Misconceptions about Inverter Applications (II)

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This post was last edited by zyz8438 on 2010-10-13 at 16:08. Misconception 2: The capacity of the frequency converter is determined based on the motor’s rated power. Compared to motors, frequency converters are relatively expensive; therefore, it is very important to reduce their capacity appropriately, while still ensuring safe and reliable operation. The power of an inverter-driven speed controller refers to the power of the 4-pole AC asynchronous motor it is designed for. For motors of the same capacity, different pole numbers result in different rated currents. As the number of poles in the motor increases, the motor’s rated current increases. The capacity selection of an variable frequency drive should not be based on the motor’s rated power. At the same time, for renovation projects that did not originally use frequency converters, the capacity selection of the frequency conversion speed regulators cannot be based on the motor’s rated current either. This is because the capacity selection of motors takes into account factors such as the maximum load, margin factor, and motor specifications; often a large margin is provided, and industrial motors typically operate at 50% to 60% of their rated load. If the capacity of the variable frequency drive is selected based on the motor’s rated current, an excessive margin is left, resulting in economic waste without any improvement in reliability. For squirrel-cage motors, the capacity selection of the variable frequency drive should be based on the principle that the rated current of the drive is 1.1 times or greater than the motor’s maximum normal operating current, thereby maximizing cost savings. For conditions such as heavy-load starting, high-temperature environments, wound-rotor motors, and synchronous motors, the capacity of the variable frequency drive should be increased appropriately. In designs that use frequency converters from the outset, it is reasonable to base the selection of the frequency converter’s capacity on the motor’s rated current. This is because the inverter capacity cannot be selected based on the actual operating conditions at this time. Of course, in order to reduce investment, in some cases it is also possible not to determine the capacity of the frequency converter upfront; instead, this can be decided after the equipment has been in operation for a while, based on the actual current consumption. In the secondary grinding system of a 2?4×13m cement mill at a cement company in Inner Mongolia, there is one domestically produced N-1500 type O-Sepa high-efficiency powder separator. The motor used is of the Y2-315M-4 type, with a power rating of 132kW; however, a FRN160-P9S-4E type frequency converter is used instead. This type of frequency converter is suitable for 4-pole motors with a power rating of 160kW. Once in operation, the maximum operating frequency is 48 Hz, and the current is only 180 A, which is less than 70% of the motor’s rated current; thus, the motor has a considerable margin of safety. Moreover, the specification chosen for the frequency converter is one grade higher than that required for the drive motor, resulting in unnecessary waste without any improvement in reliability. The No. 3 limestone crusher at Anhui Chaohu Cement Plant uses a 1500×12000 plate feeder for its feeding system; the drive motor is of the Y225M-4 type AC motor, with a rated power of 45 kW and a rated current of 84.6 A. Before carrying out the variable frequency speed control modification, tests showed that when the plate feeder motor was operating normally, the average three-phase current was only 30 A, which is 35.5% of the motor’s rated current. To save on investment, the ACS601‑0060‑3 type inverter was selected; it has a rated output current of 76A and is suitable for 4-pole motors with a power rating of 37kW, achieving good performance in practical use. These two examples, one negative and one positive, show that for renovation projects that did not originally use frequency converters, selecting the capacity of the frequency converter based on actual operating conditions can significantly reduce investment costs. More information: http://www.lingkun.net