The energy feedback unit supplier reminds you that the main function of the frequency converter is to control the control equipment of the AC motor by changing the frequency of the motor's working power supply. Do you know the types of frequency converters? What are the differences between vector specific frequency converters and general-purpose frequency converters?
There are two main differences between vector specific frequency converters and ordinary frequency converters. The first is high control accuracy, and the second is high output torque at low speeds.
Vector specific frequency converter:
The working principle of a vector specific frequency converter is to first rectify it, and then invert it to obtain the desired frequency and voltage.
Vector control technology uses coordinate transformation to equivalently transform a three-phase system into an M-T two-phase system, decomposing the stator current vector of an AC motor into two DC components (i.e. magnetic flux component and torque component), thereby achieving the goal of separately controlling the magnetic flux and torque of the AC motor, and thus achieving the same good control effect as a DC speed control system.
Vector control, also known as' speed control ', has some differences from its literal meaning.
V/F control mode: Just like when driving, the throttle opening on your feet remains constant, while the speed of the car is definitely changing! Because the road on which the car travels is uneven, the resistance on the road is also changing. When going uphill, the speed will slow down, and when going downhill, the speed will increase, right? For a frequency converter, your frequency setting value is equivalent to the throttle opening on your foot while driving, and the throttle opening is fixed during V/F control.
Vector control method: It can control the vehicle to maintain a constant speed as much as possible under changes in road conditions, resistance, uphill, downhill, and other conditions, improving speed control accuracy.
Universal frequency converter:
A universal frequency converter is one that can be applied to all loads. But if there is a dedicated frequency converter, it is still recommended to use a dedicated frequency converter. Dedicated frequency converters are optimized according to the characteristics of the load, with the characteristics of simple parameter settings, better speed regulation, and energy-saving effects.
The correct selection of frequency converter is crucial for the normal operation of the control system. When choosing a frequency converter, it is necessary to fully understand the load characteristics driven by the frequency converter. People often divide production machinery into three types in practice: constant torque load, constant power load, and fan/pump load.
Constant torque load:
The load torque TL is independent of the speed n, and TL always remains constant or almost constant at any speed. For example, friction loads such as conveyor belts, mixers, extruders, as well as potential loads such as cranes and hoists, all belong to constant torque loads.
When a frequency converter drives a load with constant torque properties, the torque at low speeds should be sufficiently large and have sufficient overload capacity. If stable operation at low speeds is required, the heat dissipation capacity of standard asynchronous motors should be considered to avoid excessive temperature rise of the motor.
Constant power load:
The required torque for machine tool spindles, rolling mills, paper machines, and plastic film production lines such as coilers and uncoiler is generally inversely proportional to the rotational speed, which is known as constant power load. The constant power property of the load should be limited to a certain range of speed changes. When the speed is very low, due to the limitation of mechanical strength, TL cannot increase infinitely and transforms into a constant torque property at low speeds. The constant power and constant torque regions of the load have a significant impact on the selection of transmission schemes. When the motor is in constant flux speed regulation, the maximum allowable output torque remains unchanged, which belongs to constant torque speed regulation; In weak magnetic speed regulation, the maximum allowable output torque is inversely proportional to the speed, which belongs to constant power speed regulation. If the range of constant torque and constant power speed regulation of the electric motor is consistent with the range of constant torque and constant power of the load, that is, in the case of "matching", the capacity of the electric motor and the capacity of the frequency converter are both minimized.
Fan and pump loads:
In various fans, water pumps, and oil pumps, the resistance generated by air or liquid within a certain speed range with the rotation of the impeller is roughly proportional to the second power of the speed n. As the rotational speed decreases, the rotational speed decreases to the power of 2. The power required for this load is proportional to the third power of the speed. When the required air volume and flow rate decrease, using a frequency converter to adjust the air volume and flow rate through speed regulation can significantly save electricity. Due to the rapid increase in required power with the speed at high speeds, which is proportional to the third power of the speed, it is generally not advisable to operate loads such as fans and pumps beyond the power frequency.