Three-phase motor open-phase dynamic response judgment method

Abstract

The invention relates to a three-phase motor open-phase dynamic response judgment method, which comprises the following steps of: setting a current open-phase threshold and detecting three-phase current, if the current of a judged phase is greater than the current open-phase threshold, determining that the current open-phase probability of the judged phase is 0 and the total accumulated open-phase probability of the phase is also 0, otherwise, executing the next step; whether the modulation depth of the judged phase crosses a zero point or not is judged, if the modulation depth crosses the zero point, the current open-phase probability of the judged phase is zero, and the next step is carried out, otherwise, the current open-phase probability of the judged phase is calculated according to the remaining two-phase current and the current operation frequency, the current open-phase probability and the historical open-phase probability are accumulated, and the next step is carried out; whether the phase loss probability is larger than 100% is judged, if the phase loss probability is smaller than 100%, the judged phase is not in phase loss, and if not, the judged phase is in phase loss. The open-phase detection speed can be dynamically adjusted according to the current output current and rotating speed state of the frequency converter.

Description

Technical Field

[0001] This invention belongs to the field of motor control technology, and in particular, it is a novel method for judging the dynamic response of a three-phase motor in the event of a phase loss. Background Technology

[0002] A frequency converter controls motor rotation by outputting three-phase voltage and detecting the motor current. If one phase of the frequency converter's output voltage suddenly disconnects before the motor starts or during motor operation, the frequency converter must immediately stop outputting voltage. Otherwise, it could lead to accidental injury to personnel or equipment.

[0003] Traditional motor phase loss detection involves the inverter detecting current through its current sensor. If the current in this circuit remains below a certain limit for a specific period, it is assumed that the connection between that phase of the motor and the inverter is broken, triggering a phase loss alarm. However, this method requires the inverter to perform phase loss detection for a fixed period. During low-speed motor operation, the inverter may trigger an overcurrent alarm instead of a phase loss alarm. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings of the prior art and propose a novel dynamic response judgment method for phase loss in three-phase motors, which can dynamically adjust the detection speed of phase loss according to the current output current and frequency of the inverter.

[0005] The technical problem solved by this invention is achieved through the following technical solution: A novel method for judging the dynamic response of a three-phase motor in the event of a phase loss includes the following steps: Step 1: Set the current phase loss threshold and detect the three-phase current. If the detected current value of the phase to be judged is greater than the current phase loss threshold, the probability of phase loss of the phase to be judged is 0, and the total cumulative phase loss probability of this phase is also 0; otherwise, proceed to Step 2. Step 2: Check if the modulation depth of the phase being judged has crossed zero. If it has crossed zero, the probability of phase loss in this judgment of the phase being judged is 0, and proceed to Step 3. Otherwise, calculate the probability of phase loss in this judgment based on the absolute values ​​of the currents of the two phases other than the phase being judged and the current output frequency of the inverter. Add the probability of phase loss in this judgment to the historical probability of phase loss, and proceed to Step 3. Step 3: Determine if the phase loss probability is greater than 100%. If the phase loss probability is less than 100%, the phase being judged is not missing; otherwise, the phase being judged is missing.

[0006] Furthermore, the method for setting the current phase loss probability is as follows: the absolute values ​​of the remaining two phase currents are averaged, and the percentage of this average value to the motor's rated current is used to calculate the phase loss probability. The higher this percentage, the greater the probability of phase loss at the current moment. This allows for dynamic adjustment of the output phase loss probability based on current changes.

[0007] Furthermore, in step 1, during the current detection process, a low ratio between the switching frequency and the inverter output frequency reduces the probability of phase loss; conversely, a high ratio results in smaller motor current spikes, maintaining the current probability of phase loss. Furthermore, the specific implementation method for determining whether the modulation depth of the phase being judged crosses zero in step 2 is as follows: A certain proportional coefficient is taken for the amplitude of the modulation depth output by the frequency converter. When the modulation depth of the detected phase is between (negative proportional coefficient, positive proportional coefficient), it indicates that the modulation depth is near the zero-crossing point, and the motor current of this phase is also near zero. In this case, no phase loss detection is performed to avoid incorrect judgments during normal motor operation. During testing, this method uses (-5%, 5%) as the zero-crossing point, and after multiple tests, no false alarms were found.

[0008] The advantages and positive effects of this invention are: This invention sets a current phase loss threshold and detects the three-phase current. If the detected current value of the phase to be judged is greater than the phase loss threshold, then the current phase loss probability of this phase is 0, and the cumulative total phase loss probability of this phase is also 0; otherwise, proceed to the next step. It checks if the modulation depth of the phase to be judged crosses zero. If it does, the current phase loss probability of the phase to be judged is 0, and proceed to the next step; otherwise, it calculates the current phase loss probability based on the magnitude of the remaining two phase currents, the current switching frequency, and the inverter output frequency. This current phase loss probability is then added to the historical phase loss probabilities, and the next step is performed. Finally, it checks if the phase loss probability is greater than 100%. If the phase loss probability is less than 100%, the phase to be judged is not experiencing a phase loss; otherwise, the phase to be judged is experiencing a phase loss. This invention can dynamically adjust the phase loss detection speed based on the current inverter output current and frequency. Attached Figure Description

[0009] Figure 1 This is the logic diagram for determining the C-phase loss in this invention; Figure 2 This is a schematic diagram of the normal three-phase current of the present invention; Figure 3 This is a schematic diagram of the three-phase current loss of the present invention; Figure 4 This is a schematic diagram showing the small output current of the frequency converter of the present invention; Figure 5 This is a schematic diagram showing the large output current of the frequency converter of the present invention. Figure 6 The curves showing the sampling current ratio and phase loss probability of this invention are shown. Figure 7 This is a schematic diagram of the burr-free current of the present invention; Figure 8 This is a schematic diagram of the burr current of the present invention; Figure 9 This is a curve showing the gain coefficient and ratio of the present invention; Figure 10 This is a schematic diagram of the current near the zero-crossing point of the present invention; Figure 11 The present invention provides three-phase current waveforms and three-phase modulation waveforms. Figure 12 This is a schematic diagram of the general algorithm (a) of an embodiment of the present invention; Figure 13 This is a schematic diagram of the general algorithm (b) of an embodiment of the present invention; Figure 14 This is a schematic diagram of the general algorithm (c) of an embodiment of the present invention; Figure 15 This is a schematic diagram of the general algorithm (d) of an embodiment of the present invention; Figure 16 This is a schematic diagram of the dynamic response algorithm (a) according to an embodiment of the present invention; Figure 17 This is a schematic diagram of the dynamic response algorithm (b) according to an embodiment of the present invention; Figure 18 This is a schematic diagram of the dynamic response algorithm (c) according to an embodiment of the present invention; Figure 19 This is a schematic diagram of the dynamic response algorithm (d) in an embodiment of the present invention. Detailed Implementation

[0010] The present invention will be further described in detail below with reference to the accompanying drawings.

[0011] A novel method for judging the dynamic response of a three-phase motor in the event of a phase loss, such as... Figure 1 As shown, it includes the following steps: Step 1: Set the current phase loss threshold and detect the three-phase current. If the current of the currently detected phase is greater than the current phase loss threshold, the probability of the detected phase being phase-loss is 0, and the cumulative total probability of phase loss for this phase is also 0. (For phase loss detection, a phase loss must be continuously detected until the alarm limit of 100% is triggered). Otherwise, proceed to Step 2.

[0012] If the permanent magnet synchronous motor has a small load, the motor current is small. Due to limitations in current sampling accuracy, the frequency converter will not accurately sample small currents. This results in large fluctuations in the small current when the frequency converter controls the motor's rotation. Figure 4 The image shows the current in one phase of the motor during normal operation. If, at this time, one of the three phase currents (A / B / C) is close to 0, and the sum of the currents in the other two phases is also close to 0, the reason could be due to a small current, or it could be due to a missing phase. Using this small current to determine the missing phase should result in a low probability of each missing phase.

[0013] If the permanent magnet synchronous motor has a large load, the motor current will be large. For example... Figure 5The image shows the current in one phase of the motor during normal operation. Current sampling provides greater accuracy, allowing the frequency converter to better control the motor's rotation. If phase C is missing, the currents in phases A and B are approximately equal in magnitude but opposite in direction, and their sum is close to zero. However, both phases A and B have significant output currents controlled by the frequency converter. Therefore, the larger the currents in phases A and B, the higher the probability of phase C being missing.

[0014] The percentage of phase A current and phase B current relative to the motor's rated current can be used as a parameter to determine the probability of phase loss. For example... Figure 6 The figure shows the curves of the sampling current percentage and the phase loss probability. The absolute values ​​of phase A and phase B currents are taken, and then an average current is calculated based on these two absolute values. The percentage of this average current to the motor's rated current is then used to determine the relationship between the sampling current percentage and the phase loss probability. Figure 6 The probability of phase loss is calculated. The higher the ratio of this average value to the motor's rated value, the greater the probability of phase loss at the current moment. This allows for dynamic adjustment of the output phase loss probability based on current changes. Assuming... Figure 6 The upper limit for the vertical axis phase loss probability is set at 10%, and the lower limit is set at 1%. This means that when the ratio of the average value of phase A and phase B on the horizontal axis to the motor's rated current is 100%, the probability of a phase loss in this motor is 10%, and at least 10 such occurrences are required to trigger a phase loss alarm.

[0015] Meanwhile, during the operation of a motor controlled by a frequency converter, the switching frequency of the frequency converter is usually fixed. With a fixed switching frequency, the lower the output frequency of the frequency converter, the better the sinusoidal effect of the motor current, and the smaller the current spikes generated during control (see the current waveform at 50Hz below). Conversely, the higher the frequency, the worse the sinusoidal effect of the motor current, and the larger the current spikes generated during control (see the current waveform at 200Hz below).

[0016] like Figure 7 The image shows a current waveform at a switching frequency of 4kHz and an electrical frequency of 50Hz; Figure 8 The image shows the current waveform at a switching frequency of 4KHz and an electrical frequency of 200Hz.

[0017] If a spike current is sampled during current sampling, the current in that phase will be too high. Using this higher current to determine if a phase is missing will increase the probability of a phase loss. Therefore, when the ratio of switching frequency to inverter output frequency is low, the probability of the current result needs to be reduced. Figure 9 The curves showing the gain coefficient and ratio are provided. The probability of detecting a phase loss each time is: current detection phase loss probability * gain coefficient.

[0018] Step 2: Check if the modulation depth of the phase being judged crosses zero. If it does, the probability of phase loss of the phase being judged in the current state is 0, and proceed to Step 3. Otherwise, calculate the probability of phase loss in the current state based on the sum of the currents of the remaining two phases and the current operating frequency, and proceed to Step 3.

[0019] Formula for three-phase current of motor:

[0020] It can be known that: When the current in phase A crosses zero, the sum of the currents in phases B and C is 0. When the current in phase B crosses zero, the sum of the currents in phases A and B is 0. When the current in phase C crosses zero, the sum of the currents in phases A and B is 0. Therefore, when the current is near the zero-crossing point, its state is similar to that of a motor in phase loss. Therefore, phase loss detection is not performed at this time to avoid generating an error probability that would ultimately be added to the overall phase loss probability. Figure 10 As shown.

[0021] The motor algorithm generates a modulation depth (Indexdepth), a parameter that controls the motor voltage output waveform, thereby controlling the motor current. The modulation depth value can be used to determine if the current waveform is near a zero-crossing point. When the modulation depth is near a zero-crossing point, phase loss detection is not performed, avoiding incorrect judgments during normal motor operation. Figure 11 As shown, the upper part is the three-phase current waveform, and the lower part is the three-phase modulation waveform.

[0022] Step 3: Determine if the probability of phase loss is greater than 100%. If the probability of phase loss is less than 100%, the phase being judged is not missing; otherwise, the phase being judged is missing.

[0023] Example Based on the above-mentioned novel three-phase motor phase loss dynamic response judgment method, the effect of the present invention is illustrated by comparing the general algorithm with the algorithm of the present invention.

[0024] General algorithm: If the current in phase C remains below a certain threshold for a certain period of time, then phase C is considered missing. The general algorithm uses a fixed detection time to ensure the reliability of the test.

[0025] During testing, when the motor is running at low speed, its inertia is small. In the event of a phase loss, because one of the three-phase currents, which are 120° apart, is lost, the force on the motor rotor is unbalanced, resulting in large current disturbances. In this process, if the current disturbance is large enough, it may trigger an overcurrent alarm.

[0026] The algorithm of this invention: During low-speed, light-load operation, when a phase loss occurs, the motor current disturbance is large. This large disturbance current, namely the current of the two remaining phases other than the phase being judged (mentioned in this paper), accelerates the judgment of the phase loss through the change of these two phase currents. A phase loss alarm is triggered within 100ms. This is significantly better than general algorithms. It also reduces the probability of triggering an overcurrent alarm.

[0027] Low-speed heavy-load operation: When a phase of the motor is lost, the motor load is provided by the current of the two remaining phases. This load current accelerates the phase loss detection, triggering the phase loss alarm in just 6ms.

[0028] During high-speed, light-load operation, when a phase loss occurs, although the motor rotor experiences uneven force, its high-speed motion results in significant inertia and small current fluctuations. At this time, the current in the remaining two phases is much smaller than the phase loss current under low-speed, light-load conditions. Therefore, the 196ms alarm trigger time is longer than that under low-speed, light-load conditions. However, this alarm time is still significantly better than typical algorithms.

[0029] High-speed heavy-load operation: When a phase of the motor is lost, the motor load is supplied by the current of the remaining two phases. Similar to low-speed load, the detection of a phase loss due to accelerated load current takes 9.2ms to trigger an alarm.

[0030] like Figure 12 As shown, using a general algorithm, (a) electrical frequency: 20Hz, no load. Phase loss alarm is triggered in 1s.

[0031] like Figure 13 As shown, using a general algorithm, (b) electrical frequency: 20Hz, load: motor current 10A: phase loss alarm triggered in 1s.

[0032] like Figure 14 As shown, using a general algorithm, (c) electrical frequency: 100Hz, no load. Phase loss alarm is triggered in 1 second.

[0033] like Figure 15 As shown, using a general algorithm, (d) electrical frequency: 100Hz. Load: motor current 10A. Phase loss alarm is triggered in 1s.

[0034] like Figure 16 As shown, the dynamic response algorithm of the present invention is adopted. (a) Electrical frequency: 20Hz, no load, trigger phase loss 100.0ms.

[0035] like Figure 17 As shown, the dynamic response algorithm of this invention is adopted. (b) Electrical frequency: 20Hz, load: motor current 10A: phase loss alarm triggered: 6.0ms.

[0036] like Figure 18 As shown, the dynamic response algorithm of this invention is adopted. (c) Electrical frequency: 100Hz, no load, alarm triggered 196.0ms.

[0037] like Figure 19 As shown, the dynamic response algorithm of this invention is adopted, (d) electrical frequency: 100Hz, load: motor current 10A, alarm trigger 9.2ms.

[0038] It should be emphasized that the embodiments described in this invention are illustrative rather than limiting. Therefore, this invention includes, but is not limited to, the embodiments described in the specific implementation. Any other implementations derived by those skilled in the art based on the technical solutions of this invention are also within the scope of protection of this invention.

Claims

1. A method for judging the dynamic response of a three-phase motor in the event of a phase loss, characterized in that: Includes the following steps: Step 1: Set the current phase loss threshold and detect the three-phase current. If the detected current value of the phase to be judged is greater than the current phase loss threshold, the probability of phase loss for the phase to be judged this time is 0, and the total cumulative phase loss probability of this phase is also 0. Otherwise, proceed to Step 2. Step 2: Check if the modulation depth of the phase being judged crosses zero. If it does, the probability of phase loss in this judgment of the phase being judged is 0, and proceed to Step 3. Otherwise, calculate the current probability of phase loss based on the absolute values ​​of the currents of the two phases other than the phase being judged and the current operating frequency. Add the current probability of phase loss to the historical probability of phase loss and proceed to Step 3. Step 3: Determine if the probability of phase loss is less than 100%. If so, the phase being judged is not missing a phase; otherwise, the phase being judged is missing a phase.

2. The novel method for judging the dynamic response of a three-phase motor in case of phase loss according to claim 1, characterized in that: The calculation method for determining the probability of a phase loss is as follows: Take the absolute values ​​of the currents of the two phases other than the phase being judged, and then calculate an average current based on these two absolute values. The percentage of this average current to the rated current of the motor is used to calculate the probability of a phase loss. The higher this percentage, the greater the probability of a phase loss at the current moment. This allows for dynamic adjustment of the output phase loss probability based on current changes.

3. The novel method for judging the dynamic response of a three-phase motor in case of phase loss according to claim 1, characterized in that: In step 1, during the current detection process, if the ratio of the switching frequency or the inverter output frequency is low, the probability of phase loss is reduced; if the ratio of the switching frequency or the inverter output frequency is high, the motor current spikes are small, and the current probability of phase loss is maintained.

4. The novel method for judging the dynamic response of a three-phase motor in phase loss according to claim 1, characterized in that: The specific implementation method for determining whether the modulation depth of the phase being judged has crossed the zero point in step 2 is as follows: take a certain proportional coefficient for the amplitude of the modulation depth output by the frequency converter. When the modulation depth of the phase being detected is between the negative proportional coefficient and the positive proportional coefficient, it indicates that the modulation depth is near the zero point. At this time, the motor current of this phase is also near zero. In this case, the motor phase loss is not detected to avoid incorrect judgment during normal operation of the motor.

5. A novel method for judging the dynamic response of a three-phase motor in case of phase loss according to claim 1, characterized in that: The phase loss detection principle in step 3 is that when a phase of the motor is missing, the current value information of the other two phases can be extracted, which can effectively shorten the phase loss detection time. At the same time, by combining the ratio of the inverter switching frequency to the current electrical frequency and the modulation depth in the motor control information, the misjudgment of phase loss detection can be effectively avoided.

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