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Improving Machinery Reliability
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Motor Part Load Efficiency
as a Function of % Full Load Elficiency

Figure 6-5. Motor part load efficiency as a function of percent of full load efficiency.
(Source: DOE E€& Handbook.)

This notion may not necessarily be correct, since motor efficiency in the working
range (50%-100%) is relatively flat for motors over 1 HP. This is illustrated in Fig-
ure 6-5. Also, because larger motors are inherently more efficient than smaller ones,
a 50 HP motor constantly running at 50% load consumes even less energy than a 25
HP motor running at 100% load. Here is our proof

Performance Characteristics of Typical 50 HP
and 25 HP Motors-230 VAC (E)

50 HP 0 1A Load 25 HP @, Full
Load
Amps (1) 75.0 64.0
Power Factor (PF) .70 .827
Efficiency 89% 88.5%

Energy Consumption (kW) =fix Ex I x PF + 1000
50 HP kW = fix 230 X 75 x .70 + 1000=20.91 kW
25 HP kW = fix 230 X 64 x 327 f 1000 = 21.08 kW

On the other hand, in applications in which the motor load cycle includes high
idle time, energy costs will be greater with the 50 HP motor that will consume 70%
more power.