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# Application Error of Inverter in Practical

1, use inverter can save electricity

Some literature claims that VVVF is a power-saving control product, giving the impression that as long as the use of VVVF can save power.

In fact, frequency conversion governor can save electricity, because it can be on the motor speed.If the frequency conversion governor is energy-saving control products, then all the speed regulating equipment can also be said to be energy-saving control products.The variable frequency governor is only slightly higher than other speed regulating equipment efficiency and power factor.

Whether the variable frequency governor can achieve power saving is determined by the speed regulation characteristics of its load.For centrifugal fan, centrifugal pump load, torque and speed of the square proportional to the power and speed of the cube proportional to the cube.As long as the original use of valve control flow, and not full load work, instead of speed regulation operation, can achieve power saving.When the speed drops to 80% of the original, the power is only 51.2% of the original.Visible, frequency conversion governor in the application of this type of load, the effect of saving electricity is the most obvious.For the load of Roots blower, the torque is independent of the speed, that is, the constant torque load.If the original use of the air valve to let off excess air volume of the method to regulate the air volume, to speed regulation operation, can also achieve power saving.When the speed drops to 80% of the original, the power is 80% of the original.Compared with the centrifugal fan, centrifugal pump in the application of electricity saving effect is much smaller.For a constant power load, the power is independent of the speed.Constant power load of cement plant, such as batched belt scale, under the condition of set flow rate, when the material layer is thick, the belt speed slows down;When the material is thin, the belt speeds up.Variable frequency governor in the application of this type of load, can not save electricity.

Compared with dc speed regulating system, dc motor is more efficient than AC motor and has higher power factor. Digital DC speed governor is almost the same as variable frequency speed governor, and even digital DC speed governor is slightly more efficient than variable frequency speed governor.Therefore, the claim that the use of ac asynchronous motor and frequency conversion governor than the use of DC motor and DC governor to save electricity, theory and practice proved that this is not correct.

2. Capacity selection of frequency converter is based on rated power of motor

Relative to the motor, the price of VVVF is more expensive, so in the premise of ensuring safe and reliable operation, it is very meaningful to reasonably reduce the capacity of VVVF.The power of the variable-frequency governor refers to the power of the 4-pole AC asynchronous motor for which it is applicable.

Because of the same capacity motor, the number of poles is different, the rated current of the motor is different.With the increase of the number of poles, the rated current of the motor increases.The capacity selection of the variable frequency governor cannot be based on the rated power of the motor.At the same time, for the original conversion project did not use the converter, the capacity of the variable frequency governor can not be based on the rated current of the motor.This is because, the capacity selection of the motor to consider the maximum load, affluance coefficient, motor specifications and other factors, often large affluance, industrial electric motor is often operating under 50% ~ 60% rated load.If the rated current of the motor is the basis to choose the capacity of the variable frequency governor, there is too much wealth, resulting in economic waste, and the reliability has not been improved.

For squirrel-cage motors, the capacity selection of variable frequency governor should be based on the principle that the rated current of the frequency converter is greater than or equal to the maximum normal operating current of the motor, 1.1 times, so as to save money to the greatest extent.For heavy load starting, high temperature environment, winding motor, synchronous motor and other conditions, the capacity of inverter governor should be appropriately increased.

In the design of frequency converter at the very beginning, the capacity of frequency converter is chosen according to the rated current of the motor.This is because the capacity of the converter cannot be selected according to the actual operation.Of course, in order to reduce the investment, in some occasions, you can also not determine the capacity of the inverter, such as the actual operation of the equipment for a period of time, then according to the actual current selection.