Capacity control system and method for screw compressor capable of switching speed at fixed frequency

Abstract

The invention discloses a screw compressor capacity control system and method capable of switching speed at a fixed frequency, comprising a screw compressor with a fixed frequency motor, a bypass slide valve with adjustable capacity installed on the screw compressor, a bypass slide valve action driver, a motor driver, a controller and a load measuring device, the bypass slide valve is connected to the bypass slide valve action driver signal and is controlled by the bypass slide valve action driver, a bypass slide valve position sensor is also provided, the bypass slide valve position sensor is connected to the screw compressor for measuring the slide valve position, the load measuring device is used to detect the compressor operation load state, the motor driver is connected to the fixed frequency compressor; the bypass slide valve position sensor, the bypass slide valve action driver, the load measuring device, the motor driver and the controller are connected by signal. A higher performance coefficient COP is achieved when running at full load and partial load, adapting to the site with frequent starting, and suppressing the starting current peak.

Description

Technical Field

[0001] The present invention belongs to the technical field of refrigeration and air-conditioning power control, and in particular relates to a screw compressor capacity control system and method. Background Art

[0002] Screw refrigeration air-conditioning units are generally divided into air-conditioning units that use fixed-frequency and fixed-speed drive screw compressors (referred to as fixed-frequency screw units) and air-conditioning units that use variable-frequency and variable-speed drive screw compressors (referred to as variable-frequency screw units). Fixed-frequency screw units have high full-load performance coefficients, but their performance is lower when they are reduced to partial load operation; variable-frequency screw units have high performance when they are partially loaded, but their full-load performance coefficients are reduced, but they are more expensive than fixed-frequency screw units.

[0003] For a fixed-frequency screw unit using a fixed-frequency and fixed-speed screw compressor, when the load is reduced from full load to part load, the fixed-frequency screw compressor generally adjusts its capacity through bypass devices such as sliding valves. Since the gas for bypass unloading has been compressed before bypassing, there is invalid work and its partial load performance coefficient COP is low.

[0004] The variable frequency screw unit using variable frequency and variable speed screw compressor realizes capacity regulation by adjusting the speed of the screw compressor through the inverter when the load is reduced from full load to partial load. There is no invalid work caused by bypass, and its partial load performance coefficient is relatively high. Due to the power loss of the inverter (about 3-5%), when running at full load, its performance coefficient COP is about 3-5% lower than that of the fixed frequency screw air-conditioning unit. However, the high cost of the inverter leads to high cost of the whole variable frequency screw unit.

[0005] Therefore, it will become a technical problem to be solved urgently in the field of refrigeration technology to carry out appropriate speed control of fixed-frequency screw units, especially to achieve a screw refrigeration and air-conditioning unit with a high coefficient of performance (COP) at full load and part load operation at a cost comparable to that of fixed-frequency and fixed-speed screw units.

[0006] In industrial applications, a motor is often required to run at two speeds (or multiple speeds). Two speeds (or multiple speeds) refer to the speed change gears such as high speed, low speed or medium speed. Commonly used starting and running methods for two speed (or multiple speed) motors include contactor drive, inverter drive, and soft starter drive. The contactor drive realizes the star-delta starting and high-speed and low-speed switching of the motor. This scheme has secondary current impact during the starting process, high cost, complex structure, and high failure rate; while the inverter drive realizes the motor starting and high-speed and low-speed switching. Although there is no secondary current impact during the starting process, the starting current is small, and the starting is smooth, the cost of this scheme is too high; the soft starter drive realizes the motor soft starting and high-speed and low-speed switching. This scheme is widely used due to its low cost, simple structure, and no impact when starting.

[0007] The traditional soft starter drive solution generally uses a soft starter and configures two (or more) contactors. When starting, close one contactor first, then start the soft starter. When stopping, stop the soft starter first and then release the contactor. Since the soft starter has requirements for the number of starts per hour, if the site requires frequent starts, this solution may cause damage to the soft starter. The soft starter requires a large current when driving at high speed and a small current when driving at low speed. The starting parameters and starting current limit are different, and the protection parameters are also different. Therefore, the traditional soft starter drive solution cannot reliably protect the motor and control the starting curve. Summary of the invention

[0008] The technical problem to be solved by the present invention is to provide a screw compressor capacity control system and method capable of fixed frequency and switchable speed operation, which can achieve a higher performance coefficient COP during full load and part load operation at a cost comparable to that of a fixed frequency and fixed speed screw unit, and can adapt to sites with frequent starts and have the ability to suppress the starting current peak.

[0009] The present invention provides the following technical solutions to the technical problems that need to be solved:

[0010] A screw compressor capacity control system capable of switching speed operation at a fixed frequency comprises a screw compressor including a fixed frequency motor, a bypass slide valve with adjustable capacity installed on the screw compressor, a bypass slide valve action driver, a motor driver, a controller and a load measuring device, the bypass slide valve is connected to the bypass slide valve action driver signal and is controlled by the bypass slide valve action driver, characterized in that: a bypass slide valve position sensor is also provided, the bypass slide valve position sensor is connected to the screw compressor for measuring the slide valve position, the load measuring device is used to detect the compressor operation load demand, the motor driver is connected to the fixed frequency compressor; the bypass slide valve position sensor, the bypass slide valve action driver, the load measuring device, the motor driver and the controller are connected by signal.

[0011] Furthermore, the motor driver is configured to enable dual-speed or multi-speed operation of the fixed-frequency motor and to switch between speed gears.

[0012] Furthermore, the fixed-frequency motor includes at least one winding, and the number of poles and the speed of the winding are switched by wiring; the motor driver includes at least one soft starter and two / more contactors; the soft starter selectively connects the motor winding through two / more contactors.

[0013] Furthermore, the fixed-frequency motor includes at least one group of high-speed windings and at least one group of low-speed windings; correspondingly, the motor driver includes a high-speed soft starter and a low-speed soft starter, and at least one circuit breaker; the circuit breaker is connected to the low-speed soft starter and the high-speed soft starter respectively; the high-speed soft starter is connected to the high-speed winding of the fixed-frequency motor; and the low-speed soft starter is connected to the low-speed winding of the fixed-frequency motor.

[0014] Furthermore, when the fixed-frequency compressor also includes at least one set of medium-speed windings whose rotation speed is between the high-speed windings and the low-speed windings, a plurality of starting contactors are also arranged in the soft starter electric control cabinet; wherein the output end of the high-speed soft starter is connected to the high-speed winding of the fixed-frequency compressor through a starting contactor; and a plurality of parallel starting contactors are arranged on the output end of the low-speed soft starter, and the plurality of parallel starting contactors are all connected to the low-speed winding and each medium-speed winding in a one-to-one docking manner.

[0015] Furthermore, a circuit breaker is respectively arranged in front of the high-speed soft starter and the low-speed soft starter; the two circuit breakers are connected in parallel to the power supply.

[0016] Furthermore, an isolating switch QS is connected to the main circuit in front of the two circuit breakers; the two circuit breakers are selectively connected to their respective soft starters through the isolating switch QS.

[0017] The control method of the screw compressor capable of switching speed at a fixed frequency is characterized in that: when the motor is running at different speeds, the controller obtains the load demand according to the load measuring device, the bypass slide valve position sensor detects the slide valve opening position signal, and the controller adjusts the motor running speed through the motor driver according to the load demand and the slide valve position signal to achieve multi-speed output and / or switching, and adjusts the bypass slide valve opening position through the bypass slide valve action driver so that the bypass slide valve opening position is within a set opening range, and the opening range makes the slide valve opening position always keep the motor speed in line with the trend of the motor running at a low speed and the slide valve at a high opening position.

[0018] Furthermore, the motor driver is used to adjust the motor speed to achieve three-speed output and / or switching. The specific process is as follows:

[0019] When the fixed-frequency compressor is running at high speed, the slide valve opening position is detected. If the slide valve opening position is ≤H% and there is a need to reduce load, the fixed-frequency compressor will switch to medium-speed operation, otherwise it will continue to run at high speed; H% is the lower limit of the slide valve opening position when the compressor is running at high speed set in the controller;

[0020] During medium-speed operation, the slide valve opening position is detected. If the slide valve opening position is ≤M% and there is a need to reduce load, the fixed-frequency compressor will enter low-speed operation; if the slide valve position is 100% and there is a need to increase load, the fixed-frequency compressor will enter high-speed operation, so that the slide valve opening position is between H% and 100%; otherwise, it will continue to operate at medium speed, and the slide valve opening position of medium-speed operation will be kept at M% to 100%; where M% is the lower limit of the slide valve opening position when the compressor is running at medium speed;

[0021] During low-speed operation, the slide valve opening position is detected. If the slide valve position is 100% and there is a demand for increased load, the operation is switched to medium-speed operation, and the slide valve opening position of medium-speed operation is maintained at M%~100%; otherwise, the low-speed operation is continued, and the slide valve opening position of low-speed operation is maintained at N%~100%; among which, N% is the lower limit of the slide valve opening position when the compressor is running at low speed.

[0022] Furthermore, the motor driver is used to adjust the motor speed to achieve dual-speed output and / or switching. The specific process is as follows:

[0023] The controller sets the lower limit of the slide valve opening position of the bypass slide valve to H%; that is, the slide valve opening position range is H%~100%, corresponding to the compressor output capacity range of 50%~100%. At high speed, when the slide valve opening position moves to H%, and the screw compressor capacity needs to be further adjusted, the screw compressor will switch to low speed operation. When running at low speed, the slide valve opening position range is 0~100%, corresponding to the compressor output capacity range: minimum load~50%.

[0024] Furthermore, when the motor driver power start main circuit is configured with two soft starters, the high-speed soft starter controls the high-speed winding of the motor, and the low-speed soft starter controls the low-speed winding of the motor; the outputs of the high-speed soft starter and the low-speed soft starter are interlocked, and they jointly share the operation and starting of the motor; the system controller adjusts the bypass slide valve position through the bypass slide valve action driver according to the input signal of the load measuring device and the slide valve position signal, and adjusts the motor running speed through the motor driver; when the motor runs at different speeds, the bypass slide valve opening position is adjusted according to the set range.

[0025] Furthermore, the current upper limit of the high-speed soft starter is determined by the speed N0 of the highest winding, and the current upper limit of the low-speed soft starter is determined by the speed N1 of the intermediate speed winding. The winding matching rules correspond to: the speed N0 of the highest winding ≥ the speed of the winding > the speed N1 of the intermediate speed winding, both are driven by the high-speed soft starter; the winding speed ≤ the speed N1 of the intermediate speed winding; both are driven by the low-speed soft starter.

[0026] Furthermore, the motor starting method is soft start, and the starting mode is a ramp current mode. The motor starting current peak is limited, the starting voltage is adjusted, and the current rises smoothly so that the motor speed reaches the rated speed to complete the motor starting. The high-speed winding of the motor corresponds to a high-speed soft starter, and the protection parameters are limited according to the high-speed current protection. The starting current is also limited according to the high-speed current to ensure smooth starting current; the low-speed winding of the motor corresponds to a low-speed soft starter, and the protection parameters are limited according to the low-speed current protection. The starting current is also limited according to the low-speed current to ensure smooth starting current.

[0027] Furthermore, the motor speed switching method is as follows: when the high speed is switched to the low speed or medium speed, the high speed soft starter stops the output first, and after the motor stops freely for a few seconds, the low speed soft starter is started to start the motor so that the motor runs at a low speed or medium speed; when the medium speed is switched to the low speed, the low speed soft starter stops the output first, and after the motor stops freely for a few seconds, the motor winding switches from the medium speed winding to the low speed winding, and then the low speed soft starter is started again to start the motor so that the motor runs at a low speed; when the low speed is switched to the high speed, the low speed soft starter stops the output first, and after the motor stops freely for a few seconds, the high speed soft starter is started to start the motor so that the motor runs at a high speed.

[0028] The beneficial effects of the present invention are:

[0029] 1. Use a fixed-frequency screw compressor with fixed-frequency dual-speed or multi-speed drive, bypass slide valve, slide valve position sensor, and main controller to control the slide valve within a reasonable stroke range when the compressor runs at different speeds, achieving high-efficiency operation at full load and partial load;

[0030] 2. When used for fixed-frequency screw air-conditioning units, there is no need to configure a frequency conversion cabinet. Under the premise of the same full-load COP and partial-load COP, the overall cost of the unit is reduced by more than 10% compared with the variable-frequency screw air-conditioning unit;

[0031] 3. When used in fixed-frequency screw air-conditioning units, the partial load COP of 50% and below is 20-30% higher than that of fixed-frequency and fixed-speed screw air-conditioning units, with obvious energy-saving effect.

[0032] 4. The main circuit of the motor controller is equipped with two soft starters, one soft starter drives the high-speed winding, and the other soft starter drives the low-speed winding. The outputs of the soft starters are interlocked to share the operation and starting of the motor, which can meet the frequent starting of the motor and extend the life of the drive system.

[0033] 5. The soft starter is directly connected to the motor terminals. The dual-speed motor has six terminals. The soft starter only needs to connect 6 sets of power cables to the motor respectively, which makes the wiring simple.

[0034] 6. The parameters of the two soft starters configured in the main circuit are matched with the dual-speed motor windings. The high-current soft starter drives the high-speed motor windings, and the low-current soft starter drives the low-speed motor windings. The parameters correspond to the high-speed and low-speed motors, which provide safer protection and better performance.

[0035] 7. Use a soft starter to drive the motor. The current is smooth during the startup process. The peak value of the starting current can be adjusted. The starting mode uses a ramp current solution to make the starting current smoother, which is more suitable for the starting curves of high-speed and low-speed motors. There is no secondary impact. The soft starter body has a relatively comprehensive protection function (over / undervoltage protection, reverse phase protection, three-phase current unbalance protection, input / output phase loss, etc.) to protect the motor more comprehensively and safely. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 The figure is a schematic structural diagram of a screw compressor capacity control system capable of operating at a fixed frequency and switching speed according to the present invention.

[0037] Figure 2 This is a control logic diagram taking the three-speed operation and switching of the screw compressor capacity control system of the present invention as an example.

[0038] Figure 1-2 The reference numerals in the accompanying drawings are: fixed-frequency compressor 1, bypass slide valve action driver 4, motor driver 5, load measuring device 6, system controller 7, bypass slide valve 8, slide valve position sensor 9.

[0039] Figure 3 The schematic diagram of the structure of the motor driver of the present invention is based on a dual-speed motor drive system.

[0040] Figure 4 This is a circuit structure diagram of a specific embodiment of the motor driver of the present invention based on a dual-speed motor drive system.

[0041] Figure 5 This is a main circuit structure diagram of the motor driver of the present invention driven by a single soft starter.

[0042] Figure 6 This is a main circuit structure diagram of another embodiment of the motor driver of the present invention driven by a single soft starter.

[0043] Figure 7 This is a flow chart of the dual-speed operation and switching control method of the motor driver of the present invention.

[0044] Figure 3-7 The reference numerals in the accompanying drawings are as follows: 1. Fixed-frequency compressor (wherein motor windings 1, 2, and 3 are high-speed windings, and motor windings 4, 5, and 6 are low-speed windings), 2. Soft starter electric control cabinet (QM1 is a high-speed soft starter, QM2 is a low-speed soft starter, and QF is a circuit breaker), 3. Controller (CB).

[0045] Figure 8 It is a schematic diagram of the structure of the motor driver 5 of the present invention based on multi-speed motor drive.

[0046] Fig. 9 It is a circuit structure diagram of a specific embodiment of the motor driver 5 of the present invention based on multi-speed motor driving.

[0047] Fig.10 The present invention is a flow chart of a three-speed operation and switching control method of a motor driver 5 based on a multi-speed motor drive. DETAILED DESCRIPTION

[0048] Figure 1 The structural principle diagram of the screw compressor capacity control system capable of fixed-frequency switching speed operation according to the present invention. It includes a fixed-frequency compressor 1, a bypass slide valve action driver 4, a motor driver 5, a load measuring device 6, a system controller 7, and a bypass slide valve 8; the fixed-frequency compressor 1 is a combination of a screw compressor and a fixed-frequency motor, and the motor speed is adjusted by switching the pole pairs. The screw compressor is equipped with a bypass slide valve 8 with adjustable capacity, which is connected to and controlled by the bypass slide valve action driver 4; a slide valve position sensor 9 is also provided, which is connected to the screw compressor for measuring the slide valve position; the motor driver 5 is functionally connected to the motor to realize dual-speed or multi-speed operation; the load measuring device 6 is used to detect the load demand of the compressor operation; the bypass slide valve position sensor 9, the bypass slide valve action driver 4, the load measuring device 6, the motor driver 5 are connected to the system controller 7 by signal.

[0049] The control method of the screw compressor and capacity control system capable of switching speed at a fixed frequency is characterized in that: the bypass slide valve position sensor 9 detects the slide valve position signal, the system controller 7 adjusts the position of the bypass slide valve 8 through the bypass slide valve action driver 4 according to the input signal of the load measuring device 6 and the slide valve position signal, and adjusts the motor running speed through the motor driver 5 to achieve multi-speed output; or when the motor runs at different speeds, the system controller 7 sets different movable position ranges for the bypass slide valve 8.

[0050] The characteristic of fixed frequency screw compressor is that according to the designed pressure ratio, the closer the slide valve position is to the full position, the higher the energy efficiency. When outputting at a small capacity, the compressor should be operated at a lower speed and a higher slide valve position as much as possible to obtain higher energy efficiency.

[0051] The purpose of controlling the speed and matching the slide valve position is to put the compressor in the lowest power (highest energy efficiency) state. For example, if a three-speed compressor wants to output 30% of its capacity, there are theoretically three output modes: ① high speed (3000r / min) + slide valve 30%; ② medium speed (1500r / min) + slide valve 60%; ③ low speed (1000r / min) + slide valve 90.9%; After many studies and experiments of the present invention, it is found that the output energy efficiency is the highest in the third mode.

[0052] In order to further implement this control method and achieve the highest energy efficiency output of the third method, the present invention adopts the following Figure 2 The control flow shown (control logic taking three-speed operation as an example):

[0053] When the fixed-frequency compressor 1 is running at high speed, the slide valve position is detected. If the slide valve position is ≤H% and there is a need to reduce load, the fixed-frequency compressor 1 is switched to medium-speed operation, otherwise it continues to operate at high speed; H% is the lower limit of the valve position movement when the compressor is running at high speed;

[0054] During medium-speed operation, the slide valve position is detected. If the slide valve position is ≤M% and there is a need to reduce load, the fixed-frequency compressor 1 is switched to low-speed operation; if the slide valve position is 100% and there is a need to increase load, the fixed-frequency compressor 1 is switched to high-speed operation, so that the slide valve position is between H% and 100%; otherwise, it continues to operate at medium speed, and the slide valve position of medium-speed operation is kept at M% to 100%; where M% is the lower limit of the valve position movement when the compressor is running at medium speed;

[0055] When running at low speed, the slide valve position is detected. If the slide valve position is at 100% and there is a demand for load increase, it will switch to medium speed operation and keep the slide valve position at medium speed operation at M%~100%; otherwise, it will continue to run at low speed and keep the slide valve position at low speed operation at N%~100%; among which, N% is the lower limit of the valve position movement when the compressor runs at low speed.

[0056] The control logic can also be Figure 2 Whether it is high speed or low speed, as long as it is running, press Figure 2 The load demand shown is adjusted in combination with the position of the slide valve.

[0057] A specific embodiment of starting the compressor according to the above method is as follows: Taking the operation of the high-speed motor as an example, the slide valve is positioned at 50%. At this time, the output capacity is 50%, which is higher than the target load. The load measuring device 6 feeds back to the system controller 7, indicating that the load needs to be reduced. The system controller 7 sends a command to the motor driver 5: stop the high-speed motor and start the medium-speed motor. During startup, the slide valve will move to 66%. At this time, the load is 33%, which is higher than the target load. The load measuring device 6 feeds back to the system controller 7, indicating that the load needs to be further reduced. The system controller 7 sends a command to the motor driver 5: stop the medium-speed motor and start the low-speed motor. The bypass slide valve 8 will be based on the feedback of the load measuring device 6. The system controller 7 sends a command to the slide valve driver 4, so that the bypass slide valve 8 will eventually stop at the 90.9% position to achieve a load output of 30%. (This process is the process of the compressor capacity changing from high to low, and the process of switching from high speed to low speed. It is the most common in actual operation.)

[0058] Taking the bypass slide valve 8 position of 0% corresponding to the minimum capacity of the screw compressor and the bypass slide valve 8 position of 100% corresponding to the maximum capacity of the screw compressor as an example, the system controller 7 detects the position of the bypass slide valve 8 through the bypass slide valve position sensor 9, and controls the movable position of the slide valve 8 within a certain range at different speeds to achieve a high performance coefficient of the screw compressor from full load to minimum load. The compressor speeds and recommended slide valve positions for two-speed and three-speed operation are shown in Table 1 and Table 2.

[0059] As shown in Table 1, for a three-speed drive screw compressor with high, medium and low speed pole numbers of 2, 4 and 6 respectively, when operating at a high speed with a pole number of 2, it is defined that when operating at a high speed, the lower limit of the slide valve opening position H% is 40-60%, preferably H% is about 50% (that is, the movable position range of the bypass slide valve is H%-100% when operating at a high speed, corresponding to the compressor output capacity range of 50%-100%). When the slide valve position moves to H%, and the screw compressor capacity needs to be further adjusted, the screw compressor will switch to operating at a medium speed with a pole number of 4; when operating at a medium speed, the lower limit of the slide valve opening position movement is M%, generally M% is 50-75%, preferably About 66% (that is, the movable range of the slide valve opening position is M%~100%, corresponding to the compressor output capacity range of 33%~50%). At medium speed, when the slide valve position moves to M%, and the screw compressor capacity needs to be further adjusted, the screw compressor will switch to low speed operation with a pole number of 6. When running at low speed, the lower limit of the slide valve opening position movement is N%. Generally, N% is between 0 and 50% (that is, the movable range of the slide valve opening position is N%~100%, corresponding to the compressor output capacity range: minimum load~33%). The smaller the N value, the better. The smaller the N value, the smaller the minimum load of the compressor, which is affected by the motor temperature rise and the reliability of the compressor.

[0060] Table 1 Relationship between compressor output capacity, speed and slide valve position for three-speed operation

[0061] Compressor output capacity Speed Bypass slide valve movable position range 50%～100% High (3000r / min) (H% 40~60%)H%~100% 33%～50% Medium (1500r / min) (M% 50~75%)M%~100% min~33% Low (1000r / min) (N% 0~50%) N%~100%

[0062] Table 1 explains: The compressor capacity corresponding to the 100% slide valve position at medium speed is equivalent to that at the H% slide valve position at high speed; the compressor capacity corresponding to the 100% slide valve position at low speed is equivalent to that at the M% slide valve position at medium speed.

[0063] As shown in Table 2, for a dual-speed drive screw compressor with 2 and 4 high-speed and low-speed pole numbers respectively, when the pole pair number is 1 (pole number is 2) and the high-speed operation is performed, the controller sets the lower limit of the bypass slide valve position movement to H% (that is, the movable range of the bypass slide valve position is H%~100%, corresponding to the compressor output capacity range of 50%~100%). Generally, H% is around 50%. At high speed, when the slide valve position moves ≤H% and the screw compressor capacity needs to be further adjusted, the screw compressor will switch to low-speed operation (the movable range of the bypass slide valve position is N%~100%, corresponding to the compressor output capacity range: minimum load~50%).

[0064] Table 2 Relationship between compressor output capacity, speed and slide valve position in dual-speed operation

[0065] Compressor output capacity Speed Bypass slide valve movable position range 50%～100% High (3000r / min) (H% 40~60%)H%~100% min~50% Low (1500r / min) (N% 0~50%) N%~100%

[0066] Table 2 shows that the compressor capacity corresponding to the 100% slide valve position at low speed and the H% slide valve position at high speed is equivalent.

[0067] There are many implementation methods for the motor driver 5 used in the present invention. Conventional driving methods, such as "one soft starter + two / multiple contactor drives", can also be applied to the present invention and are also included in the protection scope. In addition, a further feature of the present invention is that the main circuit is equipped with two soft starters, one soft starter drives the high-speed winding of the motor, and the other soft starter drives the medium-speed and low-speed windings of the motor. The outputs of the soft starters are interlocked, and the operation and starting of the motor are shared, which can meet the frequent starting of the motor and extend the life of the drive system; the parameters of the two soft starters configured in the main circuit are matched with the dual-speed motor windings, respectively, the high-current soft starter drives the high-speed motor windings, and the low-current soft starter drives the low-speed motor windings. The parameters correspond to the high-speed and low-speed motors, and the protection is safer and the performance is better. When the fixed-frequency compressor 1 runs at different speeds, the slide valve is controlled within a reasonable stroke range to achieve high-efficiency operation at full load and partial load.

[0068] like Figure 3The figure shows the structural principle diagram of the motor driver of the present invention based on the dual-speed motor drive system. The controller 3 is connected to the soft starter electric control cabinet 2, and the soft starter electric control cabinet 2 is connected to the fixed frequency compressor 1; a circuit breaker QF, a high-speed soft starter QM1, and a low-speed soft starter QM2 are arranged in the soft starter electric control cabinet 2; the fixed frequency compressor 1 includes a group of high-speed windings (motor windings 1, 2, 3) and a group of low-speed windings (motor windings 4, 5, 6).

[0069] like Figure 4 Shown Figure 3 The figure is a circuit diagram of a specific embodiment of the motor driver of the present invention based on a dual-speed motor drive system. Among them, the control ends of the high-speed soft starter QM1 and the low-speed soft starter QM2 are respectively connected in parallel to the controller 3; the input ends (terminals L1, L2, and L3) of the high-speed soft starter QM1 and the low-speed soft starter QM2 are respectively connected in parallel to the three-phase terminals behind the circuit breaker QF; the output end (terminals L4, L5, and L6) of the high-speed soft starter QM1 is sequentially connected to the high-speed winding windings of the fixed-frequency compressor 1 (motor windings 1, 2, and 3); the output end (terminals L4, L5, and L6) of the low-speed soft starter QM2 is sequentially connected to the low-speed winding windings of the fixed-frequency compressor 1 (motor windings 4, 5, and 6).

[0070] Figure 5 This is a main circuit structure diagram of the motor driver 5 of the present invention based on a single soft starter drive. Figure 4 A variant embodiment of the embodiment in Figure 4 The difference is that two circuit breakers QF1 and QF2 are used, each circuit breaker is connected to a soft starter, and then the two soft starters drive the dual-speed motor. The high-speed soft starter QM1 and the low-speed soft starter QM2 are started according to the on and off of the circuit breakers QF1 and QF2, respectively, to achieve the dual-speed motor driven by a single soft starter.

[0071] Figure 6 FIG. 5 is a main circuit structure diagram of another embodiment of the motor driver 5 of the present invention based on a single soft starter drive. Figure 5 An isolating switch QS is connected to the main road in front of the two circuit breakers QF1 and QF2; two soft starters are formed through the isolating switch QS to selectively realize driving a dual-speed motor based on a single soft starter.

[0072] Figure 7 This is a flow chart of the motor controller of the present invention based on the above-mentioned dual-speed operation and switching control method. Among them:

[0073] The motor starting mode is soft start, and the starting mode is ramp current mode. The peak current of the motor starting current is limited, the starting voltage is adjusted, and the current rises smoothly so that the motor speed reaches the rated speed to complete the motor starting. The high-speed windings 1, 2, and 3 of the motor correspond to a high-current soft starter (QM1). The protection parameters are based on high-speed current protection, and the starting current is also limited according to the high-speed current to ensure smooth starting current; the low-speed windings 4, 5, and 6 of the motor correspond to a low-speed current soft starter (QM2). The protection parameters are based on low-speed current protection, and the starting current is also limited according to the low-speed current to ensure smooth starting current;

[0074] Motor high-speed and low-speed switching mode: when switching from high speed to low speed, first high-speed soft starter QM1 stops output, the motor stops for a few seconds, then low-speed soft starter QM2 is started to start the motor to make the motor run at low speed; when switching from low speed to high speed, first low-speed soft starter QM2 stops output, the motor stops for a few seconds, then high-speed soft starter QM1 is started to start the motor to make the motor run at high speed;

[0075] Two soft starters share the starting and running of a motor. When one soft starter is running, the other one stops, which can meet the needs of frequent starting and extend the service life.

[0076] During all starting and running processes, the soft starter feeds back the current signal to the controller, and the controller protects the starting and running conditions of the motor according to the current value.

[0077] Figure 8 The schematic diagram of the structure of the motor controller of the present invention based on multi-speed motor drive. The difference between it and the dual-speed start is that a soft starter is equipped with multiple contactors. Specifically: the controller 3 is connected to the soft starter electric control cabinet 2, and the soft starter electric control cabinet 2 is connected to the fixed-frequency compressor 1; a circuit breaker QF, a high-speed soft starter QM1, a low-speed soft starter QM2, and N starting contactors (KM1~KMN are starting contactors) are set in the soft starter electric control cabinet 2; the fixed-frequency compressor 1 includes multiple groups of windings, at least one group of high-speed windings (motor windings 1, 2, 3) and one group of low-speed windings (motor windings N, N+1, N+2).

[0078] Fig. 9It is a circuit structure diagram of a specific embodiment of the motor controller of the present invention based on multi-speed motor drive. Among them, the control ends of the high-speed soft starter QM1 and the low-speed soft starter QM2 are respectively connected in parallel to the controller 3; the input ends (terminals L1, L2, and L3) of the high-speed soft starter QM1 and the low-speed soft starter QM2 are respectively connected in parallel to the three-phase terminals behind the circuit breaker QF; the output end (terminals L4, L5, and L6) of the high-speed soft starter QM1 is connected to the high-speed winding windings of the fixed-frequency compressor 1 (motor windings 1, 2, and 3) in sequence through the starting contactor KM1; the output end (terminals L4, L5, and L6) of the low-speed soft starter QM2 is output in parallel with at least two ends, one of which is connected to the low-speed winding windings of the fixed-frequency compressor 1 in sequence (N, N+1, and N+2) through the starting contactor KMN; the other end is connected to the medium-speed winding windings of the fixed-frequency compressor 1 in sequence through the starting contactor KM2 (4, 5, and 6); and so on.

[0079] Fig.10 The invention is a flow chart of the three-speed operation and switching control method of the motor driver 5 based on the multi-speed motor drive of the present invention. The motor speed switching mode is as follows: when the high speed is switched to the low speed or medium speed, the high speed soft starter stops the output first, and the motor stops for a few seconds, and then the low speed soft starter is started to start the motor so that the motor runs at a low speed or medium speed; when the medium speed is switched to the low speed, the low speed soft starter stops the output first, and the motor stops for a few seconds, and then the motor winding switches from the medium speed winding to the low speed winding, and then the low speed soft starter is started again to start the motor so that the motor runs at a low speed; when the low speed is switched to the high speed, the low speed soft starter stops the output first, and the motor stops for a few seconds, and then the high speed soft starter is started to start the motor so that the motor runs at a high speed.

[0080] Before the motor starts, the controller 3 controls the contactor KMN corresponding to the winding to be started to be attracted first, and then controls the soft starter QM to start.

[0081] When switching, the controller first stops the working starter QM, then releases the contactor KM corresponding to the winding, then starts the starter QM that needs to run, and then connects the contactor KM corresponding to the winding.

[0082] First, the high-speed soft starter QM1 stops output, and the motor stops freely for a few seconds before starting the low-speed soft starter QM2 to start the motor and make the motor run at low speed; when the high speed is switched to the medium speed, the medium-speed soft starter QM2 starts the motor and makes the motor run at medium speed; when switching from medium speed to low speed, the low-speed soft starter QM2 starts the motor and makes the motor run at low speed.

[0083] The matching rules between multi-speed (speed gear greater than 2) motor and soft starter are: high-current soft starter drives high-speed motor winding, and low-current soft starter drives low-speed motor winding.

[0084] The current upper limit of the high-speed soft starter is determined by the speed N0 of the highest winding, and the current upper limit of the low-speed soft starter is determined by the speed N1 of the intermediate speed winding. The winding matching rules correspond to: the speed of the highest winding N0 ≥ winding speed > the speed of the intermediate speed winding N1, both are driven by the high-speed soft starter; the winding speed ≤ the speed of the intermediate speed winding N1; both are driven by the low-speed soft starter.

[0085] The main circuit configuration of the dual-speed and multi-speed motor drive system is not limited to the above forms, and any solution that uses a soft starter to drive a dual-speed or multi-speed motor is within the protection scope of the present invention.

[0086] Therefore, the beneficial effects of the present invention are as follows:

[0087] 1. Use a fixed-frequency screw compressor with fixed-frequency dual-speed or multi-speed drive, bypass slide valve, slide valve position sensor, and main controller to control the slide valve within a reasonable stroke range when the compressor runs at different speeds, achieving high-efficiency operation at full load and partial load;

[0088] 2. When used for fixed-frequency screw air-conditioning units, there is no need to configure a frequency conversion cabinet. Under the premise of the same full-load COP and partial-load COP, the overall cost of the unit is reduced by more than 10% compared with the variable-frequency screw air-conditioning unit;

[0089] 3. When used in fixed-frequency screw air-conditioning units, the partial load COP of 50% and below is 20-30% higher than that of fixed-frequency and fixed-speed screw air-conditioning units, with obvious energy-saving effect.

[0090] 4. The main circuit of the motor controller is equipped with two soft starters, one soft starter drives the high-speed winding, and the other soft starter drives the low-speed winding. The outputs of the soft starters are interlocked to share the operation and starting of the motor, which can meet the frequent starting of the motor and extend the life of the drive system.

[0091] 5. The soft starter is directly connected to the motor terminals. The dual-speed motor has six terminals. The soft starter only needs to connect 6 sets of power cables to the motor respectively, which makes the wiring simple.

[0092] 6. The parameters of the two soft starters configured in the main circuit are matched with the dual-speed motor windings. The high-current soft starter drives the high-speed motor windings, and the low-current soft starter drives the low-speed motor windings. The parameters correspond to the high-speed and low-speed motors, which provide safer protection and better performance.

[0093] 7. Use a soft starter to drive the motor. The current is smooth during the startup process. The peak value of the starting current can be adjusted. The starting mode uses a ramp current solution to make the starting current smoother, which is more suitable for the starting curves of high-speed and low-speed motors. There is no secondary impact. The soft starter body has a relatively comprehensive protection function (over / undervoltage protection, reverse phase protection, three-phase current unbalance protection, input / output phase loss, etc.) to protect the motor more comprehensively and safely.

Claims

1. A screw compressor capacity control system capable of switching speed at a fixed frequency, comprising a screw compressor with a fixed frequency motor, a bypass slide valve with adjustable capacity installed on the screw compressor, a bypass slide valve action driver, a motor driver, a controller and a load measuring device, the bypass slide valve is connected to the bypass slide valve action driver signal and is controlled by the bypass slide valve action driver, characterized in that: A bypass slide valve position sensor is also provided, which is connected to the screw compressor to measure the bypass slide valve position, a load measuring device is used to detect the operating load demand of the screw compressor, and a motor driver is connected to the fixed-frequency screw compressor; the bypass slide valve position sensor, the bypass slide valve action driver, the load measuring device, and the motor driver are connected to the controller signal; The motor driver includes at least one soft starter and at least two contactors; The soft starter selectively connects the motor windings via at least two contactors: The fixed frequency motor includes at least one set of high speed windings and at least one set of low speed windings; Correspondingly, the motor driver includes a high-speed soft starter and a low-speed soft starter, and at least one circuit breaker; The circuit breaker is connected to the low-speed soft starter and the high-speed soft starter respectively; the high-speed soft starter is connected to the high-speed winding of the fixed-frequency motor; The low-speed soft starter is connected to the low-speed winding of the fixed-frequency motor; and, the fixed-frequency motor includes at least one set of medium-speed windings whose speed is between the high-speed windings and the low-speed windings, and the soft starter is provided with a plurality of starting contactors in addition to the high-speed soft starter and the low-speed soft starter; The output end of the high-speed soft starter is connected to the high-speed winding of the fixed-frequency screw compressor through a starting contactor; a plurality of parallel starting contactors are arranged on the output end of the low-speed soft starter, and the plurality of parallel starting contactors are connected to the low-speed winding and each medium-speed winding in a one-to-one docking manner.

2. The screw compressor capacity control system capable of fixed frequency switching speed operation according to claim 1, characterized in that: The motor driver is configured to enable the fixed-frequency motor to run at multiple speeds and switch between speed gears; the fixed-frequency motor includes at least one winding, and the number of poles and the speed of the winding are switched by wiring.

3. The screw compressor capacity control system capable of fixed frequency and switching speed operation according to claim 1, characterized in that: A circuit breaker is respectively arranged in front of a high-speed soft starter and a low-speed soft starter; the two circuit breakers are connected in parallel to the power supply.

4. The screw compressor capacity control system capable of fixed frequency switching speed operation according to claim 3 is characterized in that: An isolating switch QS is connected to the main circuit in front of the two circuit breakers; the two circuit breakers can be selectively connected to their respective soft starters through the isolating switch QS.

5. A capacity control method for a screw compressor capacity control system capable of fixed frequency and switchable speed operation according to any one of claims 1 to 4, characterized in that: When the fixed-frequency motor is running at different speeds, the controller obtains the load demand according to the load measuring device, and the bypass slide valve position sensor detects the bypass slide valve opening position signal. The controller adjusts the fixed-frequency motor running speed through the motor driver to achieve multi-speed output and / or switching according to the load demand and the bypass slide valve opening position signal, and adjusts the bypass slide valve opening position through the bypass slide valve action driver to make the bypass slide valve opening position within a set opening range. The opening range makes the bypass slide valve opening position always keep the fixed-frequency motor speed in a trend toward the fixed-frequency motor running at a low speed and the bypass slide valve at a high opening position.

6. A capacity control method using the screw compressor capacity control system capable of fixed frequency and switchable speed operation according to claim 5, characterized in that: The specific process of adjusting the running speed of the fixed-frequency motor through the motor driver to achieve three-speed output and / or switching is as follows: When the fixed-frequency screw compressor is running at high speed, the bypass slide valve opening position is detected. If the bypass slide valve opening position is ≤H% and there is a need to reduce load, the fixed-frequency screw compressor will switch to medium-speed operation, otherwise it will continue to operate at high speed; H% is the lower limit of the bypass slide valve opening position when the screw compressor is running at high speed set in the controller; During medium-speed operation, the bypass slide valve opening position is detected. If the bypass slide valve opening position is ≤M% and there is a need to reduce load, the fixed-frequency screw compressor is switched to low-speed operation; if the bypass slide valve position is at 100% and there is a need to increase load, the fixed-frequency screw compressor is switched to high-speed operation, so that the bypass slide valve opening position is between H% and 100%; otherwise, it continues to operate at medium speed, and the bypass slide valve opening position of medium-speed operation is maintained at M% to 100%; where M% is the lower limit of the bypass slide valve opening position when the screw compressor is running at medium speed; During low-speed operation, the bypass slide valve opening position is detected. If the bypass slide valve is at 100% and there is a demand for load increase, the operation is switched to medium-speed operation, and the bypass slide valve opening position of medium-speed operation is maintained at M%~100%; otherwise, the low-speed operation is continued, and the bypass slide valve opening position of low-speed operation is maintained at N%~100%; among which, N% is the lower limit of the bypass slide valve opening position when the screw compressor is running at low speed.

7. A capacity control method using the screw compressor capacity control system capable of fixed frequency and switchable speed operation according to claim 5, characterized in that: The specific process of adjusting the running speed of the fixed-frequency motor through the motor driver to achieve dual-speed output and / or switching is as follows: When the fixed-frequency screw compressor is running at high speed, the bypass slide valve opening position is detected. If the bypass slide valve opening position is ≤H% and there is a demand for load reduction, the fixed-frequency screw compressor is switched to low-speed operation, otherwise it continues to operate at high speed; H% is the lower limit of the bypass slide valve opening position when the screw compressor is running at high speed set in the controller; when running at low speed, the bypass slide valve opening position is detected. If the bypass slide valve is at 100% and there is a demand for load increase, it is switched to high-speed operation, and the bypass slide valve opening position of high-speed operation is maintained at H%~100%; otherwise, it continues to run at low speed, and the bypass slide valve opening position of low-speed operation is maintained at N%~100%; where N% is the lower limit of the bypass slide valve opening position when the screw compressor is running at low speed.

8. A capacity control method using a screw compressor capacity control system capable of fixed frequency and switchable speed operation according to any one of claims 5 to 7, characterized in that: When the motor driver power start main circuit is configured with two soft starters, the high-speed soft starter controls the high-speed winding of the fixed-frequency motor, and the low-speed soft starter controls the low-speed winding of the fixed-frequency motor; the outputs of the high-speed soft starter and the low-speed soft starter are interlocked, and they share the operation and start-up of the fixed-frequency motor; the controller adjusts the bypass slide valve opening position through the bypass slide valve action driver according to the input signal of the load measurement device and the bypass slide valve opening position signal, and adjusts the operating speed of the fixed-frequency motor through the motor driver; when the fixed-frequency motor runs at different speeds, the bypass slide valve opening position is adjusted according to the set range.

9. A capacity control method using a screw compressor capacity control system capable of fixed frequency and switchable speed operation according to any one of claims 5 to 7, characterized in that: The current upper limit of the high-speed soft starter is determined by the speed N0 of the high-speed winding, and the current upper limit of the low-speed soft starter is determined by the speed N1 of the medium-speed winding. The winding matching rules correspond to: the speed N0 of the high-speed winding ≥ the speed of the winding > the speed N1 of the medium-speed winding, both are driven by the high-speed soft starter; the winding speed ≤ the speed N1 of the medium-speed winding, both are driven by the low-speed soft starter.

10. A capacity control method using a screw compressor capacity control system capable of fixed frequency and switchable speed operation according to any one of claims 5 to 7, characterized in that: The starting mode of the fixed-frequency motor is soft start, and the starting mode is ramp current mode. The starting current peak of the fixed-frequency motor is limited, the starting voltage is adjusted, and the current rises smoothly so that the fixed-frequency motor speed reaches the rated speed to complete the starting of the fixed-frequency motor. The high-speed winding of the fixed-frequency motor corresponds to a high-speed soft starter, and the protection parameters are limited by high-speed current protection. The starting current is also limited by high-speed current to ensure smooth starting current; the low-speed winding of the fixed-frequency motor corresponds to a low-speed soft starter, and the protection parameters are limited by low-speed current protection. The starting current is also limited by low-speed current to ensure smooth starting current.

11. A capacity control method using a screw compressor capacity control system capable of fixed frequency and switchable speed operation according to any one of claims 5 to 7, characterized in that: The speed switching mode of the fixed-frequency motor is as follows: when switching from high speed to low speed or medium speed, the high-speed soft starter stops output first, and the fixed-frequency motor stops freely for a few seconds, then the low-speed soft starter is started to start the fixed-frequency motor to make it run at low speed or medium speed; when switching from medium speed to low speed, the low-speed soft starter stops output first, and the fixed-frequency motor stops freely for a few seconds, then the winding of the fixed-frequency motor switches from the medium-speed winding to the low-speed winding, and then the low-speed soft starter is started again to start the fixed-frequency motor to make it run at low speed; When switching from low speed to high speed, the low-speed soft starter stops output first, and after the fixed-frequency motor stops freely for a few seconds, the high-speed soft starter is started to start the fixed-frequency motor and make it run at high speed.

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