Pressure switch for compressor: device, marking, connection and adjustment

The use of an air pneumatic relay allows you to automate the filling of the compressor receiver with compressed gas. The operator of equipment with a pressure switch does not need to monitor the process, trying to fix the limit parameters. As a result, engine damage is prevented. Significant results, right?

If you are planning to purchase a pressure switch for your compressor, then you have come to the right place. Here you will find a vast amount of extremely useful information about the principles of operation of the device, its configuration and connection methods.

We have described in detail the existing types of pneumatic relays. They provided options for connecting to a household and industrial network with extremely clear diagrams. We looked at typical breakdowns and ways to prevent them. The information and useful tips we provide are supplemented with graphic, photo and video applications.

Operating principle of a pressure switch

The name of the relay is determined by its purpose - controlling a piston compressor to maintain the required atmospheric pressure in the receiver. It is rarely found on a screw type device responsible for compressing and supplying air.

I take into account the magnitude of the pressing force in pneumatic automation; the device acts on the voltage line, closing or opening it. Thus, insufficient pressure in the compressor starts the motor, and when the required level is reached, it turns it off.

This standard operating principle, based on connecting a normal closed loop to a circuit, is used to control the motor.

The design of all ejectors contains a cylinder containing air at a certain pressure. Reducing it requires turning on the engine to replenish the supply. If the situation is the opposite and an excess is detected, the supply is stopped so that the container does not burst. These processes are controlled by a pressure switch

Modifications with the opposite operating algorithm are also presented: when reaching minimum values in the compression circuit, the pressure switch turns off the electric motor, and at maximum values it activates. Here the system operates in a normally open loop.

The operating system is made up of spring mechanisms with varying degrees of rigidity, reproducing the response to fluctuations in the air pressure unit.

During operation, the indicators formed as a result of the elastic force of tension or compression of the springs and the pressure of the atmosphere pressed by the device are compared. Any changes automatically activate the action of the spiral and the relay unit connects or disconnects the electricity supply line.

However, it is worth considering that the design of the review model does not provide for regulatory influence. Exceptional impact on the engine. In this case, the user has the opportunity to set a peak value, upon reaching which the spring will fire.

Purpose

The function of air compressors is to produce a stream of air with a certain pressure; it must be stable and uniform. It should also be possible to change the parameters of this jet. Each compressor has a reservoir (cylinder) for air. It must have the required pressure. When it drops, turn on the engine to replenish the air supply. If there is excessive pressure, the air supply should be stopped to prevent the container from bursting. This process is controlled by a pressure switch.

Pressure switch device RDM-5

When it functions correctly, the engine is preserved, it is protected from frequent switching on and off, and the operation of the system is uniform and stable. The container membrane is connected to the pressure switch. By moving, she can turn the relay on and off.

Principle of operation

Taking into account the amount of pressure in the system, the relay serves to open and close the voltage circuit; if the pressure is insufficient, it starts the compressor and turns it off when the parameter rises to a predetermined level. This is a normally closed loop operating principle for motor control.

The opposite principle of operation is also encountered, when the relay turns off the electric motor at minimum pressure in the circuit, and turns it on at maximum. This is a normally open loop circuit.

The working system consists of springs of different levels of rigidity that respond to changes in pressure. During operation, the deformation forces of springs and compressed air pressure are compared. When the pressure changes, the spring mechanism is activated and the relay closes or opens the electrical circuit.

Accessories

The air compressor relay may contain the following components:

Unloading valve. It is located between the compression chamber and the compressor check valve. When the engine stops, this component is activated and removes excess pressure from the piston block. When the engine starts, the pressure generated closes the valve, making it easier to start the unit. Some unloader valves have delayed activation. When starting the engine, it assists the engine by remaining open until the system reaches a predetermined value. During this time, the engine reaches maximum speed.
Mechanical switch.
Serves to enable and disable automation. The switch usually has two positions. When the mode is turned on, the automation is activated, the compressor is connected to the network and turned off, taking into account the specified pressure parameters in the system. In the OFF position, no power is supplied to the drive. Pressure switch for air compressor
Thermal relay. It protects the electric motor by limiting the current so that the motor windings do not burn out. The required current strength is set using a regulator. If the set value is exceeded, the engine will be disconnected from the network.
Safety valve. Protects the system in case of improper functioning of the pressure switch. When the pressure is exceeded, if the relay does not operate, the safety valve is activated, which relieves the pressure. This allows you to avoid accidents when the control breaks down.

Complete set of compressor automation unit

The relay design is a small-sized block equipped with receiving pipes, a sensing element (spring) and a membrane. Mandatory subassemblies include an unloading valve and a mechanical switch.

The pressure switch sensing unit is made up of a spring mechanism, the compression force of which is changed by a screw. According to the factory standardized settings, the elasticity coefficient is set to a pressure in the pneumatic chain of 4-6 at, as reported in the instructions for the device.

Inexpensive models of ejectors are not always equipped with relay automation since such devices are mounted on the receiver. However, during long-term operation, to eliminate the problem of overheating of engine elements, it makes sense to install a pressure switch

The degree of rigidity and flexibility of the spring elements is subject to the temperature of the environment, therefore absolutely all models of industrial devices are designed for stable operation in an environment from -5 to +80 ºC.

The reservoir membrane is connected to the relay switch. During movement, it turns the pressure switch on and off.

The unloading unit is connected to the air supply line, which allows excess pressure to be released into the atmosphere from the piston compartment. This relieves the moving parts of the compressor from excessive force.

The unloading element is located between the ejector check valve and the compression block. If the motor drive stops working, the unloading section is activated, through which excess pressure (up to 2 atm) is released from the piston compartment.

With further start or acceleration of the electric motor, a pressure is created that closes the valve. This prevents overloading of the drive and simplifies starting the device in switched off mode.

There is an unloading system with a time interval of activation. The mechanism remains in the open position when the engine starts for a specified period. This range is enough for the engine to achieve maximum torque.

A mechanical switch is required to start and stop the automatic system options. As a rule, it has two positions: “on.” and "off". The first mode turns on the drive and the compressor operates according to the established automatic principle. The second one prevents accidental starting of the engine, even when the pressure in the pneumatic system is low.

Shut-off valves allow you to avoid emergency situations when elements of the control circuit fail, for example, a breakdown of the piston unit or a sudden stop of the motor

Safety in industrial structures must be at a high level. For these purposes, the compressor regulator is equipped with a safety valve. This ensures system protection in case of incorrect relay operation.

In emergency situations, when the pressure level is higher than the permissible norm, and the telepressostat does not work, the safety unit comes into operation and vents the air. Safety valves in heating systems operate according to a similar scheme, the operating principles and devices of which are described in the article we recommend.

Optionally, a thermal relay can be used as additional protective equipment in the review device. With its help, the strength of the supply current is monitored for timely disconnection from the network when parameters increase.

To avoid burnout of the motor windings, the power is turned off. The nominal values are set using a special control device.

What is a pressure switch?

Pressostat is a pressure switch for a compressor, the main component of compressor automation. He is responsible for starting and stopping the system if it reaches the specified air mass supply parameters. As the required values are obtained, the following operations occur:

the engine turns on when the pressure is critically high;
The system shuts down if the pressure is unacceptably low.

That is, the compressor pressure switch regulates the operating process of the system from the moment it is turned on until it is completely turned off. It does not allow the compressor to operate under increased loads and prevents its premature wear. In many ways, the success of the system depends on the coherence of all its components: the air reducer, pressure gauges, supply-type communications (fittings, tubes), the relay itself.

Types of pressure switch devices

There are only two variations in the design of the automatic compressor unit. The determination is made based on their operating principle. In the first version, the mechanism turns off the electric motor when the established limits of the air mass pressure level in the pneumatic network are exceeded. These devices are called normally open.

Schematic structure of a membrane pressure switch: 1 – pressure converter; 2 and 3 – contacts; 4 – piston; 5 – spring; 6 – membrane; 7 – threaded connection

Another model with the opposite principle - turns on the engine if a drop in pressure is detected below the permissible level. Devices of this type are called normally closed.

2.5.5. Control systems

All compressors are equipped with some type of monitoring equipment to protect the compressor and prevent unproductive downtime. Sensors are used to determine the current state of the compressor unit. Information coming from the sensors is processed by a control system, which sends a signal, for example, to an actuator. Temperature or pressure sensors often consist of a sensing element and a signal converter. The perceiving element (sensor) determines the measured quantity. The converter converts the sensor output signal into an electrical signal suitable for further processing by the control system.

2.5.5.1. Temperature measurement.

An RTD is usually used to measure temperature. A metal resistor is used as a converter, the resistance of which changes with increasing temperature. The change in resistance is measured and converted into a 4–20 mA signal. The most common resistance thermometer is Pt100. The nominal resistance at 0°C is 100 Ohms. A thermistor is a semiconductor whose resistance changes with temperature. It can be used as a temperature control device, for example in an electric motor. The most commonly used thermistors are positive temperature coefficient (PTC) thermistors. The resistance of a PTC thermistor changes slightly with increasing temperature up to a special point at which the resistance increases abruptly. The thermistor is connected to a controller, which senses this “resistance jump” and issues a signal, for example, to stop the engine.

2.5.5.2. Pressure measurement.

To measure pressure, a pressure-sensing object, such as a membrane, is used. The mechanical signal from the membrane is then converted into an electrical signal of 4-20 mA or 0-5 V. The conversion of a mechanical signal into an electrical signal can occur in various measuring systems. In a capacitive system, pressure is transferred to a membrane. The position of the measuring membrane is sensed by the capacitor plate and converted by the converter into a pressure-proportional constant voltage or direct current. The resistive sensing system consists of a strain gauge connected to a bridge system and attached to a membrane. When the membrane is subjected to pressure, the sensor produces a low voltage signal (mV). It is then amplified to the required level. The piezoelectric system uses special crystals (for example, quartz) that generate electrical charges on their surfaces. The amount of charge is proportional to the force applied to the surface of the crystal (i.e. pressure).

2.5.5.3. Control.

The control instruments are adapted to the specific compressor type, which entails the use of a wide range of instruments. Small piston compressors are equipped only with a conventional safety switch that switches off the motor in case of overload, while large screw compressors have a number of safety switches - converters that react to overload, temperature, pressure, etc. In small, most simple machines, when the protection issues an alarm, the control device turns off the compressor and the machine is blocked from starting again. In some cases, the alarm light may indicate the cause of the alarm. In modern compressors, their compressor performance can be monitored from the control panel, for example, by directly reading pressure, temperature, compressor status, etc. When the transmitter output approaches the alarm limit, the control equipment issues a warning signal. This is done so that action can be taken before the compressor switches off. If the compressor is stopped by an alarm, restarting is blocked until the fault is corrected or the protection is manually reset. Troubleshooting is made much easier with compressors equipped with a memory that records parameters such as temperature, pressure and operating mode. The memory capacity allows you to remember data, for example, for the last 24 hours. Using this data, you can determine the trend over the last 24 hours, and then, using fault finding logic, quickly track the reason for the compressor stopping.

Structure of pneumatic relay symbols

The marking of the air pressure switch indicates the entire optional set of the device, design features, including information about the factory settings for the pressure differential.

Condor's production models offer a wide range of pressure control equipment. The MDR series is aimed at using ejectors of various powers

Let us examine the designations in more detail using the example of devices for air ejectors RDK – (*) (****) – (*)/(*):

RDK – series of relays for compressors;
(*) – number of threaded ports: 1 – one port with 1/4”NPT internal thread; 4 – four connectors;
(****) - type of housing design: T10P - version 10 with a “lever” switch; T10K – “button” switch; T18P – execution 18 with a “switch” switch; T19P - 19 s;
(*) – factory settings of the threshold response: 1 – 4…6 bar; 2 – 6…8 bar; 3 – 8…10 bar;
(*) – diameter of the unloading valve: the absence of a symbol means a standardized parameter of 6 mm; 6.5 mm – 6.5 mm.

The difference between the minimum and maximum pressure thresholds is set by the manufacturer and, as a rule, has a value of 2 bar.

However, it is also possible to manually adjust the range of two values – maximum and minimum, but only downward.

The specifics of setting up pressure switches for pumping stations are outlined in the following article, the contents of which we recommend that you familiarize yourself with.

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Air relay connection diagrams

The compressor pressure switch is manufactured for connection to electrical circuits of different loads. In accordance with the rating of the power supply line, the appropriate model of the relay unit is selected.

Option #1: to a network with a nominal value of 220 V

If the drive motor is a single-phase device, then a 220 V relay with two groups of contacts is installed.

To work with a single-phase load, manufacturers recommend equipping the unit using models of the RDK series: xT10R-x; xT10K-x; xT19P-x, since these devices have two contact groups

Option #2: to a three-phase network with a voltage of 380 V

For a three-phase load of a 380 V circuit, one of the options can be used: a modification of the relay for 220 V or 380 V, with three contact lines, to simultaneously disconnect all three phases.

Both methods have different schemes. Let's consider the first option:

To operate in a three-phase electrical circuit, a pressure switch RDK-xT18P-x is used. This model is equipped with three contacts and facilitates simultaneous switching of all phases

By choosing the second method, power is supplied from one phase (zero) and in this case the relay rating should be 220 V. For more details, see the following diagram:

It is allowed to use telepressostats of the RDK series: xT10R-x, xT10K-x and xT19P-x with a three-phase load, however, the use of such a circuit requires incomplete disconnection from the supply network. More specifically, one phase will be permanently connected to the load

After connecting to the power supply, you need to understand the additional capabilities provided in the air blocks for ejectors.

Connection diagram for a refrigerator compressor with a condenser and directly

Connection according to instructions

The electric motor used to drive the pump is equipped with a double excitation winding. To start the equipment, increased power is required, so the motor design includes a starting winding. After starting work, the power is automatically switched to the working winding, which reduces energy consumption. Additional relays that maintain the required temperature background are located upstream of the compressor housing.

To connect the refrigerator compressor according to the factory circuit, you will need to use a cable equipped with a plug socket. The wires are connected to the terminals on the relay body; since alternating current is used for power, the polarity of the connection is not taken into account. To ensure reliable contact, terminals are installed on the cables; the type of elements depends on the modification and manufacturer of the relay. After plugging the plug into the socket, the motor should start; if the start is unsuccessful, then you should start checking the components in the power circuit.

How to connect without a relay

The equipment design uses a relay that switches the current supply depending on the operating mode. The product provides protection for the windings of the electric motor; if it is broken or missing, normal starting of the motor is impossible. The owner of the equipment can simulate the operation of the relay, which allows you to check the performance of the compressor. It is strictly prohibited to operate a refrigerator with a missing relay.

To turn on the equipment, it is necessary to provide an alternating current supply of 220 V to both windings of the motor. To connect the product, a copper cable with a cross-section of at least 0.75 mm² is required (solid or stranded wire can be used). To ensure contact, connecting terminals are installed at the ends of the wire, which are fixed with solder or crimping with a special tool. The power supply is switched to the terminals of the common point and the working winding (the location of the elements is indicated on the compressor housing).

On some compressors, to provide access to the contact elements, you will need to remove a special plastic container into which condensate and melt water are collected.

To supply a short pulse to the starting winding, use an electrical screwdriver (with a handle made of special plastic) or a separate toggle switch. The button is placed in the gap in the wire that connects the terminals of the windings. If the windings and bearings are in good condition, the motor starts to work; the starting winding is turned off by removing the screwdriver or pressing the switch again.

How to connect without a capacitor

The classic condenser in refrigeration equipment is used to cool and convert gaseous refrigerant into the liquid phase. The refrigerant pump allows short-term operation without a condensing unit, but long-term operation of the unit is not recommended (due to lack of oil supply). There is an electrolytic capacitor in the compressor itself, which provides an additional current pulse at the moment the equipment starts up. The capacitor was used in refrigerators produced in the 60s and 70s. last century.

What brand of equipment do you use at home?

The capacitor works in conjunction with the control relay and is placed in the gap between the power line and the starting winding. When checking the performance of the motor, you can connect the power directly, bypassing additional circuit components. In equipment manufactured after the 90s, the element is not used. The capacitor is used to start 3-phase electric motors connected to a household AC network. The installed element simulates the missing phase, but such motors are not used in household refrigeration equipment.

If there was a capacitor in the circuit, then it is removed (soldered off), and subsequent start-up is carried out through a standard relay.

If the motor does not respond to power supply, then you will need to remove the relay. If, when power is applied, a monotonous hum is heard from the compressor housing, then the cause of the breakdown is jammed rolling bearings or a broken piston pump. If the motor does not work and there is no extraneous hum, then the cause of the loss of performance should be sought in a broken wire inside the compressor. Such a unit cannot be repaired, but must be disposed of.

Installation of relays and auxiliary elements

In some modifications of pressure switches, you can find additional equipment in the form of flange connections, through which additional equipment is connected. These are basically three-way parts, with a ¼-inch diameter.

By means of several flange connectors, additional elements can be introduced into the system: safety valve, pressure gauge and other necessary mechanisms

To put the device into operation, it must be connected to the receiver. Installation consists of the following steps:

The device is connected to the compressor through the main outlet.
A pressure gauge is connected to the device with flanges. There may also be other auxiliary mechanisms that require activation: a safety or unloading valve.
Channels that are not used for connection must be closed with plugs.
Next, according to the electrical diagram, the relay is connected to the contacts of the motor control circuit.

Motors with low power can be connected directly; in other cases, additional installation of an electromagnetic starter of appropriate power is required.

Before moving on to setting the threshold response parameters, it is worth paying attention to the operating conditions. First, adjustments are made under pressure. Secondly, the electrical supply to the engine must be cut off.

2.5.7. Central Administration

Central control of compressors usually involves the use of intelligent control systems. The main requirements for the system are the ability to maintain a given pressure within narrow limits and the most economical operation of the entire installation. To perform these tasks, the system must be able to predict events in the compressor system and at the same time monitor the load on the compressor. The control system perceives the rate of pressure change in the direction of increase or decrease (i.e., the derivative of pressure with respect to time). Using these values, the system can perform calculations that make it possible to predict compressed air consumption and, for example, unload and load or start and stop machines. In properly designed installations, pressure is maintained to within ±0.2 bar. To ensure economical operation, it is essential that the central control system can select the compressor or combination of compressors when compressors with different capacities are combined in the system. Compressors must operate virtually continuously under load in order to minimize idling time and therefore achieve the greatest economy. Another advantage of a comprehensive control system is that older machines can be connected to the system and thus modernize the entire installation in a relatively easy way. Operation becomes more economical and reliability increases.

Adjustment and commissioning process

Factory set parameters do not always meet consumer requirements. In most cases, this is due to insufficient compression force at the highest point of disassembly.

The operating range of the pressure switch may also not be suitable. In this case, independent adjustment of the actuator will be relevant.

Standard factory settings: upper limit 2.8 atmospheres, lower limit 1.4 bar. The parameters are monitored visually using a pressure gauge included in the standard pressure switch kit. New models, for example, Italtecnica, have a transparent body and are equipped with a compression scale indicator directly on the relay

To begin setting the operating compression value, you will need to inspect the engraved plate, which indicates the parameters of the electric motor and compressor.

We only need the largest value that the device produces. This indicator indicates the maximum pressure force that can be set on the relay for the correct operation of the entire pneumatic system.

If you set the specified value (in the figure 4.2 atm), then taking into account all factors - differences in power supply, exhaustion of the service life of parts, etc. - the compressor may not reach the maximum pressure, and accordingly it will not turn off.

In this mode, the working elements of the equipment will begin to overheat, then deform and eventually melt.

The maximum ejector value must be taken into account when determining the maximum relay value. This indicator should be less than the rated pressure of the compressor. In this case, all elements of the system will work uninterruptedly

For reliable operation without shutdowns, it is necessary to set the highest shutdown pressure on the relay, which does not reach the nominal value engraved on the compressor, namely 0.4-0.5 atm lower. According to our example - 3.7-3.8 atm.

The pressure limits at which the compressor is turned on/off are regulated by a single bolt. In order not to make a mistake with the choice of direction for increasing/decreasing, arrows are placed on the metal base

Having determined the level that will be set, it is necessary to remove the relay housing. Under it there are two adjusting elements - a small and a large nut (in Figure 1.3).

Nearby there are arrow indicators for the direction in which the twists will be made - thereby compressing and unclenching the spring mechanism (2.4).

A large screw clamp and spring are provided to control compression settings. When twisted clockwise, the spiral compresses - the compressor switch-off pressure increases. Reverse adjustment - weakens, and accordingly, the pressure level for shutdown decreases.

It is worth remembering: by increasing the shutdown compression strength, we are changing the factory settings, which were set taking into account the regulatory requirements for the operation of the equipment. Before making adjustments, check the technical documentation of the device so as not to exceed the limits stated by the manufacturer

When reproducing settings, the receiver must be at least 2/3 full.

Having understood the purpose of the elements, let's proceed:

To ensure the proper level of safety, we turn off the power supply.
Changing the compression level of the springs is done by turning the nut several turns in the required direction. On the board, near the large-diameter adjusting screw, according to the standards, there is a symbol in Latin letters P (Pressure), a smaller one - ΔР.
The adjustment process is monitored visually on a pressure gauge.

For convenience, some manufacturers place the adjusting fittings for changing the nominal value on the surface of the device body.

Technical specifications

The technical parameters of the control device are designed to visualize the maximum and minimum gas pressure, as well as the flow rate of the working medium.
The highest value at the inlet/outlet for a liquefied medium is 250 atm, for liquefied fuel - 25 atm. At the output, the indicator varies between 1−16 atm. In the design, the 220 V electric gas pressure regulator contains a sensitive mechanism that can compare the signal from the set pointer with the current value and converts the command pulse into mechanical work to move the movable plate to the neutral position. If the switching force is exceeded, the sensing element, or pilot, transmits a command to switch off to the sensors.

The pilot regulator can be astatic, static, isodromic.

Astatic

During operation, an astatic type relay experiences two types of load: active (acting) and passive (reacting). It is recommended to connect the device with a sensitive membrane to the equipment for sampling gas from the central pipeline. A device of this type adjusts the pressure of the system medium according to specified parameters, regardless of the degree of workload on the control element.

Static

The static pressure switch design kit includes process stabilizers that counteract friction and play at the system joints. Static devices form equilibrium indicators that differ from the permissible values of the rated load. The control process is activated by an acting force along a damped amplitude.

Izodromny

The isodromic industrial relay is automatically switched on when the pressure deviates from the set value. The 380 V pilot element reacts to real pressure gauge readings that differ from the permissible norm. To relieve the pressure, the regulating element independently reduces the indicators to the optimal operating parameter.

Possible malfunctions of the device

Several malfunctions characteristic of pressure switches are noted. In most cases, they are simply replaced with new devices. However, there are minor problems that you can fix yourself without the help of a repairman.

If the cause of the malfunction was determined to be a pressure switch, the technician will insist on replacing the device. All service actions for cleaning and replacing contacts will cost the user more than purchasing and installing a new device

The most common malfunction is characterized by air leakage from the relay when the receiver is turned on. In this case, the culprit may be the start valve. It is enough to replace the gasket and the problem will be eliminated.

Frequent starting of the compressor indicates loosening and displacement of the adjusting bolts. Here you will need to double-check the threshold for turning on and off the relay and adjust them according to the instructions in the previous section.

Troubleshooting methods

A more difficult problem lies ahead if the compressor does not work. There may be several sources. Let's consider one of them - melting of the pressure switch contacts due to erosion arising from electrical sparks.

Burning of the contact group occurs due to electric spark erosion, which is formed as a result of the opening of the contacts. However, it is not always possible to replace elements - some modifications are no longer available for sale

To eliminate this type of malfunction, you can use one of the following methods: clean the surface, which extends the service life by at least 3 months, or repair it by replacing the contacts in the terminal clamps.

Step-by-step instructions for the second option:

Bleed all air from the receiver and turn off the power to the ejector. Remove the pressure switch.
Having removed the protective housing, disconnect the wiring connected to the group of contacts.
Using a screwdriver, you need to remove the terminal with contacts and drill out the burnt lines from it.
You can replace the wire with copper wire. It is necessary to select it taking into account the diameter of the hole, since it must fit tightly into the seat. It is inserted into the hole and pressed on both sides.
Similar actions are performed with the remaining burnt lines.
After the contact group is assembled, it is mounted in its original place and the pressure switch cover is screwed on.

The compressor relay operates in difficult conditions, subject to wear and failure.

Although the repair is not cost-effective, those familiar with the device can perform the repair themselves. However, the option of replacing it with a new device still remains profitable.