Efficiency - Danfoss FC 300 Design Manual

FC 300 Design Guide
FC 300 Selection

Efficiency

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Efficiency of FC 300 Series (η )
VLT
The load on the adjustable frequency drive has little effect on its efficiency. In general, the
efficiency is the same at the rated motor frequency f , even if the motor supplies 100% of
M,N
the rated shaft torque or only 75%, i.e., in case of part loads.
This also means that the efficiency of the adjustable frequency drive 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 line voltage is 500 V, or if the motor cable is longer than 98.43 ft. [30 m].
Efficiency of the motor (η )
MOTOR
The efficiency of a motor connected to the adjustable frequency drive depends on magnetizing level. In general,
the efficiency is just as good as with line 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 adjustable frequency drive and when it runs directly on line power.
In small motors, the influence from the U/f characteristic on efficiency is marginal. However, in
motors from 14.75 HP [11 kW] and up, the advantages are significant.
In general, the switching frequency does not affect the efficiency of small motors. Motors from
14.75 HP [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 FC 300 Series efficiency (η ) is multiplied
VLT
by the efficiency of the motor (η ):
MOTOR
η ) = η x η
SYSTEM VLT MOTOR
Calculate the efficiency of the system at different loads based on the graph above.
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MG.33.B6.22 - VLT is a registered Danfoss trademark