Page 350 - Handbook of Electrical Engineering
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PROTECTIVE RELAY COORDINATION 337
Figure 12.15 Protection devices for high voltage motors.
12.7.1 Overloading or Thermal Image
2
As with most electrical power equipment the thermal characteristic is based on an I t law. The
equation for the thermal image as given by IEC60255 Part 8 when the motor is cold is,
2
I
t c = T th log e 2 seconds
2
I − I o
Where I = Relay current as a multiple of the nominal current, pu.
I o = Reference current in pu that determines the position of the asymptote, e.g.
at t c → 10,000 seconds. I o has a typical value between 1.015 and 1.065.
T th = Thermal time constant in seconds, usually given in minutes for a particular motor.
Note: The equation is only valid for I> I o .
A similar equation is used for the hot condition of the motor,
2 2
I − I p
t h = T th log seconds
e 2 2
I − I o
Where I p = motor load current before the overload, pu.