Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 118)

4.1.2.4.1 Protection during starting, monitoring of starting time, start interlocking
In addition to protection in continuous duty, protection during motor starting is a central require-
ment because of the high starting currents. Protective response of the protection device before
the motor danger zone is reached is advantageous as long as normal starting is not prevented.
In the case of disturbances (for example locked rotor) the motor would not be loaded up to its
thermal limit and the waiting time for cooling down after removal of the cause of the fault is
reduced. Temperature-measuring protective equipment does not switch off until the trip
temperature is reached, while current-measuring protective devices may trip earlier depending
on their trip characteristic (with electronic relays often adjustable).
Electronic relays offer the option to monitor the starting time. The thermal protection can in this
case be set to the danger limit of the motor, while the starting time monitoring warns or switches
off, if the motor current has not fallen to the operating level in the expected time.
With prolonged start-ups in which the thermal capacity of the motor can be largely exploited, it
may be desirable to not allow starting until the necessary thermal reserve is available. Such a
start interlocking can be implemented with electronic relays that have an output that displays the
simulated heating of the motor, or with temperature sensors. Analog sensors (for example
Pt 100) offer the possibility of setting the desired temperature threshold; in sensors with a fixed
threshold of response (for example PTC) a second sensor unit is required with the desired
response temperature.
Fig. 4.1-8
Unsuccessful starts can be avoided if the motor start is not enabled unless the motor has an adequate
temperature-rise reserve.
1 Trip
temperature
2 Temperature rise during starting
3 If the windings temperature is too high, the start is not enabled as it would lead to tripping
4 Temperature threshold, under which starts are enabled
6 Temperature at the end of a starting that just did not lead to tripping
4.1.2.4.2 Asymmetry protection
When asynchronous motors are supplied with an asymmetrical voltage – with the extreme case
of loss of a phase – a thermal risk results because of the differing windings currents that are
caused by the negative sequence system of the supply voltage. An asymmetric supply voltage
contains such a negative sequence system, that turns in the opposite direction to the normal
symmetrical supply, the positive sequence system, and hence has a frequency of around 100
Hz relative to the rotor of a running motor. These voltage components create in the rotor – and
hence also in the stator – a comparatively large current, similar to the conditions during motor
start-up, when the 50Hz/60Hz supply voltage with the rotor at standstill generates the high
starting current. The current distribution in the rotor due to the high frequency results in heavily
increased losses that can in the long term thermally endanger the motor even at relatively small
voltage asymmetries. In accordance with IEC 60034-1 the rated data of motors are based on a
LVSAM-WP001A-EN-P - April 2009 4-10