Please keep in mind that you are dealing with people who deal with inverters in their day job.
Something it is evident that you have no direct experience of whatsoever.
I don't mean that unkindly, but bear it in mind when you are posting here.You might make fewer gaffes.
You are being given opportunities to learn from those of us who do have direct hands on experience.
For example, the waveforms I posted were captured by me personally.
That's not what the waveforms I gave show. They are repeated* voltage overshoots to use the wording in your NEMA article. These waveforms are not worst case.
Nor are they for long conductors. I already told you that in post #14.
Going back to the NEMA 3.1voltage insulation level. If the original motor is built just to that requirement the insulation withstand requirement would be, as you stated in post #8, 713V.
Good enough for a motor being run at 455V from an inverter? I don't think so. Look at post #12. A repetitive peak of nearly 900V from a 400V inverter.
Now look at post #12. This is fairly typical and certainly by no means the worst I have seen.
Yep, it seems he's just reading things. I can't agree with you more here.
It's not being rude to be imparting knowledge to others in the trade, I guess. Since he failed in his first "guess", I'll just give the correct reasons why "3.1" factor came into that NEMA MG-1 specs.
The idea behind that stipulation (3.1 times) is that term highlighted above by @besoeker: "
overshoot voltage".
The maximum overshoot voltage is that voltage that can possibly occur on the motor line/ motor terminals due to fast-triggering IGBT switches employed in 3rd generation VFDs. See it like this:
The maximum input voltage will be 110% (allowed line voltage fluctuations) Vmax.
Or, we take it as Vmax = 1.1 X Vac.
The maximum DC Bus voltage in the rectifier portion of a VFD will then be = Vmax X SQRT(2) = (Vac X 1.1) X SQRT(2).
V
DCbus = 1.1 X 1.414 X Vac = 1.55 Vac
With motor leads of a certain length, there is an increase in line capacitance (capacitance being a function of frequency) such that if a voltage spike is impressed, the capacitance charges up and later discharges when the polarity of the wave reverses. However, due to the fast frequency of switching, the line cannot possibly discharge what charge it has on a specific period and voltage will pile up. As the voltage repeatedly spikes, the maximum overshoot voltage could be twice that of the maximum DC bus voltage available from the DC link of the VFD. Hence:
Maximum overshoot voltage = 1.55 Vac X 2 =
3.11 X Vac!
Vac, being the input voltage!
Take note that the 0.1 ms wave front is basically the speed at which most 3rd generation IGBT's run. Older types like the SCRs have switching speeds of around 100ms. Didn't you ever wonder why the older types of VFDs do not have problems with dv/dt? That simply is the reason.
With regards to the OP, it is safe to say that 230V motors can be operated at higher than rated voltages because of the existing motor standards manufacturers have agreed upon.
You see, you just have to listen and ask from others. And you get better education here. This is one of the things we do, day-in, day-out and we don't earn if we make mistakes in our jobs.
