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2
Pressure Switches
• Pressure switches are used to start and stop electrical loads such as motors or coils.
• Pressure controls contain a bellows, a diaphragm, or a Bourdon tube to create movement when the pressure is changed.
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Pressure Switches
• When used as a switch the bellows or tube is connected to linkage that operates the electrical contacts.
• When used as a valve they operate the valve directly.
• The electrical contacts are the components that actually open and close the circuit.
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Pressure Switch
• The electrical contacts either open or close with a snap action on a rise in pressure.
• The pressure switch can also sense a pressure differential and be designed to open or close a set of electrical contacts based on this differential.
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Pressure Switch
• The pressure control can either be an operating type control or a safety type control.
• It can operate either at low or high pressures depending on the type of design.
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Pressure Switch
• Pressure controls can sometimes be recognized by the small line running to them for measuring fluid pressures.
• Pressure switches are manufactured to handle control voltages or line currents to start a compressor up to 3 hp maximum.
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Pressure Switch
• The high pressure and low pressure controls in the refrigeration and air conditioning industry are the most two widely used pressure controls in the industry.
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Pressure Switch
• Some pressure switches are adjustable and some are not.
• Some controls are automatic reset and some are manual reset.
• In some pressure controls the high-pressure and low-pressure controls are built into one housing.
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Pressure Switch
• The point or pressure setting at which the control interrupts the electrical circuit is known as the cut-out.
• The point or pressure setting at which the electrical circuit is made is known as the cut-in.
• The difference in the two settings is known as the differential.
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Pressure Controls - High
• High Pressure controls open on a rise in pressure.
• If you hold a high pressure control in your hand at room pressure it is closed.
• Primarily used as a safety device to shut off compressor if pressure rise to high.
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Pressure Controls - High
• Most high pressure controls have a manual reset.
• This is to prevent cycling in an unsafe condition.
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Pressure Switch - Low
• Low pressure switches are used to sense a drop in pressure.
• Most often the low switch is used to sense a loss of charge or refrigerant.
• The low pressure switch is also used to control the compressor in a pump-down system.
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Pressure Switch - Low
• It is not desirable to run a system with a low charge because:– The compressor motors use refrigerant to
cool the windings.– If the system goes into a vacuum because
of a leak it may pull air and noncondensable gases into the system and cause more damage.
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Oil Pressure Safety
• The oil pressure safety switch is a low and high pressure switch that is a pressure differential control.
• The differential control takes the high and low pressure and compares the two numbers coming up with a net pressure.
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Oil Pressure Safety
• The net oil pressure is the higher number minus the lower number.
• The net oil pressure is the useable oil pressure.
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Oil Pressure Safety
• The oil pressure is caused by a pump attached to the crankshaft of the compressor.
• When the compressor is not running there is no oil pressure.
• Because of this the oil pressure safety contains a timer that delays the action of the switch for roughly 90 seconds.
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Oil Pressure Safety
• The oil pressure is measured between the suction side (compressor crankcase) and the oil pump outlet.
• Most compressors must have a minimum of 30psi of oil pressure.
• Always follow manufactures directions.• Oil pressure safety controls have a
manual reset.