Pump knowledge -- Minimum operating frequency of slurry pump

  • 11  Aug, 2021

In the applications of Slurry Pumps, frequency conversion operation is sometimes required. For example, direct coupling connection must be achieved at some sites, or flowrate is unstable at other sites, or the transport distance is relatively long etc.. Hence, frequency converters are needed to adjust the speed of the slurry pump so that the pump discharge pressure matches the actual required one.

In the process of frequency conversion, people often consult about the lowest frequency: someone says it’s 25Hz, some says 30Hz, and some says 5Hz. Are these parameters correct? What is the exact value? The inaccurate setting of the minimum frequency in the control system will affect the normal operation of the slurry pump.

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In fact, the above three frequency values come from two aspects. One is the driving equipment of the pump, i.e. the motor and another is the slurry pump itself.

I: the minimum operating frequency of VSD motors

1. Speaking of theory alone, the lowest operating frequency that a VSD motor can run is 0Hz, but a 0HZ motor has no speed, so this cannot be regarded as the lowest operating frequency;

2. The allowable operating speed range of different VSD motors is different;

3. To put it simply, if the speed regulation range of the VSD motor is 5-50Hz, then the minimum allowable operating frequency of the variable frequency motor is 5Hz;

4. The reason why the variable frequency motor can run at multiple frequencies.

(1) The VSD motor has good heat dissipation performance. Its cooling and ventilation system are driven by independent wiring. It can be forced to dissipate heat to ensure that the VSD motor can operate at different frequencies. The motor can generate heat and dissipate in time;

(2) The insulation performance of the VSD motor is good, and it can take in the impact that has on the VSD motor, from the different current and voltage of different frequencies.

5. It is not recommended to run the variable frequency motor at low frequency. After the motor runs at low frequency for a long time, it is particularly prone to heat generation, which will cause the motor to burn out. The best operating frequency of the motor is to work close to the constant operating frequency.

6. The frequency conversion range of the commonly used frequency converter is 1-400HZ; but in practical application, considering that the standard of the Chinese  motor is planned according to the power frequency of 50HZ, the application is in fact limited within the range of 20-50HZ.

Therefore, the minimum allowable frequency of the variable-frequency motor is related to the specific operating frequency range of the variable-frequency motor. Generally, the lowest value that the VSD motor allows can be taken.

II: the minimum operating speed of slurry pumps

Each slurry pump has its own performance curve, which specifies the minimum operating speed of the pump. Only when the speed is higher than the specified speed, the pump can operate normally. The frequency at this speed is the minimum operating frequency of the slurry pump.

Of course, there are other influences such as the flow rate of pipelines. Simply put, it can be considered that the above two points, that is, the frequency determined by the minimum speed of the slurry pump and the minimum operating frequency of the variable frequency motor are the two factors that affect the minimum operating frequency of the slurry pump. Among these two factors, the highest frequency value is the minimum operating frequency of the slurry pump.