8.2 Efficiency
Efficiency of the (η
DRIVE
)
The load on the has little effect on its efficiency. In general,
the efficiency is the same at the rated motor frequency
f
M,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 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 480V,
or if the motor cable is longer than 30m.
efficiency calculation
Calculate the efficiency of the at different loads based on
Illustration 8.2. The factor in this graph must be multiplied
with the specific efficiency factor listed in the specification
tables:
Illustration 8.2 Typical Efficiency Curves
Example: Assume a 55kW, 380-480V AC at 25% load at
50% speed. The graph is showing 0,97 - rated efficiency for
a 55kW FC 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 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 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 11kW
and up, the advantages are significant.
In general, the switching frequency does not affect the
efficiency of small motors. Motors from 11kW 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 the
(η
DRIVE
) is multiplied by the efficiency of the motor (η
MOTOR
):
η
SYSTEM
= η
DRIVE
x η
MOTOR
8.3 Acoustic noise
The acoustic noise from the comes from three sources:
1. DC intermediate circuit coils.
2. Integral fan.
3. RFI filter choke.
The typical values measured at a distance of 1 m from the
unit:
Enclosure
At reduced fan
speed (50%) [dBA]
*
Full fan speed
[dBA]
A2 51 60
A3 51 60
A5 54 63
B1 61 67
B2 58 70
B3 59.4 70.5
B4 53 62.8
C1 52 62
C2 55 65
C3 56.4 67.3
C4 - -
D1/D3 74 76
D2/D4 73 74
*
**
* For frame size D, reduced fan speed is at 87%, measured at
200V.
Table 8.8
8.4 Peak Voltage on Motor
When a transistor in the inverter bridge switches, the
voltage across the motor increases by a du/dt ratio
depending on:
- the motor cable (type, cross-section, length
screened or unscreened)
- inductance
General Specifications and ...
VLT
®
Refrigeration Drive Design Guide
MG16G102 - VLT
®
is a registered Danfoss trademark 117
8
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