Dear Team,

We are using a condensing fan in a unit input voltage is 380v but wn we are running it on the site its giving 600rpm as per fan specification at delta connection it will give 900 rpm we check the connection also its was delta. When we check this fan in our factory again we 900 rpm as per our desired.
I am confused why is this happening

Where are you from? What is the correct operating voltage for your motor. Is the motor single phase or 3 phase. What is the rpm rating on the nameplate of the motor?

Motor rpm in an induction motor is in direct relationship to the poles and the frequency of the power source (50 or 60 cycle)of the motor, not the voltage. Low voltage etc, will cause the motor to overload and the overload will slow the motor down to a point where it should trip out on onverload because of current rise. I would assume you are starting the motor across the line and not using and inverter or start - run where you are starting in delta and then switching to Star to run. I need some input.

A 2 pole motors synchronous speed is 3600 rpm.
A 4 pole motors synchronous speed is 1800 rpm.
A 6 pole motors synchronous speed is 1200 rpm

These are typical motors in the US based on 60 cycle AC power If you are in another country and using 50 cycle power, the speeds will be different.

Connecting motors in Delta or Star requires that you have all 12 leads accessible. A motor that comes wired Delta is generally less common than the motors wired in Star, but is used for different reasons. The windings of a Delta motor are basically a series connection, where as a Star connection is a parallel connection. Thus currents and starting torque will be different, but the rpm rating should be the same. As a note you will not run a standard motor at its synchronous speed, but somewhat lower. The "slip" between the synchronous speed and the actual speed created torque or current rise if you want to look at it that way.