A frequency converter (VDC) is a control device that changes the frequency of the power supply to a motor to control an AC motor. To enhance your understanding of VDCs, this article will introduce the application of VDCs in motor control and the matching relationship between VDCs and motors. If you are interested in VDCs, please continue reading.
I. Application of Variable Frequency Drives and Motor Control
The widespread use of frequency converters has brought benefits to mechanical equipment in various fields! Only when used in conjunction with different types of machinery can the advantages of frequency converters be fully realized! The control of frequency converters and motors is one of the many applications in mechanical equipment! Here, we will introduce the application of frequency converters in motor control!
(1) Controlling the starting current of the motor. When the motor is started directly at the mains frequency, it will generate 7 to 8 times the rated current of the motor. This current value will greatly increase the electrical stress on the motor windings and generate heat, thereby reducing the life of the motor. Variable frequency speed control, on the other hand, can start at zero speed and zero voltage (with appropriate torque boost). Once the relationship between frequency and voltage is established, the frequency converter can drive the load according to V/F or vector control. Using variable frequency speed control can significantly reduce the starting current and improve the winding's load-bearing capacity. The most direct benefit for users is that the maintenance cost of the motor will be further reduced, and the life of the motor will be increased accordingly.
(2) Reduce voltage fluctuations in power lines. During motor startup at power frequency, the current surges, causing significant voltage fluctuations. The magnitude of the voltage drop depends on the power of the starting motor and the capacity of the power distribution network. Voltage drops can cause voltage-sensitive equipment in the same power supply network to malfunction, trip, or malfunction, such as PCs, sensors, proximity switches, and contactors. However, by using variable frequency speed control, which allows for gradual startup at zero frequency and zero voltage, voltage drops can be eliminated to the greatest extent possible.
(3) Lower power required for startup. Motor power is directly proportional to the product of current and voltage. Therefore, the power consumed by a motor directly started at the mains frequency will be significantly higher than that required for variable frequency starting. In some operating conditions, the power distribution system has already reached its maximum limit, and the power surge generated by directly starting the motor at the mains frequency will seriously affect other users on the same grid, resulting in warnings or even fines from the grid operator. Using a variable frequency drive (VFD) for motor starting and stopping avoids these problems.
(4) Controllable acceleration function. Variable frequency speed control can start at zero speed and accelerate evenly according to the user's needs, and its acceleration curve can also be selected (linear acceleration, S-shaped acceleration, or automatic acceleration). In contrast, starting at mains frequency will generate severe vibrations in the motor or connected mechanical shafts or gears. This vibration will further aggravate mechanical wear and loss, reducing the lifespan of mechanical parts and motors. In addition, variable frequency starting can also be used in applications such as filling lines to prevent bottles from tipping over or being damaged.
II. How to handle the matching relationship between the frequency converter and the motor
Using frequency converters to drive motors has become an irreversible trend. However, in actual use, improper matching between the frequency converter and the motor often leads to problems. When selecting a frequency converter, it is essential to fully understand the load characteristics of the equipment it drives. Ms. Can has compiled some relevant information to share, hoping it will be helpful in practical work.
We can classify production machinery into three types: constant power load, constant torque load, and fan and water pump load. Different load types have different requirements for frequency converters, and we should match them reasonably according to the specific situation.
1. Constant power load
The torque required by machine tool spindles and in rolling mills, paper machines, and winding/unwinding machines in plastic film production lines is generally inversely proportional to the rotational speed, classifying them as constant power loads. The constant power nature of a load refers to a certain range of speed variation. At very low speeds, due to mechanical limitations, it will change to a constant torque load. When a motor adjusts its speed using constant flux, it is a constant torque speed regulation; while when it adjusts its speed using field weakening, it is a constant power speed regulation.
2. Fan and pump loads
As the impeller of equipment such as fans, water pumps, and oil pumps rotates, the torque decreases with the square of the rotational speed. The power required by the load is directly proportional to the cube of the speed. When the required air volume or flow rate decreases, adjusting the air volume and flow rate using a frequency converter can significantly save electricity. Because the power required increases too rapidly with the rotational speed at high speeds, fans and pumps should not be operated above their operating frequency.
3. Constant torque load
TL remains constant or nearly constant at any speed. When a frequency converter drives a load with constant torque, the torque at low speeds must be large enough, and it must have sufficient overload capacity. If stable operation at low speeds is required, the motor's heat dissipation performance should be considered to prevent the motor from burning out due to excessive temperature rise.
