Inverter load characteristics
Inverter load characteristicsWhat is the most important factor for the normal operation of the inverter control system? The answer is load characteristics. Inverter selection requires a complete understanding of the characteristics of the load driven by the inverter. Therefore, in practice, production machines are often divided into three types: constant torque load, constant output load, and fan / water pump load, depending on the difference in load characteristics.
1. Constant torque load:
The load torque TL has nothing to do with speed n, and TL remains constant or nearly constant at any speed. For example, friction loads such as conveyor belts, mixers and extruders, and potential energy loads such as cranes and hoists are all constant torque loads.
If the inverter drives the load with a constant torque, the torque at low speeds must be large enough and have sufficient overload capacity. If you need to operate at a low speed and at a constant speed, you should consider the heat dissipation capacity of a standard asynchronous motor to avoid excessive temperature rise of the motor.
2. Constant power load:
The torque required for machine tool spindles, rolling mills, paper machines, and ticket dispensers and ticket dispensers on plastic film production lines is approximately inversely proportional to speed. This is the so-called constant power load. The constant power nature of the load should be in terms of a particular speed range. If the speed limited by the mechanical strength is very slow, the TL cannot be increased indefinitely, resulting in constant torque characteristics at low speeds. The constant output region and constant torque region of the load have a great influence on the selection of the transmission method. When the motor is adjusting the constant flux speed, the maximum allowable output torque does not change, which belongs to the constant torque speed adjustment. With field weakening speed adjustment, the maximum permissible output torque is inversely proportional to the speed.
This belongs to constant power rate adjustment. When the constant torque and constant output speed adjustment range of the motor matches the constant torque and constant output range of the load
If they are the same, that is, "matching", the capacity of the motor and the capacity of the frequency converter are minimized.
3. Fan and pump load:
In various fans, water pumps, and oil pumps, the resistance of air or liquid within a certain speed range is approximately proportional to the square of the speed n as the impeller rotates. When the speed decreases, the speed decreases according to the square of the speed. The power required for this load is proportional to the cube of speed. If the required airflow and flow rate are reduced, a frequency converter can be used to adjust the airflow and flow rate for speed adjustment. This can save a lot of electrical energy. Fan and pump loads are generally not allowed above the power frequency, as the power required at high speeds increases exponentially with speed and is proportional to the cube of the speed.
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