What is the max current (burst) of the inverter George? Sometimes they can support much higher currents for short period of times.
And can the inverter take a pwm control input?
Do you have links to the specs of the inverter and the spindle?
I believe it has to do with managing the variable frequencies that originate from an alternating current (AC). I believe that is what JPaddie was getting at when he was talking about handling the frequencies from “the mains.” I, honestly don’t know anything about it, and I may be way off. I know only enough about electricity to be dangerous…but I’m trying to learn (albeit very late). 
@Justin_Adie I can get them for you, Justin. That is very kind!
Apologies Justin, I called you JPaddie. I’m assuming that is you
Here are the specs for the VFD:
Description
1.5kw VFD Drive
Technical Parameter
Input Voltage: 220V(We also stock 380V, please contact us for item stock)
Output Voltage: 208-240VAC (analogous to Input Voltage)
Input Frequency: 48-63 Hz
Output Frequency: 0-400 Hz
Input Phase: 1 or 3 phase
Output Phase: 3 phase
Item Manufacture: From a well-known manufacture who manufacture VFD for more than 12 years.
Control Mode:SPWM
Input Power:220V class：220±15%(we also stock 380V item, please contact us for stock)
5-Digits Display & Status Indicator Lamp:Displaying frequency, current, revolution, voltage, counter, temperature, FOR or REV rotation, and fault, etc.
Working Temperature:-10~40℃
Humidity:0-95% Relative Humidity(without dew)
Vibration:Below 0.5G
Feature
Our item adopts the Japan Mitsubishi chip;
PID control; advanced calculus PID closed-loop arithmetic, quick responding speed and high suitability;
Easy PLC function can realize 16 legs of speed and inverter control function;
Carrier adjustable by 16KHZ, completely soundless working environment;
Auto voltage regulation (AVR) technique, for ensuring the inverter load capability;
Various control technique by both synchronization and non-synchronization, dynamically realize fast starting without changing brakes;
ISO9001 quality certifications which means this item is the top quality item;
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Here is what I have on the spindle. Not a whole lot, but here it is:
Features
steel
Ships from CA, United States. 2.2kw Water-cooled Motor Spindle
Four bearing(2C7005+2C7002) in our spindle motor
2.2kw Water-cooling High Speed High Precision Spindle
Water-cooled Er20 Collet Chuck
Pass ISO9001 and CE Quality Certification
Product information
Part Number USTS22SG
Color Silver
Shape Round
Power Source ac
Voltage 220 volts
Wattage 2200 watts
Horsepower 3 HP
Batteries Included? No
Batteries Required? No
Additional Information
ASIN: B011971H90
Customer Reviews: 4.4 out of 5 stars 3Reviews
Best Sellers Rank: 555704
Shipping Weight: 13.23 pounds
Date First Available: July 9, 2015
My best estimate on the above questions:
A VFD converts the AC input to a variable AC frequency that is proportional to the speed you want the motor to run. These are ac muti-phase motors. Higher speed = higher output frequency.
Mains frequency are typically 50, 60 & 400 hz and the drive may need to match the input freq/voltage or it can be configured to be compatible.
Most VFDS I have looked at control using analog speed controls not digital ones [a varying analog voltage promotional to speed]. Often this is 0-10vdc. For a TinyG you would need a PWM to analog converter. You can get them cheap but not sure how linear they are. Then again they probably aren’t any worse than our RIO RAND drivers :).
Not sure about power because it depends on the drives design but I would guess the motor will draw whatever current it needs for the workload its just that you could not get the full load power rating of the motor. It would likely stall above 1.5kw. The key question is what will the drive do if it sees a larger load and stalls? Blow fuse, current limit ??? I suspect that it would current limit as any drive needs to be protected for locked rotor. You could ask the vendor. Also the manuals setup variables might give you a hint.
If there is an online manual, I could take a more detailed look at the configuration specs.
Great! Thanks Don! They didn’t send me the manuals for either, but Im pretty sure I can get the VFD manual online. It’s pretty comprehensive. Fortunately, there is also a lot of videos/tutorials for these as well.
I also saw that another person was building a modified Ox for his machine, which may do the trick. I’ll have to see what y’all think of it handling the weight of the spindle and torque.
Hmm. I typed a long reply but it has been lost. Annoying.
The issue I would see is that to keep the power below the max for the controller you will need to reduce the duty cycle. I’d need a calculator and some paper to check whether that corresponds to lowering or increasing the frequency (depends on the exact wave shape that the vfd emits). Or perhaps the sine wave is fixed at a frequency of choice and the wave form is chopped by the pwm. Just don’t know without seeing the schematic or the manual. Or maybe the speed control is actually voltage and the frequency remains the same.
Whatever the situation there are three things to be cognisant of:
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The motor may not work correctly at frequencies for which it wasn’t designed. I.e. if you buy a European motor intended for 50hz then odd things may happen. Most things in Europe are tolerant from 45-66hz due to differences (historically) between UK and western Europe. And between 220-240v AC.
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Running at a lower voltage may not work as the motor may not generated enough torque. You may also get overheating issues in wires.
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It doesn’t seem you can run the device at full spindle speed since you are outside the power specs. If the vfd is well designed it will shut off rather than risk damage. Potentially cocking up your milling.
So you have to reduce the duty cycle by 2/3. And as above I don’t know the mechanisms as to how the vfds do that. By their name I guess they change the frequency!
Ok, I may just need to get a different one. I’ll get you the info when I get it online. Unfortunately, they didn’t send me the manual
Thanks for all your help. You’ve gone out of your way. I really appreciate it!
@Justin_Adie my understanding of these VFD’s operation is the 3 phase motor is running on a variable frequency (50-400hz) sine wave that is created in the drive from the mains. The speed of the motor is therefore proportional to frequency. The speed is set from the external controller and the drive converts that to a corresponding drive frequency. Power is controlled by sinusodial PWM (or something like that) feed back from the the drives output to keep the motor at that speed.
So I think the motor will run at the programmed speed until the drive is at capacity and then it will stall, fault and shut down or limit. My guess is that its propensity to stall will depend on how hard you push the mill.
That works - is this a triphase spindle?
I assume there are three big output caps to handle the excess load on start up.
And then the question is whether the 2.2kW is the max rating of the spindle (stalled) or the free spinning current. If the former then you might get away with lower consumption for milling light stuff. How are spindles usually rated?
I’d guess the vfds can be set for max current too. In which case perhaps worth setting at 1.5kW and seeing what you can get out of the spindle. I wonder whether some judicious caps between on the triphase hot lines might allow higher power to be used without prejudicing the vfd.
Can you get meaty enough transformers at sensible prices to shift up the US voltage at that current? Reliably of course…
I don’t know, but I think that was what the CNCXR7 guy had when he was converting his Precision Matthews to a CNC. I’ll have to check on it.
@Justin_Adie I just read on Amazon that the spindle is a 4-pole spindle that is converted to 3-phase through the VFD, if that makes sense. Here is the comment: A: This VFD spindle is 220VAC converted to 3-phase at the inverter using three shielded wires the ground is not used ! But they come with four pin connectors. The shield must be grounded on the inverter end of cable onl
Actually you say this is 2.2kW 3A spindle. So that should have told me right off the bat that this is triphase. Silly me.
You can wire triphase in 3 or 4 wire configurations.
@Justin_Adie yes its 3 phase. I believe there is soft start logic that keeps the current per phase low then ramping the % on time of the wave.
Yes. Actually I think that triphase vfd will innately provide soft start with just logic control, as you say.
I guess the http://system.must convert input AC to DC, smooth it then use pwm to chop the DC into a sine like wave form. Or rather three sine like wave forms 120° apart.
Seems obvious now that I think about it properly.
So the motor speed is controlled by frequency and the number of poles.
And this won’t act as a current limiter (until thermistors cut in). So the motor will probably function fine at light loads. I suspect you might see current on each phase of only 1A or so at moderate loads, increasing with the milling medium. So maybe you’ll be ok with 1.5kW if you keep pass depth low.
Is there a significant price bump to a 2.5kW VFD?