The advent of frequency converters has revolutionized industrial automation control and motor energy conservation. Frequency converters are almost indispensable in industrial production, and even in daily life, elevators and variable frequency air conditioners have become essential components. Frequency converters have begun to permeate every corner of production and daily life. However, frequency converters have also brought many unprecedented problems, among which motor damage is one of the most typical phenomena.
Many people have noticed the damage that frequency converters can cause to motors. For example, a water pump factory has seen frequent reports from its customers over the past two years of pump failures within the warranty period. Previously, this factory's products were considered highly reliable. An investigation revealed that all the damaged pumps were driven by frequency converters.
Although the phenomenon of frequency converters damaging motors is receiving increasing attention, the underlying mechanisms and preventative measures remain unclear. This article aims to address these concerns.
Damage to motors caused by frequency converters
The damage caused by frequency converters to motors includes two aspects: damage to the stator windings and damage to the bearings, as shown in Figure 1. This damage generally occurs within a few weeks to a dozen months, with the specific timing depending on many factors such as the brand of the frequency converter, the brand of the motor, the power of the motor, the carrier frequency of the frequency converter, the length of the cable between the frequency converter and the motor, and the ambient temperature. Early unexpected damage to the motor can lead to significant economic losses for businesses. These losses include not only the costs of motor repair and replacement, but more importantly, the economic losses from unexpected production downtime. Therefore, when using frequency converters to drive motors, sufficient attention must be paid to the issue of motor damage.
Damage to motors caused by frequency converters
The difference between frequency converter drive and power frequency drive
To understand why power frequency motors are more prone to damage under inverter drive conditions, we must first understand the difference between the voltage used to drive the motor by the inverter and the power frequency voltage. Then, we need to understand how this difference adversely affects the motor.
The basic structure of a frequency converter is shown in Figure 2, consisting of a rectifier circuit and an inverter circuit. The rectifier circuit is a DC voltage output circuit composed of ordinary diodes and filter capacitors. The inverter circuit converts the DC voltage into a pulse-width modulated voltage waveform (PWM voltage). Therefore, the voltage waveform of the motor driven by the frequency converter is a pulse waveform with varying pulse width, rather than a sinusoidal voltage waveform. Using a pulse voltage to drive the motor is the root cause of the motor's susceptibility to damage.
Mechanism of frequency converter damage to motor stator windings
When a pulsed voltage is transmitted through a cable, if the cable impedance does not match the load impedance, reflection will occur at the load end. As a result of this reflection, the incident and reflected waves superimpose, forming a higher voltage. Its amplitude can reach up to twice the DC bus voltage, approximately three times the inverter input voltage, as shown in Figure 3. Excessively high voltage spikes applied to the motor stator coils cause voltage surges, and frequent overvoltage surges can lead to premature motor failure.
The actual lifespan of a frequency converter-driven motor after being subjected to voltage spikes is related to many factors, including temperature, pollution, vibration, voltage, carrier frequency, and the coil insulation process.
A higher carrier frequency in a frequency converter results in an output current waveform closer to a sine wave, which reduces the motor's operating temperature and extends insulation life. However, a higher carrier frequency also means more voltage spikes per second, leading to more impacts on the motor. Figure 4 shows the change in insulation life as a function of cable length and carrier frequency. The figure shows that for a 200-foot cable, when the carrier frequency increases from 3kHz to 12kHz (a fourfold increase), the insulation life decreases from approximately 80,000 hours to 20,000 hours (a fourfold difference).
The effect of carrier frequency on insulation
The higher the motor temperature, the shorter the insulation life. As shown in Figure 5, when the temperature rises to 75°C, the motor's lifespan is only 50%. For motors driven by frequency converters, because the PWM voltage contains more high-frequency components, the motor temperature will be much higher than that driven by mains frequency voltage.
Mechanism of frequency converter damage to motor bearings
The reason why frequency converters damage motor bearings is that there is current flowing through the bearings, and this current is in an intermittent state. The intermittent circuit will generate an electric arc, which burns out the bearings.
There are two main reasons for the current flowing through the bearings of an AC motor: first, the induced voltage generated by the imbalance of the internal electromagnetic field; and second, the high-frequency current path caused by stray capacitance.
In an ideal AC induction motor, the internal magnetic field is symmetrical. When the currents in the three-phase windings are equal and 120° out of phase, no voltage is induced on the motor shaft. However, when the PWM voltage output by the inverter causes an asymmetry in the motor's internal magnetic field, a voltage is induced on the shaft. This voltage ranges from 10 to 30V, depending on the drive voltage; the higher the drive voltage, the higher the voltage on the shaft. When this voltage exceeds the insulation strength of the lubricating oil in the bearing, a current path is formed. During shaft rotation, at some point, the insulation of the lubricating oil blocks the current. This process is similar to the switching on and off of a mechanical switch, generating an electric arc that burns the surfaces of the shaft, balls, and bearing cups, forming pits. Without external vibration, these small pits have little impact, but external vibration can create grooves, which significantly affect the motor's operation.
In addition, experiments show that the voltage on the shaft is also related to the fundamental frequency of the inverter's output voltage. The lower the fundamental frequency, the higher the voltage on the shaft, and the more severe the bearing damage.
In the initial stage of motor operation, when the lubricating oil temperature is low, the current amplitude is 5-200mA, which is too small to cause any damage to the bearing. However, after the motor has been running for a period of time, as the lubricating oil temperature rises, the peak current will reach 5-10A, which will generate arcing and form small pits on the surface of the bearing components.
Protection of motor stator windings
When the cable length exceeds 30 meters, modern frequency converters will inevitably generate voltage spikes at the motor terminals, shortening the motor's lifespan. To prevent motor damage, there are two approaches: one is to use a motor with higher winding insulation dielectric strength (generally called a variable frequency motor), and the other is to take measures to reduce voltage spikes. The former is suitable for new projects, while the latter is suitable for retrofitting existing motors.
The following are four commonly used methods for motor protection:
1) Install a reactor at the output of the frequency converter: This measure is the most common, but it should be noted that this method is effective for shorter cables (less than 30 meters), but sometimes the effect is not ideal, as shown in Figure 6(c).
2) Install a dv/dt filter at the output of the frequency converter: This measure is suitable for situations where the cable length is less than 300 meters. The price is slightly higher than that of the reactor, but the effect is significantly improved, as shown in Figure 6(d).
3) Install a sine wave filter at the output of the frequency converter: This measure is ideal. Because here, the PWM pulse voltage is converted into a sine wave voltage, so that the motor operates under the same conditions as the mains frequency voltage, and the problem of voltage spikes is completely solved (no more voltage spikes will occur even if the cable is long).
4) Install a voltage spike absorber at the cable-motor interface: The drawbacks of the previous measures are that when the motor power is high, the reactor or filter becomes very large and heavy, and expensive. Furthermore, both reactors and filters cause voltage drops, affecting the motor's output torque. Using a frequency converter voltage spike absorber can overcome these drawbacks. The SVA voltage spike absorber developed by the 706 Institute of the Second Academy of China Aerospace Science and Industry Corporation employs advanced power electronics and intelligent control technologies, making it an ideal device for solving motor damage. In addition, the SVA voltage spike absorber can also protect the motor's bearings.
The spike voltage absorber is a new type of motor protection device, as shown in Figure 7 (SVA model from China Aerospace Science and Industry Corporation). It is connected in parallel to the power input terminal of the motor.
The principle block diagram of the SVA spike voltage absorber is shown in Figure 8. Its working process is as follows:
1) The spike voltage detection circuit detects the voltage amplitude on the motor power line in real time;
2) When the detected voltage amplitude exceeds the set threshold, control the spike energy buffer circuit to absorb the energy of the spike voltage;
3) When the peak voltage energy fills the peak energy buffer, the peak energy absorption control valve opens, allowing the peak energy in the buffer to be released into the peak energy absorber, converting electrical energy into heat energy.
4) The temperature monitor monitors the temperature of the peak energy absorber. When the temperature is too high, the peak energy absorption control valve is closed appropriately to reduce energy absorption (while ensuring the motor is protected) to prevent the peak voltage absorber from overheating and being damaged.
5) The function of the bearing current absorption circuit is to absorb the bearing current and protect the motor bearing.
Compared to motor protection methods such as du/dt filters and sine wave filters mentioned earlier, the biggest advantages of spike absorbers are their small size, low cost, and ease of installation (parallel installation). Especially in high-power applications, spike absorbers offer significant advantages in terms of price, size, and weight. Furthermore, because they are installed in parallel, there is no voltage drop, whereas du/dt filters and sine wave filters both exhibit a certain voltage drop, with sine wave filters experiencing a voltage drop approaching 10%, which can lead to a reduction in motor torque.
