Typical Application of Frequency Converter——Constant Pressure Water Supply

1.Constant Pressure Water Supply
Constant pressure water supply is a method of supplying water using secondary water supply equipment. It maintains a constant water pressure, ensuring a balance between water supply and usage. When there is high water usage, more water is supplied, and when there is low water usage, less water is supplied, thereby improving the quality of water supply.

2.Variable Frequency Application for Constant Pressure Water Supply
In a typical water supply system, multiple common pumps are installed. When there is high water demand, multiple pumps are operated simultaneously, and when there is low water demand, only one or two pumps are operated. When using variable frequency speed control for constant pressure water supply, there are two methods: using a single frequency converter for all pumps or using one frequency converter for each pump. The latter method adjusts the frequency converter output based on pressure feedback signals through PID calculations, changing the motor speed to achieve constant pressure in the pipeline. This method forms a closed-loop system and is relatively simpler, but it is more expensive. The former method has lower costs and comparable performance, but the control program is more complex. It represents the future development direction.

3.Variable Frequency Control Principle
LSD-H7000 uses frequency conversion speed regulation to realize constant pressure water supply. Compared with using regulating valve to realize constant pressure water supply, the energy saving effect is very significant (can be calculated according to specific conditions). Its advantages are:
(1). Starting balance, the starting current can be limited within the rated current, thus avoiding the impact on the power grid during starting;
(2). Since the average speed of the pump is reduced, the service life of the pump and valves can be extended;
(3). It can eliminate the water hammer effect when starting and stopping.

Generally, when using a LSD-H7000 frequency converter to control a single motor, the rated capacity of the frequency converter should match the actual motor capacity. When a frequency converter controls two motors simultaneously, the rated capacity of the frequency converter should ideally be equal to the sum of the two motor capacities. However, if the high peak load water usage significantly differs from the full-speed water supply of the two pumps, it may be considered to appropriately reduce the capacity of the frequency converter, but ensuring sufficient capacity is important. Although the working current of the motor is relatively low when the pump is operating at low speed, when the water usage fluctuates frequently, the motor undergoes frequent acceleration and deceleration, which can slightly exceed the rated current and result in motor overheating. Therefore, thermal protection for the motor is necessary. The electronic thermal protection function within the LSD-H7000 frequency converter is not sufficient to protect against the temperature rise accumulated due to frequent changes in speed. Hence, thermal relays should be used for motor protection.