GE AF-600 FP Design Manual page 150

AF-600 FP Design Guide
7.3 Efficiency
Efficiency of the frequency converter (η )
DRIVE
The load on the frequency converter has little effect on its efficiency. In general, the efficiency is the same at the rated motor frequency f , even if the motor
M,N
supplies 100% of the rated shaft torque or only 75%, i.e. in case of part loads.
This also means that the efficiency of the frequency converter does not change even if other U/f characteristics are chosen.
However, the U/f characteristics influence the efficiency of the motor.
The efficiency declines a little when the switching frequency is set to a value of above 5 kHz. The efficiency will also be slightly reduced if the mains voltage is 480
V, or if the motor cable is longer than 30 m.
Frequency converter efficiency calculation
Calculate the efficiency of the frequency converter at different loads based on the graph below. The factor in this graph must be multiplied with the specific
efficiency factor listed in the specification tables:
7
Illustration 7.1: Typical efficiency curves
The above example shows the efficiency of a 55 kW, 380-480 VAC frequency converter at 25% load at 25 Hz. The graph is showing 0.97 - rated efficiency for a
55 kW AF-600 FP is 0.98. The actual efficiency is then: 0.97x0.98=0.95.
Efficiency of the motor (η )
MOTOR
The efficiency of a motor connected to the frequency converter depends on magnetizing level. In general, the efficiency is just as good as with mains operation.
The efficiency of the motor depends on the type of motor.
In the range of 75-100% of the rated torque, the efficiency of the motor is practically constant, both when it is controlled by the frequency converter and when
it runs directly on mains.
In small motors, the influence from the U/f characteristic on efficiency is marginal. However, in motors from 11 kW and up, the advantages are significant.
In general, the switching frequency does not affect the efficiency of small motors. Motors from 11 kW and up have their efficiency improved (1-2%). This is because
the sine shape of the motor current is almost perfect at high switching frequency.
Efficiency of the system (η )
SYSTEM
To calculate the system efficiency, the efficiency of the frequency converter (η ) is multiplied by the efficiency of the motor (η ):
DRIVE MOTOR
η ) = η x η
SYSTEM DRIVE MOTOR
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