5A might be the safety factor. I looked around online and there are 800W spindles that are rated at 10A max. At 120V this is 1200W. What gives? The spindle speed is directly proportional to the V though, so at 12000 rpm my VFD says ~55V and is using 2.3A. Maybe that is where the power equation is applied, ie with those settings 126.5W is it’s power with 800W max. If V dictates frequency, then at that frequency current would have to increase with a load in order to maintain the same torque.
Spindle speed is created by AC frequency. We need max current at every frequency for this to all be proportional. That is the VFD promise. If there are compromises it will effect these assumptions.
Here is my spindle, it is 10A max. Smw3d preconfigures the VFD for the 800W spindle, but the spindles on their site say 5A so I am wondering if my VFD is configured for a 5A spindle. The only change I’d need to make is raise the current limit, should I raise it to 10A? @Brandon_Satterfield
Ok, so another update. D1-05 shows that the “Inverter rated current” is 8A, and the spindle itself on the case says 15A next to the model number. I don’t see anywhere you can adjust the max current, only protection settings; ie, P0-35 lets you change the motor overload protection current between 50% - 110%.
I’m not sure if there is much I can change on the VFD, and I am curious at that max current 10A on my spindle. Does that mean it is not running at full power unless it is getting 10A? If my spindle was 800W and 5A, and I hooked it up this VFD that shows inverter operating current of 8A, that it would be getting more current than the spindle was rated for?
I need to read up on VFDs and spindles, I clearly don’t know much about them.
I have not done any measurements on my spindle. And, forgive me if I am telling you something you already know but I am going to start at the beginning. Watts = Volts x Amps. We need 880 watts to be full power. The Volts and Amps are not important as long as they add up to the right watts. 88 volts at 10 amps would be fine but so would 7.3 Amps at 120 volts. They work out to the same power at the cutter.
Hmm, in that case it makes sense that my spindle stutters. I have not measured it directly, but when cutting aluminum the VFD reports that the output current is 2.3A @ 55V @ 200 Hz, and sometimes it looks like my spindle stutters (but the current doesn’t change). That would mean that it’s cutting and stuttering with 2.3A * 55V = 126.5W, not even close to it’s max capable power.
The two things I am confused at is on the outside of my VFD it says 15A output, but in the firmware in setting D1-05 it says the inverter current output is 8A. Then when it runs, it pretty much floats between 2.1 - 2.3A reported output (as read by parameter d2-3).
We both need to read up on VFDs. This is the blind leading the blind. 
Some helpful info I got from #linuxcnc on freenode, the max current is simply that; max current before the windings melt. The difference between the 5A and 10A 800W spindles is just that it can handle current spikes better. I’m still confused because as spindle speed goes up, so does voltage, so to be running at 110V it has to be 24K rpm. Does that mean it can only ever reach 880W @ 24K rpm (ignoring the fact that my output current never seems to go above 2.3A)…
The NowForever VFD has a torque boost parameter that best I can tell boosts the voltage at lower RPM for higher torque. I might try this tomorrow and see what it does.
@Brandon_Byrne The 800w spindles now ship with 1.5kW inverters. Old picture on the site.
Love reading down this.
@Colin_Kaminski and @Brandon_Byrne I am glad you guys are digging into the VFD. It actually does way more than what it is set to do. Even the small things make a difference. For example I turn up my acc and decell instantly when setting a new one.
So why did I up the amperage of the spindle and up the VFD to 1.5? I buried one of the old 5 Amps. Upset, sent an email to supplier. Issue fixed I wont run out spindle anymore… Just snap end mills… . 
Some things I just “Cowboy” up on.