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, the other 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 on the speed range In theory, high-speed pump speed control area through the frequency-efficient area around the end of two similar conditions parabolic middle area OA1A2. 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, Therefore, the minimum speed can be obtained as follows: Since the conditions at point C and point A1 are similar, according to the proportional law: (QC / Q1) 2 = HC / H1 The point C is on the curve H = H0 + S? Q2 There are: HC = H0 + S? QC2 Where, HC, QC is unknown, the solution equation: HC = H1 × H0 / (H1-S? Q12) QC = Q1 × [H0 / (H1-S? Q12) / 2 According to the proportional law: nmin = n0 × [H0 / (H1-S? Q12)] 1/2 2 speed pump on the speed range In practice, the water supply system is often multiple pumps in parallel. 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 with the same type of pump must be run side by side, although the scheduling flexibility, but can not take into account the speed pump and the constant speed pump efficient work section, therefore, in this case speed range is tiny. 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 when the right end of the head and the high-speed section of the left end of the pump lift 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 drop sharply, but if the motor output power is too much deviation from the rated power or excessive operating frequency Offset frequency, will make the motor efficiency drops too fast, eventually affecting the efficiency of the entire pump unit. And since the cold motor continuous low-speed operation, but also due to insufficient air volume affect the heat, threaten the safe operation of the motor.