FC 300 Design Guide
How to Select Your VLT
" Special Conditions
" Derating for Ambient Temperature
The ambient temperature (T
AMB,MAX
) is the ma ximum tempe rature allowed. The average (T
AMB,AVG
)
measured over 24 hours must be at least 9 ºF (5 ºC) lower.
If the adjustable frequency drive is operated at temperatures above 122 ºF (50 ºC), a
derating of the continuous output current is necessary.
" Derating for Air Pressure
Below 3000 ft (1000 m) altitude, no
derating is necessary.
Above 3000 ft (1000 m), the am bient tem peratu
re
(T
AMB
) or max. output current (I
VLT,MAX
)mustbe
deratedinaccordancewiththeshowndiagram:
1. Derating of output current versus altitude
at T
AMB
= max. 122 °F (50° C)
2. Derating of max. T
AMB
versus a ltitude
at 100% output current.
" Derating for Running at Low Speed
When a motor is connected to an adjustable frequency drive, it is necessary to check
that the cooling of the motor is ad equa te.
At low RPM values, the motor fan is not able to supply the required volume of air for cooling. This problem
occurs when the load torque is constant (e.g. a conveyor belt) across the regulating range. The reduced
ventilation available determines the size of the torque that can be permitted under a continuous load.
If the motor is to run continuously at an RPM value lower than half the rated value, the motor must be
supplied with additional air for cooling (or use a motor designed for this type of operation).
62
MG.33.B3.22 - VLT is a registered Danfoss trademark
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