Page 357 - Improving Machinery Reliability

Extending Motor Life in the Process Plant Environment 323

50 HP Q Idle 25 HP Q Idle
Amps (1) 47.8 25.0
Power Factor (PF) .OR .09

Power Consumption (kW) = 6 x E x I x PF + 1,000
50 HP kW =fix 230 X 47.8 x .08 + 1,000 = 1.523 kW
25 HP kW = fix 230 X 25.0 x .09 + 1,000 = .896 kW

Even so, looking at a typical application requiring the motors to operate 4,000 hours
per year-half under load (2,000 hours) and half at idle (2,000 hours)-with a cost per
kWh of $.06, the net energy savings with the 25 HP motor would be only $54.

Energy Cost ($) = kW x total hours x $/kWh

haded:
50 HP cost = 20.91 x 2,000 x .06 = $2,509
25 HP cost = 2 1.08 x 2,000 x -06 = $2,530
50 HP motor savings = $21

Idhg:
50 HP cost = 1 .523 x 2,000 x .06 = $183
25 HP cost = 2396 x 2,000 x .06 = $108
25 HP motor savings = $75

($75 - $21 = $54 net annual savings with 25 HP motor)
This is a small price to pay for the knowledge that the 50 HP motor has a much
larger thermal cushion than the smaller motor and will be far more able to withstand
unforeseen overloads.

Keep Bearings in Mind
Rotor shaft bearings locate the rotor and maintain the desired air gap between the
rotor and stator. In addition, they often support a superimposed radial load from a
pulley, sprocket, or rotary tool, such as a saw blade or grinding wheel, attached
directly to the motor shaft. Depending upon the size of the load and the capacity of
the bearings, motor electrical performance, heat buildup, and bearing life can be seri-
ously affected.
Bearing wear from overload or lack of lubrication leads to increased bearing fric-
tion and greater horsepower demand. The resulting heat buildup-more current
means greater watt losses-can consume the motor’s thermal cushion and lead to
insulation failure.