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Variable speed drive in pump control characteristics

Electric motor restart
Some pumps require high efficiency operation, which usually need power frequency supply and variable frequency power supply switching at any time. For example, a residential area using deep geothermal water for heating, with three motors - water pump heating is controlled by a variable speed drive, the maximum frequency is 50Hz, the largest water supply volume is two motor units operation in power frequency supply. According to diurnal temperature changes with different demands for heating, the system's two motor units can be: one motor running in power frequency, the other one running in variable frequency (50Hz less) power;  one motor outage, the other one running in variable frequency power. The third motor is controlled by soft starter to make mutual backup with other two motors. Therefore, such operation methods not only meet the heating requirements, improve operational efficiency, but also save lots of electricity energies. However, in those operation methods, it requires the variable frequency drive has speed tracking feature, particularly from the commercial power supply switches to variable frequency operation. Thus, the motor has a smooth switching when power supply cut off, from idle to restart, thereby enhance the system reliability and stability in continuous operation.

Self-diagnosis of continuous operation
Due to power disturbances, the pumps often tripped and the reasons are difficult to find out. When an abnormal condition occurs, the variable speed drive can perform self-diagnosis first, if the system has no problem it will reset and restart automatically, and find out the electric motor deceleration reasons in self-diagnosis by speed detection function during this period, and make it reaches the original speed, which requires the system should be integrated abnormal shutdown recovery function.

Free tripping operation function
Pump driving has torque decreased characteristic, effective overload protection function is important to make it running on the over-torque conditions. In this case, once the overload current reaches the set value, reduce the variable speed drive output frequency, that is stall in running. For such torque decreased loads, the variable speed drive can be running at the balance point for a short time, and restore original set frequency automatically after the load drops.

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