Hi I am trying to learn as much as I can about motors and starters!

I have a question, what can make the the overloads operate when a star delta three phase motor starts?

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,

Thanks,

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?

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.

Chris.