Frequency control of water pump variable frequency range of three factors

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.

1 pump process characteristics of the speed range

In theory, the pump speed governing high-efficiency area is the parabola middle area OA1A2 of two similar working 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, the high-speed pump governor shrank to PA1A2 [2]. Obviously, if the operating point has exceeded this area, Energy saving.)

Therefore, the minimum speed can be obtained as follows:

As C and A1 similar conditions, according to the law of proportion:

(QC / Q1) 2 = HC / H1

Point C is on the curve H = H0 + S? Q2:

HC = H0 + S? QC2

Among them, HC, QC are unknown, solution equation has:

HC = H1 × H0 / (H1-S? Q12)

QC = Q1 × [H0 / (H1-S? Q12)] 1/2

According to the law of proportion:

nmin = n0 × [H0 / (H1-S? Q12)] 1/2

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 [2]. The main points of the following two situations:

2.1 The same type of pump must be run side by side while running, although the scheduling flexibility, but can not take into account the high-speed pump and constant speed pump work section, therefore, this case speed operation is very small.

2.2 different models of a pump must be run side by side, if it can reach the speed of the pump at the rated speed of the right end of the high end of the head and high-speed section of the pump head is equal. 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.

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.

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