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3.11.1 Motor Thermal Protection
To protect the application from serious damage, VLT AutomationDrive offers several dedicated features
Torque Limit:
The torque limit feature: the motor is protected for being overloaded independent of the speed. Torque limit is controlled in
par. 4-16
Torque Limit Motor Mode
and or par. 4-17
Torque Limit Generator Mode
and the time before the torque limit warning shall trip is controlled in
par. 14-25
Trip Delay at Torque Limit
.
Current Limit:
The current limit is controlled in par. 4-18
Current Limit
and the time before the current limit warning shall trip is controlled in
par. 14-24
Trip Delay at Current Limit
.
Min Speed Limit:
(par. 4-11
Motor Speed Low Limit [RPM]
or par. 4-12
Motor Speed Low Limit [Hz]
) limit the operating speed range to for instance
between 30 and 50/60 Hz. Max Speed Limit: (par. 4-13
Motor Speed High Limit [RPM]
or par. 4-19
Max Output Frequency
) limit the max output speed
the drive can provide
ETR (Electronic Thermal relay):
The adjustable frequency drive ETR function measures actual current, speed and time to calculate motor temperature
and protect the motor from being overheated (Warning or trip). An external thermistor input is also available. ETR is an electronic feature that simulates
a bimetal relay based on internal measurements. The characteristic is shown in the following figure:
Figure 3.7: Figure ETR: The X-axis shows the ratio between I
motor
and I
motor
nominal. The Y-axis shows the time in seconds before the ETR
cuts off and trips the drive. The curves show the characteristic nominal speed, at twice the nominal speed and at 0.2 x the nominal speed.
At lower speed the ETR cuts off at lower heat due to less cooling of the motor. In that way, the motors are protected from being overheated
even at low speeds. The ETR feature calculates 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 AutomationDrive FC 300.
3 Introduction to AutomationDrive FC 300
FC 300 Design Guide
3-36
MG.33.BC.22 - VLT
®
is a registered Danfoss trademark
3