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Star/delta And Operation Of Overloads


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Hello andym79


When a motor is started, it draws a very high current for the duration of the start period. The actual current and length of start time is a function of the motor design, the load torque and inertia and the starting method used, but under full voltage starting conditions, the start current will typically be between 550% and 900% of the motor rating.

This overload current will cause a temperature rise in the motor and also a temperature rise in the thermal model employed by the overload relay. If the overload relay thinks that th motor is too hot, it will trip.


Best regards,

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Hi, as i know, overload relays will operate every time that the seting current is overpassed. The thermal protection in a overload relay will act following a curve, so if the current is not to high, it will take a time to act. The magnetic protection act inmediatley wehn a fault occurs, like an earth fault.








Is it also right that overload relays can also operate due to an earth fault, undercurrent and phase imbalance? Or am I confusing my self, as on start up overload/overcurrent is the likely cause?


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