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

General Specifications and ...
I (%)
OUT
100
95
90
85
80
0 500
Illustration 8.3 Derating of Output Current Versus Altitude at
T
AMB, MAX
(°C)
45
8 8
40
35
30
0 500
Illustration 8.4 Derating of Output Current Versus Altitude at
T
AMB, MAX

8.5.3 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
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.
186
Design Guide
1000 1500 2000 2500
Altitude (meters above sea level)*
1000 1500 2000 2500
Altitude (meters above sea level)*
Danfoss A/S © Rev. 06/2014 All rights reserved.
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
3000
100
80
60
40
20
0
0 10 20
Illustration 8.5 Maximum Load for a Standard Motor at 40 °C
HO
NO
─ ─ ─ ─
─•─•─•─
‒‒‒‒‒
3000
Table 8.22 Legend to Illustration 8.5
NOTICE
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.
30 40 50 60 70 80 90
v %
Typical torque at VT load
Max torque with forced cooling
Max torque
1)
100 110
MG20Z202