Derating For Running At Low Speed - Danfoss VLT AQUA Drive FC 202 Design Manual

Selection
An alternative is to lower the ambient temperature at high
altitudes and ensure 100% output current at high altitudes.
As an example of how to read the graph, the situation at 2
km is elaborated. At a temperature of 45 °C (T
K), 91% of the rated output current is available. At a
temperature of 41.7 °C, 100% of the rated output current
is available.
I (%)
OUT
100
95
90
85
80
0 500 1000 1500
Altitude (meters above sea level)*
Illustration 3.3 Derating of Output Current Versus Altitude at
T
AMB, MAX
(°C)
45
40
35
30
0 500 1000 1500
Altitude (meters above sea level)*
Illustration 3.4 Derating of Output Current Versus Altitude at
T
AMB, MAX

3.5.3 Derating for Running at Low Speed

When a motor is connected to a frequency converter, it is
necessary to check that the cooling of the motor is
adequate.
The level of heating depends on the load on the motor, as
well as the operating speed and time.
Constant torque applications (CT mode)
In constant torque applications, it is possible for a motor
to draw full current while operating at slow speeds. In
such cases, the cooling fins do not adequately cool the
motor, causing it to overheat. When the motor is operating
®
VLT AQUA Drive FC 202 Design Guide
- 3.3
AMB, MAX
2000 2500 3000
HO
NO
2000 2500 3000
®
MG20Z102 - VLT is a registered Danfoss trademark
continuously at less than half its rated speed, apply more
cooling.
Alternately, an oversized motor can be used to reduce the
load level. However, the size of the motor is limited to one
size larger than that specified by the frequency converter.
An alternative is to reduce the load level of the motor by
choosing a larger motor. However, the design of the
frequency converter puts a limit to the motor size.
Variable (Quadratic) torque applications (VT)
In VT applications such as centrifugal pumps and fans,
where the torque is proportional to the square of the
speed and the power is proportional to the cube of the
speed, there is no need for more cooling or de-rating of
the motor.
In the graphs shown below, the typical VT curve is below
the maximum torque with de-rating and maximum torque
with forced cooling at all speeds.
120
100
80
60
40
20
0
0 10 20 30 40 50
v %
Illustration 3.5 Maximum Load for a Standard Motor at 40 °C
─ ─ ─ ─
Typical torque at VT load
─•─•─•─
Max torque with forced cooling
‒‒‒‒‒
Max torque
Table 3.15 Legend to Illustration 3.5
NOTE
Over-synchronous speed operation results in the available
motor torque decreasing inversely proportional with the
increase in speed. Consider this decrease during the design
phase to avoid overloading the motor.
1)
60 70 80 90 100 110
53
3 3