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Motors and Motor Controls for Air Compressors
Electric Motors
Electric Motors are the most common type of driver applied to industrial air compressors. The motors are designed to NEMA standards and usually are NEMA T frame construction. Open drip proof (ODP) and Totally Enclosed Fan Cooled (TEFC) are the most popular enclosure types. ODP motors are applied in indoor applications. TEFC motors are used in outdoor or wet environments. Motor speeds are normally 1800 rpm, however 3600 rpm may be used.
Motor horsepower ratings are standard NEMA however it is common industry practice to apply the motors at 110% of the nameplate rating. The motors usually have service factors of at least 15%. For example, a motor nameplate for 50 hp (37kW) will actually produce 50 hp X 1.15 = 57.5 hp (37 kW X 1.15 = 42.5 kW). Compressor manufacturers will normally apply this motor at 50 hp X 1.10 = 55 hp (37 X 1.10 = 40.7 kW).
Mounting arrangements for the motors will either be foot mounted for v-belt drives or flanged mounted for direct drive.
Motor Controls
Due to the size of the electric motors utilized on industrial air compressors, motor starters must be used to start and run the motor and to protect the motor from overload conditions. Some very small compressors, operating at 115 volt/ 1 phase/ 60 hertz, may not require a motor starter. Usually, a pressure switch rated for the start and running amp load, is sufficient to safely operate the compressor.
Full voltage motor starters are commonly applied to air compressors. The motor starters are usually either full voltage or reduced voltage. Soft start and variable frequency starters can also be applied to air compressors.
Three phase full voltage starters utilize a three (3) pole contactor to connect the motor to line voltage. A coil, energized from the compressor control circuit, is used to close the three-pole contactor. An overload relay, with three overload heaters, is installed to protect the motor from overload conditions. The overload heaters are sized basis the motor's full load amp draw. The overload relay will normally have a time delay built in to prevent nuisance shutdowns when the motor is starting.
Reduced voltage starters are utilized when it is desirable to reduce the motor's inrush current during start-up. When starting a motor, with a full voltage starter, the inrush current can be more than six (6) times the motor's full load amps. For example, a 50 hp (37 kW) motor, operating on 460 volt will draw approximately full load 60 amps. At start-up, the inrush current will increase to 60 X 6 = 360 amps or higher. Reduced voltage starters are available in a number of configurations including auto transformer, star-delta, part winding. Selection of the proper reduced voltage starter must involve the compressor manufacturer since the torque developed during start-up must be sufficient to start the compressor.
Proper selection of the motor and motor control involves a number of critical factors:
1) Select the required operating voltage based on the voltage available in your facility.
2) Select the type of motor and starter enclosure based on the expected conditions in the location the compressor will be installed.
3) Select the motor starter type based on the expected impact the motor will have on your electrical system. If reduced starters are required, be sure to consult with our application engineers at eCompressedair.com.
Installation of motors and motor controls must be done in accordance with all applicable federal, state and local codes.
Installation
Proper installation of the air compressor is essential for safe and trouble-free operation. Most modern air compressors are easy to install provided the equipment is installed in accordance with the manufacturer's recommendations and with all applicable federal, state and local requirements.
Some common installation recommendations:
- Rotary and tank mounted reciprocating compressors require a floor capable of supporting the static weight of the compressor package. Larger reciprocating compressors may require a concrete foundation or structural steel support designed to offset unbalanced forces that are present during normal operation.
- Locate the compressor in an area where the added noise will not interfere with factory and office personnel or violate OSHA regulations.
- All compressors produce heat during the compression process. This heat must be removed from the compressor room for proper operation of the compressor. Be sure to provide sufficient ventilation for all equipment that may be installed in the compressor room. All compressor manufacturers publish allowable operating temperatures.
- Properly vent the exhaust from engine driven compressors. Be sure that the exhaust cannot be redrawn into the plant through HVAC systems or drawn into the air inlet of compressors.
- Leave sufficient space around the compressor to permit routine maintenance. It is also suggested to provide space for the removal of major components during compressor overhauls.
- Select piping systems that have low pressure drop and provide corrosion free operation. When selecting the main air header, size for a maximum pressure drop of 1 to 2 psi (0.07 to 0.14 bar). A good rule is to use a header pipe size at least one size larger than calculated. This will provide additional air storage capacity and allow for future expansion.
- Provide means to drain moisture that may accumulate in piping. Headers should be pitched to enhance drainage and drip legs installed in all low points.
- Connect all drain valve outlets to an approved drain. Be sure the drain is vented. Do not pipe drain valves into a common closed pipe or header.
Note: Condensed moisture may contain compressor lubricants and other chemicals that must be disposed of in accordance with all federal, state and local regulations. Contact the application engineers at eCompressedair for assistance in selecting the correct oil and water separator.
- Remember water freezes at 32ºF (0ºC). If equipment, piping or drains can be exposed to freezing temperatures be sure to insulate and heat trace to prevent freezing. Follow the manufacturers guidelines.
Contact our application engineers at eCompressedair.com for installation assistance.
Compressor Maintenance
Air compressors, like all mechanical equipment, require periodic maintenance to provide a long and reliable life. Proper maintenance can also increase operating efficiency, reduce long term operating costs and minimize downtime.
Small reciprocating compressors, typically 1.5 to 30 hp (1.1 to 22 kW), require the periodic replacement of inlet air filters, oil filters and oil. Oil level must be checked daily as a minimum. Compressor intake and discharge valves must also be replaced. The frequency of the valve replacement depends on the number of operating hours and temperature. Electric motors and engines will also require periodic maintenance.
Rotary compressors also require periodic maintenance. This usually involves the checking of lubricant levels, addition of lubricants, changing of inlet air filter elements, lubricant filters and air/oil separator elements. Lubricant change-out depends on the type lubricant and can vary from 1000 hours to 8000 hours.
Recommended maintenance parts can be purchased on the model specific "Product Information" pages.
Electric motors and engines will also require periodic maintenance.
Routine preventative maintenance is a useful tool to conduct necessary inspections and maintenance on a schedule that will minimize disruption to plant operations. Maintenance contracts have become a popular method for compressor users to plan and budget for equipment maintenance. eCompressedair can arrange to maintain your equipment through our network of local service providers. Click on the "Service Information" button on the "Product Information" page for additional information or contact our application engineers.
Selection of the air compressor is only the first step in designing an efficient and reliable compressed air system. The air exiting the compressor is saturated with moisture and will have compressor lubricants (lubricated compressors only). Other chemicals that may have been drawn into the compressor intake may also be present. This contamination is harmful to many processes, pneumatic tools, instruments and equipment.
Air purification equipment, filters, air dryers, breathing air purifiers, monitoring equipment, used alone or in combination will remove these contaminants.
Selection and purchase of the compressor and necessary purification equipment can be easily done on the eCompressedair site. Our application engineers are ready to answer all of your questions and to assist you in placing your order.
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