Industrial inverters use seven major misunderstandings (I)

With the implementation of policies and the vigorous advancement of frequency conversion technology, coupled with the vigorous publicity of inverter manufacturers, some industrial enterprises have used the inverter and energy-saving power-saving paintings in the subconscious. However, in actual use, due to different situations, many companies have gradually realized in practice that not all places where inverters are applied can save energy and save electricity. So what are the reasons for this situation and what are the misunderstandings about the inverter?

Misunderstanding: Power saving can be achieved by using the inverter

Some literature claims that the frequency converter is a power-saving control product, giving the impression that power can be saved by using the inverter.

In fact, the reason why the inverter can save electricity is because it can regulate the motor. If the inverter is a power-saving control product, then all the speed control equipment can also be said to be a power-saving control product. The inverter is only slightly higher in efficiency and power factor than other speed control devices.

Whether the inverter can achieve power saving is determined by the speed regulation characteristics of its load. For loads such as centrifugal fans and centrifugal pumps, the torque is proportional to the square of the speed, and the power is proportional to the cube of the speed. As long as the original valve is used to control the flow, and it is not working at full load, it can be saved to speed control. When the speed drops to 80% of the original, the power is only 51.2% of the original. It can be seen that the application of the frequency converter in this type of load has the most obvious power saving effect. For loads such as Roots blowers, the torque is independent of the magnitude of the speed, ie the constant torque load. If the air volume is adjusted by using the venting valve to remove the excess air volume, and the speed control operation is changed, the power saving can also be realized. When the speed drops to 80% of the original, the power is 80% of the original. It is much less efficient than the application in centrifugal fans and centrifugal pumps. For constant power loads, the power is independent of the magnitude of the speed. Constant power load of cement plant, such as batching belt scale, under the condition of set flow rate, when the material layer is thick, the belt speed is slowed down; when the material layer is thin, the belt speed is accelerated. The application of the inverter in this type of load cannot save power.

Compared with the DC speed control system, the DC motor has higher efficiency and higher power factor than the AC motor. The digital DC governor is as efficient as the inverter. Even the digital DC converter is slightly more efficient than the inverter. Therefore, it is claimed that the use of AC asynchronous motors and frequency converters is more energy efficient than the use of DC motors and DC converters. Theory and practice have proved that this is not true.

Misunderstanding: The capacity selection of the inverter is based on the rated power of the motor.

Compared with the electric motor, the price of the frequency converter is relatively expensive, so it is very meaningful to reasonably reduce the capacity of the frequency converter under the premise of ensuring safe and reliable operation.

The power of the inverter refers to the power of the 4-pole AC asynchronous motor to which it is applied.

Due to the same capacity motor, the number of poles is different and the rated current of the motor is different. As the number of poles of the motor increases, the rated current of the motor increases. The capacity selection of the inverter cannot be based on the rated motor power. At the same time, for the transformation project that did not use the inverter, the capacity selection of the inverter can not be based on the rated current of the motor. This is because the capacity selection of the motor should take into account factors such as maximum load, richness factor, motor specifications, etc., and often have a large margin, and industrial motors often operate at 50% to 60% of rated load. If the capacity of the inverter is selected based on the rated current of the motor, the margin is too large, resulting in economic waste, and the reliability is not improved.

For squirrel-cage motors, the capacity of the inverter should be selected based on the principle that the rated current of the inverter is greater than or equal to 1.1 times the maximum normal operating current of the motor, which can save money. For heavy-duty starting, high temperature environment, wound motor, synchronous motor, etc., the capacity of the inverter should be appropriately increased.

For the design of the inverter from the beginning, the choice of the inverter capacity is based on the rated current of the motor. This is because the inverter capacity cannot be selected in actual operation at this time. Of course, in order to reduce investment, in some occasions, the capacity of the inverter can be determined first. After the equipment is actually running for a period of time, it is selected according to the actual current.

In a Φ24m×13m cement mill secondary grinding system of a cement company in Inner Mongolia, there is a domestically produced N-1500 O-Sepa high-efficiency classifier with a motor type Y2-315M-4 and a motor power of 132kW. However, the FRN160-P9S-4E inverter is used. This inverter is suitable for 4-pole, 160kW electric motor. After being put into operation, the maximum working frequency is 48Hz, the current is only 180A, less than 70% of the rated current of the motor, and the motor itself has a considerable margin. The inverter selection specification is one level larger than the drag motor, which causes unnecessary waste and the reliability will not be improved.

The No. 3 limestone crusher of Anhui Chaohu Cement Plant uses a 1500×12000 plate feeder for the feeding system. The Y225M-4 AC motor is used for the drag motor. The rated power of the motor is 45kW and the rated current is 84.6A. Before the frequency conversion speed regulation transformation, it was found through the test that when the plate feeder was running normally, the average current of the three phases was only 30A, only 35.5% of the rated current of the motor. In order to save investment, the ACS601-0060-3 type inverter is used. The rated output current of the inverter is 76A. It is suitable for 4-pole, 37kW electric motor, and has achieved good results.

The two examples are reversed and explained. For the transformation project that did not use the inverter, the capacity of the inverter can be greatly reduced according to the actual working conditions.