Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 121)

Multiples of current setting Type of overload relay
A B
Reference ambient air
temperature
Thermal, compensated for
ambient air temperature
variations or electronic
Not phase loss sensitive
3 poles
1.0
2 poles
1.32
1 pole
0
+20 °C
Thermal, not compensated
for ambient air tempera-
ture variations
Not phase loss sensitive
3 poles
1.0
2 poles
1.25
1 pole
0
+40 °C
Thermal, compensated for
ambient air temperature
variations or electronic
Phase loss sensitive
2 poles
1.0
1 pole
0.9
2 poles
1.15
1 pole
0
+20 °C
Tab. 4.1-5
Tripping threshold values of three-pole overload relays with phase-loss protection in accordance with IEC
60947-4-1 am 2
Column A
No tripping within 2 hours (from cold state)
Column B tripping within 2 hours subsequently to the test according Column A
Copyright © IEC, Geneva, Switzerland, www.iec.ch
Protective shutdown with the outage of a supplying phase is desirable to prevent damage to the
motor and because continuation of normal operation is in any case impossible. A motor start
with phase-loss is not possible, because three-phase induction motors do not develop any
torque with phase loss at standstill.
Phase failure protection is largely standard with current-measuring protective relays. Tripping is
usually accelerated (bimetal relay with sensitivity to phase failure) or briefly delayed (electronic
relays). Complex electronic protective relays are often equipped with a sensitive asymmetry
protection that takes into consideration the risk to the motor from the negative sequence
system.
Protective equipment with sensors in the windings protects against overheating of the windings
due to phase-loss and also to asymmetry. As they measure the temperature in the stator
windings, they are not able to detect a specific risk to the rotor. They do not respond until the
threshold temperature is reached.
4.1.2.4.4 Stalling protection
In applications, in which drives are stalling from normal operation or can be heavily overloaded
(for example stone crushers, calenders), it may be required that the application immediately
shuts down or issues a warning signal when a high load occurs before the thermal protection is
acting. This is to protect the mechanical transmission elements or to early rectify the fault and
prevent long waiting times after a thermal protective shutdown.
Electronic relays measure the motor current and often offer a stalling protection function with an
adjustable threshold with respect to the current and trip delay. In order to make motor start-up
possible, stalling protection is not activated until after start-up.
LVSAM-WP001A-EN-P - April 2009 4-13