JPS6240927B2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to an overload detection device for an AC motor,
In particular, the present invention relates to an overload detection device suitable as a detection device for a hysteresis motor having constant current characteristics.

When an AC motor falls into an overload state, it becomes so-called step-out, which increases secondary loss and may cause burnout of the motor windings. In order to prevent such accidents, overload detection devices are generally provided.
An overload detection device for a normal synchronous motor or induction motor detects the magnitude of the motor input current value, and when the magnitude exceeds a reference level, it sends out an overload signal to prevent input interruption, etc. It is designed to perform a protective operation. However, such a normal overload detection device that detects input current cannot detect an overload of a motor having constant current characteristics such as a hysteresis motor.

Hysteresis motors (hereinafter referred to as H motors) are a type of synchronous motor, but compared to general synchronous motors, they have a simpler structure, are easier to start, and can be easily operated in parallel. There is. Therefore, it is very useful as an electric motor that drives a large number of loads with the same characteristic, such as in a spinning machine. However, since this H motor has a constant current characteristic as described above, there is a problem in that a conventional overload detection device of the input current detection type cannot be used.

As an overload detection means for an H motor, there is an input power factor detection type overload detection device. This takes advantage of the fact that when the H motor is overloaded, the input power factor increases. However, the input power factor detection type device is extremely expensive. Therefore, applying the above-mentioned overload detection device to a system using a very large number of H motors poses a problem in terms of equipment cost, and is not preferable. Note that the conventionally widely known rotation speed detection type overload detection device is
It is necessary to attach a tachometer to each motor, which has the same drawback as the above-mentioned problem of increased equipment costs, and depending on the structure of the H motor, it may be difficult to attach the tachometer.

The present invention has been made in consideration of such circumstances, and its purpose is to be able to accurately detect overload of an AC motor having constant current characteristics.
Moreover, it is an object of the present invention to provide an overload detection device for an AC motor that is inexpensive and has a simple configuration and is very suitable as an overload detection means for a large number of motors operated in parallel.

Hereinafter, before clarifying the present invention with reference to the embodiment shown in FIG. The load detection device will be explained with reference to FIG. In the figure, 1 is an AC power supply, and 2A and 2B are H motors that are supplied with the AC power and operated in parallel. 3A and 3B are current transformers for detecting the current flowing into the two H motors, respectively, and the line currents I ₁ and I ₂ of the same phase are detected.
It is designed to detect with reverse polarity. That is, the positive output end of current transformer 3A and the negative output end of current transformer 3B are connected, and the negative output end of current transformer 3A and the positive output end of current transformer 3B are connected. 4 is the current transformer 3
This is an impedance element connected to the connection points P and Q between the output ends of A and 3B. Reference numeral 5 denotes an overload detector which detects a change in the voltage V _Z occurring across the impedance element 4 and sends out an overload signal when the magnitude of the change exceeds a reference level.

FIG. 2 is a diagram showing the circuit configuration of the overload detector 5. As shown in FIG. In FIG. 2, 7 is a full-wave rectifier, which converts the voltage _V〓Z across the impedance element applied to the terminals 6a and 6b into direct current. A filter 8 is connected to the output end of the full-wave rectifier, and removes ripples contained in the direct current. Reference numeral 9 denotes a comparator composed of an operational amplifier, which compares the level of the output signal of the filter 8 with the level of the reference voltage signal, and sets the output level to a low level "0" when the output signal level exceeds the reference voltage signal level. ” to a high level “1” and sends this out from the terminal 10 as an overload signal. Note that the reference voltage signal level can be variably set as appropriate by a reference voltage setter 11 comprising a potentiometer connected to the reference input terminal of the comparator 9.

Next, the operation of the apparatus configured as described above will be explained. In order to simplify the explanation, H motor 2
It is assumed that A and 2B have the same characteristics and drive loads with the same characteristics. Further, the current transformation ratio of the current transformers 3A and 3B is 1:n (n<1). Now, if a current of I〓 ₁ flows through the H motor 2A and a current of I〓 ₂ flows through the H motor 2B, then V〓 ₂ = nR (I〓 ₁ - I〓 ₂ ) ...(1) A voltage is generated. However, R is a resistance value when the impedance element 4 is considered as a resistor. In the steady state, the above I〓 ₁ and I〓 ₂ are expressed as I〓 ₁ =I〓 ₂ =I ^-j 〓 (2) using AC representation. (2) In formula (2), I is the magnitude of the current, and θ is the delayed phase of the current with respect to the voltage. Substituting equation (2) into equation (1) yields V〓 _Z =0.

Now, suppose that the H motor 2A has fallen into an overload state. In this case, the magnitude of the current _I〓1 hardly changes from the constant current characteristic of the H motor, but the phase θ changes. For example, if θ becomes θ'(θ'<θ), the current I〓 ₁ becomes I〓 ₁ =I ^-j 〓' (3). Substituting equation (3) into equation (1) yields V〓 _Z = nRI ( ^-j 〓′− ^-j 〓) ……(4). Therefore, when calculating the magnitude of V〓 _Z , we get ｜V〓 _Z ｜=nRI｜ ^-j 〓′− ^-j 〓｜ =nRI√2−2(−′) =2nRI｜sin(θ−θ′/2) | ...(5) becomes. Therefore, the magnitude of V〓 _Z changes according to the change in phase. Note that the same equation as equation (5) is also used when the H motor 2B becomes overloaded.

The above voltage V〓 _Z is the terminal 6a, 6 of the overload detector 5
When applied across b, this voltage V〓 _Z is full-wave rectified by a rectifier 7, smoothed by a filter 8, and then input to a signal input terminal of a comparator 9. A positive reference voltage signal 0<V _r <√2 |V〓 _Z | set by a setter 11 is applied to the reference input terminal of the comparator 9 . Therefore, if H motors 2A and 2B are in normal condition, V〓 _Z
= 0, the output of comparator 9 maintains a low level. As mentioned above, H motor 2A or 2
When B falls into an overload state, the voltage |V〓 _Z | shown by equation (5) is converted to direct current and input to the signal input terminal of the comparator 9. The magnitude of the above DC signal is
If the voltage drop of the full-wave rectifier 7 and the filter 8 is ignored, it becomes √2|V〓 _Z |. Therefore, the reference voltage signal V _r is exceeded, and the output level of the comparator 9 changes from a low level "0" state to a high level "1" state. This change is sent out from terminal 10 as an overload signal. Therefore, by activating an alarm device, a warning lamp, etc. in response to the above signal, it is possible to know that the H motor 2A or 2B has become overloaded. Furthermore, by activating a power source, disconnector, etc., in response to the above signal, it is possible to stop the power supply to the H motors 2A and 2B, thereby protecting the H motors.

Note that even when an H motor with the same characteristics drives a load with the same characteristics, some variation still exists. Therefore, even when both H motors 2A and 2B are operating normally, V〓 _Z = 0.
There may be cases where this is not the case. In such a case, if there is a concern that the output level of the comparator 9 will be "1", overload detection can be performed stably by adjusting the set value by the setting device 11, that is, the reference voltage signal level. can be done.

On the other hand, if the H motors 2A and 2B are overloaded at the same time, the phases of _I〓1 and _I〓2 change in the same way, so that no change in _V〓Z occurs. Therefore, overload detection cannot be performed in this case. However, in general, the probability that two H motors are overloaded at the same time is extremely low. Moreover, if there is even a slight difference in time, V〓 _Z will change for a period corresponding to the time difference. Therefore, if the instantaneous overload signal is used to activate and store a self-holding relay, flip-flop circuit, etc., overload detection can be performed. Therefore, even if the two H motors 2A and 2B are overloaded at the same time, overload detection can be performed without any practical problem.

However, in the above overload detection device, the H motor 2
Due to variations in the characteristics and load characteristics of A and 2B.
The condition of equation (3) may not be satisfied. In order to eliminate the disadvantages caused by the above-mentioned variations, the present invention connects the current transformers 3A and 3B with the same polarity as shown in the embodiment of _FIG . This is what I did. That is, the positive output end of the current transformer 3A and the positive output end of the current transformer 3B are connected, and the variable terminal of the variable resistor 12 is connected to the connection point, and one end of the resistance element of the variable resistor 12 is connected to the positive output end of the current transformer 3B. It is connected to the negative output end of the current transformer 3A, and the other end of the same resistance element is connected to the negative output end of the current transformer 3B. . Therefore, the detection current of each current transformer flows in opposite directions between the point where the variable terminal of the variable resistor 12 is located and both ends of the resistance element.
Note that everything other than the above is the same as that shown in FIG. 1 described above. Therefore, the same parts are given the same reference numerals and detailed explanations will be omitted.

Next, the operation of this apparatus configured as shown in FIG. 3 will be explained. It should be noted that the characteristics and load conditions of the H motors 2A and 2B, as well as the current transformation ratios of the current transformers 3A and 3B, etc. are all the same as in the case of FIG. 1, and the resistance value of the resistance element in the variable resistor 12 is R _v , the load resistance value seen from the current transformer 3A is KR _V. Therefore, the resistance value seen from the current transformer 3B is (1-K) _RV . Thus, the voltage V〓 _Z appearing across the variable resistor 12 is: V〓 _Z = nI〓 ₁ KR _V - nI〓 ₂ (1-K) R _V = nR _V [KI〓 ₁ - (1-K) I〓 ₂ ]...(6) becomes. Now, if K=1/2, the above equation (6) becomes V〓 _Z = 1/2nR _V [I〓 ₁ −I〓 ₂ ] ...(7). If R _V =2R here, equation (7) becomes the same as equation (1). Therefore, when the above conditions are given, the device shown in FIG. 3 operates in exactly the same way as the device shown in FIG. 1.

Note that if the condition of equation (3) is not satisfied due to variations in the characteristics and load characteristics of the H motors 2A and 2B, the adverse effects caused by the above variations can be eliminated by adjusting the position of the variable terminal of the variable resistor 12. Can be removed. This point will be explained below.

Letting I〓 ₁ and I〓 ₂ as I〓 ₁ =I ₁ ^-j 〓 ¹ ...(8) I〓 ₂ =I ₂ ^-j 〓 ² ...(9) and substituting this into equation (6), we get V〓 _Z = nK _V [KI ₁ ^-j 〓 ¹ − (1-K) I ₂ ^-j 〓 ² ] ...(10). Therefore, by adjusting the variable resistor 12 and setting KI ₁ = (1-K) I ₂ ∴K = I ₂ /I ₁ + I ₂ ...(11), the equation (10) becomes V〓 _Z = nR _V・I ₁・I ₂ /I ₁ +I ₂ [ ^-j 〓 ¹ − ^-j
〓 ² 〕. Therefore, the magnitude of V〓 _Z is |V〓 _Z |=nR _V I ₁・I ₂ /I ₁ +I ₂・2 | sin(θ
1-θ2/2) |... (12)

H motor 2A becomes overloaded, and the power factor angle θ1 becomes θ.
1′, the magnitude of V〓 _Z is |V〓 _Z |=nR _V I ₁・I ₂ /I ₁ +I ₂・2｜sin (θ
1'-θ2/2) |... (13) Furthermore, when the H motor 2B is overloaded and the power factor angle θ2 becomes θ2', the magnitude of V〓 _Z is |V〓 _Z |=nR _V I ₁・I ₂ /I ₁ +I ₂・2 | sin (θ
1-θ2'/2) |... (14) Generally, the difference between θ1 and θ2 is θ1' and θ2
and the difference between θ1 and θ2'. Therefore, |V〓 _Z | during overload
is extremely large compared to |V〓 _Z | under normal conditions. Therefore, the overload of the H motor can be detected in the same way as the device shown in FIG.

As is clear from equation (10), the circuit shown in Figure 3 compensates for the generation of voltage at R _V due to the difference in current value among the differences in current between H motors 2A and 2B during normal operation. There is.

Although the above description has been made under the condition that the characteristics and load characteristics of the H motors 2A and 2B are the same, it is possible to detect overload even if the characteristics are different. In other words, when I ₁ and _I By adjusting the resistor 12, the voltage generation caused by the difference in current value can be compensated for. Note that even if such measures are taken, the voltage generated due to the phase difference based on the difference in characteristics will not become zero, but the overload detector 5
Since it detects the magnitude of the voltage change based on the change in phase difference when A and 2B are overloaded, it is the voltage value itself that is generated due to the fixed phase difference caused by the difference in characteristics. Overload detection can be performed regardless of the

Figure 4 shows this device configured as shown in Figure 3.
It is a figure which showed the example of use in the case of using for overload detection of three H motors 2A, 2B, and 2C. In FIG. 4, currents I ₁ and I ₂ flowing through the H motors 2A and 2B are detected by current transformers 31A and 31B, and converted into voltage signals by a variable resistor 21. The overload detector 51 detects an overload on the H motor 2A or 2B. Similarly, currents I ₂ and I ₃ flowing through the H motors 2B and 2C are detected by current transformers 32B and 32C, respectively, and converted into voltage signals by a variable resistor 22. Then, the H motor 2 is detected by the overload detector 52.
B or 2C overload detection is performed. Similarly, the current I flowing through H motors 2A and 2C
₁ and I〓 ₃ are detected by current transformers 33A and 33C, respectively, and converted into voltage signals by variable resistor 23.
It is designed to detect overload of A or 2C. It should be noted that, except for the above points, the structure is exactly the same as that shown in FIG. 3, so the same parts are given the same reference numerals and a detailed explanation will be omitted.

Using this device as described above has the following advantages. It is now assumed that the H motor 2A is overloaded and the other H motors 2B and 2C are normal. Then, overload detectors 51 and 53 perform overload detection, and only overload detector 52 does not perform overload detection. Therefore, by monitoring the output status of each of the detectors 51, 52, and 53, it is possible to know which H motor has fallen into an overload state. In the above example, since the overload detector 52 does not perform overload detection, the H motor 2B,
Since motor 2C is found to be normal, it is determined that the H motor 2A has fallen into an overload state. Similarly, when the H motor 2B or 2C becomes overloaded, the overload detectors 51, 52, 53
It can be determined which H motor has become overloaded from the output status.

By the way, when two of the three H motors 2A, 2B, and 2C are overloaded, the overload detectors 51, 52, and 53 all perform overload detection, and the three H motors 2A, 2B, and 2C are overloaded. Motor 2A, 2B, 2C
If all of them become overloaded, all of the overload detectors 51, 52, and 53 will not perform overload detection. Therefore, the detector 5
From the output status of Nos. 1, 52, and 53, it can be known that at least two H motors have fallen into an overload state. Also, all H motors 2A, 2B, 2C
The probability that the H motors 2A, 2B, and Even if all 2Cs are overloaded at the same time, there is no practical problem.

Note that the present invention is not limited to the embodiments described above. For example, in the usage example shown in FIG. 4, a case has been described in which overload detection is performed for three H motors, but it may be configured to perform overload detection for three or more H motors. The connection method is such that current detection for one H motor is performed by one current transformer in each of the two overcurrent detection devices. In this case, if there are N H motors, overload detection for (N-1) H motors can be performed. Of course, if only one unit is overloaded, overload detection can be performed for that one unit. Furthermore, although the above embodiments illustrate the case where the present invention is used as an overload detection device for an H motor, that is, a hysteresis motor, it is of course possible to use the present invention for other motors having constant current characteristics, as well as for general motors. Can be used.
However, when used for a general motor, the signal based on the difference in current value is detected simultaneously with the signal based on the phase difference, so the detection signal level and reference signal level etc. should be adjusted to match the characteristics of the motor being detected. It is necessary to make adjustments in advance. Of course, various other modifications can be made without departing from the gist of the present invention.

As detailed above, according to the present invention, the first and second current transformers are connected to the same polarity, one end of the variable terminal is connected to one connection point of these current transformers, and the other end is connected to the first and second current transformers connected to the same polarity. By combining a variable resistor with both ends of a resistive element connected to a point, changes in the phase difference between the currents flowing through two motors can be detected. Overload detection can be performed accurately even if there are variations in motor characteristics and load characteristics. Moreover, it can be manufactured using general-purpose parts such as current transformers, variable resistors, rectifiers, filters, and comparators, and has a simple configuration and can be manufactured at low cost, making it very useful as an overload detection means for multiple motors operated in parallel. A preferable AC motor overload detection device can be provided.

[Brief explanation of the drawing]

Figure 1 is a circuit diagram showing the configuration of an overload detection device that combines a current transformer and an impedance element as a means for detecting changes in the phase difference between the currents flowing through two motors, and Figure 2 is a circuit diagram that shows the configuration of an overload detection device that combines a current transformer and an impedance element. FIG. 3 is a circuit diagram showing an embodiment of the overload detection device according to the present invention, and FIG. FIG. 2 is a circuit diagram schematically showing an example of use of the overload detection device of the invention. 1...AC power supply, 2A, 2B, 2C...H motor (hysteresis motor), 3A, 3B, 31
A, 31B, 32B, 32C, 33A, 33C...
...Current transformer, 4...Impedance element, 5,5
1, 52, 53... Overload detector, 7... Full wave rectifier, 8... Filter, 9... Comparator, 11...
Reference voltage setter, 12, 21, 22, 23...variable resistor.

Claims (1)

[Claims] 1 2 of multiple AC motors operated in parallel
A first and a second current transformer connected to the same polarity are used to detect the in-phase current flowing into the AC motor of the stand, and one end of the variable terminal is connected to the connection point of one of these current transformers. A variable resistor with both ends of a resistance element connected to the other connection point, and an overload sensor that detects the voltage generated across the resistance element of this variable resistor and sends an overload signal when the voltage level exceeds the reference voltage level. An overload detection device for an AC motor, characterized by comprising a load detector.