introduction
With the increasing global awareness of environmental protection and the rapid development of energy-saving requirements, especially in industrial motor control, frequency converters assembled with power semiconductor components are becoming the mainstream application.
However, when the wiring distance between the frequency converter and the motor is long, the micro-surge voltage caused by the high-speed switching process of the frequency converter at the motor terminals can affect the motor insulation and cause damage to the motor. Here, the surge is called a micro-surge to distinguish it from sudden and powerful surges such as lightning. Micro-surges appear as dense, continuous, and very narrow voltage spikes on an oscilloscope.
This article analyzes the generation mechanism of micro-surge voltage and its impact on motors, introduces the technology for suppressing micro-surge voltage, and discusses the working principle of recently developed products that attenuate micro-surge voltage and micro-surge suppression components characterized by fine wire diameter transmission.
1. Mechanism of micro-surge voltage generation
1.1 Output voltage waveform of the frequency converter
A frequency converter mainly consists of a rectifier that converts AC mains power into DC, capacitors that smooth voltage ripples, and an inverter composed of six switching devices. As shown in Figure 1, the inverter output is an equivalent sinusoidal AC voltage generated by changing the pulse width modulation (PWM) wave to drive the motor. In recent years, to reduce motor noise, frequency converters have adopted IGBTs for high-speed switching in the inverter section. Therefore, during the rise and fall of each PWM pulse, i.e., when the switching time is very short (t = 0.1 ~ 0.3 ms), the rate of change of the DC voltage Ed inside the inverter (Ed = approximately 600V for inverters in a 400V power system) due to the switching becomes very large, dv/dt. This is one of the main sources of micro-surges.
1.2 Micro-surge voltage
Micro-surge voltage is an extremely fine spike-like surge voltage generated at the motor terminals when the wiring length between the inverter and the motor is very long. As shown in Figure 2, the inverter's output voltage is pulsed, and a micro-surge voltage spike is superimposed on the pulsed waveform at the motor terminals. Typically, the peak value of the micro-surge voltage will be twice the DC voltage inside the inverter.
1.3 Reflection caused by impedance mismatch
The output pulses of a frequency converter have very short rise or fall times, representing high-frequency components superimposed on the drive frequency (fundamental wave) and pulse modulation frequency (modulated wave) supplied to the motor by the frequency converter. Typically, the impedance ZL of the cable connecting the frequency converter and the motor is 50~100Ω, while the impedance ZM of the motor itself, especially for motors of several hundred kW, exceeds 1kΩ, more than 10 times the cable impedance. This impedance mismatch occurs at the motor terminals, causing reflection of high-frequency wave components. The reflection coefficient M at the mismatched impedance connection is...
The inverter's output pulses, reflected waves of the same polarity and almost the same magnitude, superimpose to form a micro-surge voltage spike. Figure 3 vividly illustrates the reflection, where the micro-surge voltage is raised high, much like a wave encountering an obstacle. Figure 4 shows the impedance difference between the actual cable and the motor. Generally, the impedance of the motor is more than 10 times the characteristic impedance of the cable, so reflection always exists.
1.4 Examples of Microsurge Occurrence
A certain frequency converter and motor are both rated for AC400V input and 3.7kW power, operating on a mains voltage of AC460V, with a power cable length of 50m. Under no-load conditions, the internal DC intermediate voltage of the frequency converter is measured to be 620V. The micro-surge waveform observed on the motor terminals using an oscilloscope is shown in the figure. In the figure, the micro-surge voltage value is as high as 1250V DC, which damages the motor insulation and accelerates its aging.
Figure 6 shows the surge voltage measurements taken for different cable lengths between the inverter and the motor. These measurements were taken under common conditions: an IGBT modulation frequency of 2kHz and a pulse rise time of t=0.1µs . It can be seen that when the cable length exceeds 100m, the surge voltage remains constant at twice the internal DC voltage of the inverter. However, when the cable length exceeds 20m, it is important to be aware that the surge voltage may have exceeded 1.8 times the internal DC voltage of the inverter.
2. The impact of micro-surge voltage on motors
Figure 7 shows the internal cross-section of the motor. The motor has a stator and a rotor, and the stator contains slots for housing the three-phase coils. If the interior of the slots is magnified, many coils (enameled wire) can be seen. Insulation exists between each coil and ground, between each phase, and between the coil turns. Usually, insulating paper is inserted between the coil and ground and between phases, but no insulating paper is inserted between the coil turns; insulation is achieved through the robust enamel layer of the enameled wire. Micro-surge voltages affect all of these insulations, with the most damage occurring between the coil turns. Table 1 lists the voltage values, also known as voltage stress, experienced by the various insulation components inside the motor, providing data comparing driving the motor with mains power and driving it with a frequency converter.
2.1 Insulation failure between coil turns
Figure 8 shows the simulation results of how much voltage is applied between the coil turns when a surge voltage seeps into the motor. This simulation involves placing the measurement point on each coil of the motor (the enameled coil inside the motor slot) and measuring a surge voltage with a rise time of 0.14 ms applied between U and S1. The voltage between U and S1 is shared by the first coil, measuring 65% to 75% of the total voltage. The other coils, S1-S2, S2-S3, and S3-V, share 10% to 20% of the voltage. This is because the internal impedance of the motor is high, causing the micro-surge voltage to gradually decay.
In the manufacturing process of electric motors, it is difficult to completely separate the enameled wire coil from start to finish; in most cases, it is wound haphazardly, with the wire start and end possibly touching in the slot. This can lead to corona discharge (partial discharge) between the turns due to micro-surge voltage. Even if the discharge is extremely brief, it can still reach 10,000 lbs locally. The high temperature gradually erodes the insulation, eventually causing it to break down. Figure 9 shows the lifespan characteristics of 0.85 mm diameter, 33 μm thick enamel, F-class insulation, 155 lb enameled wire.
The horizontal axis of the lifetime characteristic represents the number of applied failure pulses and the failure time; the vertical axis represents the failure voltage. The two curves represent the results measured under two conditions: 20°C and 155°C for the enameled wire.
The lifespan characteristics are represented by two lines with different slopes, and the point where the two lines meet is called the partial discharge initiation voltage. The steep slope indicates the region where discharge actually occurs, and the enameled wire is damaged within 2 hours. The gently sloping region rarely experiences partial discharge. Based on this conclusion, if the partial discharge initiation voltage of the first coil is controlled, insulation damage due to micro-surge voltages will not occur. Furthermore, if the phase-to-phase (UV-to-phase) voltage is controlled below 1000V and the voltage sharing rate of the first coil is controlled at around 750V, a 20-year lifespan can be ensured.
2.2 The actual situation of motor damage caused by micro-surges
In Japan, with the widespread adoption of inverters, motor manufacturers have strengthened motor insulation, with most achieving insulation levels exceeding 1200V. Technical data from JEMA (Japan Electrical Manufacturers' Association) shows that from 1989 to 1993, the number of micro-surge damage incidents, based on statistics of motor shipments, was only 0.013 %, a very low probability. However, the risk of insulation failure remains high for older motors with aging insulation and those considered to have low insulation levels. Furthermore, due to recent measures to counteract high-order harmonics in power supplies and the promotion of high-power-factor power supplies targeting elevator regenerative braking, the number of PWM inverter systems installed is continuously increasing. For PWM inverters to regenerate energy and return it to the mains power supply, raising the DC intermediate voltage to a higher value is crucial, resulting in an increased possibility of insulation failure due to micro-surge voltage. In China and other AC 440~380V regions, the mains voltage is twice that of Japan; therefore, the hazards of micro-surge voltage are even more significant.
3. Micro-surge suppression technology
For the reasons mentioned above, inverter manufacturers are committed to overcoming micro-surge problems and developing and selling a variety of products to suppress micro-surges.
3.1 Filter used in the output circuit
The output circuit filter consists of input/output terminals, resistors, capacitors, and reactors, as shown in Figure 10. The reactors are particularly heavy components. Key specifications include a phase-to-phase surge voltage below 1000V, a wiring length of 400m between the inverter and the motor, a product series up to 500kW, and an IP00 protection rating.
3.1.1 Working Principle
The working principle of the output filter is shown in Figure 11. Micro-surge voltage is caused by excessive dv/dt during the rise time of the inverter's output pulse, and is further amplified by reflection due to impedance mismatch. Therefore, the output circuit uses a filter to suppress dv/dt, thus suppressing the micro-surge caused by impedance mismatch in high-frequency components. Therefore, the output filter is a dv/dt suppression filter. This type of filter can produce products with excellent performance, exhibiting micro-surge voltages below 1000V, when the inverter's modulation frequency is 15kHz and the wiring length is 400m. However, to allow the inverter's output current to pass through the reactor, this type of filter must be made with a large capacity, resulting in large size, high price, and heavy weight, some exceeding 50kg, placing a real burden on users.
3.1.2 Inhibitory effect
Figure 12 shows the suppression effect of a 3.7kW inverter powered by a 440V power supply supplying a motor ( 3.7kW , 400V), with a wiring length of 100m, on the micro-surge voltage between the motor terminals UV. Without an output filter, the micro-surge voltage reaches 1360V, equivalent to 200% of the inverter's internal DC voltage of 680V. With an output filter, the peak voltage is 756V, equivalent to 111% of the inverter's internal DC voltage of 680V. The difference between this and the peak voltage without an output filter is 604V, achieving a suppression effect of 89%.
3.2 Surge Suppression Components
Figure 13 shows the appearance of the surge suppression component. Compared to the output filter, the surge suppression component is a miniaturized product. Its technical specifications include a phase-to-phase micro-surge voltage of less than 1000V and a protection rating of IP20. The surge suppression component does not require selection of the inverter capacity, but the wiring distance does. Furthermore, the wiring method is very simple; just connect the input cable of the surge suppression component to the motor terminals U, V, and W.
3.2.1 Working Principle
The working principle of the surge suppression component is shown in Figure 14. The surge suppression wire wound inside the surge suppression component has the same impedance ZL as the cable. Therefore, when connected to the motor terminal, it reduces the impedance of the motor terminal, thereby reducing reflected waves due to impedance mismatch. Typically, the impedance ZL of the high-frequency wave component on the cable is 50-100 ohms, and the designed impedance ZS of the surge suppression wire is 50-60 ohms.
The cross-sectional view of the surge suppression line is shown in Figure 14. The surge suppression line is made of 1.2mm diameter wire. The inner copper wire is plated with a high-resistivity material and covered with a high-dielectric-constant material as insulation. The outer layer is a shielded coaxial cable. The distributed capacitance between the copper wire, the high-resistivity plating core wire, and the shield wire reduces the high-frequency impedance, thus absorbing the surge. In addition to surge suppression components, products using this surge suppression line also include surge suppression cables. These cables have the surge suppression line installed parallel to the main current-carrying cable of the frequency converter. Its cross-sectional view and connection method are shown in Figure 15.
3.2.2 Characteristics of Surge Suppression Components
Simply connect it to the motor terminals to significantly reduce surge voltage;
When using a PWM inverter, the phase-to-phase voltage can be controlled to below 1000V;
No additional construction is required, and setup is easy for already installed and running equipment.
It is not related to the capacity of the frequency converter and can be applied to all types of motors (however, motors exceeding 75kW need to be configured accordingly).
The length of the wiring cable between the frequency converter and the motor needs to be matched; it is available in two specifications: 50m and 100m.
Compliant with RoHS Directive;
Compared to output filters, it is smaller and lighter.
3.2.3 Surge Suppression Principle Derived from Transmission Line Theory
According to transmission line theory, surge suppression uses surge absorption, surge attenuation, and surge suppression line reflection reduction methods.
(Member) Surge absorption: Surges are high-frequency wave components. A low-impedance surge suppression line is connected to the motor terminal to allow the surge current to flow into the suppression line, as shown in Figure 16.
Surge attenuation: Surge current is a high-frequency wave component. According to the skin effect, the surge current is concentrated on the outer surface of the conductor. Because the conductor is coated with a high resistivity material, the energy of the surge current is consumed in the resistance, as shown in Figure 17.
The reflection from the surge suppression line reduces the high-frequency components of the surge current, which are bypassed and attenuated within the line, making the surge shape blunter and shifting the surge frequency band center towards lower frequencies. Furthermore, from the perspective of the surge current, it appears as if the characteristic impedance of the surge suppression line gradually increases, making it less likely for the end of the line to be reflected back. (See Figure 18.)
3.2.4 Inhibitory effect
Figure 19 shows the effect of suppressing micro-surge voltage when the inverter's power supply voltage is 400V, for a 3.7kW motor with a wiring length of 50m, and for a 75kW motor with a wiring length of 100m. For the 3.7kW motor, without surge suppression components, the micro-surge voltage is 1036V, equivalent to 192% of the inverter's internal DC voltage of 540V; with surge suppression components, the peak voltage of the 50m cable is 733V, equivalent to 136% of the inverter's internal DC voltage of 540V. The voltage spike difference is 303V, indicating a 61% suppression effect. For the 75kW motor, without surge suppression components, the micro-surge voltage is 1040V, equivalent to 200% of the inverter's internal DC voltage of 520V; with surge suppression components, the peak voltage of the cable is 785V, equivalent to 151% of the inverter's internal DC voltage of 520V. With a voltage spike difference of 255V, it has a 49% suppression effect.
4 Conclusion
This paper addresses the hazards of micro-surges to motors caused by inverters in practical applications. It introduces micro-surge suppression technology and its principles, and compares the suppression effects of different suppressors with examples, aiming to draw the attention of inverter manufacturers and users to this issue.
