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Efficiency of the frequency converter (η VLT)
The load on the frequency converter has little effect on its
efficiency. In general, the efficiency is the same at the
rated motor frequency fM,N, even if the motor 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.
Efficiency of the motor (ηMOTOR )
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 (ηSYSTEM)
To calculate the system efficiency, the efficiency of the
frequency converter (ηVLT) is multiplied by the efficiency
of the motor (ηMOTOR):
ηSYSTEM) = η VLT x ηMOTOR
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