I. Overview.
Electric actuator is converted into mechanical energy to electrical energy, drag a variety of production machinery of the most important and most basic tools, due to production requirements and the need for energy saving,glue stick adjustable speed electric actuator has been the object of study to become science and technology personnel Its applications are increasingly widespread.
Adjustable speed electric actuator in the development process there had been DC converter, voltage control, electromagnetic speed,glue gun electro-hydraulic coupling and slip speed speed speed and other methods, but the tone of the above rate method in practical applications there are limitations and shortcomings. With the power electronics, automatic control technology, computer technology, the rapid development of a new, intelligent, and set a variety of sophisticated technologies in one speed electric actuator can be produced, which is frequency electric actuator.
With inverter technology matures, the frequency of its unique electric actuator performance and the increasingly wide application, and other than adjustable speed electric actuator has the following advantages:
a. starting current, no impact on production machinery, we can greatly extend the life of the equipment.
b. high intelligence, self-diagnostic function and protection function, the device's safety, reliability improved.
c. Installation is simple, easy to use, greatly reduced maintenance costs.
d. good mechanical properties, dynamic response speed can be adjusted to improve the quality control system.
e. fans, pumps and other power-hungry devices after using VVVF reduce energy consumption and save costs and improve efficiency.
Second, the frequency converter principle.
frequency electric actuator is a variable frequency device (drive) and electric motor can be speed drag bodies, motor speed and steering controlled by a frequency converter, in accordance with the requirements of the production process work. The theory is based on the following formula:
n = no (1-s).
no = 60f / p.
which:
n - motor speed.
no - synchronous speed.
s - slip.
f - input voltage frequency.
D - Motor pole pairs.
be seen from the above formula, changing the motor speed in three ways: First, the change in slip: the second is to change the number of motor pole pairs; third is to change the input voltage frequency. By introducing a fixed frequency device to convert electrical grid frequency required to achieve the frequency conversion electric motor speed control is the theoretical basis of the work of the implementing agencies.
AC inverter device is generally divided into a cross, a cross-cross has been implemented in two ways. AC inverter for a pay adjustment range of small, low power factor, harmonic polluting. Now most of the conversion devices have been used to pay for a delivery of the implementation, the main form consists of four parts: rectifier, filters, inverters and controls to protect the CPU as the core part.
rectifier elements connected in a controllable three-phase full-controlled bridge, the input voltage adjustable three-phase alternating current into direct current.
filter is mainly to get more straight DC, absorbing the energy of the motor feedback realization of the emergency braking function.
inverter a "gate turn-off thyristor" GTO, "power transistor" GTR, "two-level insulated gate transistor" IGBT and other components, using pulse width modulation or PWM vector control to DC inverter, etc. for the adjustable frequency AC.
control of the CPU as the core part of the frequency converter protection of the command center, the software program by entering the CPU to perform control, protection and other functions, and the realization and DCS, PLC and other control system interface.
There inverter device according to high voltage, low voltage of the points, high-voltage inverter can be divided into direct high pressure and high and one low and one high type. There are different types of drive applications, even the same type of drive because of technical differences also have different quality and price. Thus in the frequency of use of the electric actuator to the drive and motor selection of science, and strive to the best cost performance.
frequency electric actuator motor is the induction motor in, but the use of the occasion and the frequency converter technology used in different, some can use ordinary asynchronous motor, and some need to use a dedicated motor. Therefore, the original constant speed induction motor used to run the implementing agencies should be careful when the frequency of transformation: it can keep the original motor, if the motor needs to retain the original scientific choice frequency devices.
Third, the variable frequency electric actuators in thermal power plant.
Frequency electric actuator is the first successful development of foreign and domestic application of the earlier is the petrochemical, mining machinery industry, power industry was first started from a powder Frequency Control, this is because the use of low-voltage motors to the powder, Low frequency mature earlier, simple installation, transformation frequency of the motor easier. Frequency control to the powder not only achieved after the significant economic benefits, and use than traditional mechanical speed control device as well as many advantages: a homogeneous powder of silt, the phenomenon of reduced gravity coal, combustion stability; fuel widening the scope of the control system regulating, protection and accident handling capacity increased. For this reason, most power plants have on the motor and direct blow to the powder-type combustion system to the transformation of coal to the frequency, and have achieved good operating results.
With the increasing maturity of high-voltage inverter technology, high frequency electric actuator in the power industry beginning to use, taking into account the energy saving effect, domestic use of high voltage inverter device focused on the blower, Fan, mortar pump, drain pump, Boost pumps and condensate pumps, and foreign have been used in the feed pump high voltage inverter example.
For example, in the absence of prior frequency electric actuator, thermal power has been used to run the motor at constant speed, by adjusting the delivery, suction fan guide vane or baffle opening to control the boiler furnace air flow. The 200MW or more units to send, and induced draft fan motors are high-pressure high-power, power consumption, resulting throttling losses and waste a lot of auxiliary power consumption, very economic, but also because non-linear and the slow response of the automatic input and adjust the quality. Thus, some of the old power plants have been or are being considered frequency electric actuator control of air flow to transform, taking into account the overall frequency with electric actuator Although there are many advantages, but the renovation costs are high, and the original delivery, induced draft fan motor capacity level, protection level and other indicators can meet the requirements of frequency control, so reservations are based on the original equipment increased frequency devices in two ways: One way is to increase the frequency converter and the original electrical control circuit in parallel, through the switch to run the channel select control motor, shown in Fig. In this way, when the frequency converter failure or maintenance, can return to the original motor control, does not affect the operation of the unit, but between the two control channels need to add chains to avoid short circuit. The second method is to remove the existing electrical control circuits, direct frequency converter to control the motor running.
high energy consumption of the old units with high-voltage electrical equipment, modified the above method not only improved the quality of the control system of regulation and improve the control system stability and reliability, more importantly, saving auxiliary power.
So more and more attention to the plant, such as Daqing, Xinhua Huaneng Power of Fan and mortar pump frequency were modified: Anhui Tongling power plant on the drain pump for a frequency transformation; Qinghe Power Plant conducted a frequency of induced draft fan transformation. After the transformation has achieved good economic and social benefits.
Fourth, conclusions.
is clear, with variable frequency electric actuators replace many conventional actuators are energy conservation, improve the process, improve the quality of the effective means of control. However, at present most of the imported frequency conversion device, expensive, one-time engagement, the widespread use of variable frequency electric actuator is not the reality, the old unit frequency control in significant energy savings after implementing agencies to transform, for high performance and low cost The solution should become the focus of the study.
Transformation of the old unit in the frequency control devices at the same time, as far as possible in the new unit design variable frequency electric actuator, to avoid the waste of the second input; the same time, variable frequency electric actuator designed for the special frequency motor has cooling effect Well, the electromagnetic loss less, more energy efficient than conventional motors, etc., make the whole frequency electric actuator work more efficiently.