Efficiency - Danfoss VLT FC 102 Series Design Manual

General Specifications and ...
Protection and Features
•
Electronic thermal motor protection against
overload.
•
Temperature monitoring of the heatsink ensures
that the adjustable frequency drive trips, if the
temperature reaches 203 °F ± 9 °F [95 °C ± 5 °C].
An overload temperature cannot be reset until
the temperature of the heatsink is below
158 °F ± 9 °F [70 °C ± 5 °C] (guideline - these
temperatures may vary for different power sizes,
enclosures, etc.). The has an auto derating
function to avoid it's heatsink reaching
203 °F [95 °C].
•
The adjustable frequency drive is protected
against short-circuits on motor terminals U, V, W.
•
If a line phase is missing, the adjustable
frequency drive trips or issues a warning
(depending on the load).
•
Monitoring of the intermediate circuit voltage
ensures that the adjustable frequency drive trips
if the intermediate circuit voltage is too low or
too high.
•
The adjustable frequency drive is protected
against ground faults on motor terminals U, V, W.

9.3 Efficiency

Efficiency of the adjustable frequency drive (η
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
M,N
100% of 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 charac-
teristics 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 AC line
voltage is 480 V.
MG11BC22
Design Guide
)
VLT
, even if the motor supplies
Danfoss A/S © Rev. 06/2014 All rights reserved.
Adjustable frequency drive efficiency calculation
Calculate the efficiency of the adjustable frequency drive at
different loads based on Figure 9.2. The factor in this graph
must be multiplied with the specific efficiency factor listed
in the specification tables:
1.01
1.0
0.99
0.98
0.97
0.96
0.95
0.94
0.93
0.92
0% 50%
% Speed
100% load 75% load
Figure 9.2 Typical Efficiency Curves
Example: Assume a 30 hp [22 kW], 380–480 V AC
adjustable frequency drive runs at 25% load at 50% speed.
The graph shows 0.97 - rated efficiency for a 30 hp [22 kW]
adjustable frequency drive is 0.98. The actual efficiency is
then: 0.97x0.98=0.95.
Efficiency of the motor (η
MOTOR
The efficiency of a motor connected to the adjustable
frequency drive depends on the magnetizing level. In
general, the efficiency is just as good as with line power
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
SYSTEM
To calculate the system efficiency, the efficiency of the
adjustable frequency drive (η
VLT
efficiency of the motor (η
MOTOR
η = η x η
SYSTEM VLT MOTOR
100% 150% 200%
50% load 25% load
)
)
) is multiplied by the
):
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