A system, method, and device for protecting an induction motor are
disclosed. The exemplary system may have a module for determining the
current drawn by the motor and a module for determining the state of the
motor. The system may calculate a used thermal capacity based on a first
formula when the motor is in an active state. The system also calculates
the used thermal capacity based on a second formula when the motor is in
an inactive state. When the used thermal capacity attains a threshold,
the relay is tripped thus removing current to the motor and preventing
motor from overheating. A method to derive thermal time constants from
desired trip time limits (such as those defined by IEC standards or the
thermal limit curves provided by motor manufacturers) is also presented.
For example, the thermal time constants can then be used in tacking motor
used thermal capacity throughout various motor states.