A motor will produce the same power in star and delta provided the correct voltage is applied for the configuration. Typically a motor running 415v in star will need about 240v in delta, there is often a bit of variation on these exact figures as the line voltage across Europe varies although we nominally agree on 230v this is rarely adhered to as the tolerance is fiddled to embrace 220v to 240v. The new building I worked in was set to 230v but 254v was in some older areas, I remember a nut feeder did not like the elevated voltage and we had to install a transformer for it. Although it may have been possible to adjust the tapping of the main transformer it would have caused chaos to a lot of other equipment.

Mike

Wiring a motor Star or Delta does not affect the rated power output of the motor. It can affect its starting performance under load but that was not the question. The key point is that the current per phase is lower for the star connection than the delta.

With a VFD it i important to note that the mechanical output power is reduced when running the motor at lower speeds. This is because power is proportional to speed an torque and torque in a motor is proportional to current. You can't drive more current than the rating so power drops with speed. Conversly power increases with speed for the same torque / current.
Any motor being run on a VFD should be geared for the highest speed possible.
The limiting factor for this is the insulation of the windings. This is because the voltage must be boosted to mainntain the same current and torque at higher speeds.
Some motors desigined for VFDs have improved insulation to allow for this or are designed for maximum output at 400Hz and line voltage this gives higher output speed and a much smaller motor for the same output. Aircraft use 400Hz or higher to take advantage of the significant weight savings (transformers for 400Hz are much smaller too).

Robert G8RPI.

One thing to note with VFDs and 3 phase motors is that the number of poles the stator has determines the natural RPM of the motor at 50Hz, pro rata if frequency changes. So nominal speed of a 2 pole motor is 3000 (actually slightly lower as some slipage is required for the magnetic field to drive the rotor round). A four pole nominally runs at 1500rpm and a 6 pole at 1000rpm. It is important to match the motor to its requirement, if you get a 2 pole motor and end up running the VFD at 20hz to get a useable speed at the workpiece you have the wrong motor.

The √3 is the ratio of 240V to 415V (approximate voltages). If you draw 3 equal lines from a single point with each line at 120° to the next the ratio of the line length to the end to end length is √3. The 3 lines like this is the star wired motor. This means that putting 415V across the end points will result in 240V across each winding. When the motor is set up as a delta wired motor the 240V across the terminals is now applied directly to each coil. This is why the 240/415V motors we home workshop users use with UK mains are wired in delta as the single phase supplied VFDs output is 240V across each terminal (unless you pay more for something different). If you have 3 phase as supplied by the mains it will have 415V between each pair of terminals so the motor gets wired in star configuration.

Small 3 phase motors often have direct on line (dol) starters but larger ones have star/delta starters to reduce inrush currents at start up. It just gets more complicated when things get bigger.

Martin C