HV Motor Tripped

As motor startup with coupled load, it is tripped on Long Time Startup while it is running smoothly on without Load (No Load/Solo Test), what could be reasons??
I am presuming this is a Fan Motor taking time to run up. (more than 30 seconds)
This will cause the overload relay to trip.
Suggest either use a Star/Delta Starter, or better still an Inverter.
The Inverter can be arranged to provide a startup period e.g. 60 seconds.
Some allow a 'boost' to overcome the initial inertia.