Danfoss VLT AQUA Drive FC 202 Design Manual page 62

System Integration
Example: Assume a 55 kW, 380-480 V AC frequency
converter with 25% load at 50% speed. The graph is
showing 0.97 rated efficiency for a 55 kW frequency
converter is 0.98. The actual efficiency is then: 0.97 x
0.98=0.95.
3 3
Motor efficiency
The efficiency of a motor connected to the frequency
converter depends on magnetising level. 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.
•
The influence from the U/f characteristic on small
motors is marginal. However, in motors from 11
kW and up, the efficiency advantage is significant.
•
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.
System efficiency
To calculate the system efficiency, multiply the efficiency of
the frequency converter by the efficiency of the motor.
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Design Guide
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