Motor Thermal Time Constants

I would write to Siemens giving motor name plate details and request for complete design data. Generally, they oblige.

Alternative is adopting typical values that can be obtained from some text books, relay manuals. One such reference is SPAM150C (ABB make) relay manual. Depending on the motor rating and cooling arrangement, the manual indicates typical heating & cooling time constants.

But, be aware that this data is only approximate and accordingly, the protection settings based on such data will be a compromise.

This cooling time constant can be obtained from your motor manufacturer.

It can be something like 60-120mins depending on the design.

is the heating time constant of 3 minute i calculated ok?

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I'm not familiar with this type of motor protection system....

Wrong! For a 3.3kV, 450kW motor, the typical heating time constant would be around 30 minutes.

You may get access to motot data sheet. You will get all the required details therein.

Raina

Heating and cooling time constants are generally given by the motor manufacturer.

This is the time required by the motor to reach 63.2% of its steady state temperature T when run with full load.

Heating time constant (t) is given by the formula (1-1/e) T= (1-1/2.718) T= 0.632T

If the steady temperature T attained by the motor is 90 degree C (say) when started from cold condition, the heating time constant in minutes will be the time required by the motor to reach 57 degree C. This can be noted during the heat run test on the motor under rated condition.

Also refer http://store.gedigitalenergy.com/FAQ/Documents/M60/GET-8420.pdf