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  • IEC 60034-31:2009

  • 25 –

2/1554/CD

7.8

Applications where load-torque is increasing with speed (pumps, fans, compressors, …)

250

Torque [Nm]

200

150

100

IE1 IE2 IE3 Square-Load

50

0 1000

1100

1200

1300

1400

speed [1/min]

1500

Figure 10

Typical torque versus speed curves for 11kW, 4-pole, three-phase, cage- induction motors and load versus speed curves for -square-loads

As a general rule, high-efficiency cage-induction motors have a lower slip (see table 3), i.e. a higher speed of rotation, than motors of lower efficiency. When the torque of the application is a function of the square of the speed, like in pumps, fans, compressors etc., the increase in speed will lead to an increase in output power (torque) which could in some circumstances defeat the benefits from the improved energy efficiency (see Figure 10).

eff [%]

n [1/min]

M [Nm]

_ P out [kW]

_ P in [kW]

IE1

87,6

1464

75,4

11,559

13,195

IE2

89,8

1474

76,4

11,792

13,131

IE3

91,4

1480

77,1

11,948

13,073

Table 3

Example of changing of efficiency, speed and torque demand with energy efficiency class of three 11 kW, 50 Hz motors in the same application

Therefore, in such applications when a motor of lower efficiency is retrofitted by a motor of increased efficiency, the input power should not reduce as much as anticipated when comparing the efficiencies of the two motors.

In some cases the input power of the high-efficiency motor may actually increase compared to the motor of lower efficiency.

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