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.

2.16.1. Motor Thermal Protection

This is the way 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 2.5: The X-axis is showing the ratio between I

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

The thermistor cut-out value is > 3 kΩ.

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).

and I

nominal. The Y- axis is showing the time in seconds before the ETR cuts off

motor

Thermal

in the frequency converter.