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Application of frequency converter on mixer

With the maturity of frequency conversion technology, frequency converters are more and more widely used in the field of electric drive. The diversification of its control methods, the perfect motor protection function and its unique advantages are currently unparalleled in the field of industrial control.

One: the principle of transformation

According to the working principle of the fluid transfer equipment mixer, the flow rate (air volume) of the mixer is proportional to its rotation speed; the pressure (head) of the mixer is proportional to the square of the rotation speed, and the shaft power of the mixer is equal to the product of the flow rate and the pressure, so the shaft of the mixer The power is proportional to the cube of its rotational speed (ie, proportional to the cube of the power supply frequency). According to the above principle, it can be known that by reducing the rotational speed of the agitator, its power can be reduced more. For example, if the power supply frequency is reduced from 50Hz to 45Hz, then P45/P50=(45/50)3=0.729, ie P45=0.729P50 (P is the motor shaft power); if the power supply frequency is reduced from 50Hz to 40Hz, then P40/ P50=(40/50)3=0.512, ie P40=0.512P50 (P is the motor shaft power).

The power consumption of the mixer is proportional to the cube of the speed. That is, N=KN3 N: power consumption for the mixer; n: rotation speed when the mixer is running; K is a proportional coefficient. The design of the mixer is designed according to the power frequency operation. However, except for the high speed during the mixing, the flow rate is small for most of the time. Because the frequency conversion technology and the microcomputer technology are controlled by the microcomputer, the rotation speed of the mixer can be changed with the flow rate. Change, and ultimately achieve the goal of energy saving. Practice has proved that the use of frequency conversion equipment can reduce the average running speed of the mixer by 20% compared with the power frequency, thus greatly reducing energy consumption, and the energy saving rate can reach 20%-40%.

Since the mixer is a typical square torque load type, its power (shaft power), torque (pressure), and speed (air volume) satisfy the following relationship (similarity theorem):

Pressure = P axis = QH

Q’/Q=N’/N then Q’=QN’/N

P'/P=(N'/N)3 then P'=P(N'/N)3

Asynchronous motor speed formula n=60f(1-s)/p

Where: N, Q, H, P - rated speed of the mixer, air volume, shaft power

N', Q', H', P' - rated speed, water volume, shaft power of the mixer after speed regulation

Under the premise that a certain range meets the production requirements, the stirring speed can be flexibly adjusted by changing the rotation speed to facilitate uniform stirring without changing the working cycle. This characteristic indicates that the speed of the mixer is adjusted and the output of the motor is changed so that the stirring speed always meets the process requirements.

In summary: the use of frequency conversion technology to change the motor speed to adjust the flow and speed changes to replace the traditional power frequency circuit control, can achieve significant energy savings

Second: the transformation plan

Problems in the original system conditions

1. Although the main motor starts at the star-angle decompression, the current at the start is still large, which will affect the stability of the power grid and the safe operation of other electrical equipment.

2. The main motor runs at a power frequency, which is a non-economic operation, and the power is wasted seriously.

3. The main motor power frequency operation causes the mixer to operate with great noise.

4. The main motor power frequency starting equipment has a large impact, and the motor bearing wears a lot, so the equipment maintenance work is cumbersome.

Based on the above characteristics, our company adopts the following system solutions:

Adopting Kenken WK9600 series heavy-duty inverter

The main features of this product are:

1. Voltage vector control technology.

2. Automatic torque boost function, low frequency and large torque output.

3. Carrier frequency 0-16K adjustable, silent operation.

4. Diversified control methods, versatility, multi-speed and program running functions.

5. Built-in brake unit for quick stop.

6. Built-in PID adjustment function, closed loop control is simple.

7. Built-in RS-485 interface, computer network control,

8. With time accumulation function, can display a single power-on operation

Time, cumulative running time.

9. Microprocessor for motor control, frequency resolution up to 0.01HZ.

In the actual work process, the multi-stage speed can be set according to the characteristics of the agitation material and the time of adding other chemicals, and up to 16 different speeds can be achieved. The automatic program operation mode at different times greatly reduces the labor intensity of workers and improves production efficiency.

Three: Performance after system transformation:

(1) Using inverter control, the maximum energy saving is achieved on the premise of meeting the requirements of use.

(2) Due to the slow speed operation and soft start, vibration, noise and wear are reduced, the equipment maintenance period and service life are prolonged, the MTBF (mean fault repair time) value of the equipment is increased, and the impact on the power grid is reduced, and the power is increased. System reliability.

(3) The system has various protection measures to greatly improve the operating rate and safety reliability of the system.

(4) The variable frequency speed control closed-loop control system and the power frequency control system are mutually interlocked, which does not affect the operation of the original system, and the original power frequency control system can still operate normally when the variable frequency speed control closed-loop control system is inspected for maintenance or failure.