To maintain the pressure in the receiver at a certain level, most air compressors have an automation unit, the pressostat.
This piece of equipment turns the motor on and off at the right moment, preventing the compression level in the storage tank from being too high or too low.
The pressure switch for a compressor is a block containing the following elements.
- Terminals. Designed for connection of electrical cables to the relay.
- Springs. Mounted on adjusting screws. Their compression force determines the pressure level in the reservoir.
- Diaphragm. Mounted under the spring and compressed by the compressed air.
- Trigger button. Designed for starting and forcibly stopping the unit.
- Connection flanges. There are between 1 and 3. Flanges for connection of compressor starting relay to air receiver and for connection of pressure relief valve with pressure gauge.
In addition to all these, the automation for compressor can be supplemented with.
- Relief valve. Designed to relieve pressure after the motor has been forced to stop, making it easier to restart.
- Thermal relay. This sensor protects the motor windings from overheating by limiting the current.
- Time relay. Fitted on compressors with three-phase motors. Relay switches off starting capacitor a few seconds after the engine starts.
- Relief valve. If the timer fails and the compression level in the air reservoir rises to a critical value, a pressure relief valve will trip to prevent an accident and relieve the air supply.
- Reducer. The gauges for measuring the air pressure are installed on this element. The pressure reducer lets you set the compression level required for the air drawn into the hose.
The pressostat works as follows. When the compressor motor starts, the pressure starts to rise in the air reservoir. Because the air pressure regulator is connected to the air reservoir, compressed air from the air reservoir flows to the diaphragm unit. The air causes the diaphragm to deflect upwards and compress the spring. The spring, when compressed, actuates a switch which opens contacts and stops the motor of the unit. When the compression level in the receiver decreases, the diaphragm installed in the pressure regulator flexes downward. The spring opens and the switch closes the contact so that the motor can be started.
Types of pressostat devices
There are two basic versions of the device. The pneumomechanical part is identical, the difference is in the way the contacts close when the piston rod moves:
- Normally closed (NC). Used for direct control of small and medium motor circuit.
- Normally open (NC). The movement of the piston rod encloses the contacts when the pressure limit is reached. Reverse motion opens them when it decreases. Contacts are used to control a more powerful relay that starts and stops the electric motor. The circuit is more complicated but the load on the pressostat contacts is reduced, the service life is increased.
When replacing a pressure switch, care must be taken to ensure that its type matches that of the compressor wiring diagram. its type.
How to connect and adjust the pressure switch?
In a general circuit of the compressor unit, the pressure switch is located between the unloading valve and the secondary circuit of the motor control. The pressostat is normally provided with four threaded heads. One of them is intended for connection of the device to receiver, and the second one is for connection of control manometer. One of the other connectors can be used to install a safety valve and the remaining one is fitted with a conventional threaded plug ? in. The presence of a free connector allows you to install the test pressure gauge in a place convenient to the user.
Connect the pressostat in the following sequence:
The setting is made with at least two-thirds of the receiver full. To perform this operation, disconnect the relay from the power supply and, after removing the upper cover, change the compression of the two springs. The adjusting screw, on which the axis of the larger diameter spring is mounted, is responsible for the upper limit of working pressure. A commonly used pressure symbol (P. pressure) is usually found on the side of the board, along with the direction of rotation of the screw that reduces or increases the pressure. The second, smaller adjusting screw is responsible for setting the necessary pressure range (differential). It is identified by a symbol ?P, and is also provided with a direction indicator.
To reduce setting time, some designs have the adjusting screw for changing the upper pressure limit set on the outside of the pressostat housing. The result is controlled by the manometer.
How to connect and adjust the pressure switch?
In a general circuit diagram of a compressor system, the pressure switch is located between the relief valve and the secondary engine control circuit. The pressure switch is generally fitted with four threaded heads. One of them is intended for connection of the device to an air reservoir, and the second is for connection of the control manometer. One of the remaining connectors can be used to install a pressure relief valve and the remaining connector can be fitted with a standard ¼” threaded plug. Free connectors allow installing test pressure gauge at the place, convenient for user.
The pressostat is connected in the following sequence:
At connection it is necessary to check up, whether the voltage in a network corresponds to default settings of the pressure switch of the compressor. For example, in a three-phase network voltage of 380 V, the relay must have a three-pin group (two-phase), and for 220 V. two-pin.
Adjustment is carried out with at least two-thirds filled receiver. To do this, disconnect the relay from the power supply and after removing the upper cover, change the compression of the two springs. The adjusting screw, on which the axis of the larger diameter spring is mounted, is responsible for the upper limit of the working pressure. The commonly used pressure symbol (P. pressure) is usually indicated on the board next to it, as well as the direction of rotation of the screw, which decreases or increases the pressure. The second, smaller adjusting screw is responsible for setting the necessary pressure range. It is identified by the symbol ΔP and also has an arrow indicating the direction of rotation.
To reduce setting time, some designs have the adjusting screw for changing the upper pressure limit set on the outside of the pressostat. The result is monitored on the basis of the pressure gauge.
The design and diagram of a pressure switch for a compressor
All compressor pressure switches are divided into two types:
- Switching off the compressor motor when the air pressure in the network exceeds the allowable limits (such designs are called normally open);
- These switches switch on the compressor motor when the line pressure falls below the allowable limits (such designs are referred to as normally closed).
The executive element of the pressure switch for the compressor are springs, the compression force of which is changed by a special screw. The factory settings usually set the compression force of the springs to 4 to 6 atm, which is reported in the user’s manual. Since the stiffness and flexibility of the spring elements depends on the ambient temperature, all industrial pressostats are designed to operate reliably at ambient temperatures of.5 to 80ºC.
The pressure switch has two essential components. a pressure relief valve and a mechanical switch. The pressure relief valve is connected to the air line between the air receiver and the compressor. It controls the operation of the electric motor. If the compressor driver is turned off, an unloading valve on the receiver relieves excess compressed air (up to 2 atm) to the atmosphere thus relieving the compressor moving parts from the excessive force they will have to develop when the compressor is restarted. This prevents critical overloading of the motor in terms of the permissible torque. When the unloaded motor starts, the valve closes and does not put unnecessary load on the drive.
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A mechanical switch performs a stand-by function, preventing the motor from starting accidentally. By pushing a button the actuator is switched on and the compressor operates automatically. If the button is released, the compressor motor will not start even if the pressure in the compressed air supply system is below the required pressure.
In order to increase the safety of operations, industrial designs of compressor pressure switches are also equipped with a safety valve. It is useful, for example, in case of a sudden stop of the engine, piston breakage or other abnormal situation.
As an option, a thermal relay can also be integrated into the pressostat control cabinet to monitor the primary circuit current. If for any reason this parameter increases, to prevent overheating and subsequent breakdown of windings, the thermal relay will switch off the electric motor.