In order to obtain the transient hydraulic characteristics of the bulb pump unit in a pump station with VFD during startup,the essential features of lowspeed synchronous startup controlled by VFD were analyzed initially.Then the moment equilibrium equation of the rotor of pump unit was established in the startup transient period.The mathematically expressing approach for the electromagnetic torque in the synchronous motor in terms of time was investigated.The resistance moments,inclusive of hydraulic resistance moment,bearing frictional moment and motor cooling fan resistance moment etc,and their mathematical expressions were clarified.Finally,the flow rate allocation and conservation between the rapiddrop gate and flap valve during the transient period were determined.Taking Huaiyin No.3 Pumping Station as a study case,the startup transient process of the pump unit under the design condition was simulated to obtain the primary electrical and hydraulic parameters.In the simulation,the pump started running when the instant exciting current reached 0.63 times its design value and the frequency was about 6.5 Hz,causing a 15 s lag delay.As a result,a best match was obtained between the motor rotational speed frequency and the lag time for opening the rapiddrop gate.At the design condition,when the rotational speed of the synchronous motor is linearly increased from 0 to 125 r/min in 100 s interval,the optimal lag time is 70 s for opening the rapiddrop gate
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