Medium-voltage motor soft starters are now widely used in metallurgy, petrochemicals, pharmaceuticals, municipal engineering, and other fields. Traditional starting methods have many shortcomings, and these problems are even more pronounced for large-capacity equipment. With the increasing application of soft starters in production practice, choosing soft-start motors will be an inevitable trend in the future.
I. Introduction
Power electronics technology is an electronic technology applied to the field of electronic technology. It is a discipline closely related to electronics, control, and power. The invention of the thyristor in 1957 brought about a revolution in power electronics, and power electronics technology has developed rapidly.
A thyristor, short for silicon controlled rectifier, is a semi-controlled device characterized by its controllable turn-on time and zero-current turn-off. When first invented, the thyristor was widely welcomed due to its significantly superior performance compared to earlier mercury arc rectifiers, ushering in a new era of rapid development in power electronics technology. Since the 1980s, some applications of thyristors have been replaced by various fully controlled devices with better performance. However, because its voltage and current capacity remains the highest among current power devices, it still holds an important position in high-capacity applications. Its main characteristics are as follows:
(1) High withstand voltage: Currently, there are products with a withstand voltage of 8500V.
(2) High current: There are currently products with a current of 5000A.
(3) Voltage drop: When the tube is turned on, the voltage drop is <2V.
(4) Low power loss and long lifespan
(5) Strong resistance to transient overcurrent impact, etc.
II. Commonly Used Starting Methods for Medium-Voltage (6-10kV) Motors
AC motors are the most widely used type of motor in various fields. To address the impact of AC motors on the power grid and machinery during startup, many methods have been adopted, including traditional methods such as series resistance starting, series reactor starting, star-delta switching starting, autotransformer starting, and frequency conversion starting.
Thyristor-controlled motor soft starters appeared in the 19th century. At that time, an engineer at NASA named Frank obtained a patent for a "power frequency controller," which used anti-parallel thyristors and adjusted their conduction angle to achieve AC voltage regulation. This technology was applied to solve the problem of low power factor in AC asynchronous motors under no-load and light-load conditions. Later, current feedback technology was introduced, significantly improving the control level and leading to its widespread adoption. The United States named devices using this technology "energy-saving devices."
III. Advantages of Soft Starting Motors
(1) It can reduce the impact on the power grid and reduce the capacity of the transformer.
When a typical squirrel-cage motor is started directly under no-load full voltage, the starting current can reach 5 to 7 times the rated current. When the motor capacity is relatively large, this starting current will cause a sharp drop in grid voltage, which can disrupt the normal operation of other equipment on the grid and even cause grid instability, leading to larger accidents. Therefore, it is generally required that voltage fluctuations caused by frequently starting motors should not exceed 10%, and voltage fluctuations caused by occasionally starting motors should not exceed 15%. Using soft starting can reduce the starting current to 1.5 to 3 times the rated current, significantly reducing grid voltage fluctuations.
For motors powered by a separate transformer, when using direct full-voltage starting, the motor capacity should not exceed 80% of the transformer capacity. When using soft starting, the transformer capacity can be the same as the apparent power of the motor (generally S=1.1PN).
The equivalent circuit of the transformer-motor unit is shown in Figure 1. U0 is the infinite power grid. U1 is the motor terminal voltage. XT is the short-circuit impedance of the transformer. Under rated conditions, when the motor current equals the transformer's rated current, the voltage drop across XT is the transformer's short-circuit voltage value UK. If the maximum current of the motor during soft start is 3IN, then the voltage drop across XT is 3UK. For small and medium-sized power transformers, UK = (4~10.5)%UN; for large transformers, UK = (12.5~17.5)%UN. Under normal circumstances, as long as the terminal voltage of the motor at startup is not lower than 65% of the rated voltage (at maximum current), startup will be successful.
Under rated operation, I=IN, U0-U1=UK, U1=UN. If calculated under extreme conditions: UK=17.5%; I=3IN, then: U1=U0-3UK=UN-2×17.5%UN=65%UN. This also meets the starting requirements. In other cases, U1 will definitely > 65%, posing no further problem.
When a transformer is shared by a motor and other loads, it is generally required that the capacity of a frequently started motor should not exceed 20% of the transformer capacity, and the capacity of a motor that is occasionally started should not exceed 30% of the transformer capacity.
If Imax=6IN for full-voltage direct start and Imax=3IN for soft start, then considering equal current and line voltage drop, the capacity of frequently started motors can reach 40% of the transformer capacity; the capacity of occasionally started motors can reach 60% of the transformer capacity.
(2) It can reduce damage to the motor and extend the motor's lifespan.
1. The large current during direct full-voltage starting of an electric motor generates significant impact forces on the stator coils and rotor squirrel cage bars, increasing wear on the stator coils (especially at the ends) and the core, potentially damaging the winding insulation. The impact force on the squirrel cage bars can also easily cause breakage, leading to motor malfunction. The magnitude of the electrodynamic force is directly proportional to the square of the current. The electrodynamic force during direct full-voltage starting is 36 times the normal rated operating force (assuming Imax = 6IN). The electrodynamic force during soft starting is 9 times the normal rated operating force. The difference is clearly very significant.
2. The large current during direct full-voltage starting of an electric motor generates significant Joule heat in the stator and rotor windings. This can burn out the winding insulation and reduce the motor's lifespan. Soft starting can greatly reduce heat generation and extend motor life. High heat is generated at the point of maximum current. Joule heat is proportional to the square of the current, thus reducing the concentrated heat generation significantly. Generally, soft starting occurs under no-load or light-load conditions, with the maximum current typically around 2IN. At this point, the high heat generation is only at the initial starting point, undoubtedly greatly benefiting the extension of motor lifespan.
3. When a motor is started directly at full voltage, the rated voltage is instantaneously applied to the motor windings. This generates an operational overvoltage, which, in the worst-case scenario, can reach five times the rated voltage, causing significant damage to the motor insulation. Many motor failures occur during closing due to this operational overvoltage.
(3) It can reduce damage to machinery and extend the service life of machinery.
When an electric motor is started directly at full voltage, the maximum torque is approximately twice its rated torque. For gear-driven equipment, this significant impact force can accelerate gear wear or even cause breakage. For belt-driven equipment, it increases belt wear and may even lead to belt breakage. For pumps, it can cause a "water hammer effect," damaging pipes and impellers. Soft starting, on the other hand, allows the motor to accelerate slowly, gradually increasing torque and completely eliminating these hazards. The slow increase in motor speed during a soft start also facilitates proper lubrication and prevents dry friction. All of these factors greatly reduce damage to the motor and extend the lifespan of the machinery.
IV. Challenges in Power Electronics Technology
(1) As the single-unit capacity of electrical loads increases, the capacity of power electronic devices also increases, which in turn increases the requirements for the voltage and current capacity of power electronic devices. This is a difficult problem in current power electronics technology. At present, people can only solve this problem by using series and parallel connection technology, but reliability has always been a problem that hinders the widespread application of series and parallel connection technology.
If the voltage, current, and capacity limitations of power electronic devices can be resolved, a very promising future awaits: the uncontrollability of AC power grids will be solved, freeing people from tedious calculations and the fear of accidents; all electrical equipment will operate in an orderly manner: without shocks, negative sequences, or harmonics…
(2) Series and parallel applications
For larger power electronic devices, when the voltage or current rating of a single power electronic device cannot meet the requirements, it is often necessary to connect the power electronic devices in series or in parallel to work.
2.1 Series connection of thyristors
When the rated voltage of a thyristor is lower than the actual requirement, two or more devices of the same type can be connected in series. Ideally, a series connection should result in equal voltage across all devices, but in reality, due to differences in device characteristics, uneven voltage distribution is common. The leakage current flowing through series-connected devices is always the same, but due to the dispersion of their static voltage-current characteristics, the voltage across each device is unequal. When two thyristors are connected in series, the forward voltage they withstand under the same leakage current IR is different. If the applied voltage continues to increase, the device with the higher voltage will reach its break-through voltage first and conduct, causing the other device to bear the full voltage and also conduct, resulting in both devices losing their control function. Similarly, in reverse, due to the uneven voltage distribution caused by the different voltage-current characteristics, one device may break down in reverse first, followed by the other. This voltage distribution problem caused by differences in the static characteristics of the devices is called static voltage imbalance.
To achieve static voltage equalization, devices with parameters and characteristics that are as consistent as possible should be selected. Alternatively, resistor voltage equalization can be used. The resistance of RP should be much smaller than the forward and reverse resistance of any device when it is blocking, so that the voltage shared by each thyristor depends on the voltage division of the resistor.
Similarly, voltage unevenness caused by differences in the dynamic parameters and characteristics of devices is called dynamic voltage unevenness. To achieve dynamic voltage equalization, devices with similar dynamic parameters and characteristics should be selected first. Alternatively, RC parallel branches can be used for dynamic voltage equalization, as shown in Figure 2b. For thyristors, using a strong gate pulse trigger can significantly reduce the difference in device turn-on time.
2.2 Parallel connection of thyristors
In high-power thyristor devices, multiple devices are often connected in parallel to handle large currents. When thyristors are connected in parallel, uneven current distribution can occur due to differences in their static and dynamic characteristics. Poor current sharing leads to some devices receiving insufficient current while others are overloaded, hindering the overall output of the device and even causing damage to the devices and the device itself. The primary measure for current sharing is to select devices with similar characteristic parameters. Additionally, current-sharing reactors can be used. Similarly, using gate pulse triggering also helps with dynamic current sharing. When thyristors need to be connected in both series and parallel simultaneously, the series connection is usually followed by the parallel connection. Comparatively, even with redundant design, series connection is riskier than parallel connection. Therefore, in circuit design, if both methods can achieve the desired result through circuit modification, parallel connection should be preferred, as current sharing is relatively easier to achieve.
V. Comparison of the performance of soft starters
Currently, there are two main types of medium and high voltage soft starter products available both domestically and internationally: one uses a high voltage frequency converter for soft starting, and the other uses a thyristor for soft starting. Here, I will briefly introduce the performance of these two devices.
(1) Soft start of high voltage frequency converter
Variable frequency drives (VFDs) are primarily used for speed control of AC motors, offering significant energy savings. However, if used for soft starting of motors, their speed control performance should not be compared to our company's products. Using a VFD for soft starting prevents overcurrent in the motor during the entire starting process, provides high starting torque, and exhibits excellent starting performance. However, this advantage is not apparent for loads with low starting torque, such as fans and pumps. Furthermore, the following drawbacks also exist:
1. Variable frequency drive (VFD) technology is still in its developmental stage. Due to relatively high switching losses, its reliability is relatively low, and its failure rate is relatively high, classifying it as a maintainable device. Various units often experience long downtimes due to insufficient maintenance technology. For example, a steel company once experienced a situation where a 35,000 KW blast furnace blower took more than a week to start. There are also cases where it is simply left unused.
2. The high harmonic content in the output voltage of a frequency converter can cause localized overheating in the motor's tooth slots, burning out the insulation and affecting the motor's lifespan. This is why specially designed frequency converter motors are required for speed control applications.
3. When using a frequency converter for soft starting, a good synchronization function is required when switching from the frequency converter to the mains frequency power supply to reach the sub-synchronous speed (some frequency converters do not have this function). Otherwise, mechanical shock will occur, damaging the mechanical equipment.
4. Variable frequency drives are expensive.
5. The circuit principle of frequency converters is complex, requiring a high level of maintenance skills and taking a long time to repair.
VI. Conclusion
Medium-voltage motor soft starters are now widely used in metallurgy, petrochemicals, pharmaceuticals, municipal engineering, and other fields. Traditional starting methods have many shortcomings, and these problems are even more pronounced for large-capacity equipment. With the increasing application of soft starters in production practice, choosing soft-start motors will be an inevitable trend in the future.
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