Automatic Adaptations To Ensure Performance - Danfoss VLT AQUA Drive Design Manual

VLT ® AQUA Drive Design Guide
3.5.4. Derating for Running at Low Speed
When a motor is connected to an adjustable frequency drive, it is necessary to make sure that
the cooling of the motor is adequate.
A problem may occur at low RPM values in constant torque applications. The motor fan may not
be able to supply the required volume of air for cooling, which limits the torque that can be sup-
ported. 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 adjustable frequency drive limits the motor size.
3.5.5. Derating for Installing Long Motor Cables or Cables with Larger
Cross-Section
The maximum cable length for this adjustable frequency drive is 984 ft [300 m] for unshielded
cable, and 492 ft [150 m] for shielded cable.
The adjustable frequency drive has been designed to work using a motor cable with a rated cross-
section. If a cable with a larger cross-section is used, reduce the output current by 5% for every
step the cross-section is increased.
(Increased cable cross-section leads to increased capacity to ground, and thus an increased
ground leakage current).

3.5.6. Automatic adaptations to ensure performance

The adjustable frequency drive constantly checks for critical levels of internal temperature, load
current, high voltage on the intermediate circuit and low motor speeds. As a response to a critical
level, the adjustable frequency drive can adjust the switching frequency and/or change the
switching pattern in order to ensure the performance of the drive. The capability to automatically
reduce the output current extends the acceptable operating conditions even further.
MG.20.N2.22 - VLT ® is a registered Danfoss trademark.
3. VLT AQUA Selection
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