Start delta starter of an extraction fan is cutting power when it suppose to switch to delta after a certain time. I think my timer relay has a problem, so I want to replace it. I understand that the switching over supposed to be done when a motor reached 80% of its rpm, but I don't know how to set timer relay to achieve that. If are there any calculations involved, please assist. How can I set the time?
The switch-over time could be calculated by calculating the time to accelerate the load. To do that, you need to know the load inertia and torque vs. speed profile. You also need to no the motor torque vs. speed capability with the delta connection. Even a crude estimate would provide a starting point.
Without calculating, you could use trial and error. Set the timer to something less than the time it takes for the overload relay to shut the motor off. Initiate start while measuring current and observing the speed. Use a stopwatch to measure the time it takes for the current to drop significantly and the speed to reach 80%.
You could use a current sensing relay, but a fan is not likely to change characteristics over time, so all you really need to do is determine the time once. The starter should work fine with the proper time setting without needing to change the design to a current sensing system.
With full motor nameplate data, you could make a good estimate of the time required using the assumption that the motor capability is matched to the fan requirement.