Motor Hot withstand time 1

[SSKLC]

Motor Specification from one of the reputed consultant in India states following :

For motors with starting time upto 20 secs at minimum permissible voltage during starting, the locked rotor withstand time under hot condition at highest voltage limit shall be at least 2.5 sec more than starting time.
For motors with starting time more than 20 secs and upto 45 secs at minimum permissible voltage during starting, the locked rotor withstand time under hot condition at highest voltage limit shall be at least 5 secs. more than starting time.
For motors with starting time more than 45 secs. at minimum permissible voltage during starting, the locked rotor withstand time under hot condition at highest voltage limit shall be more than starting time by at least 10% of the starting time.
Speed switches mounted on the motor shaft shall be provided in cases where above requirements are not met.

Can someone throw some light on this? Is this coming from IEC/IEEE standard ? because I am not able to find this in any of the standard.

 

[RRaghunath]

IEEE C37.96 Motor protection guide has the following:
"...the time-delay setting should be selected to provide 2-s to 5-s margin above the starting time at rated voltage. Due consideration should be given the integrating effect of the motor-starting current on the relay. When the start time is in the 5-s to 10-s range, a 2-s margin is satisfactory. For a start time of 40 s to 50 s, a margin of 5 s is more appropriate.
When the time delay cannot be set to obtain the desired margin above starting current and still protect the motor (i.e., trip before the motor thermal limit is exceeded), it may be desirable to supervise the TOC with another device (see 5.2.8)."
Hope it helps.