Motors Delta/Star connections to VFD

For a given motor, the fundamental difference between the star and delta connections is that the rated voltage for the star connection is 1.731 X the rated voltage for the delta connection. That way a motor can be designed to run at either of two voltages, 240 V, delta or 415 V, star for example. If you connect the motor delta and connect it to 415 V, that is too much voltage for the design. The motor will draw excessive current and overheat. If you connect the voltage star and connect it to 240 volts, that is not enough voltage for the design. Since torque capability is proportional to the square of speed, the motor will only be able to produce about 33% of rated torque without running slow and drawing too much current. With a VFD the effect would be the same, the motor will produce less torque per amp of current.

For normal operation of a motor with a VFD, there is no choice, you must connect the motor for the voltage that matches the VFD voltage rating. The maximum VFD output voltage rating is normally the same as the input power voltage rating.

What could be done with a VFD is to use the delta motor connection with a 415 V VFD, but set the VFD to have an output of 240 V at 50 Hz and 415 V at 86 Hz. That would increase the power rating of the motor X 1.731 because the motor the nearly the same torque capability at a high speed. There would be some increase in motor losses, so the motor's torque capability is reduced somewhat. There would be some concern about bearing life and motor vibration, so it is not a good idea to operate a motor that way without checking with the motor manufacturer.

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.