Some explanations regarding frequency converters and magnetic couplers
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Recently, I’ve read some articles on permanent magnets; this is a rather extreme topic. Let’s see what the experts in this field have to say. 1. Introduction: China’s economy is currently in a period of rapid development. As the annual industrial output value continues to rise, energy consumption also increases significantly. Due to an unbalanced distribution of industrial development in the country, the shortages of electricity and high electricity prices in industrial production have become serious obstacles to economic growth. Therefore, policies aimed at energy conservation and emission reduction have been introduced. Currently, thermal power generation plants have high energy consumption for their auxiliary equipment. Moreover, the power grid places increasing demands on generator sets to assist in peak load management, which further keeps the energy consumption of these auxiliary devices high and severely hinders improvements in economic efficiency. Converting the fans among the main auxiliary equipment in power plants using variable frequency technology yields a very significant energy-saving effect. Therefore, high-voltage variable-frequency energy-saving technology, with its excellent speed control performance, comprehensive protection functions, significant energy-saving effects, and the ability to interface easily with DCS automatic control systems for automated regulation (while also enabling the optimization of the unit’s control performance and contributing to its stable operation), will undoubtedly be widely used in the renovation of high-pressure, high-capacity rotating equipment such as exhaust fans in power plants. In addition to saving energy, the use of frequency converters offers the following benefits for the machinery: (1) The high-voltage frequency converters have an excellent soft start/stop function (they can start at zero speed), and the maximum current during startup is less than the rated current, **which reduces the impact of the starting surge current on the motor and the power grid. It effectively reduces motor failures. Thus, **it extends the maintenance interval and service life of the motor. It can also effectively prevent the adverse effects of shock loads on the power grid ； (2) After the frequency conversion upgrade, the original control dampers were kept fully open, **which reduced their wear and extended their service life, lowered maintenance costs, and further decreased the resistance in the air ducts ； (3) After the frequency conversion upgrade, the power factor can be improved (the frequency-conversion power factor can reach 0.96), reducing line losses ； (4) The smooth regulation unique to high-voltage frequency converters reduces the mechanical wear of fans and motors, while also lowering the temperature of bearings and bushings. This effectively cuts down on maintenance costs and extends the service life of the equipment. 2. Regarding magnetic couplers, the common method of adjusting flow rate by controlling the opening degree serves to regulate both flow rate and pressure. However, this approach results in the motor operating at a low load, which constitutes inefficient operation; moreover, the motor’s power factor is low, leading to negative effects on the quality of electricity in the power grid, as well as some waste of electrical energy. Speed control is achieved using a hydraulic coupler, which is an inefficient method for speed regulation; it involves significant coupling losses and slip losses, results in high energy consumption, poor speed control accuracy, as well as severe non-linearity. It also leads to unreliable operation and requires extensive maintenance. Unlike methods that use variable frequency drive to adjust the motor’s speed, a magnetic coupler (also known as a permanent magnet drive) is installed between the motor and the pump, transmitting torque through magnetism rather than through a conventional mechanical connection. During operation, the motor always runs at its rated speed; the speed of the pump is adjusted directly through a permanent magnet speed regulator, thereby enabling control over the flow rate output by the water pump. A magnetic coupler (also known as a permanent magnet speed regulator; it is actually an upgraded version of a hydraulic coupler) is used to transmit torque through magnetism (while a hydraulic coupler transmits torque through fluid) It is claimed to have been introduced into the domestic market from the United States in 2006. Looking at its background, it can be seen that magnetic couplers were actually phased out in the U.S. market due to the advancement of frequency converters; as a result, magnetic couplers had to seek opportunities in the Chinese market! It was renamed a permanent magnet speed regulator (a misrepresentation of the concept), but it has never been accepted by the market due to the following five main reasons: 1. Limited modification options. It is used for speed control between the motor and the load; during renovation, it is necessary to remove the foundations of the motor or the load, and new foundations have to be poured (the scope of work for renovation is enormous!) ), however, the displacement of most motors is limited, making renovation impossible! 2. Energy savings are limited. (Its transmission efficiency is very low at low speeds; however, true energy savings are only achieved at low speeds, so although it can adjust its speed, it does not save energy!) It is not as energy-efficient as the adjustment method of \"axial flow fans with adjustable static blades\". 3. The cost of modification is high; since it is claimed to be introduced from the United States and is indeed more advanced in technology compared to hydraulic couplers, its price in the Chinese market is very high – in fact, its market price is even higher than that of hydraulic couplers in terms of manufacturing costs! It’s extremely uneconomical; if choosing a magnetic coupling, it’s more cost-effective to opt for a hydraulic coupling instead! ) 4. Power factor. (Since inverters can raise the power factor of electrical systems to over 0.95, magnetic couplers simply cannot achieve such an improvement in the power factor!) ). 5. Startup method. (Since the magnetic coupler uses direct starting at the motor’s no-load power frequency, in some high-power motor applications, the starting inrush current remains very high!) Since the inverter can start under full load, its starting capability is clearly superior to that of a magnetic coupler). In recent years, there have been many projects involving the removal of hydraulic couplings to change the frequency; it is believed that soon there will also be projects involving the removal of magnetic couplings for this purpose! With the continuous development of the power electronics industry, and the ongoing accumulation and improvement of domestic high-voltage frequency converter technology as well as field operation experience, devices such as magnetic couplers will surely be phased out of the market!
The bottom determines the head; permanent magnet couplings have nothing to do with hydraulic couplings at all. Any talk of an upgraded version is nonsense. The lifespan and reliability of frequency converters are issues that trouble users. I’ve also seen some units that have been in use for several years; when it’s time to upgrade to systems without frequency converters, people are reluctant to do so due to the fear of breakdowns and frequent shutdowns, eventually leading to the need to discard those units after their useful life expires. Hydraulic couplings are highly mature technologies with a history of over a century. Ordinary domestic versions are very cheap, and there is fierce price competition; in order to reduce costs, aluminum castings are sometimes not even annealed, resulting in average quality. If one is reluctant to use high-quality bearings and seals, they are more likely to break down – a typical characteristic of products made in China. Of course, discerning customers will choose imported brands; as for the price, it’s roughly ten times that of domestic ones. The efficiency of a hydraulic coupling is not that low; it is at least around 85%. When the speed is adjusted to 67%, or if the speed is higher or lower, the efficiency increases accordingly. At full speed, it’s about 95%, and there is test data to support this; the remaining 3% is due to heat generation caused by slip and mechanical losses. The speed and power of fluid couplings can be very high, exceeding 10,000 revolutions per minute and 10,000 kilowatts. Permanent magnet speed regulators use air cooling for low-power applications; their structure and installation are simpler than those of hydraulic couplings, and no additional cooling system is required. However, they generate relatively high noise levels, which can exceed 100 decibels if not properly managed. Air-cooled high-speed operation also isn’t feasible; currently, two-pole motors are used very rarely, still due to high noise levels and poor bearing lifespan. High-power units require water or oil cooling; the system is complex, and their speed control performance is inferior to that of fluid couplings. The force required for speed control is much greater in such units compared to fluid couplings. High power and fluid coupling ratio offer no advantages. Hydraulic couplings and permanent magnets can be used as clutches, something that frequency conversion cannot achieve. Hydraulic couplings and permanent magnets result in a high apparent current since the motor operates at a low power factor after speed regulation, with no power compensation; however, after compensation, the difference between them and variable frequency drives becomes minimal. Hydraulic couplings and permanent magnets are mechanical components; they operate reliably under normal conditions and will not stop functioning due to circuit components or electrical interference. Hydraulic couplings can be used for over a decade without any problems; bearings only need to be replaced during major repairs every few years. The permanent magnet product is indeed a characteristic of China’s industry – it’s not as good as those from other countries, nor is it superior to local alternatives. Its price is high due to the expensive materials and the costly processing involved, with so much copper and strong magnets used in its production. The permanent magnet speed regulator was not invented by Megna; it was only granted permission to produce it, and the actual company that invented it has closed down. Megna is not doing well in the United States and incurs losses every year, which is why it was sold to the Chinese. Liquid couplings are made of ordinary steel or aluminum alloys; thanks to mature manufacturing processes and mass production, their price is naturally low. In terms of sales volume, although many liquid couplings have now been replaced by variable-frequency devices, their annual output value still amounts to hundreds of millions, and that figure is real – after all, prices are extremely competitive. The annual output value of permanent magnets will not exceed 50 million, and the amount that needs to be repaired is significant. Frequency conversion: I don’t have any data. I started working with hydraulic couplings in 1997, and began working with permanent magnets in 2010; the differences between these two types of products are quite clear. I believe that the market for hydraulic actuators will not disappear; they will not be phased out. It’s hard to say about permanent magnets. Voith has developed permanent magnet retarders for the soft braking of large vehicles; they represent another competitor to hydraulic and electromagnetic retarders, so it’s worth keeping an eye on them.