The correct selection of the inverter is very important for the normal operation of the control system. When selecting an inverter, it is necessary to have a thorough understanding of the load characteristics driven by the inverter. In practice, people often divide production machinery into three types: constant torque load, constant power load and fan and pump load.
1. Constant torque load
The load torque TL is independent of the speed n, and the TL always insists to be constant or fundamentally constant at any speed. For example, friction loads such as conveyor belts, mixers, and extruders, as well as potential energy loads such as cranes and hoists, are all constant torque loads. When the inverter drags a load with constant torque properties, the torque at low speed should be large, and there is a satisfactory overload capacity. If you need to run at a steady speed at low speed, you should consider the heat dissipation capacity of the standard asynchronous motor to avoid excessive temperature rise of the motor.
2. Constant power load
The torque required by the spindle of the machine tool and the coiler and uncoiler in the rolling mill, paper machine, plastic film production line, etc., is roughly inversely proportional to the speed, which is the so-called constant power load. The constant power nature of the load should be in terms of a definite range of speed variations. When the speed is very low, due to the limitation of mechanical strength, TL may not increase indefinitely and change to constant torque properties at low speeds. The constant power zone and constant torque zone of the load have a great influence on the selection of the drive plan. When the motor is regulated by constant flux, the output torque remains unchanged, which belongs to constant torque regulation; In the case of weak magnetic speed regulation, the output torque is inversely proportional to the speed, which belongs to constant power speed regulation. If the constant torque and constant power speed regulation range of the motor are consistent with the constant torque and constant power range of the load, that is, the so-called "matching" situation, the capacity of the motor and the capacity of the inverter are small.
3. Square rotation load
In various fans, water pumps, and oil pumps, the resistance generated by air or liquid in a certain speed range is roughly proportional to the second power of velocity n as the impeller rolls. With the decrease of the speed, the torque decreases by the 2nd power of the speed. The power required for such a load is proportional to the 3rd power of speed. When the required air volume and flow rate are reduced, the inverter can be used to adjust the air volume and flow rate through speed regulation, which can greatly save electric energy. Because the required power at high speed increases too fast with the speed and is proportional to the cubic of the speed, the fan and pump load should not be operated beyond the power frequency.
When selecting the inverter, you should refer to the load characteristics, and the actual motor current value should be used as the basis for the selection of the inverter, and the rated power of the motor can only be used as a reference. In addition, it should be fully considered that the output of the inverter contains abundant higher harmonics, which will deteriorate the power factor and efficiency of the motor. Therefore, the power supply to the motor with the inverter is more than the power supply to the industrial frequency grid, and the current of the motor will increase by 10% and the temperature rise will increase by about 20%. Therefore, when selecting motors and inverters, this situation should be taken into account and a margin should be left appropriately to avoid excessive temperature rise and affect the service life of the motor.
Fan and pump load conversion selection
Generally, fans and pumps have simple requirements for inverters, and only the capacity of the inverter is equal to the capacity of the motor.
Roots blowers, deep water pumps, sediment pumps, and fast-changing music fountains can be selected with a larger capacity.
Crane loads
This type of load is characterized by a large impact at start-up, which requires a certain margin of the inverter. At the same time, when the heavy object is lowered, there will be energy feedback, so it is necessary to use the braking unit or choose the common bus method. The type of inverter can be selected E180-CH series crane special inverter
Large inertial loads
This type of load has a large inertia, so it may oscillate when it is started, and there is energy feedback when the motor decelerates. A slightly larger inverter should be used to speed up start-up and avoid oscillation. Cooperate with the braking unit to eliminate the regenerative energy.
The load is not balanced
The load is sometimes light and sometimes heavy, and the inverter capacity should be selected according to the condition of the heavy load.
Select the type when the wiring interval with the motor is long
If the cable between the inverter and the motor is too long (more than 50 meters), it is necessary to install a communication output reactor at the output end of the inverter to avoid damage to the motor and the inverter caused by the coupling capacitance between the long cable to the ground and the distributed capacitance between the cable. In addition, the output cable is too long, the voltage drop is large, and the output torque of the inverter may not be formed, so the capacity of the inverter can be expanded.
Type selection in the case of parallel operation of multiple motors
When the inverter is used to operate several motors connected in parallel, it must be considered that the sum of the length of the inverter-to-motor cables is within the allowable range of the inverter. If the specified value is exceeded, it is necessary to expand one or two gears to select the inverter. In other cases, the control method of the inverter can only be the V/F control method, and the inverter cannot complete the overcurrent and overload maintenance of the motor, so it is necessary to install a thermal relay for each motor for maintenance.
The load is selected for the type of high-speed motor
When using an inverter to control a high-speed motor, a large number of high-order harmonics will be generated because the reactance of the high-speed motor is small. These higher harmonics will increase the output current value of the inverter. Therefore, when selecting the inverter for high-speed motors, it should be slightly larger than the inverter of general motors.
The load is selected for the capacity of the winding motor
When the inverter drives the winding rotor asynchronous motor, because the winding motor is small compared with the general squirrel cage motor, the impedance of the winding motor of the winding motor is small, therefore, it is easy to overcurrent tripping caused by ripple current, so the inverter with a slightly larger capacity than the general capacity should be selected.
The load is selected for the type of synchronous motor
When the inverter drives the synchronous motor, compared with the power frequency power supply, the output capacity will be reduced by 10%-20%, and the continuous output current of the inverter is greater than the product of the rated current of the synchronous motor and the standard unit value of the synchronous intake current.
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