Figure out the type of inverter load inverter correct selection for the normal operation of the control system is very critical. Selection of frequency converter must fully understand the characteristics of the load driven by the frequency converter. People in practice often production machinery is divided into three types: constant torque loads, constant power loads and fans, pump loads.
(01) Constant torque loads are loads in which the torque TL is independent of the rotational speed n, and TL is always constant or essentially constant at any rotational speed. For example, conveyor belt, mixer, extruder and other friction loads, as well as cranes, hoists and other potential energy loads are constant torque loads. When the frequency converter drags loads with constant torque, the torque at low speed should be large enough and have enough overload capacity. If it is necessary to run steadily at low speeds, the heat dissipation capacity of the standard asynchronous motor should be considered to avoid too high a temperature rise of the motor.
(02) Constant power loads The torque required by machine tool spindles and rolling mills, paper mills, winders and unwinders in plastic film production lines is roughly inversely proportional to the rotational speed, which is known as a constant power load. The constant power nature of the load should be in terms of a certain range of speed changes. When the speed is very low, by the mechanical strength of the limitations, TL can not be increased indefinitely, in the low-speed transformation into a constant torque nature. If the range of constant torque and constant power speed control of the motor and the range of constant torque and constant power of the load are consistent, that is, the so-called "matching" situation, the capacity of the motor and the capacity of the inverter are minimized.
(03) Fan and pump loads In various fans, pumps and oil pumps, with the rotation of the impeller, the resistance of air or liquid within a certain speed range is roughly proportional to the second power of the speed n. As the speed decreases, the torque is proportional to the second power of the speed. As the speed decreases, the torque decreases by the second power of the speed. The power required for this type of load is proportional to the third power of the speed. When the required air volume and flow rate decreases, the use of frequency converter to adjust the air volume and flow rate by means of speed regulation can significantly save power. Due to the high speed when the power required with the speed growth is too fast, and the speed of the third power is directly proportional to the speed, so usually should not make fans, pumps, loads over frequency operation.
Familiar with the principles of frequency conversion selection
(01) Combined with the overall framework of the project, starting from the process characteristics and electrical control, the load type, the use of the environment, communication architecture and interface types must be considered, such as whether the serial port, DP or PN communication interface.
(02) Select the frequency converter according to the load characteristics, such as the load for constant torque load can choose Siemens G120 frequency converter, such as the load for fans, pumps load can choose Siemens G120XA frequency converter.
(03) When selecting inverter, the actual motor current value should be taken as the basis for selecting inverter, and the rated power of the motor can only be used as a reference. In addition, due consideration should be given to the fact that the output of the frequency converter contains high harmonics, which will cause the power factor and efficiency of the motor to deteriorate. Therefore, with inverter to motor power supply and industrial frequency grid power supply comparison, the motor current increased by 10% and temperature rise increased by about 20%. So in the choice of motor and frequency converter, should take into account this situation, appropriate margin, in order to prevent the temperature rise is too high, affecting the service life of the motor.
(04) frequency converter to long cable operation, at this time should take measures to inhibit the influence of long cable coupling capacitance on the ground, to avoid the frequency converter is not enough, so the frequency converter should be enlarged by one grade selection or the output reactor installed at the output of the frequency converter.
(05) for some special applications, such as high ambient temperature, high switching frequency (especially in building automation and other noise restrictions on the use of higher applications need to pay attention to), high altitude, etc., at this time will cause the inverter's capacity reduction, the inverter needs to be enlarged by one gear selection.
(06) When the inverter is used to control several motors connected in parallel, the total length of the cable from the inverter to the motor must be considered within the tolerance of the inverter. If it exceeds the specified value, amplify the inverter by one or two stops to select the inverter. In addition, in this case, the control mode of the frequency converter can only be V / F control mode, and the frequency converter can not protect the motor overcurrent, overload protection, at this time need to be added to each motor fuse to realize the protection.
(07) for some special applications, such as high ambient temperature, high switching frequency, high altitude, etc., at this time will cause the frequency converter capacity reduction, the frequency converter needs to be enlarged by one gear selection.
(08) When using inverter to control high speed motor, due to the small reactance of high speed motor, the high harmonics also increase the output current value. Therefore, when choosing the inverter for high-speed motor, it should be slightly larger than the inverter for ordinary motor.
(09) When the frequency converter is used for variable pole motor, due attention should be paid to selecting the capacity of the frequency converter so that its maximum rated current is below the rated output current of the frequency converter. In addition, when converting poles in operation, the motor should be stopped first, otherwise it will cause the motor to idle, which will cause the inverter to be damaged in bad times.
(10) When driving an explosion-proof motor, the inverter does not have an explosion-proof structure, and the inverter should be set outside the hazardous place.
(11) When an inverter is used to drive a geared motor, the range of use is limited by the lubrication method of the rotating part of the gear. There is no restriction on lubrication in the low-speed range, but there is a danger of running out of lubricant in the high-speed range that exceeds the rated speed. Therefore, do not exceed the maximum speed tolerance.
(12) When the inverter drives a wound rotor asynchronous motor, it mostly utilizes an existing motor. The impedance of the winding of a wound motor is small compared with that of an ordinary squirrel cage motor. Therefore, overcurrent tripping due to ripple current is prone to occur, so an inverter with a slightly larger capacity than usual should be selected. Generally, wire-wound motor is mostly used in the occasion of larger flywheel torque GD2, and more attention should be paid to the setting of acceleration and deceleration time.
(13) inverter drive synchronous motor, compared with the industrial frequency power supply, reduce the output capacity of 10% to 20%, the continuous output current of the frequency converter should be greater than the synchronous motor rated current and synchronous draw current of the product of the standard value.
(14) For compressors, vibration machines and other loads with large torque fluctuations and oil pressure pumps and other peak loads, if the inverter is selected in accordance with the rated current or power value of the motor, overcurrent protection may be activated due to the peak current. Therefore, it is necessary to select an inverter with a rated output current that is larger than the maximum current, knowing the operating conditions of the industrial frequency. When the inverter drives the submersible pump motor, because the rated current of the submersible pump motor is larger than that of the usual motor, the rated current of the inverter should be larger than that of the submersible pump motor when selecting the inverter.
(15) When the frequency converter controls the Roots fan, because its starting current is large, so when selecting the frequency converter, be sure to pay attention to whether the capacity of the frequency converter is large enough.
(16) When selecting the frequency converter, be sure to pay attention to whether its protection level matches the situation on site. Otherwise, the dust and water vapor on the site will affect the long-term operation of the inverter.
(17) Single-phase motors are not suitable for inverter drive.
(18) When the motor load is very light, even if the motor load current is within the rated current of the frequency converter, the frequency converter that is much smaller than the motor capacity cannot be used. This is because the reactance of the motor is different with the capacity of the motor, even if the motor load is the same, the larger the capacity of the motor the larger the value of its pulsating current, and thus may exceed the allowable value of the current of the inverter. (19) If the power supply of inverter is self-supplied, it is better to add inlet reactor.