VVVF

It's difficult to read your post without proper punctuation.

It appears you are asking what voltage to wire your motors for when you have 415V mains? Why would you think to wire them for anything other than 415V?

sorry about the punctuation, what we have is 415 into the drive and 240 out(all three phase) these motors are 240 delta.

I was wondering will it be better if its 415v in and 415V out and change the motors to star configiration.

¿¿Didn't you say before that the motors were 415Y, 240Δ??

Maybe I'm confused, but maybe not the only one....

My post did look a bit ambiguous. lets start again. MOTOR NAMEPLATE: 415 star/240 Delta. supply voltage: 415V. motor voltage setting on drive: 240V. Motor connection :Delta. I want to change this to: motor voltage setting on drive:415 Motor connection:Star Will this method improve torque through the range.

"My post did look a bit ambiguous. lets start again. MOTOR NAMEPLATE: 415 star/240 Delta. supply voltage: 415V. motor voltage setting on drive: 240V. Motor connection :Delta. I want to change this to: motor voltage setting on drive:415 Motor connection:Star Will this method improve torque through the range."

OK, now we are getting somewhere.

As a general rule, it's not a good idea to run a motor at a low voltage by using the VFD to reduce it. The reason is that although the VFD is perfectly capable of doing it, the actual peak voltage of the PWM pulses is at the 415V conversion level, usually around 585VDC, so the PWM pattern must compensate to lower the RMS to the motor, even at full speed / voltage, by switching a lot more often. This puts a little more thermal stress on the motor windings and increases the switching losses in the VFD. Not enough to cause de-rating, but it is needless stress... in most cases.

But there is a valid use for this concept, if the design of the system is using the VFD's ability to use any output frequency to over speed the motor and you don't want to lose torque in the process. This takes some explaining if you are unfamiliar with VFD concepts.

AC motors create torque based on the design voltage and desired speed, and that torque/speed relationship is the mechanical power, what we call Horsepower or what you IEC folks call mechanical kW (as opposed to electrical kW or absorbed power). Torque then is created by the applied voltage and the current it induces. Speed in the motor is a relationship to the number of poles in the windings and the applied frequency of the AC current. Design torque in a motor is therefore based on the relationship between the voltage and current, what is called the V/Hz ratio. As long as the V/Hz ratio remains constant, the motor torque remains constant. This is the magic behind a VFD being able to maintain "Constant Torque" regardless of speed, it changes the frequency to change the speed and also changes the output RMS voltage to maintain the V/Hz relationship so that torque is not lost.

Now enter the fact that a VFD can increase the output frequency way beyond the original motor design frequency; some VFDs can increase the output frequency to 400Hz for example. To keep it simple, I'll use an example of 120Hz, double the speed of a 60Hz motor. Now I can tell the motor to spin at 120Hz, but what happens to the V/Hz relationship? If I had a 460V motor, the motor design was looking for a V/Hz ratio of 460/60 = 7.67:1 for normal operation. Once I increase the frequency above 60Hz, and my line supply is still only 460V, how can I keep increasing the voltage to maintain the V/Hz relationship? The answer is I can't, so above the base motor speed, I begin to lower the V/Hz relationship and at 120Hz, it is now down to 460/120 = 3.83:1. I have lost half of my running torque and my starting torque, being related to the square of the effective voltage, is reduced to 1/4th of what it would have been normally. So not only have I weakened the motor, it may not even be able to start itself under load!

All this gets us to where you might be and why your motors may be wired for 240V Delta. A "trick-of-the-trade" is to take advantage of a dual voltage motor to "fix" this problem if, for some reason, it is necessary to run the motor at higher than standard speed. So in your case, the motor was designed for 415V 50Hz, a V/Hz ratio of 8.3:1. If connected in Delta, the V/Hz ratio becomes 240/50 = 4.8:1. Now if I use the 415V supply, I can increase my output voltage up to 415V, which when applied at the lower ratio means I can maintain full torque up to 415 / 4.8 = 86Hz. So for example a 4 pole 1450RPM motor can be effectively run up to 2500RPM without losing torque. This also of course increases the mechanical power output of the motor, and the VFD must be sized to deliver the increased current at that speed, but it's no problem for the motor, which is why people do it.

Bottom line then, unless you understand WHY someone was running your motors at 240V Delta from a 415V drive and supply, I suggest not messing with it.

Thanks Jraef, that will explain why the max freq. was set on 80Hz.