JP2013096321A - Oilless pneumatic compressor and method of controlling the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an oilless pneumatic compressor controllable properly even if the cable of a delivery compressed air temperature detection sensor is broken.SOLUTION: This oilless pneumatic compressor includes a compressor body which compresses air without oil supply, a thermistor which detects a temperature of the discharged compressed air, and a compressor control device for controlling the operation of the compressor. The breakage of the cable of the sensor is not determined immediately after the start of operation of the compressor body, and the determination of the breaking of the cable is started after the satisfaction of the requirements for determining the breakage of the cable is determined. Consequently, an erroneous determination which may occur when the temperature is low can be avoided.

Description

  The present invention relates to an oil-free air compressor and a control method thereof.

  The thing of patent document 1 is known as an oilless type air compressor using a thermistor as a temperature sensor. In this example, there is described an example in which a disconnection judgment is made when one of the two is disconnected by installing two sensors at substantially the same location for detecting the discharge temperature and comparing the temperatures.

JP 2000-345969 A

  The oil-free air compressor compresses air without supplying oil to the compressed air path. When air is compressed, the temperature rises. However, in the case of an oil-free air compressor, since a medium for cooling the temperature is not mixed, it is generally very high as compared with the oil-lubricated type.

  The temperature of the compressed air is measured by a temperature sensor provided in the air compressor. When the temperature measured by the temperature sensor becomes higher than a preset value, for example, control is performed such as issuing an alarm or determining that a failure has occurred and stopping the compressor.

  As the temperature sensor, a thermistor element is widely used (see Patent Document 1 described above). The thermistor element is one that is inexpensive and highly available and is suitable for use as a temperature sensor for an air compressor.

  In the thermistor, the resistance value changes depending on the temperature, and the thermistor element and the control board are connected by wiring in order to detect the change in the resistance value as a temperature change. However, if the wiring itself is disconnected, failure determination based on temperature information becomes impossible, and therefore, determination of disconnection is required for control. Patent Document 1 discloses an example in which two sensors are installed at substantially the same location for detecting the discharge temperature, and when one of them is disconnected by comparing the temperatures, the disconnection determination is made.

  On the other hand, it is possible to determine disconnection using the characteristics of the thermistor. The thermistor uses a characteristic that the resistance value of a semiconductor element changes with temperature. In other words, the property that the resistance value decreases when the temperature is high and the resistance value increases when the temperature is low is used as a temperature sensor. Therefore, when the resistance value grasped by the control board is clearly abnormal in view of the use environment of the air compressor, it can be determined that the circuit is disconnected.

  For example, in the case of an air compressor, since the outside air temperature is 0 to 45 ° C. in the normal use mode, when a resistance value (for example, equivalent to minus 20 ° C.) that is significantly outside this temperature range is detected, Since it is an obvious abnormal value, it can be determined that there is a disconnection.

  However, the operating environment of the compressor varies. For example, when the compressor is used in a cold region in winter, the ambient temperature assumed to be significantly different from the product specification and the ambient temperature will be minus 20 ° C. There is. At this time, the compressor makes a disconnection determination even though it is not actually disconnected.

  For example, the following phenomenon may occur. Since the compressor repeats automatic operation and automatic stop according to the use state of the compressed air of the user, the detected temperature is also lowered when the compressor is automatically stopped in an environment where the ambient temperature is low. Therefore, if minus 20 ° C. is detected during the stop, it is determined that the wire is disconnected, and the next activation is impossible.

  On the other hand, the temperature measurement range as a compressor is approximately 200 to 300 K (Kelvin) from the resolution of the control board that inputs the both-end wiring of the thermistor. Beyond this range, the temperature detection accuracy is significantly deteriorated.

  An oil-free screw compressor will be specifically described as an example. Oil-free screw compressors include single-stage compressors and multi-stage compressors, but even in the case of two-stage compressors with relatively low temperatures, the discharge temperatures of the low-pressure stage compressor body and the high-pressure stage compressor body respectively. Is about 200 ° C to 250 ° C. Therefore, a thermistor with high accuracy is selected in the vicinity of the discharge temperature.

  At this time, if the thermistor is selected based on the discharge temperature, the accuracy of temperature measurement near 0 ° C. is deteriorated. For this reason, when adopting a method of determining disconnection when minus 20 ° C. is detected, disconnection may be determined even when the ambient temperature is around 0 ° C. due to poor temperature measurement accuracy.

  As a measure for avoiding such erroneous disconnection determination, the time until the compressor warms up after the operation, that is, the time until the temperature measurement accuracy becomes high, for example, about 20 seconds without performing disconnection determination control. A method of driving can be considered.

  However, in the case of an oil-free screw compressor, immediately after the start of operation, the discharge temperature rises immediately after the air is discharged, so it is difficult to uniformly evaluate the time. For example, assuming that the operating temperature exceeds the upper limit of the compressor use range of 40 ° C. to 45 ° C., if the disconnection is not judged for 20 seconds or more, the abnormally high discharge temperature cannot be detected, and the compressor The body block can be damaged.

  The present invention has been made in view of the above problems, and provides an oil-free air compressor and a control method therefor capable of appropriately performing control even when a discharge temperature detection sensor for compressed air is disconnected. The purpose is that.

  In order to achieve the above object, the present invention employs the configuration and control described in the claims. For example, an oil-free air compressor includes a compressor body that compresses air without oil supply, a thermistor that detects the temperature of compressed air discharged from the compressor body, and the operation of the compressor. And a wiring for connecting the thermistor and the compressor control device, and after the start of the operation of the compressor body, the disconnection judgment of the wiring is started when the disconnection judgment condition is satisfied. It is characterized by doing.

  As another example, a compressor main body that compresses air without lubrication, a lubricating oil pipe through which lubricating oil supplied to the bearing of the compressor main body circulates, and a first thermistor that detects the temperature of the lubricating oil, A second thermistor for detecting the temperature of the compressed air discharged from the compressor body, a compressor controller for controlling the operation of the compressor, and the second thermistor and the compressor controller. And wiring, and when the detected temperature of the first thermistor reaches a predetermined reference temperature after the start of operation of the compressor, the determination of disconnection of the wiring is started.

  Furthermore, a method of adopting the above example is also possible when switching control between load operation and no-load operation.

  Moreover, the following control is suitable as an example of the control method. That is, the temperature of the compressed air discharged from the compressor body when the temperature detected by the first thermistor for detecting the temperature of the lubricating oil supplied to the bearing of the compressor body reaches a predetermined reference temperature. The disconnection judgment of the wiring connecting the second thermistor for detecting the oil-free air compressor and the control device for controlling the oil-free air compressor is started, and when the disconnection is not judged, the temperature of the compressed air is a predetermined temperature. When the value is reached, an alarm is issued or the oil-free air compressor is stopped.

  More specific modes in the above-described examples will be made clear from the description of the mode for carrying out the invention and the drawings. These descriptions do not limit the content of the invention, but are suitable as an embodiment of the invention.

  ADVANTAGE OF THE INVENTION According to this invention, even if the discharge temperature detection sensor of compressed air disconnects, the appropriate control which can be coped with before abnormality occurs in a compressor is attained.

The figure which shows the whole structure of the oilless type screw compressor of this embodiment. The block diagram which shows a control structure. The characteristic view of the thermistor 30a, 30b. The characteristic diagram of the thermistor 29. FIG. Temperature measurement flowchart. Results of verification experiment (when outside temperature is low). Results of verification experiment (when outside temperature is high).

  Embodiments of the present invention will be described with reference to the drawings. In the following, a compressor body having a pair of male and female screw rotors that can rotate without contact and without lubrication by a timing gear, and bearings and gear lubricating oil used in driving units inside and outside the compressor body and the compressor body An oil-free screw compressor having a cooling lubricant system and a thermistor capable of measuring the compressed air temperature and the lubricant temperature will be described as an example.

  FIG. 1 is a diagram showing an overall configuration of an oil-free screw compressor, and shows a device configuration and a flow diagram of compressed air and lubricating oil.

  In FIG. 1, the oil-free screw compressor housed in the compressor unit case 1 is a two-stage compressor, and includes a low-pressure stage compressor body (one-stage compressor) 2a and a high-pressure stage compressor body (two-stage compressor). ) 2b. A suction throttle valve 6 is provided on the upstream side of the suction gas passage of the low-pressure stage compressor body 2a. The compressor main bodies 2a and 2b each contain a male rotor 3 and a female rotor 4 which are a pair of screw rotors in a compression chamber. The male and female rotors 3 and 4 are rotatably disposed in an oil-free and non-contact state, and a gas passage groove whose volume changes is formed on the outer periphery thereof.

  The air flow of this embodiment will be described. Both compressor bodies 2a and 2b are rotationally driven via a drive gear 7 by a compressor body drive motor 8. The gas used for the compression is taken in from the outside at normal temperature via the suction filter 5. By driving the compressor body driving motor 8, air is supplied from the outside to the low pressure stage compressor body 2a, and the compressed air is supplied to the low pressure stage air-cooled heat exchanger (intercooler) 9 through the pipe 35a. After being cooled, the refrigerant is supplied to the high-pressure compressor main body 2b through the pipe 36. The air further compressed by the high-pressure stage compressor body 2b is supplied to the high-pressure stage air-cooled heat exchanger (aftercooler) 11 through the pipe 35b, and after cooling, is discharged outside the compressor unit.

  Next, the circulation of the lubricating oil will be described. The lubricating oil filled in the gear case 12 that houses the drive gear 7 passes through the lubricating oil pipe 21 and is cooled to an appropriate temperature by the air-cooled heat exchanger (oil cooler) 13 for the compressor lubricating oil. After passing through the oil filter 22 connected to the compressor body 2, the compressor bearings and the drive gear 7 including the inside of the compressor main bodies 2 a and 2 b are supplied for cooling and rotational lubrication, and recovered to the gear case 12 again. .

  Inside the compressor unit case 1 (lower left side in FIG. 1), a start control panel 24 is provided, and a compressor controller 31 for controlling the operation of the screw compressor of the present embodiment is provided inside.

  In addition, a cooling fan 25 is provided inside the casing constituting the compressor unit case 1 (upper right side in FIG. 1). The cooling fan 25 is rotationally controlled by a motor 26. Due to the rotation of the cooling fan 25, outside air is taken in from the intake holes 27 formed in the compressor unit case 1. This outside air cools the inside of the unit, and after exchanging heat with the intercooler 9, the aftercooler 11 and the oil cooler 13, it is discharged from an exhaust hole 28 formed in the compressor unit case 1.

  The lubricating oil pipe 23 on the outlet side of the oil cooler 13 is provided with a first thermistor 29 as a sensor for detecting the temperature of the lubricating oil. The temperature of the air discharged from the low-pressure stage compressor body 2a and the high-pressure stage compressor body 2b can be detected by the second thermistor 30a and the third thermistor 30b attached to the discharge pipes 35a and 35b, respectively.

  FIG. 2 is a block diagram showing a control configuration relating to temperature detection and display in the screw compressor of the present embodiment. Specifically, a configuration is shown in which the thermistors 29, 30a, and 30b detect the temperature and control the compressor.

  As shown in the figure, the signals from the thermistors 29, 30 a, 30 b are taken into the compressor control device 31. The compressor control device 31 includes a calculation unit 31a and a storage unit 31b, and displays temperature information captured from the thermistors 29, 30a, and 30b on the display unit 31c, or the calculation unit 31a stores the temperature information in advance in the storage unit 31b. When it is determined that the input temperature is higher than the set temperature, an alarm is displayed on the display unit 31c, or a malfunction is displayed to stop the compressor. Information necessary for control performed by the calculation unit 31a is stored in the storage unit 31b. For example, various reference temperatures and reference times used in control described later are stored.

  Next, characteristics of the thermistor used in this embodiment will be described. FIG. 3 is a characteristic diagram of the second thermistor 30 a and the third thermistor 30 b, and FIG. 4 is a characteristic diagram of the first thermistor 29.

  Since the discharge temperature of the low-pressure stage compressor body 2a and the high-pressure stage compressor body 2b is 200 to 250 ° C., the second thermistor 30a and the third thermistor 30b are required to have measurement accuracy within this range. As shown in FIG. 3, temperature measurement accuracy in the section A is required. When the compressor control device 31 shown in FIG. 2 is used, in the section (section B) where the resistance values of the thermistors 30a and 30b exceed 500 kΩ, the resistance value is too large on the control device side, and the change to the temperature of 1 ° C. Since the change amount of the resistance value is too large, an error in determining the temperature becomes large. There is no problem in detecting the discharge temperature during normal operation, but it becomes a problem in detecting disconnection.

  The problem of disconnection detection judgment that occurs in this embodiment will be specifically described. In screw compressors, it is generally assumed that the outside air temperature is 0 to 45 ° C. Therefore, when a resistance value (for example, minus 20 ° C.) that is significantly out of this temperature range is detected, the compressor control device 31 determines that a disconnection has occurred. However, when this compressor is used in a cold region in winter, the ambient temperature may be minus 20 ° C., and the temperature corresponding to the thermistor disconnection judgment is near (0 ° C., the lower limit of the compressor operating temperature range). It is difficult to distinguish between minus 20 ° C. and a temperature of 0 ° C., and even when used in an environment assumed in the product specifications, the control device 31 may erroneously determine that the wire is broken.

  On the other hand, the oil-free screw compressor of this embodiment has a compressor bearing including the inside of the compressor main body and the drive gear 7, and therefore, lubricating oil is used to lubricate the compressor bearing. When the temperature of the lubricating oil becomes high, there is a risk of equipment burn-in, so the first thermistor 29 detects the lubricating oil temperature.

  The lubricating oil temperature does not become as high as the discharge air temperature, and the temperature for judging an abnormality or the like is around 70 ° C. For this reason, in order to display a failure and stop the compressor, a thermistor having characteristics as shown in FIG. 4 is employed.

  For this reason, compared with the thermistor characteristics (characteristics of the second thermistor 30a and the third thermistor 30b) shown in FIG. 3, the detection accuracy on the low temperature side can be maintained high. As in the range shown in section C, the resistance value near 0 ° C. is 100 kΩ or less, and the resistance value near −20 ° C. is also about 200 kΩ. Therefore, since the first thermistor 29 can accurately distinguish between 0 ° C. and −20 ° C., when detecting a temperature of −20 ° C. or less that greatly falls below the compressor operating temperature range (0 to 45 ° C.), It can be determined that the thermistor 29 is disconnected.

  In view of the above situation, in the present embodiment, after starting the operation of the compressor, it is determined whether or not the disconnection determination condition is satisfied, and the disconnection determination is started when the disconnection determination condition is satisfied. Specifically, disconnection detection of the thermistors 30a and 30b is performed according to the flow shown in FIG. FIG. 5 is a temperature measurement flow chart according to the screw compressor of this embodiment.

  As shown in FIG. 5, the thermistor temperature is determined almost simultaneously with the start of operation. In order to prevent erroneous determination of the apparatus due to switching noise or the like, the compressor control means 31 does not perform disconnection determination for ts seconds from the start of operation, or does not affect the other control on the result of disconnection determination. (However, it is needless to say that the condition is not limited as long as there is no problem such as switching noise).

  As shown in FIG. 2, information on the thermistors 29, 30a, and 30b is transmitted to the compressor control device 31, and the detected temperature of the first thermistor 29 is equal to or higher than a predetermined reference temperature To1 ° C. If it is detected, the disconnection judgment is started. The disconnection judgment is started under this condition for the following reason.

  As described above, the disconnection misjudgment that may occur at low temperatures is due to the low temperature detection accuracy of the thermistors 30a and 30b near 0 ° C. On the other hand, the first thermistor 29 has better measurement accuracy around 0 ° C. than the second to third thermistors 30a and 30b. Therefore, it is determined whether the temperature is such that the disconnection can be detected by the second to third thermistors 30a and 30b using the accuracy of the first thermistor 29. That is, if the detected temperature of the first thermistor 29 is equal to or higher than To1 ° C., it means that the disconnection detection of the second to third thermistors 30a and 30b can be performed without erroneous determination.

  Thus, the disconnection determination is started after the temperature detected by the thermistor 29 for measuring the lubricant temperature reaches To1 ° C. or higher. Thereafter, when the resistance value of the thermistor 30a or 30b becomes equal to or higher than a preset resistance value Ra (for example, a resistance value corresponding to minus 20 ° C.), it is determined that the wire is disconnected and the compressor is stopped.

  When the resistance value Ra does not exceed the value and the disconnection judgment is not performed, the normal temperature measurement is performed, and the compressor control device 31 has the resistance value of the thermistor 30a or 30b equal to or less than the preset resistance value R2. In such a case (shown in FIG. 5 as whether or not the discharge temperature is equal to or higher than the discharge temperature Tsd), it is determined that the temperature is abnormally high, and the compressor is stopped. At this time, control may be performed to issue an alarm before stopping. In these normal measurements, the thermistors 30a and 30b are originally selected to measure the discharge air temperature at 200 ° C. or higher, so that accurate determination can be made.

  According to the above-described embodiment, it is possible to prevent erroneous disconnection determination due to the poor temperature detection accuracy at the time of low temperature of the second thermistor 30a and the third thermistor 30b. However, since the flow shown in FIG. 5 is a control in which the disconnection detection of the second to third thermistors 30a and 30b is not performed before the first thermistor 29 detects the oil temperature To1, the disconnection is actually performed during this period. This cannot be determined even if

  Therefore, verification was performed from the relationship with troubles in using the air compressor that may occur when the disconnection of the discharge temperature sensor cannot be detected. Hereinafter, description will be given with reference to the drawings.

  The verification also serves as verification of the validity of the control according to the present embodiment. That is, in the control flow of FIG. 5, it is assumed that after the first thermistor 29 detects the oil temperature To1, the second to third thermistors 30a and 30b can detect disconnection without any problem. The validity of the control is also verified.

  6 and 7 show the results of verification experiments by the inventors. FIG. 6 shows verification of control at low temperatures assumed in this embodiment, and FIG. 7 shows verification when the control of this embodiment is performed at high temperatures.

  FIG. 6 shows detection temperature and disconnection detection by the thermistor when the temperature of the outside air is low (assuming 0 ° C.) and the discharge pressure is low and the temperature of the compressed air flowing through the thermistors 30a and 30b is low and the temperature is difficult to rise. It is a verification experiment result which shows the relationship. In the figure, the disconnection detection start time ts is 5 seconds, the disconnection detection start temperature To1 is 5 ° C., and the failure determination temperature Tsd by the discharge temperature sensors (thermistors 30a and 30b) is 210 ° C.

Problems that may occur when disconnection of the discharge temperature sensor cannot be detected are the following situations (1) to (4).
(1) Although it is actually disconnected, disconnection cannot be detected.
(2) From the above (1), the control device 31 erroneously determines that the detected temperature is minus 20 ° C.
(3) Due to the above (2), the actual discharge temperature rises while the temperature is erroneously determined.
(4) Even if the discharge temperature rises and exceeds the failure determination temperature, the controller 31 cannot determine the abnormality.

  The verification result of FIG. 6 will be described. The compressor is started from an ambient temperature of 0 ° C., ts seconds elapse, and the time when the disconnection detection start lubricating oil temperature exceeds To1 ° C. is reached (illustrated by a vertical line). At this time, it can be understood that the temperatures detected by the second thermistor 30a and the third thermistor 30b, which are discharge temperature sensors, greatly exceed 50 ° C.

  As shown in FIG. 3, since the temperature can be detected sufficiently accurately at 50 ° C. or higher, no malfunction occurs in the section V after the start of disconnection determination. At this time, in order to start disconnection detection earlier, it is desirable that the disconnection detection start lubricating oil temperature To1 be equal to or lower than the compressor use lower limit temperature + 10 ° C.

  Further, as shown in FIG. 6, the discharge air temperature at the time when disconnection detection is started in the section V is 100 ° C. or less. Even when the two-stage discharge pressure is as high as 0.7 MPa (G) and the discharge air temperature is high, the two-stage discharge temperature is also 150 ° C. or less when the ambient temperature is as low as around 0 ° C. Therefore, even if both the second thermistor 30a and the third thermistor 3b are disconnected before the disconnection is detected, the actual discharge temperature is sufficiently lower than the failure determination temperature, so that no problem occurs. In this case, since the disconnection is detected after section V and the compressor stops, the trouble in use assumed in the above (4) is avoided.

  FIG. 7 shows experimental verification results when the ambient temperature is high, the discharge pressure is high, and the temperature of the compressed air flowing through the thermistors 30a and 30b is high. For example, when the ambient temperature is 45 ° C., the detected temperature of the thermistor 29 is about 45 ° C. when the compressor is started. For this reason, the disconnection detection start temperature To1 has already been exceeded after the disconnection detection start time ts seconds has elapsed since the start of operation. Therefore, the disconnection detection condition is satisfied after the elapse of ts seconds, and the disconnection detection of the thermistors 30a and 30b is started (section W). Therefore, if the thermistor is disconnected, the compressor can be stopped at that time, and if the detected temperature of the thermistors 30a and 30b exceeds the failure determination temperature Tsd, the compressor can be stopped. Therefore, it can be said that no problem occurs in this case.

  When the outside air temperature is high, the time to reach the failure determination temperature is short (for example, it reaches in 32 seconds in the example of FIG. 7), so the trouble (4) is likely to occur, but as shown in FIG. In addition, when the outside air temperature is high, the disconnection detection condition can be satisfied early (in FIG. 7, the condition is satisfied in 5 seconds from the start of operation).

  Thus, although this embodiment is the control resulting from the problem which may arise when outside temperature is low, it can respond suitably also when outside temperature is high, and can avoid a trouble in a wide temperature environment.

  In the above description of the embodiment, a two-stage screw compressor is taken as an example, but it goes without saying that it can be solved by the same means as long as it is an air compressor that causes the same problems as in this embodiment.

  In the above description of the embodiment, the operation start of the compressor has been described as an example. However, the operation is not limited to the operation start as long as the discharge of compressed air is started. For example, no-load operation of the air compressor (load reduction operation performed while the compressor drive motor 8 is rotated when the amount of air used is small. For example, the operation of closing the suction throttle valve 6 or the discharge pipe 35 This is also applicable to the case where the detection temperature of the thermistor 29 for measuring the lubricating oil temperature is lower than To1 after the provided air release valve (not shown) is opened and the compressed air is released. is there.

  A description of a specific example at the time of transition from no-load operation to load operation is omitted because it overlaps with the above description, but the compressor operation start in the above description is referred to as “load operation transition” or “load operation restart”. It is only necessary to replace the reading and the compressor stop state with the “no load operation state”. For example, when the temperature decreases during no-load operation and the temperature detection accuracy of the thermistors 30a and 30b is low, the disconnection detection is stopped, and the control after restarting the load operation is performed as described above. Similar effects can be obtained.

  As mentioned above, according to embodiment of this invention, even if it is a case where the discharge temperature detection sensor of compressed air is disconnected, a failure display and a compressor can be stopped appropriately.

  DESCRIPTION OF SYMBOLS 1 ... Compressor unit case, 2a ... Low pressure stage compressor main body, 2b ... High pressure stage compressor main body, 6 ... Suction throttle valve, 7 ... Drive gear, 8 ... Compressor drive motor, 9 ... Low pressure stage air cooling type heat exchange 21 ... lubricating oil piping, 23 ... lubricating oil piping, 24 ... start control panel, 29 ... first thermistor, 30a ... second thermistor, 30b ... third thermistor, 31 ... compressor control device, 35 ... Piping.

Claims (8)

A compressor body that compresses air without lubrication, a thermistor that detects the temperature of compressed air discharged from the compressor body, a compressor controller that controls the operation of the compressor, the thermistor, and the compressor control Wiring to connect the device,
After the start of operation of the compressor main body, the disconnection judgment of the wiring is started when the disconnection judgment condition is satisfied. From the compressor main body that compresses air without oil supply, the lubricating oil pipe through which the lubricating oil supplied to the bearing of the compressor main body circulates, the first thermistor that detects the temperature of the lubricating oil, and the compressor main body A second thermistor for detecting the temperature of the compressed air to be discharged; a compressor control device for controlling the operation of the compressor; and a wiring for connecting the second thermistor and the compressor control device,
After the start of operation of the compressor, when the temperature detected by the first thermistor reaches a predetermined reference temperature, the determination of disconnection of the wiring is started. A compressor body that compresses air without lubrication, a thermistor that detects the temperature of compressed air discharged from the compressor body, a compressor controller that controls the operation of the compressor, the thermistor, and the compressor control In an oil-free air compressor that includes wiring for connecting the device, and switches between load operation and no-load operation according to the amount of compressed air used,
An oil-free air compressor characterized in that after the transition from no-load operation to load operation, the disconnection judgment of the wiring is started when the disconnection judgment condition is satisfied. From the compressor main body that compresses air without oil supply, the lubricating oil pipe through which the lubricating oil supplied to the bearing of the compressor main body circulates, the first thermistor that detects the temperature of the lubricating oil, and the compressor main body A second thermistor for detecting the temperature of the compressed air to be discharged; a compressor control device for controlling the operation of the compressor; and a wiring for connecting the second thermistor and the compressor control device. In an oil-free air compressor that switches between load operation and no-load operation according to the amount of air used,
After shifting from no-load operation to load operation, when the detected temperature of the first thermistor reaches a predetermined reference temperature, the disconnection judgment of the wiring is started. .   The oilless air compressor according to any one of claims 1 to 4, wherein the compressor body includes a low pressure stage compressor body and a high pressure stage compressor. .   The oilless air compressor according to any one of claims 1 to 4, wherein the determination of the disconnection of the wiring is performed after a predetermined set time has elapsed.   5. The oilless air compressor according to claim 2, wherein the reference temperature is a compressor lower limit temperature of + 10 ° C. or less. A control method for an oil-free air compressor,
Detects the temperature of compressed air discharged from the compressor body when the temperature detected by the first thermistor that detects the temperature of the lubricating oil supplied to the bearing of the compressor body reaches a predetermined reference temperature. Starting the disconnection judgment of the wiring connecting the second thermistor and the control device for controlling the oil-free air compressor,
When the disconnection is not judged, an alarm is issued when the temperature of the compressed air reaches a predetermined temperature, or the oilless air compressor is stopped. Control method.

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