Danfoss VLT Series Design Manual page 66

3. VLT HVAC Selection
3.2. Efficiency
Efficiency of VLT HVAC (η
The load on the adjustable frequency drive has little effect on its efficiency. In general, the effi-
ciency is the same at the rated motor frequency f
3
shaft torque, or only 75% 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 greater than 5 kHz.
The efficiency will also be slightly reduced if the line voltage is 480 V, or if the motor cable is longer
than 98.43 ft. [30 m].
Efficiency of the motor (η
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 15 hp [11 kW] and up, the advantages are significant.
In general, the switching frequency does not affect the efficiency of small motors. The efficiency
of motors from 15 hp [11 kW] and up improves by 1-2%. This is because the sine shape of the
motor current is almost perfect at high switching frequency.
Efficiency of the system (η
To calculate the system efficiency, the efficiency of VLT HVAC (η
of the motor (η
η
SYSTEM
Calculate the efficiency of the system at different loads based on the graph below.
66
)
VLT
MOTOR
SYSTEM
):
MOTOR
) = η x η
VLT MOTOR
MG.11.B4.22 - VLT
, even if the motor supplies 100% of the rated
M,N
)
)
VLT
® is a registered Danfoss trademark.
VLT ® HVAC Drive Design Guide
) is multiplied by the efficiency