The servo motor and variable frequency drive (VFD) typically consist of two main components: the motor itself and the controller that powers it, also known as the amplifier, servo drive, or inverter. These components are connected via cables, with the controller drawing power from an alternating current source. Servo motors are equipped with a feedback circuit to maintain precise positioning, which means they remain active even when not in motion. In contrast, VFDs regulate motor speed by adjusting the frequency of the drive signal. Both systems rely on pulse-width modulation (PWM) signals for operation. 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 incorporate such drives, with up to 20 units in some cases.
There are numerous challenges associated with VFDs and servo motors, but we will focus on key issues. For a comprehensive understanding of the full range of problems—including bearing damage, overvoltage, and electromagnetic interference—readers can refer to online resources.
Pulse drive signal characteristics play a crucial role in these systems. To reduce costs, drive inverters use fast-switching components to generate pulses with rise and fall times in the nanosecond range, spreading the signal spectrum to a few megahertz. This method is simpler, cheaper, and more efficient than producing stepped voltage outputs. However, these sharp pulses are a major source of various issues. If the connection between the controller and motor is properly designed as an RF transmission line—matching impedances, using high-quality cables, etc.—many of these problems could be avoided. Unfortunately, motors are not designed for signal transmission but for mechanical work, so their high-frequency characteristics are often overlooked.
Some of the problems caused by these sharp pulses include:
- Motor bearing damage
- Overvoltage and insulation degradation
- High-level conducted electromagnetic interference (EMI)
- Electrical overload due to ground plane currents
- Radiated EMI from cables
- Mechanical noise
- Motor overheating
In the following sections, we will explore these issues and discuss potential solutions.
**Motor Bearing Damage**
When considering the motor as an electronic system, a steep drive pulse is applied to the stator. The stator has strong capacitive coupling with the rotor, which creates a path for high-frequency currents. Although the fundamental frequency of the drive pulse is low (typically below 20 kHz), the high-frequency components of the pulse edge have low impedance through this coupling. This results in high-frequency voltages that cause current to flow through the rotor, bearings, and into the ground plane. This current can arc across the bearing, causing EDM (electrical discharge machining), which gradually damages the bearing surface. Even small voltages, such as 200 mV, can trigger arcing, leading to pitting and eventual failure. This issue is common and often goes unnoticed until significant damage occurs.
**Pulse Edge Overvoltage**
Mismatched impedances between the motor controller, motor input, and connecting cable can lead to ringing and overvoltage. While ideal matching would result in a clean square wave, real-world conditions often lead to voltage spikes. These overvoltages can exceed 60% of the normal pulse amplitude, damaging motor insulation and increasing ground currents through the bearings. According to the U.S. Department of Energy, even small overvoltages can cause harmful currents in the bearings. Such issues are well-documented, and standards like IEC/TS 60034-25 limit the allowable pulse voltage based on rise time.
**Electromagnetic Interference (EMI) from Equipment**
The EMI generated by VFDs and servo motors can affect other sensitive equipment. High-frequency interference may cause:
- Non-compliance with EMC standards
- Disruption of electronic devices
- Sensor errors and measurement inaccuracies
- Electrical overload (EOS) for sensitive components
The rapid changes in supply current during pulse generation produce conducted emissions that return to the power grid. Power line filters are commonly used to mitigate these effects, but internal EMI remains a challenge. Capacitive coupling between the drive cable and the device ground can introduce noise, reducing signal integrity and potentially causing safety hazards.
**Electromagnetic Interference Test**
To address EMI issues, testing is essential. One effective method is measuring the current in the return path of the drive signal, such as the ground connection between the controller and motor. A broadband current probe can capture this current, providing insight into the bearing current. For example, a peak current of 1.72A measured at 10,000 cycles per second can cause significant bearing wear, even when the motor is stationary.
Safety is critical when working with high-voltage systems. A battery-powered oscilloscope with a bandwidth of at least 200MHz and a high-voltage probe is recommended. Avoid using AC-powered oscilloscopes due to ground loops, and ensure proper grounding to prevent damage to equipment or injury.
**Eliminate Electromagnetic Interference Problems**
The root cause of many issues lies in the sharp edges of the PWM signal. Solutions involve slowing the rise and fall times to minimize capacitive coupling while maintaining motor performance. Optimizing trace layouts, altering current paths, or blocking them entirely are viable strategies. While there are many solutions available, users should prioritize technical analysis over marketing claims when selecting a remedy. Proper cable selection, shielding, and layout optimization can significantly reduce EMI and improve system reliability.
iPad Wireless Keyboard Case,LED Display Magic Keyboard Case,Detachable Magnetic Keyboard Case,Magnetic Keyboard Case,Rotatable Wireless Keyboard
Shenzhen Ruidian Technology CO., Ltd , https://www.wisonen.com