Many people have already realized the damaging effects of inverters on motors. For example, in a water pump factory, over the past two years, users frequently reported that pumps were damaged during the warranty period. Previously, the quality of these pumps was considered reliable. After investigation, it was discovered that the damaged pumps were driven by inverters.
Although the issue of inverters damaging motors is gaining more attention, many still lack a clear understanding of the underlying mechanisms and how to prevent such damage. This article aims to clarify these uncertainties and provide practical solutions.
The damage caused by inverters to motors can be categorized into two main types: stator winding damage and bearing damage, as shown in Figure 1. These issues typically occur within a few weeks to ten months, and the exact timing depends on various factors such as the inverter brand, motor brand, motor power, inverter carrier frequency, cable length between the inverter and motor, and ambient temperature.
Early motor failures due to inverter usage result in significant economic losses for enterprises. These losses are not only from repair and replacement costs but also from unexpected downtime. Therefore, when using an inverter to drive a motor, it is essential to pay close attention to the risk of motor damage.
Inverter-driven motors face unique challenges compared to those operated at power frequency. Understanding the differences between inverter voltage and standard power frequency voltage is crucial to grasping why this can lead to motor failure.
An inverter consists of a rectifier circuit and an inverter circuit. The rectifier converts AC to DC, while the inverter circuit modulates this DC into a pulse-width modulated (PWM) voltage waveform. This results in a pulsed voltage waveform rather than a smooth sine wave, which can stress the motor’s insulation and cause damage over time.
When this pulsed voltage travels through the cable, mismatches in impedance can cause reflections. These reflections can combine with the original signal, creating higher voltage spikes—up to twice the DC bus voltage or even three times the input voltage. These spikes can damage the stator windings, leading to premature motor failure.
The lifespan of the motor after being exposed to such voltage spikes depends on several factors, including temperature, pollution, vibration, voltage levels, carrier frequency, and the quality of the coil insulation.
Higher inverter carrier frequencies produce a waveform closer to a sine wave, which reduces motor operating temperature and extends insulation life. However, they also generate more voltage spikes per second, increasing stress on the motor. As shown in Figure 4, increasing the carrier frequency from 3 kHz to 12 kHz significantly reduces insulation life.
Temperature also plays a critical role. Higher temperatures shorten the life of motor insulation. Inverters introduce high-frequency components into the voltage, causing the motor to run hotter than under power frequency conditions.
Another major issue is the damage to motor bearings. This occurs because current flows through the bearing, creating intermittent arcs that burn the bearing surface. Two main causes of this current are electromagnetic field imbalance and stray capacitance.
When the magnetic field inside the motor becomes asymmetrical due to PWM voltage, a shaft voltage is induced. If this voltage exceeds the insulation strength of the lubricating oil, a current path is formed. During rotation, the lubricant may block the current, creating an arc that damages the bearing.
Over time, repeated arcing can create small pits, and if there's external vibration, grooves may form, severely affecting motor performance.
To protect the motor, especially the stator winding, two approaches are commonly used: installing a variable frequency motor with higher insulation resistance or reducing peak voltage. The former is ideal for new installations, while the latter is suitable for retrofitting existing systems.
There are four common protection methods:
1. Installing a reactor at the inverter output: Effective for short cables, but less so for longer ones.
2. Using a dv/dt filter: Better for medium-length cables, offering improved performance.
3. Installing a sine wave filter: Converts PWM to a sine wave, eliminating voltage spikes.
4. Adding a peak voltage absorber: Offers compact, cost-effective protection without voltage drop.
The SVA spike voltage absorber developed by the Second Hospital of the Aerospace Science and Industry Group is an advanced solution. It uses power electronics and intelligent control to absorb voltage spikes and protect both the motor windings and bearings.
This device is installed in parallel with the motor power input and works by detecting voltage peaks, absorbing their energy, and converting it into heat. It also includes a bearing current absorption circuit to protect against electrical erosion.
Compared to other protection methods, the SVA offers advantages like smaller size, lower cost, and easy installation. It avoids voltage drops that affect motor torque, making it particularly effective for high-power applications.
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