9.2.5 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 frequen-
cy 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.
Maximum load for a standard motor at 40
°
C driven by a frequency converter 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.
Specifications
VLT
®
HVAC Drive Operating Instructions
146
MG.11.AD.02 - VLT
®
is a registered Danfoss trademark
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