Sizing Drive Motors for Industrial Compressors, Blowers and Fans - Performance curves & Motor sizing PDF Print E-mail
Written by Gert Dam   
Thursday, 01 December 2011 22:23
Article Index
Sizing Drive Motors for Industrial Compressors, Blowers and Fans
Different conditions effecting required Shaft Power
Performance curves & Motor sizing
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Effect of Inlet Temperature on Centrifugal Compressor/Blower/Fan Performance Curves:

In addition to the density effects presented above, there is an effect of inlet conditions on centrifugal compressor/blower/fan performance curve shape and location.

Something that may not be obvious to users is: vendor-supplied centrifugal compressor/blower/fan performance curves of discharge pressure vs inlet mass flow or volume flow are not "invariant" when inlet conditions change. For example, when buying an air compressor, the curves move with inlet temperature, so a compressor that has a high enough motor HP rating on a hot day may exceed the motor's rating on a cold day. Or increases in inlet and cooling water temperature can reduce turndown and rise-to-surge capability, leading to operational problems. For an air compressor, the biggest driver on performance curve shifts is temperature, which is presented here. For a process machine, molecular weight, Cp/Cv and inlet pressure changes can have even larger effects on performance curve shape and location.

Here is a group of curves showing the effect of inlet and cooling water temperature changes on typical centrifugal plant air compressor performance. Inlet temperature controls the operating conditions into the first stage, but cooling water temperature controls inlet conditions to the following stages. So, inlet temperature (Tin) and cooling water temperature (Tcw) changes are shown here, but it can be assumed that increases in either temperature separately will have a similar, but smaller effect.


From these head vs flow curves it's clear, operating flow range and maximum available discharge pressure are severely impacted with a hot inlet, relative to an average condition. While this is not a motor issue, it does affect the operability of the machine and the ability to potentially save power on a cold day.

Here are the power curves that correspond to the above head curves:


From these curves, it's clear stage inlet temperatures can have a very large affect on centrifugal compressor head / performance curves and power consumption.

For centrifugal compressors and blowers, the vendor can provide performance curves what give discharge pressure and power requirements vs flow, like the above. Even though plant designers may plan to normally operate at a certain point on the performance curve, there may be temporary conditions (startup, shutdown, purge, transient, upset, etc.) were there will be very little resistance downstream of the machine. This leads to operation at the extreme high-flow end of the curve with low discharge pressure and high power. This is called running out to the end of the curve. If there is a chance this will happen, then the motor should be size for the power at the end of the curve and not the power at the normal operating point.

This is why it's highly recommended that all foreseeable operating circumstances and ambient conditions are covered in the machine and motor specification operating point table. It’s very important that users obtain the performance and power curves from the vendor at all the expected operating conditions to be assured the motor is sized for all conditions. If a motor is chosen that cannot run the machine at all conditions, then it is possible to determine from the group of curves what conditions can be run without overloading the motor.

Motor Sizing and Positive Displacement Machine Torque:

Positive displacement compressors (recips., diaphragms, screws, rotary lobes, rotary vanes, liquid rings, scrolls, ,etc.) by their very nature produce an oscillating load on the motor. As each volume of gas is compressed, the torque required of the motor increase to a maximum and then decreases. Often, motors are sized for average torque, so that means the motor will be running over its design load during the peak of the compression cycle. Due to their design, recips and diaphragm compressors can have very large variations in torque over one revolution. Here is an example of a reciprocating compressor curve. Note that the motor rating is below the peak torque for about one quarter of its rotation.


Vendor data sheets will often give the crankshaft power, but not indicate how the torque or power various during rotation or how this relates to the motor torque rating. It is recommended for small positive displacement machines with motors below 100 HP 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 - the motor will run into the service factor for some part of each revolution. Often, vendors do not supply this sort of information, so a specific request should be made to get torque, power or current information to assure proper motor sizing.

Motor Sizing and Integrally Geared Centrifugal, Reciprocating and Diaphragm Compressor Starting Torque:

As many engineers know, reciprocating and diaphragm compressors have large flywheels that require sustained high motor torque during starting to accelerate to full speed. Integrally geared compressors have large bullgears, and high speed pinions, that together act like a large flywheel. Because of these high rotational moments of inertia (high WR^2) and resulting high starting loads, motor selection has to be done carefully.

These large rotational loads, also affect the choice for any soft start methods. With reduced voltage starts, the motor torque goes as the square of voltage, so an 80% reduced voltage start cuts motor output torque to 64% of its normal rating. Due to the reduction in torque available from the motor, and high rotational load, these compressors can take a long time to accelerate. Reduced voltage starts often take 10-30 seconds. This is particularly critical for "captive transformers" used for reduced voltage starting. During this long starting time, the motor inrush current is many times it's normal operating current, so the windings and rotor are subjected to substantial heating. Because of this, acceleration studies for larger compressors need to be carried out when selecting a motor or soft start technology. These studies should also include starts with the highest foreseeable gas load. For centrifugal compressors this is of particular interest as the compressor aerodynamic load goes with the square of speed, which can create a pinch point as the compressor load curve approaches the motor available torque curve, often around 75-80% speed. This is in addition to any system voltage dip studies that would be done for a large motor.

Effect of Motor Sizing on Multiple Compressor Lineup:

There is one other consideration for multiple same-service compressors that see inlet temperature or inlet density variations, like a plant air compressor lineup. If the compressor's motor is designed to run the machine with a cold or dense inlet, then it can handle more mass flow (or SCFM) than with average or hot inlet conditions. This allows for the possibility of shutting down one machine and letting the remaining machines handle the flow. For instance, assume an air compressor lineup of 6 duplicate machines rated for 1000 ACFM, with a plant demand of 4600 SCFM. On an average day (where SCFM ~ ACFM) this would require 4 machines running at 1000 SCFM and 1 running turned-down to 600 SCFM or all 5 compressors running at 920 SCFM. From the table in the section above on inlet temperature effects on power, it can be seen that with a motor sized for a 116% of a 20 degr.C (68 degr.F) inlet will run the machine at - 20 degr.C (-4 degr.F). At this cold day condition, each machine is capable of 1160 SCFM (at 1000 ACFM). So, you can meet the plant demand with only 4 machines running (4 x 1160 = 4640 SCFM). The conclusion from this: it's a good idea for the motor to be sized for the coldest or densest inlet condition.

Motor Sizing and Motor Life:

Slightly oversized motors have other benefits. A larger motor will run cooler and last longer. The usual rule of thumb, is every 10 degr.C (18 degr.F) drop in winding temperature can extend motor insulation life by a factor of two.

It is the recommendation of many electrical engineers that a motor not be sized so that it runs regularly into its service factor. Running above the motor nameplate rating, even with a service factor above 1.0, will sacrifice insulation life. In the next chart, which uses data from the noted IEEE paper, the 1000 HP rated motor has its life cut by 74% when running continuously at 1150 HP, which is the top of it's service factor. And it's clear the larger motor with the 1.00 service factor is the better choice for motor life, given an application that requires 900-1150 HP. This also shows the life benefits for running any motor de-rated.


It is not unusual for vendors to "right size" their motors in standard packaged machines like plant air compressors, which are sold in a very competitive market. Typically, packaged air compressors operate at their rated flow, at Standard Day inlet conditions, and rated discharge pressure with the motor running right at its nameplate rating. That is, there is no extra margin in the motor and the motor will run above nameplate rating under cold inlet conditions with the machine running "wide-open".

The 1250 HP motor in this IEEE paper is only 11% more expensive than the 1000 HP motor, so the extra life is purchased fairly inexpensively. Also, there is relatively little efficiency penalty Relative Winding Insulation Life, factor for running a motor de-rated. Here is the same data, showing motor efficiency. Note that the efficiency loss running de-rated is 1% or less, which is small relative to efficiency gains that can be achieved operating the driven machine more efficiently.