Is the Variable Frequency Drive the Reason Why the Motor Breaks down Quickly?

Introduce

The advent of the variable frequency drive (VFD), also known as a frequency inverter or variable speed drive, has brought about revolutionary changes in industrial automation and energy efficiency for motors. In modern industrial production, frequency inverters are almost indispensable, and their applications have even extended to daily life—elevators and variable speed air conditioners are prime examples. However, despite these advantages, VFDs have introduced new challenges, one of the most prominent being their potential to damage motors.

This raises a crucial question: are variable frequency inverters the reason why motors break down quickly? Although this issue has gained attention, the mechanisms causing such failures remain unclear to many. The purpose of this article is to explore these mechanisms and suggest preventive measures.

What Are the Damages of the Inverter to the Motor?

The damage caused by a variable frequency drive to a motor can be categorized into two main areas: damage to the motor stator windings and damage to the motor bearings. This type of damage can occur over a few weeks to several months, depending on various factors such as the brand of the frequency inverter, the motor brand, motor power, carrier frequency, cable length between the inverter and motor, and ambient temperature.

Unexpected motor failures not only lead to high repair and replacement costs but also result in significant production downtime. Thus, when using inverter VFDs to drive motors, it is crucial to be aware of the potential risks and take preventive actions.

Mechanism of Inverter Damaging Motor Stator Winding

The root cause of motor damage when driven by a variable frequency inverter lies in the nature of the voltage waveform produced by the VFD. Unlike a pure sine wave, the output of a VFD is a pulse-width modulated (PWM) voltage, which can lead to increased stress on motor windings. When these PWM pulses travel through the cables, if the impedance of the cable doesn’t match the motor’s impedance, voltage reflections occur. These reflected waves combine with incoming waves, creating voltage spikes that can reach twice the magnitude of the DC bus voltage, or up to three times the input voltage of the inverter.

These high voltage spikes are applied to the motor’s stator windings, subjecting them to significant electrical stress. Repeated exposure to such overvoltage conditions shortens the insulation life of the motor windings, leading to premature failure. The life of a motor driven by a frequency inverter depends on several factors including temperature, pollution, vibration, voltage, carrier frequency, and insulation quality. While higher carrier frequencies can reduce motor operating temperatures and extend insulation life, they also increase the frequency and number of voltage spikes, which further stresses the motor.

Influence of Carrier Frequency on Insulation

Temperature is a critical factor in determining the lifespan of motor insulation. The higher the temperature, the shorter the insulation’s lifespan. Motors driven by variable frequency drives tend to operate at higher temperatures due to the high-frequency components present in the PWM voltage. As shown in studies, motor life can be reduced by 50% when operating at temperatures above 75°C. This rise in temperature, primarily due to the VFD’s high switching frequency, exacerbates insulation degradation, further accelerating motor failure.

The Mechanism of Inverter Damaging Motor Bearings

Variable frequency inverters can also damage motor bearings, primarily due to circulating currents that flow through the bearings. These currents are caused by two factors: induced voltages due to imbalances in the electromagnetic field inside the motor, and high-frequency currents resulting from stray capacitance.

In an ideal AC motor, when the three-phase currents are balanced, no voltage is induced on the motor shaft. However, the PWM voltage output from a frequency inverter distorts the magnetic field, causing a voltage to appear on the motor shaft. If this shaft voltage exceeds the breakdown voltage of the bearing lubricant, a current path is established, leading to arcing across the bearings. This arc discharge burns the bearing surfaces, creating pitting and eventually forming grooves that disrupt motor operation.

The severity of the bearing damage depends on several factors, including the motor’s operating time and temperature. Initially, the current may be too small to cause significant harm, but as the motor runs and the lubricant heats up, peak currents can increase, leading to severe arcing that accelerates bearing wear.

Protection of Motor Stator Windings

To prevent motor damage due to frequency inverters, several protective measures can be implemented:

1. Installing Reactors: Installing reactors at the inverter’s output can reduce voltage spikes but may be less effective for longer cables.
2. Using dv/dt Filters: These filters are effective for cable lengths up to 300 meters, offering better protection than reactors.
3. Sine Wave Filters: These filters convert the PWM voltage to a sine wave, completely eliminating voltage spikes and providing ideal protection for the motor.
4. Peak Voltage Absorbers: Installed at the motor terminals, these devices can absorb harmful voltage spikes, protecting both the motor windings and bearings.

In conclusion, the proper selection and installation of protective devices can significantly extend motor life when using variable frequency inverters.

Conclusion

While variable frequency drives have transformed the control and efficiency of motors in industrial and everyday applications, they can also be the reason for premature motor failure. The damage caused by inverter VFDs includes stator winding insulation breakdown and bearing failure, driven by voltage spikes, temperature rise, and circulating currents. By understanding these risks and employing appropriate protective measures, such as using inverter-duty motors, reactors, or filters, it is possible to mitigate the negative impact of VFDs and ensure longer motor life.