power supplied to the electrical power consumed during operation is called
the motor efficiency,hmotor. Therefore, the electric power consumed by the
motor and the mechanical (shaft) power supplied to the compressor are
related to each other by (Fig. 7–76)
(7–92)For example, assuming no transmission losses, a motor that is 80 percent effi-
cient will draw 1/0.8 1.25 kW of electric power for each kW of shaft
power it delivers to the compressor, whereas a motor that is 95 percent effi-
cient will draw only 1/0.95 1.05 kW to deliver 1 kW. Therefore, high-
efficiency motors cost less to operate than their standard counterparts, but
they also usually cost more to purchase. However, the energy savings usually
make up for the price differential during the first few years. This is especially
true for large compressors that operate more than one regular shift. The electric
power savedby replacing the existing standard motor of efficiency hstandardby
a high-efficiency one of efficiency hefficientis determined from(7–93)where rated poweris the nominal power of the motor listed on its label (the
power the motor delivers at full load) and the load factoris the fraction of
the rated power at which the motor normally operates. Then the annual
energy savings as a result of replacing a motor by a high-efficiency motor
instead of a comparable standard one is(7–94)The efficiencies of motors used to power compressors usually range from
about 70 percent to over 96 percent. The portion of electric energy not con-
verted to mechanical energy is converted to heat. The amount of heat gener-
ated by the motors may reach high levels, especially at part load, and it may
cause overheating if not dissipated effectively. It may also cause the air tem-
perature in the compressor room to rise to undesirable levels. For example, a
90-percent-efficient 100-kW motor generates as much heat as a 10-kW resis-
tance heater in the confined space of the compressor room, and it contributes
greatly to the heating of the air in the room. If this heated air is not vented
properly, and the air into the compressor is drawn from inside the compressor
room, the performance of the compressor will also decline, as explained later.
Important considerations in the selection of a motor for a compressor are
the operating profile of the compressor (i.e., the variation of the load with
time), and the efficiency of the motor at part-load conditions. The part-load
efficiency of a motor is as important as the full-load efficiency if the compres-
sor is expected to operate at part load during a significant portion of the total
operating time. A typical motor has a nearly flat efficiency curve between half
load and full load, and peak efficiency is usually at about 75% load. Effi-
ciency falls off pretty steeply below half load, and thus operation below 50%
load should be avoided as much as possible. For example, the efficiency of a
motor may drop from 90 percent at full load to 87 percent at half load and 80
percent at quarter load (Fig. 7–77). The efficiency of another motor of similarEnergy savingsW#
electric,savedAnnual operating hours 1 Rated power 21 Load factor 211 >hstandard 1 >hefficient 2W#
comp^11 >hstandard^1 >hefficient^2W#
electric,savedW#
electric,standardW#
electric,efficientW#
electricW#
comp>hmotor396 | ThermodynamicshmotorhmotorWelectricWelectric = Wshaft/hmotorWshaftMotor
efficiencyElectrical power
consumed per kW of
mechanical (shaft)
power output,100%
90
80
70
60
50
40
30
20
101.00 kW
1.11
1.25
1.43
1.67
2.00
2.50
3.33
5.00
10.00FIGURE 7–76
The electrical energy consumed by a
motor is inversely proportional to its
efficiency.hmotor, %10203040Motor
efficiency5060708090100(^020406080) 100 Load, %
FIGURE 7–77
The efficiency of an electric motor
decreases at part load.
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