8.5.5 Derating for Running at Low Speed
When a motor is connected to a , 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 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.
Maximum load for a standard motor at 40
°
C driven by a type
VLT FCxxx
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.
Table 8.32
8.6 Alarm and Status Messages
A warning or an alarm is signalled by the relevant LED on
the front of the frequency converter and indicated by a
code on the display.
A warning remains active until its cause is no longer
present. Under certain circumstances operation of the
motor may still be continued. Warning messages may be
critical, but are not necessarily so.
In the event of an alarm, the frequency converter will have
tripped. Alarms must be reset to restart operation once
their cause has been rectified. This may be done in four
ways:
1.
By pressing [Reset].
2.
Via a digital input with the “Reset” function.
3.
Via serial communication/optional fieldbus.
4.
By resetting automatically using the [Auto Reset]
function, which is a default setting for VLT Refrig-
eration Drive, see
14-20 Reset Mode
in
VLT
®
Refrigeration Drive Programming Guide, MG16H
NOTE
After a manual reset pressing the [Reset], [Auto On] must
be pressed to restart the motor.
If an alarm cannot be reset, the reason may be that its
cause has not been rectified, or the alarm is trip-locked
(see also table on following page).
Alarms that are trip-locked offer additional protection,
since the mains supply must be switched off before the
alarm can be reset. After being switched back on, the
frequency converter is no longer blocked and may be reset
as described above once the cause has been rectified.
Alarms that are not trip-locked can also be reset using the
automatic reset function in
14-20 Reset Mode
(Warning:
automatic wake-up is possible!)
If a warning and alarm is marked against a code in the
table on the following page, this means that either a
warning occurs before an alarm, or it can be specified
whether it is a warning or an alarm that is to be displayed
for a given fault.
This is possible, for instance, in
1-90 Motor Thermal
Protection
. After an alarm or trip, the motor carries on
coasting, and the alarm and warning flash on the
General Specifications and ...
VLT
®
Refrigeration Drive Design Guide
MG16G102 - VLT
®
is a registered Danfoss trademark
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