Motor Thermal Protection - Danfoss VLT AQUA Drive FC 202 Design Manual

Introduction
dissipated as a loss in the frequency converter,
the motor, and the installation.
2 2
3. In-correct slip compensation setting can cause
higher DC link voltage.
The control unit could attempt to correct the ramp if
possible (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 2-10 Brake Function and 2-17 Over-voltage Control to
select the method used for controlling the intermediate
circuit voltage level.
High temperature
High ambient temperature can cause the frequency
converter to overheat.
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
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
When the frequency converter is overloaded (the torque
limit in 4-16 Torque Limit Motor Mode/4-17 Torque Limit
Generator Mode is reached), the controls reduces the
output frequency to reduce the load.
If the overload is excessive, a current could 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 14-25 Trip Delay at Torque Limit.
38
Design Guide
+
mode
Danfoss A/S © Rev. 06/2014 All rights reserved.

2.16.1 Motor Thermal Protection

Danfoss uses motor thermal protection to keep the motor
from being overheated. It is an electronic feature that
simulates a bimetal relay based on internal measurements.
The characteristic is shown in Illustration 2.26
t [s]
2000
1000
600
500
400
300
200
100
60
50
40
30
20
10
1.6 1.8
1.0 1.2 1.4
Illustration 2.26 Motor Thermal Protection
In Illustration 2.26, the X-axis is showing the ratio between
I and I nominal. The Y-axis is showing the time in
motor motor
seconds before the ETR cuts off and trips the frequency
converter. The curves are showing the characteristic
nominal speed at twice the nominal speed and at 0.2x the
nominal speed.
At lower speed the ETR cuts off at a lower temperature
due to less cooling of the motor. In that way, the motor is
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 16-18 Motor Thermal in
the frequency converter.
The thermistor cut-out value is > 3kΩ.
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).
f OUT = 1 x f M,N (par. 1-23)
f OUT = 2 x f M,N
f OUT = 0. 2 x f M,N
I
M
I (par. 1-24)
2.0
MN
MG20Z202