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1 pump frequency control operation of the energy-saving principle
Figure 1 for the pump valve control, when the flow rate decreases from Q1 to Q2, the valve must be closed. At this time the valve friction increases, the pipeline curve from R to R ', head lift from Ha to Hb, operating conditions point from a point b point.
Figure 2 is the speed control, when the flow rate requirement decreases from Q1 to Q2, the characteristic of the pump depends on the speed as the resistance curve R is unchanged. If the speed is reduced from n to n ', the performance curve changes from (QH) to (QH)' and the operating point moves from point a to point c and the head drops from Ha to Hc.
According to centrifugal pump characteristic curve formula:
N = RQH / 102η?
Where: N - pump working conditions shaft power (kw)?
Q - the use of operating conditions point of flow (m3 / s) ?;
H - head of service point (m);
R - output media unit volume weight (kg / m3) ?;
η - pump efficiency (%) at the point of use.
Can find out the operation of the shaft pump power at point b and c point pump shaft power were:
Nb = RQ2Hb / 102η ??
Nc = RQ2Hc / 102η ??
The difference between the two is: ΔN = Nc-Nb = R × Q2 × (Hb-Hc) / 102η?
That is to say, when using the valve to control the flow, ΔN power is wasted and wasted, and the loss increases as the valve keeps turning off. The use of speed control, the flow Q and speed n is proportional to the first power; head H and speed n is proportional to the square; shaft power P and speed n is proportional to the cube, that is, power and speed n into the power of the relationship between the third power decline. If not the method of closing the valve, but the motor speed down, then the same flow in the case of transport, the original consumption of the valve in the power can be avoided, and achieved good energy-saving effect, which is the principle of energy-saving pumps .
2 Frequency Control of the basic principles
The basic principle of frequency control is based on the principle of AC motor speed relationship:
n = 60f (1-s) / p
Where: f - pump motor power frequency (Hz);
p - motor pole pairs;
It can be seen from the above equation that even changing the power frequency f of the stator winding of the motor can smoothly change the synchronous speed of the motor. Motor speed is slow, the corresponding reduction of shaft power, motor input power also will be reduced. This is the energy-saving pump frequency control role.
3 pump frequency control system design
At present, the domestic frequency control technology in the pump control system, most of the open-loop state, that is artificially based on changes in technology or external conditions to change the frequency of the inverter in order to achieve the purpose of speed control system mainly consists of The four components: (1) control object (2) inverter (3) pressure measurement transmitter (PT) (4) regulator (PID).
System control process is:
Measured by the pressure transmitter pipe outlet pressure measured and converted into the corresponding 4 ~ 20mA standard electrical signal sent to the regulator and the process control indicators required to be compared to draw the deviation. The deviation value is calculated by the regulator according to the pre-defined regulation rule to obtain the adjustment signal. The signal is directly sent to the inverter so that the inverter converts the AC input with 380V / 50Hz into the output with 0 ~ 380V / 0 ~ 400Hz continuously adjustable voltage and frequency AC, directly to the pump motor.
4 pump frequency control applications note
Pump speed is generally slow down the problem. When the use of frequency control, the original design by the frequency of the state of the pump and motor operating parameters have undergone major changes in addition, such as pipeline characteristics curve, speed governor pump in parallel with the constant speed pump and other factors, will be Speed ​​range has a certain impact. Out of range speed control is difficult to achieve the purpose of energy conservation. Therefore, frequency conversion speed regulation can not be unlimited. Generally believed that the frequency control speed should not be lower than the rated speed of 50%, preferably at 75% to 100%, and should be combined with the actual calculation.
4.1 pump process characteristics of the speed range
In theory, the pump speed governing high-efficiency zone is the middle of the parabola of two similar conditions passing the left and right end points of the high-frequency area. In fact, when the pump speed is too small, the efficiency of the pump will drop sharply, affected by this, high-speed pump speed atrophy, if the operating point has exceeded this area, it is not appropriate to use speed to save energy.
4.2 constant speed pump speed range
In practice, the water supply system is often multiple pumps in parallel water supply. Due to the expensive investment, it is impossible to speed up all the pumps, so the general speed pump, fixed speed pump mixed water supply. In such a system, care should be taken to ensure that both the variable speed pump and the fixed speed pump operate at the high efficiency stage and achieve the optimum system. At this moment, the constant speed pump has a great influence on the speed regulating range of the speed regulating pump running in parallel with it. The main points of the following two situations:
4.2.1 The same type of pump must be run side by side, although the scheduling flexibility, but can not take into account the high-speed pump and fixed-speed pump work section, therefore, in this case speed range is very small.
4.2.2 When different types of pumps must run side by side, if the speed regulating pump can reach the rated speed, the right end of head of high efficiency section and the left end of high speed section of constant speed pump will be equal in lift. You can achieve the maximum range of speed operation. However, at this moment, the speed regulating pump and the constant speed pump must not be allowed to run in parallel after the exchange.
4.3 motor efficiency on the speed range
In the case of similar conditions, usually Nαn3, so as the speed decreases, the shaft power will decline sharply, but if the motor output power is too much deviation from the rated power or over-frequency shift frequency, will make the motor efficiency Fall too fast, eventually affecting the efficiency of the entire pump unit. And since the cold motor continuous low-speed operation, but also due to lack of air volume affect the heat, threaten the safe operation of the motor.
Analysis and Application of Pump Frequency Conversion Energy Saving Technology
Abstract: The energy-saving principle, basic working principle and system control process of the pump frequency conversion speed regulation system are introduced, and according to the actual application, Progress, but energy waste is staggering. According to the information reported, China's fans, pumps, air compressors total about 42 million units, installed capacity of about 110 million kilowatts. However, the actual operating efficiency of the system is only 30-40%, and the power consumption of the system accounts for more than 38% of the total generating capacity. This is due to many fans, the pump drag motor at a constant speed operation, and the production of wind, water flow requirements in a variable condition operation; there are many companies in the system design, the capacity to choose a larger system to match Unreasonable, often "big car", resulting in a lot of energy waste. Therefore, to do a good job in energy conservation of fans and pumps is of great significance to the development of the national economy.