Automatic Adaptations To Ensure Performance - Danfoss VLT Series Design Manual

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VLT HVAC Drive Design Guide
An alternative is to lower the ambient temperature at high altitudes and thereby ensure 100%
output current at high altitudes.
3.6.4. 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.
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 and this limits the torque that can be
supported. 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.
3.6.5. Derating for Installing Long Motor Cables or Cables with Larger
Cross-Section
The maximum cable length for this frequency converter is 300 m unscreened and 150 m screened
cable.
The frequency converter has been designed to work using a motor cable with a rated cross-sec-
tion. 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 earth, and thus an increased earth
leakage current).

3.6.6. Automatic adaptations to ensure performance

The frequency converter 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 frequency converter 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.
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MG.11.B2.02 - VLT is a registered Danfoss trademark
3. VLT HVAC Selection
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