Tips about Suction and Discharge Valves Used In Reciprocating Air Compressor.

The performance of air compressor is mostly depending upon the condition of suction and discharge valves. These valves are very important part in any reciprocating air compressor and engineering field. That’s why due care must be required. Every valve requires a periodic inspection and maintenance. Here you will find, important information about suction and discharge valves used in Chicago Pneumatic Air Compressor.

01) Dismantling of valves:-
To avoid damage to the valve and inner dowel pins, it is suggested to use a simple fixture for dismantle and assemble valves. Never tightened the valve directly in a vice and do not hammer on spanner when loosening or tightening the nut.

02) Cleaning of valves:-
Examine all the parts thoroughly after dismantled the valve. Remove the carbon formation with the help of Trichloroethylene or diesel and light scraping paper or brush. Don’t scratch the valve plate or seat while cleaning. Completely dry out the valves before installing it on compressor.

03) Inspection and Reconditioning:-
When valve plate, damper plate, spring plate shows sign of wear, it is imperative to replace these parts, even if no breakage has occurred. A maximum wear of about 10 % of the total thickness of valve plate is allowed. For highest efficiency of the valve it is important that the seat face is flat and free from any traces of wear, thus preventing valve leakage. If any damage on the seat observed, it is recommended to replace the same. Dew to presence of dowel pins, re-machining or lapping can not be carried out. It is also suggested to use a new valve plate with new valve seat.

04) Reassembly and Installation:-

A) Do the valve assembly in required sequence. (Refer diagram from instruction manual) Use fixture for proper tightening of valve nut. Check valve plate for free movement.

B) Test valves for leakage and insure it made dry after testing.

C) Suction valve equipped with unloader, the clearance between the valve plate and unloader lifter as well as the clearance between unloading piston and lifter is checked.

D) Valve cover nuts have to be tightened with specific torque by torque wrench.

E) The hollow side of the thin part in the centre of the valve plate must be placed upward facing, valve keeper.

The above information will help you to increase life of suction and discharge valve and hence the discharge pressure of the air compressor.

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How To Start New Reciprocating Air Compressor.

Below points are most essentials in air compressor engineering.
01) Be certain leveling screw in frame base has been backed off, and foundation bolts are tightened properly.

02) Clean off all accumulated dirt from exterior of compressor and electric motor.

03) Remove covers, tapes, wrapping from openings, which had been blanked and sealed.

04) Open frame top cover and check all connecting rods, bolts etc. to see that they are correctly installed and tightened. Check crankshaft end float.

05) Check and clean thoroughly the inside of the frame. Use cloth for cleaning inside. Never use cotton waste on internal parts.

06) Remove silica gel bags if placed in valve ports, do not wipe any carbon and Teflon powder from valve or cylinder. (For Non Lubricated Compressor.)

07) Fill compressor frame with correct grade of oil up to required level. (Middle of the visual screw)

08) Rotate the compressor manually, in direction of rotation and check bearing lubrication as well as free running of crankshaft.

09) Fill oil in force feed lubricator, disconnect oil line near to cylinder and rotate handle. Check oil is coming at the end of lubricating pipe. Reconnect oil line and operate pump at least 20-25 turns.

10) Check each compressor piston rod to be certain it is smooth and rust free, especially for the section where rod is passing through packing and wiper rings.

11) Check the bumping clearance as per recommended.

12) Replace frame inspection cover on the compressor.

13) Check compressor air cleaner and suction lines for dirt and clean, if required.

14) Check all electrical connections for correct tightness.

15) Check the insulation resistance with megger. If any moisture or dirt in the winding indicated, remove it immediately.

16) Remove safety devices and start motor without v-belts. Check the incoming voltage and current drawn by motor. Check electric motor for abnormal noise and vibrations.

17) When satisfied with electric motor, fit v-belts, align motor and adjust belts tension as required.

18) Check strainer/air filter before unloader and insure pressure sensing airline is clean and free from rust, dust etc.

19) Remove all suction and discharge valves and starts motor. Check oil pressure unusual noise, vibration and safety trappings.

20) Shut down compressor, check all parts for unusual heating. Install all suction and discharge valves.

21) Restart compressor and continue to run until the unit has warmed. Check various temperature and pressure gauges for proper operations.

22) If found every thing in order, put the compressor air discharge net to supply compressed air for plant utilities.

Follow all the above instructions while commissioning every new or overhauled reciprocating air compressor.

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Which Air Compressor You Can Use In Gas Turbine Power Plant?

Gas Turbine is used in Power Plant for driving the Generator,by which we can produce Electricity with other arrangements.Compressed Air plays the vital role in every Gas Turbine Plant.This Compressed Air is obtained from Air Compressor.

Usually Rotary Air Compressor are used with a Gas Turbine.Mostly Centrifugal Compressors or Axial Compressors are used.

Now we will see the details of each Air Compressor.

01)Centrifugal Air Compressor:-

The Impeller which consist of large number of Blades,is mounted on the Compressor Shaft,inside the Stationary Casing.As the Impeller rotates,the Pressure in suction region falls and hence the Air enters through the Eye and flows Radially outwards through Impeller Blades.As a result Velocity and Pressure of Air increases.

Later this Air enters and flows through the Convergent Passages formed by the Diffuser Blades.At this stage the Velocity of Air is decreases but the Pressure increases still further.We may say that,during this stage the Velocity Energy is converted into Pressure Energy.

Finally this High Pressure Air escapes from the Compressor Delivery Portion.This is Single Stage Compression and is suitable for small pressure ratios.You can obtain High Pressure Ratios by arranging the number of Air Compressor in series.

02)Axial Air Compressor:-

This type of Compressors are more commonly used now a days.In Axial Compressor,the Air flows in an Axial Direction right from Intake to the Delivery.

The Working Principle is as illustrated below.

The Stator,which have Stator Blades encloses the Rotor,which are provided with Rotor Blades.As the Air enters from Suction Region,it flows through the alternately arranged Stator and Rotor Blade Rings.

In flowing through each pair of Blade Rings formed up of one Rotor Blade Ring and one Stator Blade Ring,the Air gets Compressed successively.The Air is finally Delivered from Delivery Region,which is smaller in size compare to Suction side.

I have tried to disclose as much as knowledge I have, but engineering is such a vast field that can not be covered in single post.You can still search from Internet for more details.

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Trouble shooting and rectification In Automatic Starters.(Part-B)

Some faults are already covered in Part-A.In this post remaining faults about Automatic Starter will be disclosed.In Air Compressor,accessories are very important to keep the machine in running condition.

Automatic Starters (Contactor type) are mainly used to start Electric Motor so the Air Compressor coupled to that motor can be started.

Lets see remaining problems,its reason and rectification of Automatic Starters (Contactor type).

(E)FAULT:-NOISY CONTACTOR.

Reason:-(01) Dirty or rusty magnet and armature faces.
Rectification:-(01) Clean the magnet and armature face with soft cloth and apply petroleum Jelly.

Reason:-(02) Restricted movement or friction in magnetic movement.
Rectification:-(02) Check magnet by hand for free movement.

Reason:-(03) Intermittent contact on control circuit.
Rectification:-(03) Check all the contacts and control switches in control circuit for effective contacts.

Reason:-(04) Magnet and /or armature faces are not true.
Rectification:-(04) Change magnet and /or armature as the case may be.

Reason:-(05) Contactor chatter after changing the "No Volt" coil because the moving contact or carrier is fitted in reverse direction.
Rectification:-(05) Correct the direction.

(F)FAULT:-COIL BURNING.

Reason:-(01) Over voltage.
Rectification:-(01) Correct the incoming supply voltage.

Reason:-(02) Coil rating wrong.
Rectification:-(02) Use proper rating coil.

Reason:-(03) Noisy contactor.
Rectification:-(03) Refer the rectification for noisy contactor as per fault (E).

Reason:-(04) Contactor fails to close.
Rectification:-(04) Refer the rectification for contactor fails to close from previous post.(Part-A)

(G)FAULT:-STARTER FAILS TO OPEN.
For this fault there are three conditions.

(A) The fault occurs,when the supply switch is "Off".Reason is,Residual magnetism in armature and magnet.Rectification is,Polish very little or put some punch on the centre line of armature.

(B) The fault occurs,when "Off" push button pressed.
Reason is,(1) Cover fixing screw are loose.
Rectification is,(1) Tighten the screw fully.
Reason:-(02) Push button length is short.
Rectification:-(02) Set the right push button.

(C) The fault occurs due to condition A & B
Reason is,(1) Contact tips welded.
Rectification is,(1) Change the defective contacts.

Reason:-(02) Moving contacts carrier sticky.
Rectification:-(02) Check the moving contacts carrier and rectify the cause of sticking.

So,the maximum possible faults,its reason and rectifications of Automatic Starters are covered in Part-A & Part-B.

You are suggested to add if you know something about this topic.My goal is to keep you informed about every field of Air Compressor and engineering.

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Trouble shooting and rectification In Automatic Starters.(Part-A)

In any Air Compressor unit,Electric Motor is the most important accessory to drive the air compressor.To start and run the electric motor starters are used you will find more details from electrical engineering books.

These starters are mainly of two types one is Automatic type and another is Manual operated starter.

Automatic Starters are Contactor type and mostly used in all Air

Compressor units now a days.The details about both Starters are already given in my earlier posts,which you may refer.

Here I will disclose some problems which may occur in Contactor type Starter (Automatic Starter) in two parts.I hope these post will help you to rectify problems of your Starter of Air Compressor.

Lets see some problems,its reason and rectification of Automatic Starters (Contactor type).

(A)FAULT:-CONTACTOR FAILS TO CLOSE.
Reason:-(01) Coil open circuited.
Rectification:-(01) Change the coil.
Reason:-(02) Loose control circuit connections.
Rectification:-(02) Tighten all the screws.
Reason:-(03) Voltage too low.
Rectification:-(03) Improve voltage conditions.
Reason:-(04) Coil rating wrong.
Rectification:-(04) Use correct rating coil.
Reason:-(05) Obstacles in moving contacts carriers movement.
Rectification:-(05) Remove the obstacles.

(B)FAULT:-FUSES BLOWS DURING STARTING.
Reason:-(01) Fuse rating is too low.
Rectification:-(01) Use correct rating of fuses.Fuse for DOL Starters should be 3 to 4 times of motor full load current and for Star-Delta 1.5 to 2.5 times.
Reason:-(02) Short circuit in the system.
Rectification:-(02) Check and rectify the faulty CCF.

(C)FAULT:-INADVERTENT TRIPPING OF STARTERS.
Reason:-(01) Overload Relay setting low or single phasing
Rectification:-(01) Check voltage and current drawn by the Electric Motor and adjust Overload Relay to motor full load current.
Reason:-(02) Motor takes very long time to accelerate due to very low incoming voltage.
Rectification:-(02) Improve the voltage condition and use proper size cables.
Reason:-(03) Motor overloaded.
Rectification:-(03) Unload the Air Compressor during the starting.

(D)FAULT:-SEVERE CONTACT BURNING OR WELDING.
Reason:-(01) Excessive voltage drop during starting preventing magnet from closing in one stroke.
Rectification:-(01) Check the cross section of the cables from supply source to motor.
Reason:-(02) Insufficient contact pressure.
Rectification:-(02) Check moving contact spring pressure,change if necessary.
Reason:-(03) Change over in Star-Delta is too early.
Rectification:-(03) Increase the timer setting.

Some more faults will be covered in next part-B.

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Details of Electro-Pneumatic control used in Reciprocating Air Compressor.

As you know,Air compressor is used to increase the air pressure by reducing its volume.Every compressor has to discharge some specific pressure so we can use this utility to operate pneumatic tools etc.Basically electro-pneumatic control is a part of instrument engineering.

To control discharge air pressure of Air Compressor,some controlling devices as like M-type unloader,W-type unloader etc.are used in old days.
The accuracy of such devices are not perfect,to overcome this problem" Electro-Pneumatic control" is introduced.

This is an automatic type capacity control used to replace mechanical unloader.The discharge pressure may be control in 2-step,3-step or 5-step depending upon the model & capacity of the compressor.

It consist of 2 way or 3 way solenoid valves operated by the pressure switches,actuating at receiver pressure.Copper or Nylon tube is used for flow of air from receiver to suction or inlet valve through pressure switch & solenoid valve.

This system also provides the compressor to operate at no-load,till the electric motor attains normal speed & lubricating oil builds up to the required pressure,at the time of initial starting.

At starting of Air Compressor,the air pressure inside the receiver is low so pressure switches energises the solenoid valves.This will stop air supply to inlet or suction valve & compressor comes to full speed,which is known as 100% load.As the receiver pressure increases than the set pressure of pressure switch-1,the contact of switch opens & cut the electric supply of respective solenoid valve & de-energises the same.

Due to this air supply flows to inlet or suction valve and one set of inlet valve open thus compressor unloades partially.i.e. 50% load.If the consumption of compressed air is low than production then air pressure in receiver further increase to the set pressure of another pressure (switch-2) again the contact of switch opens & de-energies the respective solenoid valve.

This unloads the other set of inlet or suction valve & compressor gets fully unloaded which is known as 0% load.

Similarly,fall in receiver pressure causes the pressure switches to energises the solenoid valve & stops air supply to inlet or suction valve & compressor comes on load in steps i.e. 50% load & 100% load.

The pressure switches may be acting direct at receiver pressure or at reduced pressure where process controller is fitted.

As this is modern type of capacity controller,it is essential to take following precautions for smooth operation of capacity control.

01)Keep control filters clean,by draining the moisture collected in the filter bowl.otherwise,this will mix up with dust,oil & other materials and forms paste and restrict the air flow of air or sometimes enter into the inlet valves.

02)Stop air leakage in the impulse line.

03)Do not disturb the setting of pressure switches.

04)Set minimum 0.5 kg/cm square as differential pressure to operate the switch.

05)Use only genuine quality pressure switch & solenoid valves.

Safety switches used in Air Compressor.

It is very important that the air compressor should run in good working condition to deliver best result,for this we have to take care of this important utility. This is the most important machine in engineering plant.

As I said earlier,we can not keep one operator for full time near to compressor & keep watching performance .That is why it is necessary to provide some safety switches in control panel.This safety devices will work in abnormal condition & stop electric motor and further prevent any breakdown in Air Compressor.

Many safety switches are used to avoid any accident to air compressor.Out of that the most important safety switch is "Low oil pressure switch" in short LOPS. As the name indicates,this switch will operate in condition,when oil pressure in crankcase is lower than set limit & stops the compressor.

The details of LOPS is as below Indfos make IPS-70 switch is commonly used & the function of this switch is to monitor the oil pressure in the crankcase of air compressor.This switch is connected in the oil pressure sensing line coming from the crankcase to the oil pressure gauge,through Tee connection.

The main spring can be set to balance different pressure on the bellows .When the pressure increases,the bellow is affected so that the main spindle is moved upwards until the spring pressure balance the bellow pressure.The spindle has guiding knob and differential adjusting nut which jointly transmit the spindle movement to the switch.

The differential pressure has to be set at zero (Indicator scale at extreme left,facing to LOPS) and the main scale is set to required pressure.(Minimum oil pressure required by the compressor.i.e. 14 PSIG for reciprocating air compressor & 35 PSIG for rotary air compressor).The pressure on main scale can be set by rotating the knob given at top of the switch.

How to set & test Low Oil Pressure Switch.

Connect an Ohmmeter across normal open contact of the switch to observe the switch operation.Start the compressor and watch the oil pressure at which contacts closes,then stop the compressor to see at what oil pressure contacts opens.By adjusting the knob further fine adjustment can be done to the exact pressure required.So confirm the setting by starting & stopping the compressor.

Since the switch remains open during starting,a normal close contact of the timer is connected across the normal open contact of LOPS.The timer is set for 60 sec.As the compressor starts,timer starts its timing cycle and the compressor oil pressure also start building up in 60 sec. timer contacts will becomes open.Due to developed oil pressure in sensing line,LOPS contacts closes & continued the healthy running condition of air compressor.In case the oil pressure does not build due to any reason,this contact remain open and compressor will trip in 60 sec. time span.

A indicator lamp on Solid State Annunciator or any other indication device will flash & hooter starts blowing, rush to compressor immediately & rectify the problem.

High Discharge Air Temperature Switch used for safty of Air Compressor.

It is most important to prevent your Air Compressor from any breakdown,because breakdown not only increase maintenance cost but also suffers your production.You have to bear production loss due to breakdown of machine and engineeering troubles.

Many safety switches as like LOPS,LCWPS,HDATS etc. are installed for safety of compressor.

I have written on LOPS in last post.High Discharge Air Temperature Switch (HDATS) is also one of the most essential safety major has to be taken to prevent any abnormal to your valuable utility.If the temperature of discharge air (i.e.leaving compressed air from high pressure cylinder) is more,than the requirement,it could stuck the piston inside the cylinder,which cause a breakdown of compressor.

A working HDATS will safeguard your compressor from this problem.
A INDFOS Make RT-120 high discharge air temperature switch is normally used for safety of Air Compressor.We will see the details now.This switch having 2 mtr.long capillary connecting the switch and the bulb,with thermowell to fit the bulb.Fit the thermowell in air discharge line in the direction of discharge air flow in angular direction,in between the compressor and AFTER-COOLER,closer to the discharge cylinder.So the discharge air temperature safety can be done.If it has to monitor the air temperature before AFTER-COOLER.Fit the HDATS after the AFTER-COOLER,if it is to monitor the air temperature after AFTER-COOLER.

HDATS has to be set for some high value which is recommended by manufacturer that is why it is essential to set the temperature switch outside in an oil bath,having steady required temperature at the switch to be set.keep the differential setting at zero ,connect an Ohm meter across Normal close (NC)contact of the switch.Immerse the bulb and thermometer in the oil bath,keep stirring.

Set the high Discharge Air Temperature Switch at 170 degree Centigrade,on the main scale.control the steady temperature 170 degree Centigrade of oil bath,now close contact of switch should change & the Ohm meter pointer should show infinity,if not,reduce the setting till the contact changes to open.Increase the temperature setting if the contact opened before 170 degree Centigrade till it closes and opens.Now the switch is set at 170 degree Centigrade.

Confirm the setting once again by heating oil bath after the oil attains the ambient temperature or take fresh oil & continue the process.
After setting the switch,install near the thermowell,fill compressor oil in the thermowell and insert the bulb of temperature switch slowly.Wire the control cable in control pannel to Normal Close (NC) Contact of the switch.

Your Air compressor is more safe if you provide more safety devices.It is not possible to install all the safety due to cost or technical problem but still I recommend to install Low Oil Pressure Switch,Low water Pressure Switch,High Discharge Temperature Switch & Motor Overload Relay.

This four safety switches are most essential to run your Air Compressor without trouble.

Details about Solenoid Valve used in Air Compressor.

Solenoid valve is integral part of Electro-Pneumatic Control system of engineering.This valve will control the supply of air to inlet or suction valve in load/unload operation of reciprocating air compressor .

Different types of solenoid valves are available in market.In Air Compressor Technology Normally open (N/O) or Normally Close (N/C) type solenoid valve are used to perform loading/unloading action.

Solenoid valve has three ports,these are
(1) inlet
(2) outlet
(3) exhaust.
use of each port is as below

01)Inlet Port:-Impulse air supply through tube of Dia. not less than 12 mm from the air receiver through control filter connected to inlet port of solenoid valve

02)Outlet Port:-This port is also known as cylinder port.This is connected through,at least 6 mm Dia. Copper or Nylon tube to inlet (Suction) valve of the compressor.

03)Exhaust Port:-This port is for the escape of trapped air in the line.It should always kept open to the atmosphere.

Details of Normal Open & Normal Close Solenoid Valve.

A)Normal Open (N/O):-This type solenoid valve allow air supply to flow from inlet port to outlet (cylinder ) port,in de-energised or normal condition.When energised,due to change over in pressure switch contact stops the air supply.This type of solenoid valve are used for load/unload operation of air compressor.

B)Normal Close (N/C):-This type of solenoid valve does not allow air supply from inlet port to outlet port in de-energised or normal condition.When energised due to change over in contact of pressure switch allows air supply to flow through cylinder port.This type of solenoid valve is used in 5-step control panel for increasing and decreasing the area of clearance pocket during unloading/loading operation of Air Compressor.

Precautions to take for best performance .

01)For perfect functioning of the solenoid valve,always have an air filter,installed in the air line connected to solenoid valve.

02)Always see if the nut tightening the solenoid coil is intact.If loose,tighten the top nut.

How Electro-magnetic Overload Relay works?

Overload Relays are used to protect Electric Motor from failure due to high current drawn by motor.Electric motor has 33% of it's cost in total Air Compressor unit,so it is our duty to protect Electric Motor from any type of failure.Overload Relays are the only solution to protect motor from high current.
Overload Relays are generally of two types one is "Electro-magnetic" and another is "Thermal".

I have already given details about Thermal Overload Relay in past.In this post,you will find details about "Electro-magnetic Overload Relay" which is somewhat outdated from latest engineering,but still some old Air Compressor units are having these type of Relays.

The Electro-magnetic Overload Relays consist of a coil or solenoid carrying the line current (or a portion of it through current transformers) with an armature which when attracted sufficiently operates the release circuit or latch.

A time lag feature is in corporated to guard against unnecessary operation from temporary Overloads due to the normal operation of the machines which are being driven.Time lag is obtained by means of Dash pot (otherwise the action would be practically instantaneous) fitted to the Oil Circuit Breaker (OCB) controlling the stator circuits.

Construction and Adjustment of Dash pot.

The time lag consist of BODY CASTING,MOVABLE PLUNGER,OIL RETAINING SCREW CUP.The movable plunger comprises the core of the Overload release,to which is fitted a specially shaped brass disc moving in a restriction in the body casting,by turning the screw cap the position of the disc within the restriction is varied.

The time lag are dispatched dry and must filled with special time lag oil (SAE-60 grade) provided in separate containers in the Circuit Breaker.While fitting remove screw "A".It is not necessary to remove the bottom cup "B" and on no account should screw "C" be removed.

Adjustment of the Overload setting is made by slackening screw "D" and sliding the body casting up or down,until the top face coincides with the calibration marking required on the calibration tube,screw "D' is then tightened.The bottom rotatable cup "B" is now described reset themselves after operation within few second.

This type of Overload Relays are very simple in operation but due care must taken for accurate operation.All the marks "A","B","C"&"D" are embossed on respective spaces,you have to identify first before working on "Electro-magnetic Overload Relay".

Different Types of Starters Used in Air Compressor.

Air compressor are driven by electric motor and these motors are started by different types of starters according to the requirement.These starters may be fully automatic with push button to operate or totally manual type,as per the design of the manufacturer and the choice of the customer.Here I will write about automatically operated and manually operated starters which are most important part of electrical engineering and air compressor.

01)Automatic Type Starters:-

This consist of contactors with built in coils and plunger.Pressing the starters button energises the operating coil which closes the contactor.In order that the contactor may remain closed when the "Start " push button is released,retaining contactors are required.
These are closed by the contactor lever itself and thus maintain the operating coil circuit,once it has been made and the contactors closed,irrespective of the position of the "Start" push button.Depression of the "Stop" push button or failure of supply to the operating coil immediately comes the contactor to open.
By energising,the operating coil from the same circuit as supplies the motor such an arrangement is equivalent to a no volt release.(Example-Automatic Direct-On-line (DOL) Starter and Automatic Star-Delta Starters are of these type)

02)Manual Operated Starters:-

For safety,all manual operating motor starting switch or circuit breaker return to the "Off" position in the event of failure of the incoming supply,and for this purpose an "Under voltage" or "No volt" release is fitted.The under voltage release forms an inherent part of the starting switch in all electro-magnetically operated starters,but is incorporated to the starting switch.

With drum type AC Starters having a manually operated starting handle,the starting switch is fitted with a spring which biases the switch to the "Off"position,but is retained in the "On" position by a mechanical latch fitted in the starter is a shunt-wound electro-magnet or solenoid excited by the supply voltage.
In the event of supply failure,the plunger or armature of the solenoid or electro-magnet is released and is arranged to knock off the hold-on latch and so return the switch to the "Off" position.

Mostly Star-Delta Starters are used in Air Compressor Technology.

How to Protect Electric Motor from Failure?

Mostly Air Compressors are driven by electric motor.This is the cheap option available comparing with other options such as diesel generator etc.

The operating cost of any air compressor can be reduced by using electric motor as a driving media.This is true when your electric motor runs very smoothly.But think,if your electric motor failed to start due to burning of motor winding or failure of bearings etc.

What will be the cost of repairing or replacement? This cost may be near about 33% of your Air Compressor cost.Do you believe this? For small engineering units this costs much more in the form of breakdown and production losses.To keep Air Compressor running cost at lower side,we have to protect our electric motor from failure.Most frequently motor failed due to excessive current drawn by motor.

To protect electric motor from excessive current a "Overload Protection Relay" is placed in electric circuit.

Excessive current may be due to either mechanical overload on the motor or due to electrical system,due to unbalance supply voltage or single phasing or defective starter or defective in the motor itself.In either case it is essential that the supply should be disconnected before any damage is done to the motor.An overload device thus usually operates by releasing the latching-in device by disconnecting the supply to no-volt coil or in contactor starters by operating the operating coil circuit.

Overload Relays are generally of two types.
01)Electro-magnetic
02)Thermal Electro-magnetic overload relays are old type which is not in use recent days that's why I am giving details about Thermal Overload relay.

These are mostly used in Air Compressor now a days.

Thermal Overload relay:-
This may be bi-metal strips or solder pot elements and in either case as the action is due to their heating up,a time element is always present.The action of bi-metal strips over load release depends on the movement resulting from the different rates of expansion of the two metals forming the combined strip when heated.

The bi-metal strip may be directly heated by the current or indirectly heated by a coil of resistance wire which carries the current.Thermal Overload relays are usually of the hand reset type,the reset feature is combined with the relay.

How to set Motor Overload Protection Relay.For closer protection of the motor,particularly against single phasing condition, the bi-metal overload relay is connected in the phase circuit.hence relay should be set for 0.6 times the actual line current drawn by the motor,as measured by an ammeter.In the absence of an ammeter the following procedure may be followed.

01)Start the motor with overload relay setting at 0.6 times the rated motor current for star-delta starter and for rest of the starter at motor full load current stamped on the name plate.

02)After the motor reaches the pick load condition,reduce the relay setting gradually till the relay trips.

03)Set the relay slightly higher than the tripping value.

04)To check the setting is correct or not,allow relay to cool down,then restart the motor and make sure,relay does not trip during starting.If trips,increase the relay setting slightly and check again.

05)To check the tripping,increase the unloading air pressure setting of air compressor by 1 kilogram per centimeter square from maximum rated air pressure,when the current increases than the normal peak load current,the relay should trip.(Allow air compressor to run for more than 5 minute from peak pressure condition).If relay don't trip,slightly reduce the setting.

Pre-Commissioning Checks of Electrical Motor Used to Run Air Compressor.

Air compressor is the utility which consumes maximum electric power in any engineering plant.Air compressor can be driven by the DG Set also,but it is very expensive option comparing to driven by electric motor.maximum plants used electric driven air compressor for compressed air purpose.It is also most important to know about electricals & instrumentations used in air compressor.If you have no knowledge about electricals then it will be very diffcult to run air compressor sometimes.You will get some knowledge from this post,which contains general information about the electricals used with air compressor.

Induction Motor,Squirrel cage or wound rotor (Slipring) type,of various ratings & suitable motor starting gears,are employed according to the capacity of the Air Compressor.These may be of Low Voltage (LV) or High Voltage (HV) grade,depending upon the requirment.

It is most important to install electric motor with proper check-ups.If your electric motor installed properly,then you are rest assured that you will get maximum working life and trouble free operation.Follow below

pre-commissioning checks to get best result from electric motor.

01)Check Name Plate details of electric motor & Starter,both the specification should match each other.It is most important to match Rotor Volts (RV) andRotor Amps (RA) of Slipring Motor.

02)Inspect for any transit damages.If found any damage,report the nature of damage to the nearest office of compressor manufacturer if purchased with Air Compressor or Motor manufacturer if purchased seperately.The delay in reporting the matter would involve considerable problems to claim the damages from insurance authorities or the concerned equipment manufacturers.

03)Perior to installation,the motor should be stored in a clean,dry place.The motor parts have a protective coat of anti-rust preservative which should notbe removed during normal storage period.In case of long storage,periodic examinations should be carried out and fresh preservative applied,if requiredafter any rust or moisture has been removed.

04)Before putting the electric motor into operation,measure the insulation resistance between the motor winding & frame with Megger.A 500 volt meggeris required for low voltage motor.The insulation resistance must above 2 megohms,if found below the value,motor should be "Dried Out".Insulation resistance of Starter should not be less than 20 megohms.

05)If motor is stored for more than 8 months period,then before starting the electric motor ,the bearing cover should be removed and the grease in the bearing cover should be pressed with thumbs between the race of the bearings.If any detorioration of grease is apparant,the old grease should be removed and new grease pressed into the bearings & housing.