How to Select Motor for ID Fan?

Selection of ID fan motor

Duplicate thread. See the other one.

So you use feng shui to select motors?

Somebody tampered with this thread, deleting two posts that referenced ...feng... several times each. This led to my "feng shui" remark, which has now been vitiated.

And so, from the peanut gallery, the saga of dumb and dumber continues.

Motor characteristics should match fan characteristics,refer Fan/Motor manufacturers & NEMA

NEMA design codes A,B,C,D etc for motors recommend various types of driven machines like fans(radial,axial etc).C pumps,PD pumps,compressors etc etc for each type of motor.

It is not enough to select a motor based on the load curve alone of the driven equipment. The motor is basically an electromechanical device which draws power from the network and converts electrical energy into mechanical power by developing torque at motor shaft which is transmitted to the driven equipment through coupling. Hence the selection of motor can't be viewed in isolation.

The major parameters which are associated with selection of motor are as under:

i) Starting torque or locked rotor torque: the torque developed by the motor at standstill (s=1)

ii) Rated torque: the torque developed by the motor at rated speed with rated voltage and frequency available at motor terminal

iii) Pull up torque: minimum torque developed by the motor during the process of acceleration

iv) Pull out torque or breakdown torque: maximum torque developed by the motor with rated voltage and frequency available at motor terminal

v) Speed torque characteristics of the motor based on the above four parameters

vi) Starting duty: No. of allowable cold and hot starts per hour

vii) Hot and cold thermal withstand curve of the motor.

viii) Load inertia (GD² value) with respect to motor shaft

ix) Load curve or speed torque characteristics of the driven equipment

Item no. i) to vii) are inherently motor parameters which should be taken into consideration while designing the motor to drive a load with characteristics defined by item viii) and ix).

Unless the mechanical load has a very high inertia with a typical speed torque characteristic (say ball mill, crusher, and centrifugal blower) normally standard motors (NEMA design A or B) should suffice. But these parameters should be checked before putting the motor into service. Proper setting of the overload relay of the DOL starter should also be looked into considering the starting time, starting duty and thermal withstand curve of the motor.

Will load & rotor inertia influence starting current & time taken to accelerate to rated speed.In NEMA motors of design codes A,B,C,D etc what are mechanical differences (weight,diameter,inertia etc of rotor & depth,size,resistance of rotor bar,slot shape,size etc)

Loading of the motor as well as rotor & load inertia will influence the duration of acceleration of the motor but not the starting current.

The four NEMA designs have unique speed-torque-slip relationships making them suitable for different type of applications:

NEMA design A

Maximum 5% slip , high to medium starting current, normal locked rotor torque , normal breakdown torque suitable for a broad variety of applications - as fans and pumps

NEMA design B

Maximum 5% slip , low starting current, high locked rotor torque , normal breakdown torque, normal starting torque suitable for a broad variety of applications - as fans, blowers and pumps

NEMA design C

Maximum 5% slip , low starting current, high locked rotor torque , normal breakdown torque, suitable for equipment with high inertia starts.

NEMA design D

Maximum 5-13% slip , low starting current, very high locked rotor torque suitable for equipment with very high inertia starts.

To take care of these special characteristics, the design of stator and rotor including the slot design, rotor bar design, rotor length, diameter and the weight will be different for different designs. These are proprietary design data of the motor manufacturers and not available in published catalog.

In Rockwell automation-Allen & Bradley Glossary of motor terms, equations of current, torque speed inertia,friction torque etc are given.

Easy. Pick up the phone and talk to the fan manufacturer, then get them to do it.

Next thread, please?