Well, the range of the overload has to embrace the full load current of the motor at the operating voltage, Boss, which is given on the rating plate permanently attached to the motor. Simply set the overload to that figure.

Should we assume this is a homework question?

The idea of homework is you do it yourself.

The Basics Of Selecting Overload Relays

The ONLY correct answer is, whatever the manufacturer of the overload relay says in their instructions / manual.

Some manufacturers already factor-in the "pick-up point" and want you to set the OL dial to exactly the motor nameplate FLC, some do NOT factor in the pick-up pont and require YOU to decide on that when setting the dial. Some are ambient compensated, some are not and the adjustment must be factored by you for differences between the motor and controller ambient temperature differences.

Bimetallic overload relays respond to increased heating, but can't respond fast to rapid change in current condition. They are less sensitive to phase loss (particularly if the motor is lightly loaded) and motor stalled condition. They typically are unable to compensate for ambient temperature variations without constant adjustment. Hence these relays are not very suitable to protect motors which are critical in process plant application.

Alternatively, solid state type over current relays can be used for critical motor applications. Rather than relying on the thermal characteristics of the bimetal relay, the electronic over current relay (EOCR) measure the actual current drawn by the motor. This current is used to calculate the i²t heating condition and also eliminates the time lag inherent in thermal devices.

The EOCR has inherently inverse definite minimum time (IDMT) characteristics and can be set to trip the motor if it takes 105% or 110% of rated current for longer duration (typically more than one minute) and will not trip the motor during starting condition unless the locked rotor condition continues for longer duration (say more than 10 sec depending on the load inertia). This device also reacts much faster in single phasing condition. The higher sensitivity with respect to single phasing condition can be enhanced by providing a negative sequence current filter. EOCR is completely unaffected by thermal changes in the environment.

The relay is marginally costlier as compared to the bimetallic thermal overload relay. Its setting is linear (not in steps) and flexible.

The problem if the OL has instantaneous setting that will trip the motor during starting. Maybe we can extend the response time of the relay not to response with the inrush current during starting.

Thanks for the info.

sir, can you define clearily about I2T curve......