Ms. Can's friend, Mr. Z, is a company CEO. He told me about a problem his company was experiencing with frequent inter-turn and overload failures in a particular type of motor they supplied. The puzzling thing was why the same design and manufacturing process were suddenly causing so many problems. Mr. Z's market service personnel eventually discovered that almost all the faulty motors were being used with frequency converters, while their motors were originally designed for the customer's mains-frequency equipment. With this question in mind, we'll briefly discuss the issue of using mains-frequency motors with frequency converters today.
Ordinary asynchronous motors are designed for constant frequency and constant voltage operation. They operate efficiently within a narrow range near their rated operating point, and are generally not suitable for wide-range variable frequency speed control. Some customers, for cost reasons, directly use ordinary asynchronous motors as variable frequency motors, leading to frequent motor failures or shortened lifespan. In fact, many motor manufacturers have designed and developed wide-frequency motors specifically for wide-range variable frequency speed control applications. So, returning to the topic of "using ordinary asynchronous motors as variable frequency motors," what are the potential dangers of this simplistic application ? Is it necessary to upgrade to a wide-frequency motor? Today, we will start with theoretical analysis to clarify the underlying mechanisms of frequent failures and briefly explain the issues.
Efficiency decreases while temperature increases
All frequency converters generate harmonic voltages and currents to varying degrees during operation, causing the motor to operate under non-sinusoidal voltage and current conditions. The high-order harmonics of the frequency converter increase stator copper losses, rotor copper losses, iron losses, and additional losses in the motor, with rotor copper losses being the most significant. These losses cause the motor to generate extra heat, reduce efficiency, and decrease output power. If a conventional three-phase asynchronous motor is used under frequency converter conditions, the most direct consequence is an increase in temperature rise, especially for IP23 series motors, where the impact is more severe.
Cooling effect deteriorates at low speeds
When the power supply frequency is low, the losses caused by higher harmonics in the power supply are greater. Secondly, when the speed of a conventional asynchronous motor decreases, the cooling airflow decreases significantly, which worsens the motor's cooling condition. The direct result is a sharp increase in temperature rise, making it difficult to achieve constant torque output.
Insulation strength cannot meet the requirements
The carrier frequency of the frequency converter is very high (approximately several thousand to tens of kilohertz), which causes the motor stator windings to withstand very high voltages. This puts the inter-turn insulation of the motor under severe stress and threatens the motor's insulation to ground. The end result is that the motor suffers from inter-turn, phase-to-phase, and ground faults, and in severe cases, it manifests as winding overload. Because the design margins for phase-to-phase and ground insulation are relatively large, the motor is more likely to experience inter-turn and overload faults. However, for stators processed by automatic winding machines without phase-to-phase blocks, the number of faults is relatively higher.
This leads to electromagnetic noise and vibration problems.
When a conventional asynchronous motor is powered by a frequency converter, the vibration and noise problems caused by electromagnetic, mechanical, and ventilation factors become more complex. The time harmonics in the frequency converter interfere with the inherent spatial harmonics of the motor's electromagnetic components, creating various electromagnetic excitation forces. When the frequency of these electromagnetic force waves coincides with or is close to the natural vibration frequency of the motor body, resonance occurs, resulting in severe high-frequency electromagnetic noise accompanied by a certain degree of vibration.
Structural fatigue and insulation aging
Using a frequency converter for power supply allows the motor to start at very low frequencies and voltages without inrush current, and enables rapid braking using various braking methods provided by the frequency converter, creating conditions for frequent starting and braking. However, the mechanical and electromagnetic systems of the motor are under the action of cyclic alternating forces, which causes fatigue and accelerated aging of the mechanical and insulation structures.
The application of high-performance frequency converters has brought revolutionary changes to the motor industry. Most motor testing equipment has replaced voltage regulators and frequency converters with static adjustable frequency converters. However, most end customers use mains frequency power, which inevitably brings a hidden danger that cannot be ignored: the motor can run normally when powered by a static adjustable frequency converter, but may fail to start properly when powered by mains frequency. Ms. Can's friend, Xiao C, had such an experience: a multi-speed motor worked perfectly when tested with a static adjustable frequency converter (frequency converter starting), but failed to start at a certain speed when powered by the customer's mains frequency. Therefore, a frequency converter motor that passes the power supply test under simulated actual operating conditions may also have fatal defects when applied to traditional mains frequency power supply applications.
