Motor Thermal Protection - Danfoss VLT AutomationDrive FC 300 Design Manual

FC 300 Design Guide
Switching on the Output
Switching on the output between the motor and the adjustable frequency drive is fully permitted. Damage to
the adjustable frequency drive will not occur by switching on the output, however, fault messages may appear.
Motor-generated Overvoltage
The voltage in the intermediate circuit is increased when the motor acts as a generator. This occurs in two cases:
The load drives the motor (at constant output frequency from the adjustable frequency
1.
drive), i.e. the load generates energy.
During deceleration ("ramp-down"), if the moment of inertia is high, the load is low
2.
and the ramp-down time is too short for the energy to be dissipated as a loss in the
adjustable frequency drive, the motor and the installation.
The control unit attempts to correct the ramp if possible.
The inverter turns off to protect the transistors and the intermediate circuit capacitors
when a certain voltage level is reached.
See par. 2-10 and par. 2-17 to select the method used for controlling the intermediate circuit voltage level.
Mains Drop-out
During a mains drop-out, the adjustable frequency drive keeps running until the intermediate
circuit voltage drops below the minimum stop level, which is typically 15% below the
adjustable frequency drive's lowest rated supply voltage.
The mains voltage before the drop-out and the motor load determine how long it takes for the inverter to coast.
Static Overload in VVC
When the adjustable frequency drive is overloaded (the torque limit in par. 4-16/4-17 is reached),
the control reduces the output frequency to reduce the load.
If the overload is excessive, a current may occur that makes the adjustable fre-
quency drive cut out after approx. 5-10 s.
Operation within the torque limit is limited in time (0-60 s) in par. 14-25.

Motor Thermal Protection

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The motor temperature is calculated on the basis
of motor current, output frequency, and time. See
par. 1-90 in the chapter How to Program.
Introduction to FC 300
plus mode
MG.33.B3.22 - VLT is a registered Danfoss trademark
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