The servo motor and variable frequency drive (VFD) typically consist of two main components: the motor itself and the controller that powers it, often referred to as the amplifier, servo drive, or inverter. These two parts are connected via cables, and the controller receives power from an alternating current source. Servo motors feature a feedback system that ensures precise positioning, meaning they remain active even when not in motion. In contrast, VFDs regulate motor speed by adjusting the frequency of the drive signal. Both systems operate using pulse-width modulated (PWM) signals. Figure 1 illustrates a standard VFD setup, where AC power is converted into a pulse signal to drive the motor. Figure 2 shows a typical servo motor with position control feedback. Many manufacturing and machine tool systems include one such drive, and there may be up to 20 of them in a single setup.
There are numerous challenges associated with VFDs and servo motors. While we will focus on some key issues, readers can easily find more information online about problems like bearings, overvoltages, and electromagnetic interference (EMI). Understanding these issues is crucial for effective troubleshooting and maintenance.
One of the primary causes of these issues is the sharp edges of the drive pulses. These high-frequency signals can lead to various problems, including motor bearing damage, overvoltage, EMI, and mechanical noise. If the connection between the controller and motor is properly designed—such as using RF-grade cables and matching impedances—many of these issues could be avoided. However, since motors are primarily designed for mechanical work, their high-frequency characteristics are often overlooked.
Some of the common problems caused by these drive signals include:
- Motor bearing damage due to high-frequency currents passing through the bearing.
- Overvoltage that can damage insulation and cause overheating.
- High levels of conducted and radiated EMI that interfere with other equipment.
- Electrical overload (EOS) due to ground currents.
- Mechanical noise and increased motor temperature.
In the following sections, we will explore these issues in detail and discuss practical solutions to mitigate their impact.
Motor bearing damage occurs when high-frequency voltage spikes from the drive pulse create a path for current to flow through the bearing. This can result in electrical discharge machining (EDM), which erodes the bearing surface over time. Even small voltages, as low as 200 mV, can initiate this process. The repetitive nature of the drive pulse means that this damage can accumulate quickly, leading to long-term failure of the bearing.
Pulse edge overvoltage is another critical issue. When the impedance between the controller, motor, and cable is mismatched, it can cause ringing and voltage spikes. These overvoltages can exceed 60% of the normal pulse amplitude and stress the motor's insulation, leading to premature failure. Standards like IEC/TS 60034-25 and NEMA MG1-2014 emphasize the importance of limiting these voltages to prevent damage.
Electromagnetic interference (EMI) from VFDs and servo motors can also affect nearby electronic devices. This interference can cause malfunctions, measurement errors, and even safety hazards. Ground currents and capacitive coupling between the drive cable and the device ground can further exacerbate these issues. Proper filtering and shielding are essential to minimize EMI and ensure compliance with electromagnetic compatibility (EMC) standards.
Testing for bearing currents is an important step in diagnosing and addressing these issues. Although directly measuring current through a rotating bearing is challenging, testing the return path of the drive signal—such as the ground connection between the controller and motor—can provide valuable insights. Using a broadband current probe, engineers can measure the current flowing through the bearing and assess potential damage.
To effectively eliminate EMI problems, it's essential to modify the drive signal’s rise and fall times to reduce capacitive coupling between the stator and rotor. Optimizing the motor’s wiring, changing current paths, or blocking unwanted paths can also help. There are many solutions available, but choosing the right one requires careful analysis and consideration of the specific application.
In summary, while VFDs and servo motors are powerful tools in industrial automation, they come with unique challenges. Addressing these issues requires a combination of proper design, quality cabling, and effective EMI mitigation strategies. By understanding the root causes and implementing the right solutions, users can significantly improve the reliability and performance of their systems.
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