Motor-generated Overvoltage
The voltage in the intermediate circuit is increased when the motor acts as a generator. This occurs
in following cases:
1.
The load drives the motor (at constant output frequency from the frequency converter),
ie. the load generates energy.
2.
During deceleration ("ramp-down") if the moment of inertia is high, the friction is low
and the ramp-down time is too short for the energy to be dissipated as a loss in the
frequency converter, the motor and the installation.
3.
In-correct slip compensation setting may cause higher DC link voltage.
The control unit may attempt to correct the ramp if possible (par. 2-17
Over-voltage Control
.
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 frequency converter keeps running until the intermediate circuit
voltage drops below the minimum stop level, which is typically 15% below the frequency conver-
ter's lowest rated supply voltage.
The mains voltage before the drop-out and the motor load determines how long it takes for the
inverter to coast.
Static Overload in VVC
plus
mode
When the frequency converter is overloaded (the torque limit in par. 4-16/4-17 is reached), the
controls reduces the output frequency to reduce the load.
If the overload is excessive, a current may occur that makes the frequency converter cut out after
approx. 5-10 s.
Operation within the torque limit is limited in time (0-60 s) in par. 14-25.
2.15.1. Motor Thermal Protection
The motor temperature is calculated on the
basis of motor current, output frequency, and
time or thermistor. See par. 1-90 in the chap-
ter
How to Programme
.
2. Introduction to VLT HVAC Drive
VLT
®
HVAC Drive Design Guide
48
MG.11.B2.02 - VLT
®
is a registered Danfoss trademark
2
Содержание VLT series
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