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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 converter'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
This is the way Danfoss is protecting the motor from being overheated. It is an electronic feature that simulates a bimetal relay based on internal
measurements. The characteristic is shown in the following figure:
Illustration 2.6: The X-axis is showing the ratio between I
motor
and I
motor
nominal. The Y- axis is showing the time in seconds before the ETR
cuts off and trips the drive. The curves are showing the characteristic nominal speed at twice the nominal speed and at 0,2x the nominal
speed.
It is clear that at lower speed the ETR cuts of at lower heat due to less cooling of the motor. In that way the motor are protected from being over heated
even at low speed. The ETR feature is calculating the motor temperature based on actual current and speed. The calculated temperature is visible as a
read out parameter in par. 16-18
Motor Thermal in the frequency converter.
The thermistor cut-out value is > 3 k
Ω
.
Integrate a thermistor (PTC sensor) in the motor for winding protection.
Motor protection can be implemented using a range of techniques: PTC
sensor in motor windings; mechanical thermal switch (Klixon type); or
Electronic Thermal Relay (ETR).
Automation VT Drive FC322 Design Guide
2 Introduction to VLT Automation VT Drive
MG.20.X1.22 - VLT
®
is a registered Danfoss trademark
43
2
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