Efficiency
To reduce energy consumption it is very important to
optimize the efficiency of a system. The efficiency of
each single element in the system should be as high
as possible.
Efficiency of VLT 6000 HVAC (
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 f
M,N
, regardless of whether
the motor supplies 100% of the rated shaft torque or
only 75%, i.e. in case of part loads.
The efficiency declines a little when the switching fre-
quency is set to a value of above 4 kHz (parameter 407
Switching frequency
). The rate of efficiency will also be
slightly reduced if the mains voltage is 460 V, or if the
motor cable is longer than 30 m.
Efficiency of the motor (
η
MOTOR
)
The efficiency of a motor connected to the frequency
converter depends on the sine shape of the current. 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 effi-
ciency 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 character-
istic 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
VLT 6000 HVAC (VLT) is multiplied by the efficiency
of the motor (MOTOR):
η
SYSTEM
=
η
VLT
x
η
MOTOR
Based on the graph outlined above, it is possible to
calculate the system efficiency at different speeds.
VLT
®
6000 HVAC Series
202
MG.61.B5.02 - VLT
®
is a registered Danfoss trademark
