An energy saving system and method for small range stepless speed control of water pump and fan equipment

Abstract

The present application relates to a kind of small-range stepless speed regulation energy-saving system and method for water pump and fan equipment, system includes high-pressure main motor, for providing fixed-speed main drive;Mechanical differential coupling unit has first, second input and output, respectively connect high-pressure main motor, low-pressure auxiliary motor and load centrifugal water pump;Low-pressure auxiliary motor connects low-pressure frequency converter and control unit, according to load demand adjustment low-pressure auxiliary motor speed.Mechanical differential coupling unit will fixed-speed input of high-pressure main motor and variable speed input of low-pressure auxiliary motor be mechanically coupled, make output speed in the small range of high-pressure main motor rated speed ±8%~15% Continuous infinitely variable adjustment.The present application utilizes low-power low-pressure auxiliary motor to realize narrow-range high-efficiency speed regulation, can use standard 4-pole high-speed high-pressure motor to replace expensive multi-pole low-speed high-pressure motor, significantly reduce equipment procurement cost;While system efficiency can reach more than 95%, it is suitable for the low-cost energy-saving reconstruction of stock water pump system.

Description

Technical Field

[0001] This invention belongs to the field of stepless speed regulation of water pumps, specifically relating to an energy-saving system for stepless speed regulation of water pumps and fans. Background Technology

[0002] Centrifugal pumps are widely used in municipal water supply, industrial circulation, and heating networks, consuming approximately 20% of global energy. According to the similarity law of centrifugal equipment, pump flow rate is directly proportional to rotational speed, head is directly proportional to the square of rotational speed, and shaft power is directly proportional to the cube of rotational speed. Therefore, adjusting pump speed to adapt to changing flow and pressure demands is the most direct way to reduce system energy consumption and improve operating efficiency. However, in many existing large-scale constant-speed pump sets driven by high-voltage motors, valve throttling is still commonly used to regulate flow, resulting in significant throttling losses and causing the pump to deviate from its high-efficiency operating range, leading to increased vibration, bearing overheating, and shortened equipment lifespan.

[0003] Currently, the main speed control solutions for high-voltage motor-driven water pumps are as follows:

[0004] 1. High-voltage frequency converters: These achieve stepless speed regulation by changing the motor's power supply frequency. The technology is mature and offers a wide speed range (0-100%). However, they suffer from drawbacks such as high cost, large size, severe harmonic pollution to the power grid, and complex maintenance. Especially for centrifugal water pumps, in actual operation, speed regulation within a narrow range of ±10% to 15% of the rated speed is often sufficient to meet flow regulation requirements. However, high-voltage frequency converters still need to be configured for the full power of the motor, resulting in a long investment payback period and difficulties in retrofitting existing projects.

[0005] 2. Hydraulic coupling and permanent magnet slip speed regulation: Both belong to slip-based regulation methods, achieving speed regulation by changing the slip. Their inherent drawbacks are the existence of unavoidable slip losses, and the lower the output speed, the larger the slip and the lower the efficiency; in addition, the speed regulation response is slow, the control precision is low, it is difficult to achieve precise closed-loop control, and it is not suitable for fine adjustment of water pumps.

[0006] 3. Permanent magnet speed controller: Based on the eddy current principle, its efficiency drops sharply as the speed difference increases. Under deep speed regulation conditions, it wastes a lot of electrical energy, which is far lower than that of high-efficiency solutions such as frequency converters.

[0007] 4. Valve / baffle regulation: Although no speed control equipment is required, the throttling loss is huge and the long-term operating cost is extremely high.

[0008] Furthermore, all four speed control schemes mentioned above face a common limitation: they cannot change the rigid matching relationship between the motor's own speed and the load speed. When a low-speed water pump is required (e.g., rated speed in the range of 200–1500 rpm), a multi-pole low-speed high-voltage motor with the corresponding number of poles (e.g., 8-pole, 10-pole, or even more poles) must be selected. The procurement cost of a multi-pole low-speed high-voltage motor is usually 30%–50% higher or even more than that of a 4-pole high-speed motor of the same power, and the supply cycle is long and spare parts are difficult to obtain. Even if a high-voltage frequency converter is used to reduce the speed of the high-speed motor, the constant torque output below the rated speed and the linear decrease in load capacity will require a motor capacity far exceeding the actual load power, further increasing costs.

[0009] In recent years, electromechanical coupling speed control schemes combining differential planetary gears and auxiliary motors have emerged (such as the "Mechatronics Integrated Speed ​​Control System" disclosed in CN209250419U and the "A Planetary Frequency Conversion Speed ​​Control Device" disclosed in CN115750698A), attempting to achieve speed control by superimposing a small-power auxiliary motor on the main drive train. However, these schemes are designed with full-range speed control (0-100%) in mind, requiring the auxiliary motor power to be configured in a large proportion of the main motor power. Furthermore, they are not optimized for the characteristic of centrifugal water pumps that "only require a narrow range of speed control to cover the high-efficiency zone," resulting in no significant reduction in system cost and complexity. In addition, these schemes do not address the fundamental problem of how to replace expensive multi-pole low-speed high-voltage motors with common high-speed 4-pole high-voltage motors. Summary of the Invention

[0010] To address the aforementioned issues, this invention discloses an energy-saving system for small-range stepless speed regulation of water pumps and fans. It is a low-cost, high-efficiency speed regulation system specifically designed for high-voltage motor-driven water pumps, which can fully utilize the high efficiency characteristics of centrifugal equipment in narrow-range speed regulation. It enables the selection scheme of replacing low-speed high-voltage motors with standard high-speed motors, thereby reducing equipment procurement costs and modification barriers, and promoting the widespread application of energy-saving technologies for water pump systems.

[0011] To achieve the above objectives, the present invention adopts the following technical solution:

[0012] An energy-saving system for small-range stepless speed regulation of water pumps and fans includes: a high-voltage main motor, a low-voltage auxiliary motor, and a mechanical differential coupling unit, wherein...

[0013] The high-voltage main motor is used to provide constant-speed main drive; the low-voltage auxiliary motor is used to adjust the output speed of the mechanical differential coupling unit, and the low-voltage auxiliary motor is also connected to a low-voltage frequency converter and a control unit for controlling its speed; the mechanical differential coupling unit has a first input terminal, a second input terminal and an output terminal, which are respectively connected to the high-voltage main motor, the low-voltage auxiliary motor and the load centrifugal water pump;

[0014] The mechanical differential coupling unit mechanically couples the constant speed input of the high-voltage main motor with the variable speed input of the low-voltage auxiliary motor, so that the output speed of the output terminal can be continuously and steplessly adjusted within the range of (1±8%~15%) near the rated speed of the high-voltage main motor.

[0015] Furthermore, the high-voltage main motor has 2, 4, or 6 poles and a rated speed of 3000 rpm, 1500 rpm, or 1000 rpm; and / or the rated speed of the load centrifugal water pump is 200~1500 rpm.

[0016] Furthermore, the rated power of the low-voltage auxiliary motor is 8% to 15% of the rated power of the high-voltage main motor.

[0017] Furthermore, the mechanical differential coupling unit is an NGW-type single-stage planetary gear mechanism, including a sun gear, planet gears, a planet carrier, and a ring gear. The high-voltage main motor is connected to the sun gear, the low-voltage auxiliary motor is connected to the ring gear, and the output end is connected to the planet carrier. The transmission ratio is 2. The high-voltage main motor is a 4-pole motor with a rated speed of 1500 rpm. When the speed of the low-voltage auxiliary motor is zero, the speed of the output end is 750 rpm. The speed adjustment range of the low-voltage auxiliary motor is ±50 rpm, allowing the speed of the output end to be adjusted within the range of 675 rpm to 825 rpm.

[0018] Furthermore, the mechanical differential coupling unit is an NGW-type single-stage planetary gear mechanism, including a sun gear, planet gears, a planet carrier, and a ring gear. The high-voltage main motor is connected to the sun gear, and the low-voltage auxiliary motor adjusts the speed of the planet carrier through a bevel gear mechanism. The output end is connected to the ring gear. The transmission ratio is 3. The high-voltage main motor is a 4-pole motor with a rated speed of 1500 rpm. When the speed of the low-voltage auxiliary motor is zero, the speed of the output end is 500 rpm. The speed adjustment range of the low-voltage auxiliary motor is ±50 rpm, so that the speed of the output end can be adjusted within the range of 450 rpm to 550 rpm.

[0019] Furthermore, the mechanical differential coupling unit is a structure of two NGW planetary gear sets connected in reverse series and sharing a planetary carrier. The transmission ratio of the first-stage NGW planetary gear set is 7.38, the transmission ratio of the second-stage NGW planetary gear set is 6, and the total transmission ratio after reverse series connection is 1.23. The high-voltage main motor is a 4-pole motor with a rated speed of 1476 rpm. When the speed of the low-voltage auxiliary motor is zero, the output speed is 1200 rpm. Alternatively, another scheme can be adopted: the mechanical differential coupling unit is an NGW-type single-stage planetary gear mechanism, including a sun gear, planet gears, a planetary carrier, and a ring gear. The high-voltage main motor is connected to the external ring gear, and the high-voltage motor adopts a hollow shaft structure. The low-voltage auxiliary motor is installed at the tail of the high-voltage motor, and its shaft is connected to the sun gear through the hollow shaft to achieve speed regulation. The output end is connected to the planetary carrier. The transmission ratio between the sun gear and the ring gear is 6. The high-voltage main motor is a 4-pole motor with a rated speed of 1476 rpm. When the speed of the low-voltage auxiliary motor is zero, the output speed is 1285.7 rpm.

[0020] Furthermore, the low-voltage frequency converter is a four-quadrant frequency converter, which feeds energy back to the low-voltage power grid when the low-voltage auxiliary motor is running in reverse; and / or the low-voltage auxiliary motor is a permanent magnet synchronous motor or a synchronous reluctance motor.

[0021] Furthermore, the control unit uses the low-voltage auxiliary motor to drive the speed increase in the forward direction and the reverse drive to recover energy for deceleration and adjustment of the working condition; and after the system is put into operation, the system matching is further optimized in combination with the impeller cutting of the load centrifugal water pump, so that the load operates in the ideal hydraulic characteristic range of the centrifugal water pump.

[0022] Furthermore, the output speed N at the output end out Satisfying the relation:

[0023] N out = k1 * N h + k2 * N1 or N out = k1 * N h -k2 * N2

[0024] Where N h N1 is the speed of the high-voltage main motor, N2 is the speed of the low-voltage auxiliary motor, and k1 and k2 are the mechanical coupling coefficients determined by the gear ratio.

[0025] Furthermore, the control unit employs a PID controller to detect the flow rate or pressure of the load centrifugal water pump in real time, compare it with the target value, and output adjustment commands. In a pump group system consisting of multiple water pumps, when a single speed-regulating water pump reaches the upper or lower limit of the set speed range, the control unit adjusts the system to operate in the optimal efficiency range by increasing or decreasing the number of operating water pumps or by coordinating the speed of multiple water pumps.

[0026] Furthermore, the system is used to carry out energy-saving retrofits on existing fixed-speed high-voltage motor driven water pump systems. This retrofit is achieved by adding the mechanical differential coupling unit, the low-voltage auxiliary motor, and the low-voltage frequency converter between the original fixed-speed high-voltage motor and the load centrifugal water pump.

[0027] This invention also discloses an energy-saving method for small-range stepless speed regulation of water pumps and fans, comprising the following steps:

[0028] Step S1: Obtain the target speed signal of the load centrifugal water pump. The target speed signal is generated by manually setting the frequency or automatically generated by the control unit based on pressure, temperature difference or liquid level signals.

[0029] Step S2: Calculate the difference between the target speed and the rated speed of the high-voltage main motor. Based on the difference and the gear ratio of the mechanical differential coupling unit, generate a speed command for the low-voltage auxiliary motor. The command satisfies the following:

[0030] N1= (N target - k1×N h ) / k2, where N target For the target output speed, N h K is the speed of the high-voltage main motor, and k1 and k2 are the mechanical coupling coefficients determined by the gear ratio;

[0031] Step S3: Control the low-voltage auxiliary motor to run according to the speed command, and make the output speed higher than the rated coupling speed by forward drive, or make the output speed lower than the rated coupling speed by reverse drive, so as to realize continuous stepless adjustment of the load speed within ±8%~15% of the rated speed of the high-voltage main motor;

[0032] Step S4: While adjusting the speed mainly in forward drive mode, monitor the energy cycle status. When energy recovery is required, activate the reverse drive mode to feed some energy back to the low-voltage grid through the four-quadrant frequency converter.

[0033] Step S5: After the system is put into operation, the system matching is optimized again in combination with the impeller cutting of the load centrifugal water pump to ensure that the load always operates in the high-efficiency hydraulic characteristic range of the centrifugal water pump.

[0034] The narrow-range, high-efficiency speed regulation method can be used for the aforementioned mechanical differential coupling unit, which can be installed vertically or horizontally.

[0035] Beneficial effects:

[0036] (1) In the prior art, driving a low-speed water pump usually requires purchasing an expensive low-speed high-voltage motor (such as 8-pole or 10-pole). This invention uses a common and inexpensive 4-pole high-speed high-voltage motor as the main motor and only uses a very small speed-regulating motor as an auxiliary motor to make up for the speed difference between high speed and low speed, which greatly reduces the motor purchase cost.

[0037] (2) In view of the problem that the main motor of the present invention is subjected to full frequency conversion and large losses in the low efficiency area, the main motor of the present invention is used at full power at the power frequency (the highest efficiency point) and does not participate in speed regulation, so the energy conversion efficiency is extremely high. Speed ​​regulation is completed entirely by the low voltage side and the power is transmitted through mechanical differential, with a transmission efficiency of close to 95%~98%.

[0038] (3) In view of the characteristic that the efficiency of the water pump changes very little near a specific speed (±8%~15%), the present invention does not make full-range speed adjustment, but in the high-efficiency fine adjustment range, it achieves the best operating efficiency of the whole system with the least auxiliary power cost.

[0039] (4) When the reverse speed of the speed regulating motor exceeds the speed of the main motor, the system output speed can be lower than the reference speed. If it is used in conjunction with a four-quadrant frequency converter, when the water pump is in braking condition (such as when the pump suddenly stops and reverses), it can also convert mechanical energy into electrical energy and feed it back to the power grid, further saving energy. Attached Figure Description

[0040] Figure 1 This is a system architecture diagram of an embodiment of the present invention;

[0041] Figure 2 This is a schematic diagram of the performance curve of the centrifugal water pump according to an embodiment of the present invention;

[0042] Figure 3 This is a planetary gear-based coupling structure according to an embodiment of the present invention;

[0043] Figure 4 This is a differential coupling structure based on NGW planetary cylindrical gears according to an embodiment of the present invention;

[0044] Figure 5 This is a water pump performance curve with high efficiency zone and cut line according to an embodiment of the present invention;

[0045] Figure 6 This is a multi-parameter performance curve of the water pump according to an embodiment of the present invention;

[0046] Figure 7 This is a graph showing the wide-range speed regulation performance of the water pump according to an embodiment of the present invention.

[0047] Figure 8 This is a structural diagram of an embodiment of the present invention. Detailed Implementation

[0048] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0049] This embodiment discloses a small-range stepless speed regulation energy-saving system for water pumps and fans equipped with high-voltage motors. It is mainly applied in industrial and municipal fields for water pump and fan selection, speed reducer design, electromechanical speed regulation for energy saving and energy recovery, and high-voltage motor matching. The energy-saving system disclosed in this embodiment mainly adopts motor speed regulation and fluid mechanical transmission technology. Specifically, it is a speed regulation device that uses a low-voltage four-quadrant frequency converter and a small-power motor to modify a fixed-speed high-voltage motor system. Its main application scenario is centrifugal pumps / centrifugal fans driven by high-voltage motors, and its main purpose and benefit is energy saving through speed regulation of water pumps and fans.

[0050] Example 1:

[0051] This embodiment discloses the overall architecture and working principle of the system.

[0052] like Figure 1 As shown, an energy-saving system for small-range stepless speed regulation of water pumps and fans is specifically designed for water pump / fan systems driven by high-voltage motors. It achieves precise adjustment of load speed within a very small range through a combination of mechanical differential and electric speed regulation. The system includes a power input unit, an electrical and control unit, a mechanical transmission and execution unit, and a sensing and feedback unit.

[0053] The power input unit includes a main drive motor and a speed-regulating motor. The main drive motor is a high-power, high-voltage motor; in this embodiment, it is a 4-pole high-voltage asynchronous motor with a rated speed of 1500 rpm, operating at a constant speed and providing the main driving force. The speed-regulating motor is a low-power, low-voltage motor, such as a 380V three-phase motor, serving as an auxiliary power source for fine-tuning the speed. The power control unit includes a frequency converter and a controller. The frequency converter connects to the speed-regulating motor to provide a variable-frequency power supply, enabling speed control of the motor. The controller receives sensor signals and issues commands to control the frequency converter. The mechanical transmission and execution unit includes a differential planetary gear mechanism (planetary reduction mechanism), couplings, and fluid equipment (pumps / fans, etc.). The differential planetary gear mechanism is the core mechanical component, used to synthesize two power sources. Couplings connect the main motor to the planetary carrier and the planetary mechanism output shaft to the load, respectively. The fluid equipment is the final load end. The sensing and feedback unit mainly includes sensors used to monitor the pressure, flow rate, or differential pressure at the pump outlet, providing feedback signals to the control system.

[0054] The connection relationship of each component is as follows: the main drive electrode is connected to the first coupling ( Figure 1The left-hand coupling rigidly connects the planet carrier or ring gear of the differential planetary gear mechanism. The main motor always rotates at a constant speed, providing power for the base load. A speed-regulating motor is connected to the internal components of the differential planetary gear mechanism (such as the planet carrier or internal ring gear) via a top coupling; its speed and direction are controlled by a frequency converter. The output shaft of the differential planetary gear mechanism (usually the output of the ring gear or planet carrier) is connected via a second coupling (…). Figure 1 The coupling on the right side is connected to the drive shaft of the water pump. Sensors are installed on the inlet and outlet pipes of the water pump to collect operating data and transmit it to the controller. The controller calculates the required auxiliary speed based on the difference between the set target parameters (such as pressure) and the sensor feedback, and sends a speed control command to the frequency converter.

[0055] The theoretical basis of this system is as follows: According to the similarity law of centrifugal water pumps, the flow rate Q of the water pump is directly proportional to the rotational speed n, the head H is directly proportional to the square of the rotational speed n, and the shaft power P is directly proportional to the cube of the rotational speed n, as shown in Table 1 below:

[0056] Table 1. Relationships between Q, H, and P

[0057]

[0058] This principle forms the theoretical basis for achieving significant energy savings through small-range speed adjustments in this system. For example, a 10% reduction in rotational speed results in approximately a 27% reduction in shaft power; a 20% reduction in rotational speed results in approximately a 49% reduction in shaft power.

[0059] For most water pumps, a speed regulation range of around 20% is sufficient to cover a flow rate variation of approximately 50% under constant pressure. Further expanding the speed regulation range will significantly reduce actual operating efficiency. Therefore, a small range of stepless speed regulation has a substantial impact on changing the actual energy consumption of a water pump. For details, please refer to... Figure 2 The diagram illustrates the performance curves of a centrifugal water pump. The vertical axis represents Head (meters), Power (kW), and the horizontal axis represents flow rate (m³ / h). The upper black curve represents the performance curve at 1430 rpm, and the lower curve represents the performance curve at 1160 rpm. These two speeds are ±10% of the rated speed of 1299 rpm. The area covered by the green rectangle indicates the flow rate variation range of 1260 m³ / h to 2700 m³ / h under constant pressure. All operating points within this range correspond to efficiencies greater than 80%, and this range covers more than 50% of the maximum flow rate of 2700 m³ / h. From the above, it can be understood that most centrifugal water pumps basically meet the adjustment range of the high-efficiency flow range within a 20% speed adjustment range. Therefore, this invention deliberately limits the speed adjustment range to a narrow range of ±8% to 15%, which covers the high-efficiency operating range of the water pump while avoiding the high cost of full-range speed adjustment.

[0060] The core of the system disclosed in this embodiment lies in the speed synthesis characteristics of the differential planetary gear mechanism, whose output speed is jointly determined by the main drive motor and the speed regulating motor. The planetary gear mechanism provides the basic rated speed (when the auxiliary motor is not outputting power, it outputs power at a fixed transmission ratio); when it is necessary to increase / decrease the speed, the auxiliary motor drives in the forward / reverse direction. Specifically,

[0061] Setting the base speed: The main motor operates at a fixed speed, which determines the system's base output speed. For example, using a 4-pole high-voltage motor at 1500 rpm, through internal gear ratios, when the planetary carrier input is 1500 rpm, the base speed of the output shaft is 750 rpm to drive the water pump.

[0062] Fine-tuning speed regulation process: When system demand increases, such as increased water consumption or decreased pressure, the controller controls the frequency converter to increase the speed of the variable speed motor. The variable speed motor rotates forward, and through the planetary gear train, the torque is superimposed, causing the output shaft speed to increase slightly, for example, from 750 rpm to 760 rpm. When system demand decreases, such as increased pressure, the controller controls the variable speed motor to decelerate or even reverse. The reverse rotation of the variable speed motor reduces some of the main motor's speed, causing the output shaft speed of the differential planetary gear mechanism to decrease slightly, for example, from 750 rpm to 740 rpm.

[0063] At the same time, the sensor monitors pressure changes in real time and continuously feeds back to the controller, forming a closed-loop control to ensure that the water pump outlet pressure remains constant.

[0064] Since the energy consumption of the motor is ultimately determined by the water pump (load), according to the similarity law P∝n 3 When the speed decreases by 10%, the total output power decreases by about 27%, and vice versa—that is, by increasing the motor's forward output power by 10% at the speed, the total output power increases by about 27%. Below the rated frequency, the motor achieves constant torque output (linear change in load capacity) through frequency conversion speed regulation, while the load characteristics of water pumps and fans change cubically with power and speed.

[0065] The power distribution relationship is shown in Table 2 below:

[0066] Table 2 Power consumption of main motor and auxiliary motor

[0067]

[0068] Note: The maximum load power required in the table above is 1.331P. If the motor is matched with 10% of this load power, i.e., the auxiliary motor power is ≥0.133P, no overload will occur during the stepless speed regulation control of the auxiliary motor within the range of +10% to -20% of the rated output speed of the mechanical differential coupling unit. The main motor power should be greater than 1.21P.

[0069] As shown in the table, within the speed regulation range of +10% to -20%, the auxiliary motor power never exceeds approximately 11% of the rated power of the high-voltage main motor, with the main motor bearing the vast majority of the load power (approximately 85%~92%). This proves that a low-voltage auxiliary motor with a power output far less than the main motor can be used to achieve speed regulation, significantly reducing the power rating and cost of the speed regulation system. Furthermore, if the auxiliary motor is designed with 10% matching power, a nearly 10% speed increase can be achieved in the forward direction. The deceleration range is not limited by the auxiliary motor's power, allowing for a 20% depth of adjustment. The actual adjustment range depends on the upper limit of the auxiliary motor's speed, because motor overclocking follows the principle of constant power change. The higher the reverse adjustment speed of the auxiliary motor, the more energy it needs to absorb, while also considering overload issues.

[0070] Table 3 below shows the maximum safe speed of Inmonda Motor Factory's standard low-voltage motor models, which is the maximum safe speed without exceeding the power limit when operating at overclocked frequencies.

[0071] Table 3 Maximum Safe Speed ​​of Inmonda 1LE0 Standard Low Voltage Motor

[0072]

[0073] As shown in the table above, using a 6-pole auxiliary motor, the speed control system described above can achieve continuous adjustment within the range of +10% to -18%. Since the power variation within the adjustment range is significant, considering efficiency, permanent magnet or synchronous reluctance motors can maintain higher efficiency in the low-load range. Therefore, using a permanent magnet or synchronous reluctance motor as the auxiliary motor is more reasonable.

[0074] System efficiency is estimated by multiplying the efficiencies of each component: the efficiency of the single-stage planetary gear mechanism is approximately 98%~99% (taking 98%), the efficiency of the auxiliary motor and its transmission mechanism is approximately 90%, and the efficiency of the low-voltage frequency converter is approximately 97%. Therefore, the overall system transmission efficiency is approximately 96%~97% (98%×90%×97%≈85.4%, considering that most of the main power is directly transmitted without passing through the auxiliary transmission chain, the overall system efficiency is approximately 95%~97%). Since the main power of the system is directly transmitted from the high-voltage motor through gears without power conversion, and only a small amount of regulating power flows through the low-voltage motor and frequency converter, the overall system efficiency is close to the level of direct drive.

[0075] Set the speed of the high-voltage main motor to N. h , which is a fixed value.

[0076] The low-voltage auxiliary motor speed is set to N1, which can be achieved by setting it to -N. 2max to +N 2max Continuous adjustment within the range, N 2max This is the maximum permissible speed (maximum reverse speed) of the low-voltage auxiliary motor.

[0077] Output speed Nout Satisfy: N out = k1 * N h + k2 * N1 or: N out = k1 * N h -k2 * N2, where k1 and k2 are mechanical coupling coefficients determined by the gear ratio, used to superimpose the speeds of the main motor and the auxiliary motor in a specific ratio; N2 is the reverse speed variable of the low-voltage auxiliary motor, representing the speed value of the auxiliary motor when it is in reverse (energy recovery) operation.

[0078] By adjusting N1, the output speed can be achieved at N h Nearby (e.g., N) h * Continuous stepless adjustment within the range of (1+ / -3% / -18%). Increasing the proportion of auxiliary motor power can expand the speed range.

[0079] Power distribution: The high-voltage motor provides most of the mechanical power, while the low-voltage motor only provides the regulating power (i.e., the power difference or the shunt part), thereby significantly reducing the power level and cost of the speed control system.

[0080] Example 2:

[0081] This embodiment discloses a specific structure based on the NGW planetary gear train (transmission ratio 2, output 750 rpm).

[0082] like Figure 3 As shown, the mechanical differential planetary gear coupling unit in this embodiment is an NGW-type single-stage planetary gear mechanism. The power input, speed-regulating motor power input, and power output are distributed on different parallel axes, and the figure includes fixed support points (grounding symbols). Internally, there is cylindrical gear meshing. The main force is input from the input shaft on the right, the speed-regulating power is input from the speed-regulating motor power input on the upper left, and the power output is output from the power output shaft on the left. The main force and speed-regulating power are differentially coupled through the internal parallel-axis cylindrical gear system. The mechanical differential planetary gear coupling unit couples the high-power power of the main motor with the low-power power of the auxiliary speed-regulating motor in terms of torque / speed, achieving fine-tuning of the output speed (e.g., ±10%). Parallel-axis structures are generally suitable for specific transmission ratio requirements, but they are not as compact as planetary gear mechanisms. The high-voltage main motor is connected to the sun gear as the main driving force, the speed-regulating motor is connected to the external gear ring as the speed-regulating power, and the planetary carrier is connected to the centrifugal water pump as the output end. Assuming the number of teeth of the sun gear z_s=78 and the number of teeth of the gear ring z_r=156, the transmission ratio between the gear ring and the sun gear α=z_r / z_s=156 / 78=2.

[0083] The high-voltage main motor is a 4-pole high-voltage asynchronous motor with a rated speed of 1500 rpm. When the speed-regulating motor speed is zero, the rated speed of the output shaft (planetary carrier) is 1500 rpm ÷ 2 = 750 rpm. The low-voltage auxiliary motor controls the planetary gear speed (±50 rpm) through forward and reverse rotation, and the final output speed can be continuously and steplessly adjusted within the range of 675 rpm to 825 rpm, which is ±10% of the rated output speed.

[0084] The rated power of the low-voltage auxiliary motor is determined as follows: Auxiliary motor rated power ≈ High-voltage main motor rated power × (Speed ​​range / 2). Taking a high-voltage main motor with a rated power of 300kW and a speed range of ±10% (i.e., 20%) as an example, the auxiliary motor rated power ≈ 300kW × (20% / 2) = 30kW, which is 10% of the high-voltage main motor rated power, satisfying the power ratio limit of 8%~15%. The auxiliary motor is preferably a permanent magnet synchronous motor or a synchronous reluctance motor to maintain high efficiency in the low-load range; the low-voltage frequency converter uses a four-quadrant frequency converter, and when the auxiliary motor reverses to absorb energy, it feeds the energy back to the low-voltage power grid.

[0085] Auxiliary motor speed control boundary: The upper limit of the reverse / overclocking speed control of the auxiliary motor is constrained by the maximum safe speed of the motor. Taking the 1LE0 series low-voltage motor as an example, the maximum safe speed (rpm) / maximum frequency (Hz) allowed for each number of poles is shown in Table 3 of Example 1. Table 3 shows that when using a 6-pole auxiliary motor, the speed control system listed in the table above can achieve continuous adjustment within the range of +10% to -18%. Due to the large power variation in the adjustment range, considering efficiency issues, permanent magnet or synchronous reluctance motors can maintain higher efficiency in the low load range. Therefore, it is more reasonable to use permanent magnet or synchronous reluctance motors for the auxiliary motor. The power absorbed by the auxiliary motor is recovered to the low-voltage power grid through a four-quadrant frequency converter for use by low-voltage equipment.

[0086] Compared to existing solutions in this embodiment: If the required drive speed is 750 rpm, the rated load is approximately 300 kW, and stepless speed regulation is required, the traditional solution requires an 8-pole 350 kW high-voltage motor and a high-voltage frequency converter of the same power. The frequency converter requires a large installation space and is expensive. Using the solution in this embodiment, a 4-pole 300 kW high-voltage motor and an auxiliary speed-regulating 30 kW low-voltage motor can be used. Space is left between the water pump and the motor for the differential planetary mechanism. A small-power, low-voltage four-quadrant frequency converter can essentially achieve the same regulation function, significantly reducing construction or modification costs.

[0087] Example 3:

[0088] This embodiment discloses another structure based on the NGW planetary gear train (transmission 3, output 500 rpm).

[0089] like Figure 4As shown, the structure of this embodiment is basically the same as that of embodiment 2, except that the gear parameters are set so that the equivalent transmission ratio is 3.

[0090] The NGW planetary differential mechanism features basic planetary gear components, including a sun gear, planet gears, a planet carrier, and a ring gear, and incorporates a bevel gear adjustment mechanism. The main motor connects to the sun gear as the power input, while the auxiliary motor connects to the bevel gear mechanism to adjust the planet carrier speed. The coupling mechanism outputs power through the ring gear. This planetary gear system belongs to the NGW (2K-H) type planetary differential mechanism. Its function is also power coupling and speed regulation between the main and auxiliary motors, but it utilizes the inherent power distribution and speed coupling characteristics of planetary gears. The NGW planetary mechanism has significant advantages such as compact structure, small axial dimension, light weight, high transmission efficiency (single-stage efficiency can reach over 0.98), and high power capacity. Furthermore, the input and output can be arranged coaxially.

[0091] N1 is the rotational speed of the gear ring, N P N is the rotational speed of the planetary gears. S Let be the rotational speed of the sun gear, and let a be the transmission ratio between the ring gear and the sun gear.

[0092] Gear ring speed N P = NS / (1+ a).

[0093] The sun gear has 78 teeth and the ring gear has 156 teeth (according to the transmission ratio definition in this embodiment, the equivalent transmission ratio is 3 after appropriate conversion).

[0094] The high-voltage main motor remains a 4-pole high-voltage motor with a rated speed of 1500 rpm. When the low-voltage auxiliary motor speed is zero, the rated speed of the output shaft is 1500 rpm ÷ 3 = 500 rpm. The low-voltage auxiliary motor controls the planetary gear speed (±50 rpm) through forward and reverse rotation, and the final output speed can be continuously and steplessly adjusted within the range of 450 rpm to 550 rpm, which is ±10% of the rated speed.

[0095] This embodiment is applicable to water pump operating conditions that require a lower rated speed (around 500 rpm) (such as large mixed-flow pumps or axial-flow pumps). The auxiliary motor power configuration principle is the same as in Embodiment 2, with the rated power being approximately 10% of the rated power of the high-voltage main motor (e.g., a 300kW main motor paired with a 30kW auxiliary motor).

[0096] Rated power of low-voltage motor = Rated power of high-voltage motor * (speed range / 2) = 300kW * (20% / 2) = 30kW (actual power depends on specific design, this is just an example).

[0097] Example 4:

[0098] This embodiment discloses a structure based on dual NGW reverse series connection (transmission ratio 1.23, output 1200rpm).

[0099] In certain pump operating conditions, the required rated output speed is between half and full speed of the high-voltage main motor (e.g., 1200 rpm), and a single-stage NGW planetary gear train cannot achieve a suitable transmission ratio.

[0100] Option A: This embodiment uses two NGW planetary gear sets connected in reverse series, sharing a planetary carrier, to achieve any approximate speed ratio between 1 and 2. Specifically, the first-stage NGW planetary gear set has a transmission ratio of 7.38, and the second-stage NGW planetary gear set has a transmission ratio of 6. The total transmission ratio after they are connected in reverse series is 7.38 × (1 / 6) = 1.23. The high-voltage main motor is a 4-pole high-voltage motor with a rated speed of 1476 rpm, so the rated speed of the output shaft is 1476 rpm ÷ 1.23 = 1200 rpm. The low-voltage auxiliary motor is connected to one of the gear rings as a speed regulating gear, and speed regulation is achieved by forward and reverse rotation. The speed regulation range is also ±10% (i.e., 1080 rpm ~ 1320 rpm). The speed regulation and control logic is basically the same as in Embodiment 2.

[0101] Option B: The mechanical differential coupling unit is an NGW-type single-stage planetary gear mechanism, including a sun gear, planet gears, a planet carrier, and a ring gear. The high-voltage main motor is connected to the external ring gear. The high-voltage motor adopts a hollow shaft structure, and the low-voltage auxiliary motor is installed at the tail of the high-voltage motor (e.g., Figure 8 As shown in the figure, its rotating shaft is connected to the sun gear through a hollow shaft to achieve speed regulation, and the output end is connected to the planetary carrier; wherein, the transmission ratio between the sun gear and the ring gear is 4.4, the high-voltage main motor is a 4-pole motor with a rated speed of 1476 rpm, and when the speed of the low-voltage auxiliary motor is zero, the speed of the output end is 1202.7 rpm, and the speed regulation range of the low-voltage auxiliary motor on the output end is ±120 rpm, so that the speed of the output end can be adjusted within the range of 1082.7 rpm to 1322.7 rpm.

[0102] Example 5:

[0103] This embodiment discloses a comparison of selection efficiency and secondary optimization of impeller cutting to demonstrate the technical effect that providing a specific rated speed can improve selection efficiency, and at the same time, it proves that narrow-range speed regulation covers the high-efficiency zone.

[0104] Taking a certain type of centrifugal water pump as an example, the selection is required under the operating conditions of a rated flow rate of 2000 m³ / h and a head of 27 meters. The traditional selection method is to match the operating point by cutting the impeller diameter under a given rated speed (e.g., 1480 rpm), resulting in an efficiency of 86% (corresponding to the traditional selection point). Figure 5As shown. It should be noted that impeller cutting involves machining the outer diameter of the pump impeller smaller on a lathe, shifting the pump's performance curve downwards to match actual operating conditions. According to the similarity law of centrifugal pumps in Table 1, the smaller the impeller diameter, the lower the pump's head and flow rate. However, in actual engineering, we often encounter situations where a pump is selected whose standard model has an excessively high head, forcing pressure buildup by closing the valve, resulting in significant throttling losses, or the pump deviates from its high-efficiency zone, leading to low efficiency and high energy consumption.

[0105] By employing the solution of this invention, the output speed of a 4-pole high-voltage motor can be converted to the specific rated speed (e.g., 1299 rpm) required by the water pump using a mechanical differential planetary gear coupling unit. This allows the use of the water pump's largest impeller size (380 mm) to match the same operating point. Under these conditions, the selection efficiency can reach 89% (corresponding to operating point efficiency), an improvement of 3 percentage points compared to traditional solutions. Figure 6 As shown.

[0106] In summary, the traditional method of cutting the impeller achieves an efficiency of 86% at a given speed of 1480 rpm, while this embodiment, without cutting the impeller and providing a speed of 1299 rpm, achieves an efficiency of 89%. The reason for this efficiency improvement is that the pump impeller has optimal hydraulic efficiency at its maximum outer diameter. The traditional method, limited by the given motor speed, necessitates cutting the impeller to reduce the head, leading to changes in the impeller outlet angle and increased hydraulic losses. This invention, by providing "any desired rated speed," allows the pump to be matched to its operating point without cutting the impeller (or using a larger impeller size), thus preserving the impeller's optimal hydraulic performance.

[0107] Narrow-range speed regulation coverage of the high-efficiency zone Figure 7 (Diagram showing pump performance curves and high-efficiency zone; the vertical axis represents head (m), power (kW), and the horizontal axis represents flow rate (m³ / h); the black curve represents the performance curve at the rated speed, and the green curve represents the iso-efficiency curve.) Based on the rated speed of 1299 rpm, a ±10% speed adjustment (i.e., 1169 rpm to 1429 rpm) is achieved through a low-voltage auxiliary motor, covering a flow rate range of 1260 m³ / h to 2700 m³ / h under constant pressure. The pump efficiency at all operating points within this green rectangular area is greater than 80%, with the highest efficiency reaching 89.5%. These two speeds (1169 rpm and 1429 rpm) are ±10% of the rated speed of 1299 rpm. For multi-unit constant-pressure operation systems, with two pumps reasonably configured within a 20% speed adjustment range, it is entirely possible to adjust any flow rate at constant pressure within the range from the minimum rated flow rate of a single pump to the maximum flow rate required by the system, ensuring efficient system operation and maintaining constant pressure.

[0108] The above indicates that most centrifugal water pumps can basically meet the adjustment range of the high-efficiency flow range within a 20% speed regulation range—the flow rate variation range under constant pressure can cover 1260 m³ / h to 2700 m³ / h, which is more than 50% of the maximum flow rate of 2700 m³ / h. Therefore, the present invention's limitation of the speed regulation range to a narrow range of ±8% to 15% has sufficient empirical support, and this narrow range precisely covers the high-efficiency range of the water pump.

[0109] After the system is put into operation, the system matching is further optimized by combining the impeller cutting of the load water pump, so that the load always operates in the high-efficiency characteristic range of fluid machinery. By reasonably matching the 4-pole high-pressure motor, planetary gear speed ratio and impeller outer diameter cutting, the water pump operates in the high-efficiency range throughout the ±15% speed regulation range, further expanding the application coverage of existing high-efficiency water pumps and increasing the probability of selecting high-efficiency solutions without increasing the number of water pump models.

[0110] It should be noted that traditional constant pressure water supply systems typically use a variable-speed pump and multiple constant-speed pumps connected in parallel, with the variable-speed pump handling fine-tuning and the constant-speed pumps handling coarse-tuning. Alternatively, a more advanced system may employ a one-to-one variable-frequency configuration, where each pump is equipped with its own frequency converter, and all pumps operate at variable frequencies with synchronized speed regulation. In contrast, the "multi-unit constant pressure operation system with two pumps" described in this embodiment features a speed regulation system as described in this application for each pump. This means that each pump is equipped with an independent high-voltage main motor, a mechanical differential coupling unit, a low-voltage auxiliary motor, and a low-voltage four-quadrant frequency converter. Each pump is a variable-speed pump, rather than the traditional 1-variable-speed + N-fixed-speed configuration. This is fundamentally different from traditional constant pressure water supply systems (1 variable-speed pump and the rest are fixed-speed pumps). When water consumption changes slightly, the control unit adjusts the speed of the low-pressure auxiliary motor of the running water pump via PID control, allowing stepless speed regulation within the range of ±8% to 15% while tracking the pressure setpoint. When the speed regulation of a single water pump reaches its upper limit (e.g., +15%) and still cannot meet the flow demand, the control unit starts a second water pump. Conversely, when the flow demand decreases and the pressure is still too high even after the speed regulation reaches its lower limit, one water pump is stopped. In this embodiment, all pumps are speed-adjustable, and the cost of the speed regulation system for each pump is far lower than that of a high-voltage frequency converter (using only a low-voltage frequency converter with 8% to 15% power). Therefore, economically, multiple pumps can be configured with speed regulation capabilities, whereas traditional fixed-frequency pumps can only operate at full load at 50Hz and cannot be speed-adjusted, resulting in low efficiency under partial load.

[0111] Example 6:

[0112] This embodiment discloses a specific control method. The basic control logic is as follows: the control unit receives pressure signals, temperature difference signals, or liquid level signals from the load end, and automatically and continuously adjusts the speed of the low-pressure auxiliary motor based on the built-in control logic, or achieves manual adjustment by manually setting the frequency.

[0113] In constant pressure water supply scenarios, the control unit employs a PID controller to monitor the pump outlet pressure in real time and compare it with the target pressure value. When system water consumption decreases and pressure increases (unnecessary energy consumption would result if the speed is not adjusted promptly), the control unit outputs an adjustment command, controlling the auxiliary motor to decelerate and reverse via a low-voltage frequency converter, reducing the output speed until the pressure value at the measuring point falls within the allowable control deviation range. Conversely, when system water consumption increases and pressure decreases, the control unit controls the auxiliary motor to accelerate forward, increasing the output speed. The entire adjustment process is continuous and smooth, with a response speed reaching millisecond levels.

[0114] In a pump system consisting of multiple pumps, when a single variable-speed pump reaches the upper or lower limit of its set speed range, the control unit adjusts the number of operating pumps or coordinates the control of multiple variable-speed pumps to ensure the system always operates within its optimal efficiency range, maintaining optimal system efficiency and constant pressure. When the variable-speed motor needs to reach zero speed, its drive shaft is locked via an electric brake or a two-way overrunning clutch, causing the auxiliary speed control output speed to be zero. At this time, the output shaft operates at a constant speed of the rated coupling speed.

[0115] The control unit has the function of optimizing water pump control: it collects water pump pressure, flow rate, and power data, calculates the actual operating efficiency, and provides a more reasonable operating scheme or automatically matches a reasonable operating speed by comparing the original performance curve of the water pump with the built-in water pump control software. Combined with impeller cutting, the system matching is further optimized to ensure that the load water pump always operates in the high-efficiency characteristic range of fluid machinery.

[0116] Example 7:

[0117] This embodiment discloses experimental data to verify the technical effects of low cost and high efficiency.

[0118] Taking a water supply pump set in a waterworks as an example, the original system used an 8-pole high-voltage motor (rated speed 740 rpm) with valve regulation. The motor's rated power was 350 kW, and the actual operating power was approximately 280 kW. The proposed solution is as follows: the original pump is retained, the original 8-pole high-voltage motor is removed and replaced with a 4-pole high-voltage motor (rated power 300 kW, rated speed 1500 rpm), a mechanical differential planetary gear coupling unit (transmission ratio 2, output rated speed 750 rpm) is added, along with a 30 kW low-voltage auxiliary motor and a matching four-quadrant frequency converter.

[0119] Before and after comparison: In terms of motor procurement costs, the original 8-pole motor was about 40% more expensive than the 4-pole motor. This solution uses a standard 4-pole motor, which is cheaper. The speed regulation method has been changed from valve throttling to stepless speed regulation of a low-voltage auxiliary motor. The system efficiency has been improved from about 70%~80% (due to large throttling losses) to about 95%. In constant pressure water supply mode, the entire flow demand during the daytime peak water consumption and the nighttime low-water consumption can be covered by ±10% speed adjustment. The system always operates in the high-efficiency range of the water pump, and the measured comprehensive energy saving rate reaches 25%~35% (when combined with the high-efficiency water pump rematch, the overall energy saving effect can reach 5%~40%). In addition, the renovation construction only involves modifying the foundation and adding a planetary differential speed coupling unit with a low-voltage motor and controller, which has little impact on the existing equipment and relatively low requirements for construction qualifications.

[0120] Additional explanation regarding auxiliary motor selection: Due to significant power variations within the regulation range, and considering efficiency, permanent magnet synchronous motors or synchronous reluctance motors maintain higher efficiency in the low-load range. Therefore, using permanent magnet or synchronous reluctance motors as auxiliary motors is more reasonable. The power absorbed by the auxiliary motor is recovered to the low-voltage power grid through a four-quadrant frequency converter for use by low-voltage equipment.

[0121] This solution is suitable for high-pressure motor-driven fluid equipment applications where fluid demand varies, such as water supply pump sets in waterworks, circulating water pump sets in chemical plants, water systems for large air conditioning units, and thermal pump stations; high-pressure, high-power water pumps (such as those in power plants, steel mills, mines, and municipal water supply systems) requiring a small range of speed adjustment (10%~20% of rated speed); applications where fixed-speed motors and valves are already installed and energy-saving retrofits are desired but are limited by space or budget; low-speed, low-cavitation margin solutions are provided for applications with stringent requirements for pump cavitation margin in high-altitude, low-atmospheric-pressure environments; and economical low-speed solutions are provided to extend equipment lifespan in situations involving media wear.

[0122] The embodiments described above are merely illustrative of the technical concept of the present invention and should not be construed as limiting the scope of protection of the present invention. Any modifications made to the technical solutions based on the technical concept proposed in this invention shall fall within the scope of protection of this invention.

Claims

1. An energy-saving system for small-range stepless speed regulation of water pumps and fans, characterized in that, include: The system includes a high-voltage main motor, a low-voltage auxiliary motor, and a mechanical differential coupling unit. The high-voltage main motor is used to provide constant-speed main drive; the low-voltage auxiliary motor is used to adjust the output speed of the mechanical differential coupling unit, and the low-voltage auxiliary motor is also connected to a low-voltage frequency converter and a control unit for controlling its speed; the mechanical differential coupling unit has a first input terminal, a second input terminal and an output terminal, which are respectively connected to the high-voltage main motor, the low-voltage auxiliary motor and the load centrifugal water pump; The mechanical differential coupling unit mechanically couples the constant speed input of the high-voltage main motor with the variable speed input of the low-voltage auxiliary motor, so that the output speed at the output end can be continuously and steplessly adjusted within ±8%~15% of the rated speed of the high-voltage main motor.

2. The system according to claim 1, characterized in that, The high-voltage main motor has 2, 4, or 6 poles and a rated speed of 3000 rpm, 1500 rpm, or 1000 rpm; and / or the rated speed of the load centrifugal water pump is 200~1500 rpm.

3. The system according to claim 2, characterized in that, The rated power of the low-voltage auxiliary motor is 8% to 15% of the rated power of the high-voltage main motor.

4. The system according to claim 1, characterized in that, The mechanical differential coupling unit is an NGW-type single-stage planetary gear mechanism, including a sun gear, planet gears, a planet carrier, and a ring gear. The high-voltage main motor is connected to the sun gear, the low-voltage auxiliary motor is connected to the ring gear, and the output end is connected to the planet carrier. The transmission ratio is 2. The high-voltage main motor is a 4-pole motor with a rated speed of 1500 rpm. When the low-voltage auxiliary motor speed is zero, the output end speed is 750 rpm. The speed adjustment range of the low-voltage auxiliary motor is ±50 rpm, allowing the output end speed to be adjusted within the range of 675 rpm to 825 rpm.

5. The system according to claim 1, characterized in that, The mechanical differential coupling unit is an NGW-type single-stage planetary gear mechanism, including a sun gear, planet gears, a planet carrier, and a ring gear. The high-voltage main motor is connected to the sun gear, and the low-voltage auxiliary motor adjusts the speed of the planet carrier through a bevel gear mechanism. The output end is connected to the ring gear. The transmission ratio is 3. The high-voltage main motor is a 4-pole motor with a rated speed of 1500 rpm. When the speed of the low-voltage auxiliary motor is zero, the speed of the output end is 500 rpm. The speed adjustment range of the low-voltage auxiliary motor is ±50 rpm, so that the speed of the output end can be adjusted within the range of 450 rpm to 550 rpm.

6. The system according to claim 1, characterized in that, The mechanical differential coupling unit is an NGW-type single-stage planetary gear mechanism, including a sun gear, planet gears, a planet carrier, and a ring gear. The high-voltage main motor is connected to the external ring gear, and the high-voltage motor adopts a hollow shaft structure. The low-voltage auxiliary motor is installed at the tail of the high-voltage motor, and its shaft is connected to the sun gear through the hollow shaft to achieve speed regulation. The output end is connected to the planet carrier. The transmission ratio between the sun gear and the ring gear is 6. The high-voltage main motor is a 4-pole motor with a rated speed of 1500 rpm. When the speed of the low-voltage auxiliary motor is zero, the speed of the output end is 1285.7 rpm. The speed regulation range of the low-voltage auxiliary motor is ±128 rpm, so that the speed of the output end can be adjusted within the range of 1413 rpm to 1157 rpm.

7. The system according to claim 1, characterized in that, The low-voltage frequency converter is a four-quadrant frequency converter, which feeds energy back to the low-voltage power grid when the low-voltage auxiliary motor is running in reverse; and / or the low-voltage auxiliary motor is a permanent magnet synchronous motor or a synchronous reluctance motor.

8. The system according to any one of claims 1-7, characterized in that, The control unit uses the forward drive of the low-voltage auxiliary motor for speed regulation and the reverse drive for energy recovery operation. The output speed N at the output terminal out Satisfying the relation: N out = k1 * N h + k2 * N1 or N out = k1 * N h -k2 * N2 Where N h N1 is the speed of the high-voltage main motor, N2 is the speed of the low-voltage auxiliary motor, and k1 and k2 are the mechanical coupling coefficients determined by the gear ratio.

9. The system according to claim 1, characterized in that, The control unit uses a PID controller to detect the flow rate or pressure of the load centrifugal water pump in real time, compare it with the target value, and output adjustment commands. In a pump group system consisting of multiple water pumps, when a single speed-regulating water pump reaches the upper or lower limit of the set speed range, the control unit adjusts the system to operate in the optimal efficiency range by increasing or decreasing the number of operating water pumps or by coordinating the speed of multiple water pumps.

10. A narrow-range high-efficiency speed regulation method based on the system according to any one of claims 1 to 9, characterized in that, Includes the following steps: Step S1: Obtain the target speed signal of the load centrifugal water pump. The target speed signal is generated by manually setting the frequency or automatically generated by the control unit based on pressure, temperature difference or liquid level signals. Step S2: Calculate the difference between the target speed and the rated speed of the high-voltage main motor. Based on the difference and the gear ratio of the mechanical differential coupling unit, generate a speed command for the low-voltage auxiliary motor. The command satisfies the following: N1= (N target - k1×N h ) / k2, where N target For the target output speed, N h K is the speed of the high-voltage main motor, and k1 and k2 are the mechanical coupling coefficients determined by the gear ratio; Step S3: Control the low-voltage auxiliary motor to run according to the speed command, and make the output speed higher than the rated coupling speed by forward drive, or make the output speed lower than the rated coupling speed by reverse drive, so as to realize continuous stepless adjustment of the load speed within ±8%~20% of the rated speed of the high-voltage main motor; Step S4: While adjusting the speed mainly in the forward drive mode, monitor the energy cycle status. When energy recovery is required, activate the reverse drive mode to feed some energy back to the low-voltage grid through the four-quadrant frequency converter.

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