, the cooling air volume is reduced significantly, and the cooling condition of the motor is deteriorated. The direct result is that the temperature rise sharply increases, and it is difficult to achieve constant torque output.
Insulation strength cannot be satisfied
The carrier frequency of the inverter is very high (about several thousand to ten kilohertz), which will subject the stator winding of the motor to a very high voltage, which will make the inter-turn insulation of the motor bear a severe test and pose a threat to the insulation of the motor. The end result is that the motor is faulty due to inter-turn, phase-to-phase and ground faults. Because the design margin of phase insulation and ground insulation is relatively large, the motor is more characterized by inter-turn and overload, but for the stator without the inter-phase block processed by the automatic weaving machine, the relative failure will be relatively more.
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