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 Adjustable frequency drive 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.
8.3 Efficiency
Efficiency of the Adjustable frequency drive (η
VLT
)
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
, 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 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, or if the motor cable is longer than 98 ft
[30 m].
Adjustable frequency drive efficiency calculation
Calculate the efficiency of the Adjustable frequency drive
at different loads based on
Figure 8.2
. The factor in this
graph must be multiplied with the specific efficiency factor
listed in the specification tables:
1.0
0.99
0.98
0.97
0.96
0.95
0.93
0.92
0%
50%
100%
200%
0.94
Rela
tiv
e Efficienc
y
130BB252.11
1.01
150%
% Speed
100% load 75% load 50% load 25% load
Figure 8.2 Typical Efficiency Curves
Example: Assume a 75 hp [55 kW], 380–480V AC
Adjustable frequency drive at 25% load at 50% speed. The
graph is showing 0.97 - rated efficiency for a 75 hp [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 adjustable
frequency drive depends on 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. Motors from 15 hp [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 the
Adjustable frequency drive (η
VLT
) is multiplied by the
efficiency of the motor (
η
MOTOR
):
η
SYSTEM
=
η
VLT
x
η
MOTOR
8.4 Acoustic noise
The acoustic noise from the Adjustable frequency drive
comes from three sources:
1.
DC intermediate circuit coils.
2.
Integrated fan.
General Specifications and ...
VLT
®
HVAC Drive Design Guide
MG11BB22 - VLT
®
is a registered Danfoss trademark
8-21
8
8
