motor cool time constant

tem1234 · Jan 14, 2010

Hi guys,

i'm trying to set the correct cool time constant motor running and motor stopped on a GE multilin 469.

On the motor data sheet, the info is:

Thermal copper time constant (short time altern.) : 3.6 min.
Thermal time constant (long time load altern.): 20 min.
Thermal time constant for cooling down (standstill): 140 min.

As i see it, the cool time constant stopped is 140 min, but i'm not sure about the running. What's "long time load altern." standing for?

We often set the running constant 1/2 of the stopped constant where info is missing.

Any idea?

Thanks

motorman · Mar 15, 2010

Customers often ask for cooling time constant for our machine, where MULTILIN protection is used. The time constant should be defined as removing the heat to a level of 1/e (i.e. 36.8%)…or 63.2% total heat removed.

The temperature drops at an exponential rate. The time constant is based on temperature (rise) dropping to 1/e of the total rise. The time for drop in pre-defined level of temperature rise may be reverse calculated. With elapsed time, the temperature changes per the following:

Tt = Th * e(-t/(tau))

where, Th = total temperature rise
t = elapsed time
tau = cooling time constant
Tt = remaining temperature rise

Following typical cooling time constants, regardless of the motor size, can be used.

Running cooling time constant is 45 minutes (10 minutes for 20% drop in temperature rise).
Stopped cooling time constant is 100 minutes (22 minutes for 20% drop in temperature rise).

dpc · Mar 15, 2010

motorman,

So the mass of the motor is not a factor at all? That surprises me.

David Castor

http://www.cvoes.com

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motor cool time constant

tem1234

Electrical

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dpc

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