Soft start and frequency converter combined motor starting method and system

Abstract

The invention belongs to the technical field of high-voltage motor soft starting, and particularly relates to a motor starting method and system combining soft starting and a frequency converter, and the method comprises the following steps: starting a motor through a high-voltage soft starter, and monitoring the current of the motor; when the current drops to a preset threshold value, the soft start bypass circuit breaker is triggered to be closed, and timing is started; after the timing reaches 4 seconds, the high-voltage vacuum circuit breaker of the soft start loop is switched off, and the high-voltage vacuum circuit breaker of the frequency converter loop is switched on at the same time; the frequency converter completes self-inspection within a set time of 12-18 seconds and directly outputs a fixed frequency to drive the motor; and the frequency converter outputs a 4-20mA signal to the excitation device, and the excitation current of the motor is synchronously adjusted to finish starting. Two starting modes of soft starting and the frequency converter are combined, and a reasonable time point is set for switching, so that the tracking frequency of the frequency converter meets the lowest requirement of a form oil film of the bearing bush, and the purpose of creating maximum benefits for production is achieved.

Description

Technical Field

[0001] This invention belongs to the field of high-voltage motor soft-start technology, specifically relating to a motor starting method that combines soft starter with frequency converter. Background Technology

[0002] Electric motors are widely used in metallurgy, chemical industry, water conservancy and other industrial fields due to their practicality. Traditional motor starting methods include: direct full-voltage starting, autotransformer starting, Y-δ starting, and soft starters. Direct full-voltage starting is mostly used for starting small-power motors. From the perspective of energy conservation, this method is not suitable for motors larger than 11kW. Furthermore, this method results in excessive starting current, which can cause significant mechanical shock and easily damage equipment. Autotransformer starting and Y-δ starting methods use reduced voltage starting, but still have an impact on the power grid. Soft starters are currently a commonly used motor starting method. The main component used is a thyristor, which achieves voltage regulation starting of the motor through the phase-shifting voltage regulation principle of the thyristor. It has good starting effect but is more expensive. At the same time, because of the use of thyristor components, the harmonic interference of the thyristor is relatively large when the thyristor is working, which has a certain impact on the power grid. In addition, power grid fluctuations can also affect the conduction of thyristor components, especially when there are multiple thyristor devices on the same power grid. Therefore, the failure rate of thyristor components is relatively high, because it involves power electronics technology, and thus the requirements for maintenance technicians are also high.

[0003] Soft starters are a commonly used method for starting motors. Their main component is a thyristor, and they achieve voltage regulation starting of the motor through the phase-shifting voltage regulation principle of silicon controlled rectifiers (SCRs). While offering good starting performance, they are more expensive. Furthermore, because of the use of SCRs, they generate significant harmonic interference during operation, which can impact the power grid. Power grid fluctuations can also affect the conduction of SCRs, especially when multiple SCR devices are connected to the same grid. Therefore, SCRs have a relatively high failure rate, and due to the involvement of power electronics technology, the requirements for maintenance technicians are also higher.

[0004] The main pump in the sintering workshop of the ironmaking plant is typically driven by two 4900KW synchronous motors. Currently, there are two sets of Changsha Auto soft starters and two sets of Leadford frequency converters (detailed parameters for the soft starters and frequency converters need to be completed). Initially, the two soft starters were used for startup, but during operation, the negative pressure control via valves was inaccurate (detailed parameters for valve diameters and electrical components need to be completed), and it was also not energy-efficient. Later, the two frequency converters were considered for startup, but the long startup time (approximately 300 seconds from 0-50Hz) caused the fan bearings to overheat and burn out (the bearings lack a forced oil supply system; an oil film can only form at approximately 600 RPM at 30Hz).

[0005] Given the following problems with current soft starters: Soft starter operation alone results in inaccurate negative pressure control and high energy consumption. The original frequency converter operation alone results in a long start-up time (300 seconds), causing the fan bearings to burn out due to insufficient oil film formation (requires ≥30Hz / 600 rpm). The core contradiction is that soft starters offer fast start-up but high energy consumption, while frequency converters are energy-efficient but slow to start, and the formation of the bearing oil film requires a specific speed. Therefore, research on a motor starting method and system combining a soft starter and a frequency converter is of great significance. Summary of the Invention

[0006] The purpose of this invention is to provide a motor starting method and system that combines soft starter and frequency converter to address the problems existing in the prior art. By combining the two starting methods of soft starter and frequency converter, and setting reasonable switching points, the frequency converter can track the minimum requirements of the bearing's self-contained oil film, thereby maximizing the benefits for production.

[0007] The technical solution of this invention is:

[0008] A starting method combining a soft starter and a frequency converter is disclosed. When the main pumping motor meets the start-up conditions (damper closed to zero position, motor and fan oil pressure meeting requirements, frequency converter water pressure between 0.3-0.4 MPa, and no fault alarm signals and normal display in the frequency converter cabinet and excitation cabinet), the corresponding isolating switches and circuit breakers are closed sequentially. The PLC controller causes the relevant vacuum circuit breaker of the soft starter to close. After the soft starter performs a 6-second self-test without faults, it starts outputting a starting voltage of 5500V. When the detected current drops to approximately 1000A, the soft starter bypass circuit breaker closes and sends a signal... The signal is sent to the PLC controller. Four seconds later, the PLC controller issues a stop command, causing the soft-start vacuum circuit breaker to trip and the relevant circuit breakers of the frequency converter to close. The frequency converter completes the high-voltage power-on self-test within about 12-18 seconds and sends a ready signal to the PLC controller. The PLC controller issues a frequency converter start command, and the frequency converter sends a 4-20mA excitation cabinet control signal to the microcomputer-controlled synchronous motor excitation device. The excitation device outputs DC control voltage to the excitation circuit of the synchronous motor, completing the start-up. The entire power frequency start-up time is 26-30 seconds.

[0009] A motor starting system combining a soft starter and a frequency converter, comprising:

[0010] High-voltage soft starter, frequency converter, bypass circuit breaker K1, PLC controller;

[0011] The soft start circuit includes two high-voltage vacuum circuit breakers, QF6 and QF7, which are connected in series with the high-voltage soft starter.

[0012] The frequency converter circuit includes high-voltage vacuum circuit breakers QF1 and QF2 connected in series, and the frequency converter circuit is connected in parallel with the soft start circuit.

[0013] The bypass circuit breaker K1 is connected in parallel with the high-voltage soft starter.

[0014] Specifically, the PLC controller performs the following operations:

[0015] (a) Close the bypass circuit breaker K1 when the current drops to 1000A after the high-voltage soft starter starts;

[0016] (b) After 4 seconds, disconnect QF6 and QF7 and close high-voltage vacuum circuit breakers QF1 and QF2;

[0017] (c) Control the frequency converter to skip the 0-35Hz low-frequency band and directly output the 35Hz frequency.

[0018] The beneficial effects of this invention are as follows: It solves the initial bearing oil film problem through soft-start rapid startup (within 40 seconds), and switches to the frequency converter 4 seconds after startup (when the current drops to 1000A). Frequency tracking technology allows the frequency converter to directly output 35Hz (skipping the low-frequency band), balancing speed and energy saving. The PLC controller precisely coordinates the action sequence of the high-voltage circuit breaker. The frequency converter completes self-test and starts within 12-18 seconds, reducing the total startup time to 26-30 seconds. Combining the advantages of short soft-start startup time and the power saving and precise control of the frequency converter, the PLC controller controls the switching at the appropriate time to avoid bearing burnout (meeting the oil film formation speed); it ensures that the frequency converter tracks the minimum requirements of the bearing oil film, maximizing production efficiency and saving 20,000 yuan in electricity costs per day. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the circuit structure of the present invention. Detailed Implementation

[0020] The technical solution of the present invention will now be described in detail with reference to the accompanying drawings and specific embodiments.

[0021] Example 1

[0022] like Figure 1 The diagram shown is a circuit structure schematic of a starting method combining a soft starter and a frequency converter provided in this embodiment.

[0023] A motor starting method combining a soft starter and a frequency converter includes the following steps:

[0024] S1. Start the motor using a high-voltage soft starter and monitor the motor current;

[0025] S2. When the current drops to a preset threshold, the soft-start bypass circuit breaker is triggered to close, and a timer is started;

[0026] S3. After the timer reaches 4 seconds, disconnect the high-voltage vacuum circuit breaker of the soft start circuit and simultaneously close the high-voltage vacuum circuit breaker of the inverter circuit.

[0027] S4. The frequency converter completes self-testing within a set time of 12-18 seconds and directly outputs a fixed frequency to drive the motor;

[0028] S5. The frequency converter outputs a 4-20mA signal to the excitation device to synchronously adjust the motor excitation current and complete the start-up.

[0029] This embodiment uses a No. 1 main pumping motor as an example to illustrate soft start:

[0030] When the No. 1 main pumping motor meets the start-up conditions (damper closed to zero position, motor and fan oil pressure meeting requirements, variable frequency water pressure between 0.3-0.4 MPa, and no fault alarm signals and normal display in the variable frequency cabinet and excitation cabinet), sequentially close 1-QS1, 1-QS2, and 1-QF1. Pressing the start button will cause the PLC controller to output the command to close 1#QF6 and 1#QF7. If the soft starter performs a 6-second self-test without fault, it will start outputting an initial voltage of 5500V. When the detected current drops to approximately 1000A, the soft starter bypass circuit breaker K1 closes, and a signal is sent to the PLC. After 4 seconds, the PLC controller will issue a command. When the shutdown command is given, the soft-start vacuum circuit breakers QF6 and QF7 disconnect, and 1#-QF1 and 1#-QF2 close. After about 12-18 seconds (this time is determined by testing during commissioning), the inverter completes its high-voltage power-on self-test and sends a ready signal to the PLC controller. The PLC controller issues an inverter start command, and an inverter start indicator signal appears simultaneously. At the same time, the inverter sends a 4-20mA excitation cabinet control signal to the microcomputer-controlled synchronous motor excitation device. The excitation device outputs a DC control voltage to the excitation circuit of the synchronous motor according to the received signal magnitude. The entire start-up process is completed, and the power frequency start-up time is 26-30 seconds.

[0031] During this process, taking advantage of the soft starter's short 40-second startup time, the system switches to the frequency converter after 4 seconds of normal soft start-up. The frequency converter completes the charging and startup of the power unit within 15 seconds, and the motor's operating frequency drops from 50Hz to 35Hz. The frequency converter quickly skips the low frequency to 35Hz through frequency tracking power, achieving precise operation adjustment and energy saving.

[0032] To verify the critical necessity of the current threshold (1000A) and the delay (4 seconds), the following experimental scheme was designed for a 4900KW synchronous motor (main exhaust fan of the sintering workshop of an ironmaking plant). By comparing the starting effects of different parameter combinations, it was proved that the current threshold = 1000A + switching delay = 4 seconds is the optimal solution.

[0033] 1. Experimental objective: To verify that the current threshold of 1000A is the optimal critical point for switching from soft starter to frequency converter (balancing starting speed and mechanical shock).

[0034] The 4-second delay proves to be a safe window for disconnecting the soft start circuit after the bypass is closed (to avoid current backflow damaging the equipment).

[0035] 2. Experimental group design

[0036]

[0037]

[0038] 3. Key Experimental Results

[0039] (1) Criticality verification of current threshold (fixed delay of 4 seconds)

[0040]

[0041] in conclusion:

[0042] 800A switching: The motor has not reached a stable speed (about 25Hz), and the bearing oil film has not been fully formed → the temperature exceeds 90℃ (close to the critical value of 95℃ for bearing failure).

[0043] 1000A switching: Motor speed reaches 30Hz (minimum requirement for oil film formation), with minimal inrush current (24% lower than 1200A).

[0044] 1200A switching: Excessive current causes increased grid harmonics during switching, and the thyristor temperature rises by 15%.

[0045] (2) Verification of the necessity of a 4-second delay (fixed threshold 1000A)

[0046]

[0047] in conclusion:

[0048] 2-second delay: Bypass circuit breaker not fully conducting (insufficient closing time) → arc lengthening during switching, high frequency converter self-test failure rate (due to voltage fluctuations).

[0049] 4-second delay: The soft-start current is completely transferred to the bypass → the circuit breaker interrupts the arc with the shortest time and the mechanical impact torque is the smallest (57% lower than the 2-second group).

[0050] 6-second delay: No performance improvement, and the total startup time is increased (contradicting the speed objective).

[0051] 4. Verification of the effect of combined parameters (Experimental group: 1000A + 4 seconds)

[0052] Oil film formation time: After switching, the inverter outputs 35Hz (700 rpm) → oil film thickness ≥ 0.05mm (confirmed by laser thickness gauge).

[0053] Total startup time: 26.3 ± 1.2 seconds (6 seconds for soft start + 4 seconds for delay + 16 seconds for inverter self-test).

[0054] Energy saving benefits:

[0055] Power saving = (Power consumption for 40 seconds of soft start - Power consumption for 30 seconds of combined solution) + Power saving through variable frequency speed control

[0056] = (4900kW × 40s × 0.8 - 4900kW × 30s × 0.8) + (4900kW × 24h × 0.3 × 0.25)

[0057] ≈ Daily electricity cost savings of 20,000 yuan (industrial electricity price 0.8 yuan / kWh)

[0058] 5. Experimental Conclusions

[0059] 1) 1000A current threshold: This is the current rating point at which the motor completes its initial acceleration (reaching the 30Hz oil film formation speed). If the current is below this value, the switching will cause high temperature of the bearing; if the current is above this value, it will increase the impact on the power grid.

[0060] 2) 4-second delay: This is the shortest safe time required for the bypass circuit breaker to be fully turned on, ensuring that there is no residual current in the soft start circuit and avoiding interference between the tripping arc and the inverter's self-test.

[0061] 3) Indivisibility of parameters: Optimizing the current threshold or delay alone cannot simultaneously satisfy "fast response + low impact + bearing protection"; the two must work together.

[0062] Example 2

[0063] This embodiment provides a motor starting system combining a soft starter and a frequency converter, including:

[0064] High-voltage soft starter, frequency converter, bypass circuit breaker K1, PLC controller;

[0065] The soft start circuit includes two high-voltage vacuum circuit breakers, QF6 and QF7, which are connected in series with the high-voltage soft starter.

[0066] The frequency converter circuit includes high-voltage vacuum circuit breakers QF1 and QF2 connected in series, and the frequency converter circuit is connected in parallel with the soft start circuit.

[0067] The bypass circuit breaker K1 is connected in parallel with the high-voltage soft starter.

[0068] The PLC controller performs the following operations:

[0069] (a) Close the bypass circuit breaker K1 when the current drops to 1000A after the high-voltage soft starter starts;

[0070] (b) After 4 seconds, disconnect QF6 and QF7 and close high-voltage vacuum circuit breakers QF1 and QF2;

[0071] (c) Control the frequency converter to skip the 0-35Hz low frequency band and directly output the 35Hz frequency.

[0072] The following is a verification of the correlation between the inverter hopping to 35Hz and the formation of the bearing oil film.

[0073] Test subject: 4900KW synchronous motor (main exhaust fan of sintering workshop) in the ironmaking plant

[0074] Core objective: To prove that "skipping 0–35Hz and directly outputting 35Hz" is a necessary condition to avoid bearing burnout, and that 35Hz is the minimum safe frequency for oil film to form a stable oil film.

[0075] 1. The correlation between oil film formation mechanism and rotational speed

[0076]

[0077] Theoretical basis:

[0078] The motor's bearings are hydrodynamic sliding bearings; the oil film is formed by the hydrodynamic force generated by the rotor's rotation. Speed ​​formula:

[0079]

[0080] Where n is the motor speed (unit: r / min, revolutions per minute), f is the frequency (unit: Hz, Hertz), and p is the number of pole pairs of the motor. Here, p = 6 rpm is given as the safe redundancy speed for complete oil film coverage. If the power supply frequency f is known, the speed can be calculated by substituting it into the formula.

[0081] 2. Experimental Design and Data Comparison

[0082] (1) Test group design

[0083]

[0084] (2) Key Test Results

[0085]

[0086]

[0087] Data Analysis:

[0088] Control group 1 (0→35Hz frequency increase): Low speed stage (0–30Hz) lasts for 8 seconds → oil film does not form, bearing temperature exceeds 105℃ (triggering bearing failure and shutdown).

[0089] Control group 2 (0→50Hz frequency increase): At 30Hz, the oil film thickness was only 0.02mm (less than 67% of the safety threshold), resulting in local pitting wear.

[0090] Experimental group (frequency hopping to 35Hz): Direct output of 35Hz (700 rpm) → instantaneous formation of oil film (thickness 0.06mm), temperature stabilized at 78℃.

[0091] 3. The technical necessity of frequency hopping to 35Hz

[0092] (1) Avoid low-speed "danger zones"

[0093] The relationship between oil film thickness and rotational speed is as follows:

[0094] 1. Low-speed range (danger zone):

[0095] When the rotational speed is extremely low (e.g., 0–25Hz (0–500 rpm): oil film thickness ≈ 0 (boundary friction state)), the curve shows that the oil film thickness is extremely thin (even approaching 0). At this time, the hydrodynamic force generated by the rotor rotation is insufficient to form a stable oil film, and the bearing and journal may come into direct contact, leading to increased friction and wear, which is in the "danger zone".

[0096] 2. Transition section:

[0097] As the rotational speed increases and exceeds a critical value (e.g., 30Hz (600 rpm): oil film begins to form (thickness ≥ 0.03 mm)), the oil film thickness increases rapidly with increasing rotational speed. This is because the rotor rotation increases the power driving the lubricating oil, and the oil film gradually builds up and thickens, moving away from the state of dry friction or boundary friction.

[0098] 3. Stable phase:

[0099] When the rotational speed reaches a certain value (35Hz (700 rpm): the oil film completely covers the bearing (thickness ≥ 0.05 mm)), the increase in oil film thickness tends to level off and enters a stable range. At this point, the oil film thickness is sufficient to completely separate the bearing bush from the journal, forming pure fluid friction, and the bearing is in a safe operating condition.

[0100] The critical turning point (low-speed threshold) is the upper limit of the "danger zone". In actual operation, it is necessary to ensure that the motor speed is always higher than this threshold to avoid oil film failure.

[0101] (2) Economic Benefit Comparison

[0102]

[0103] Note: The cost of a single repair for tile failure is approximately 200,000 yuan (including downtime losses), while the probability of tile failure in the control group is 29.2% per year.

[0104] 4. Conclusion

[0105] 1. The irreplaceable nature of frequency hopping design:

[0106] Skipping 0–35Hz and directly outputting 35Hz is the only effective way to avoid the risk of low-speed bearing failure (traditional frequency upsampling cannot meet the requirements for instantaneous oil film formation).

[0107] The critical value of 2.35Hz is based on:

[0108] 30Hz: Theoretical lower limit for oil film formation → In practice, 35Hz is required to provide safety redundancy (to cope with sudden load changes and oil temperature fluctuations).

[0109] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them; although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications can still be made to the specific implementation of the present invention or equivalent substitutions can be made to some technical features without departing from the spirit of the technical solutions of the present invention, and all such modifications and substitutions should be covered within the scope of the technical solutions claimed in the present invention.

Claims

1. A motor starting method combining a soft starter and a frequency converter, characterized in that, Includes the following steps: S1. Start the motor using a high-voltage soft starter and monitor the motor current; S2. When the current drops to a preset threshold, the soft-start bypass circuit breaker is triggered to close, and a timer is started; S3. After the timer reaches 4 seconds, disconnect the high-voltage vacuum circuit breaker of the soft start circuit and simultaneously close the high-voltage vacuum circuit breaker of the inverter circuit. S4. The frequency converter completes self-testing within a set time of 12-18 seconds and directly outputs a fixed frequency to drive the motor; S5. The frequency converter outputs a 4-20mA signal to the excitation device to synchronously adjust the motor excitation current and complete the start-up.

2. The motor starting method combining a soft starter and a frequency converter according to claim 1, characterized in that, In step S3, the opening and closing actions of the high-voltage vacuum circuit breaker are controlled by a PLC controller. After receiving the soft-start bypass signal, the PLC controller triggers the disconnection command.

3. The motor starting method combining a soft starter and a frequency converter according to claim 1, characterized in that, The interval between the soft start circuit disconnection and the inverter circuit closing action is ≤1 second.

4. The motor starting method combining a soft starter and a frequency converter according to claim 1, characterized in that, After the inverter completes its self-test, it directly outputs a 35Hz frequency through the frequency tracking function, skipping the low-frequency start-up phase of 0-35Hz.

5. The motor starting method combining a soft starter and a frequency converter according to claim 1, characterized in that, The preset current threshold is 900-1100A.

6. A motor starting system combining a soft starter and a frequency converter, characterized in that, include: High-voltage soft starter, frequency converter, bypass circuit breaker K1, PLC controller; The soft start circuit includes two high-voltage vacuum circuit breakers, QF6 and QF7, which are connected in series with the high-voltage soft starter. The frequency converter circuit includes high-voltage vacuum circuit breakers QF1 and QF2 connected in series, and the frequency converter circuit is connected in parallel with the soft start circuit. The bypass circuit breaker K1 is connected in parallel with the high-voltage soft starter.

7. The system according to claim 6, characterized in that, The PLC controller performs the following operations: (a) Close the bypass circuit breaker K1 when the current drops to 1000A after the high-voltage soft starter starts; (b) After 4 seconds, disconnect QF6 and QF7 and close high-voltage vacuum circuit breakers QF1 and QF2; (c) Control the frequency converter to skip the 0-35Hz low frequency band and directly output the 35Hz frequency.

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