Special Conditions; Manual Derating; Derating For Running At Low Speed; Automatic Derating - Danfoss VLT AutomationDrive Design Manual

FC 300 Selection

4.9 Special Conditions

Under some special conditions, where the operation of the
drive is challenged, derating must be taken into account.
In some conditions, derating must be done manually.
In other conditions, the drive automatically performs a
degree of derating when necessary. This is done in order
to ensure the performance at critical stages where the
alternative could be a trip.
4 4

4.9.1 Manual Derating

Manual derating must be considered for:
•
Air pressure – relevant for installation at altitudes
above 1km
•
Motor speed – at continuous operation at low
RPM in constant torque applications
•
Ambient temperature – relevant for ambient
temperatures above 50°C
See application note MN.33.FX.YY for tables and
elaboration. Only the case of running at low motor speeds
is elaborated here.

4.6.1.1 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)
A problem may occur at low RPM values in constant
torque applications. In a constant torque application s a
motor may over-heat at low speeds due to less cooling air
from the motor integral fan.
Therefore, if the motor is to be run continuously at an RPM
value lower than half of the rated value, the motor must
be supplied with additional air-cooling (or a motor
designed for this type of operation may be used).
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 additional 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.
94
FC 300 Design Guide
Maximum load for a standard motor at 40 °C driven by a
120
100
80
60
40
20
0
Legend: ─ ─ ─ ─Typical torque at VT load ─•─•─•─ Max torque
with forced cooling ‒‒‒‒‒Max torque
Note 1) Over-syncronous speed operation will result in the
available motor torque decreasing inversely proportional with
the increase in speed. This must be considered during the
design phase to avoid over-loading of the motor.

4.9.2 Automatic Derating

The drive constantly checks for critical levels:
•
•
•
•
As a response to a critical level, the frequency converter
adjusts the switching frequency. For critical high internal
temperatures and low motor speed, the drive can also
force the PWM pattern to SFAVM.
NOTE
The automatic derating is different when par. 14-55 Output
Filter is set to [2] Sine-Wave Filter Fixed.
®
MG.33.BD.02 - VLT is a registered Danfoss trademark
frequency converter type VLT FCxxx
0 10 20 30 40 50 60 70 80
v %
Critical high temperature on the control card or
heatsink
High motor load
High DC link voltage
Low motor speed
1)
90 100 110