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Sizing Drive Motors for Industrial Compressors, Blowers and Fans
Brad Addison, DuPont, Sept. 2011
For rotating machinery drive motors, robust company standards or commonly adopted industry standards, like IEEE 841 and API 541 / 547, should be used to get a high quality, durable motor. Even with good standards, and sometimes when taking a vendor-standard motor offering, it's possible for vendors to provide undersized or barely adequate motors for a given duty.
So, this article is intended to inform users and project personnel about buying a properly sized motor, as well as the advantages of buying a slightly oversized motor for driving a process or air compressor, blower or fan. It is not intended to address issues associated with driving other types of equipment, particularly where shock loads are involved.
These are some recommendations that cover original compressor/blower/fan and motor procurement, as well as some comments on retrofitting existing machines with new motors:
- Motors and rotating machinery should be sized for all the foreseeable operating conditions. This includes all foreseeable inlet and discharge conditions.
- Centrifugal compressor, blower, and fan performance and power curves or positive displacement machine operating point data sheets should be obtained for all foreseeable operating conditions to assure a properly sized motor.
- For centrifugal compressors, size the motor to the high end of the horsepower curve with the densest inlet foreseeable.
- Motors should not be sized for regular or continuous operation above 1.0 service factor, even if they're rated for 1.15 service factor or higher.
- There are instances where multiple compressors feeding one header can benefit from a slightly oversized motor, such that one compressor can be turned off if the others can carry the load.
- For small reciprocating and diaphragm compressors below 100 HP it is recommended that the motors are rated so that the RMS current under the highest power operating condition does not exceed the nameplate amps. A minimum of 1.15 service factor must be used.
- Some compressor types have a large flywheel or bullgear that increases acceleration time and can be hard for a motor to start, particularly at reduced voltage. Care must be taken motor selection and starting technology.
- A slightly oversized motor can have durability benefits due reduced winding temperatures.
Most compressors, blowers, and fans don't operate at a constant operating point, the power requirements will vary. Typically, the motor is sized for the highest power condition, perhaps with some safety factor or margin (eg. 5 - 15 %). For variable speed centrifugal machines, the power requirements vary approximately with the cube of speed (eg. the "Fan Laws"), so it is possible to get large increases in power requirement with small increases in speed. The motor should be sized for all speeds and inlet conditions that can be foreseen.
Why over size motors - won't this increase motor cost ? There are some very good reasons to slightly oversize drive motors, based on lifecycle costs:
- Cooler motor windings will last for the life of the machine, which should be compared to regular motor rewind costs or buying 2 or 3 motors for the machine over its lifetime.
- Paying a little more to get the most robust motor internals and low resistance windings for a given frame size will maximize motor efficiency and life, and is inexpensive when viewed in the long term.
- What's the value of your UPtime & peak rate ? It's very hard to predict at the time of purchase how a machine or motor will be used over it's lifetime. And most trends are toward getting more product out of the same equipment, so why artificially limit the machine, and your production capability, by buying a motor that cannot run the machine with all foreseeable inlet and discharge conditions ?
Compressor, blowers and fans are all compressible flow devices where power requirements go with inlet density and mass flow. This is a function of inlet pressure, temperature and molecular weight.
Here are some conditions that effect machine power consumption and motor sizing.