[Al_The_Man]

In a conventional Brushed DC motor the commutator alternately energizes some number of coils, and the amount of current that is drawn is dictated by the supply Voltage, the winding Inductance, the winding Resistance and the period of Time any particular coil is energized.

A conventional 3-phase VFD provides a 3-phase signal to a 3-phase motor. The motor rpm is directly linked to the frequency of the drive signal. The amount of current required is dictated by "rotor slip". Rotor Slip is a measurement of the phase angle difference between where the rotor "should be" vs where it actually is. Under no-load conditions there should be very negligible slip, as the load increases, more current is required to keep the rotor "where it is suppose to be", and the "slip angle" increases.

Fish

I know I came to the game late but I read through all the posts and maybe can make some things clearer.
The brushed DC motor current relies on BEMF to limit the current, with no load, the BEMF voltage approaches the applied voltage and current is minimum, as the load increases, the rpm drops and hence the difference between BEMF and applied Voltage increases and in turn current.
A normal 3ph induction motor can never be synchronous, as it requires slip to induce a magnetic field into the rotor.
the lower the slip, the lower the current, apply a load, the rpm drops and the slip increases in turn as does the current.
The difference in construction between a AC P.M. motor and a BLDC is indistinguishable.
The difference is in the commutation, the BLDC is so named because it represents a DC brushed motor turned inside out, and only two of the 3 windings are energised at any given time.
The P.M. AC has three phases applied to the stator 120deg apart, and can be truly synchronous.
The amount of poles of a BLDC or P.M.AC indicates how many electrical revolutions or cycles there are per mechanical revolutions.
e.g. a 8 pole motor has 4 electrical revolutions per mechanical.
Al.

[JohnZ]

The difference in construction between a AC P.M. motor and a BLDC is indistinguishable.
The difference is in the commutation, the BLDC is so named because it represents a DC brushed motor turned inside out, and only two of the 3 windings are energised at any given time.
The P.M. AC has three phases applied to the stator 120deg apart, and can be truly synchronous.
The amount of poles of a BLDC or P.M.AC indicates how many electrical revolutions or cycles there are per mechanical revolutions.
e.g. a 8 pole motor has 4 electrical revolutions per mechanical.
Al.

I've got to ask a question I have been wondering about for a long time now.

Since a BLDC and P.M. AC motor are constructed in essentially the same manner, can you run a BLDC motor with a VFD designed for a three phase AC motor, and on the flip side, can you run a P.M. AC motor with an ESC designed for a BLDC?

Thanks - John Z

[JohnZ]

I had some time today, so i bread-boarded everything up. The firmware isn't finished yet, but I will prolly "hard code" something just to test the wiring. I have posted updated schematics of the AVR and Power PCBs below. The schematics do not show the RS323 interface, but that is simple enough (in fact I used a small prototyping version that plugs directly into the breadboard). The schematic shows 8 IGBTs, 8 FETs and 8 bypass Diodes, this redundancy is for the PCB so that it can accommodate TO220 OR TO247 devices (NOT BOTH) and the diodes might or might not be required depending on the choice of switch. In the case of the breadboard I am using IRFB4332 Mosfets with a built in drain-source diode, so no external diode is needed. (These Mosfets are rated @ 250V/62A; I am using them because I had some on hand). Also, not shown, is the driver power supply, I will prolly use a 6A 24V line transformer I have for the initial testing. The small power supply you see in the picture is a 30W +/-15V +5V power supply I found on ebay dirt cheap (I bought half a dozen they were sooo cheap, lol). This will supply the +5V and the +15V for the logic/drivers. I just thought I would post this update since someone is actually following, lol, thanks JohnZ ;-)

Fish

Perhaps this may be the sign of a sick mind... er, um ... but ...

Every time I logon to the Zone, the first thing I do is check on this thread to see if anyone has posted. I am very much interested in the progress of this project.

Thanks - John Z

[Al_The_Man]

Since a BLDC and P.M. AC motor are constructed in essentially the same manner, can you run a BLDC motor with a VFD designed for a three phase AC motor, and on the flip side, can you run a P.M. AC motor with an ESC designed for a BLDC?

Thanks - John Z

I have tried the PM AC and BLDC motor with a VFD with not good results!.
A normal BLDC servo motor/drive has hall effect or equivalent commutation feedback.
The only difference I have come across between the AC servo and the BLDC servo was the rotor magnets had curved tops to them on the AC servo?
But usually both can be used in either mode, with suitable commutation.
BLDC demo.
http://users.tinyworld.co.uk/flecc/4...otor031102.swf
Al.

[cd_edwards]

I definitely have an interest in this thread. I currently have a nice little Align 700MX motor sitting on a shelf with the following specs.

- Input voltage: DC11.1V-50.4V
- Max continuous current: 90A/150A(5sec)
- Max output power: 4000W/6600W(5sec)
- KV value: 530KV
- Stator Arms: 12
- Magnet Poles: 10
- Dimension: spindle Φ6xΦ52x57.5mm
- Weight: 405g (prox.)

[tskguy]

I’m sure that 700mx align motor would work very well. I think the issue you will run in to is trying to find a power supply that can give you the rpm your after.

So let’s say you want to drive a spindle like the taig or maybe even a X2 mill
head at a max 6000 rpm. With the way these motors work you would at least want a 2 to 1 ratio so that would be 12000rpm at the motor. To do this you would need a 24v dc power supply that can give you at least 100amps.
It’s for sure possible it just won’t be cheap.
I’ve seen some info about guys converting some old server power supplies but it didn’t really say if they worked or any length of time.
I am actually using a 600mxl align motor that I am driving with a 15 volt supply at 80amps for a little over a kilowatt. It works very well for me so in no way am I steering you away. In fact I even considered your exact motor but I was after over 10000 rpm at the cutting tool so I went for a higher kv motor.

My 2 cents

[ClockMaster]

I have had an idea similar to this and have done modifications to a BLDC motor before. Here is what I had in mind. A direct drive setup using these components.
Motor - HobbyKing R/C Hobby Store : TGY AerodriveXp 160 SK Series 63-64 230Kv / 3150W
Spindle - New Straight Milling Shank C10 ER8 M 150L Collet Chuck | eBay

The assembly would be simple. I have replaced the bearings and shaft in that motor for another project. RPM's would be in the lower range but pleanty of torque is available. The motor can be rewound easily to a different KV.

[Fish4Fun]

Sooooo.....4 years later.....I have fairly thoroughly investigated the hobby-BLDC motors, more to the point, I have worked on various methods of driving these motors...along with a host of other electronics related CNC topics....I am certainly NOT an expert, more of a hack, but I thought I should revisit this thread and report some of my findings for anyone who might stumble across this thread and get this far....

One of the primary reasons I started this thread was the disparity in price between hobby-class BLDC motors vs commercially available VFD Spindles.....since the OP the price/availability of commercially produced VFD Spindles/Drives has dropped significantly.....Currently the price tag on a 3kW water cooled spindle and matching drive is likely less than it would cost to DIY one of similar quality, so I cannot think of any good reason to dedicate the hundreds of hours required to actualize such a project...but, that doesn't mean I shouldn't report my findings....

Drive Methods:

There are three basic ways to drive BLDC motors:

1) Discrete Sensors (Internal or External) to indicate commutation timing
2) Use back-emf to provide "Sensorless" commutation timing
3) Drive the motors completely synchronously.

All three methods require the same Triple Half-Bridge (THB) Analog Amplifier, typically actualized using N-Channel mosfets/IGBTs and some type of High-Side // High/Low Side driver similar to the IR2110 (there are literally hundreds of choices in ICs to implement a High/Low Side half-bridge , each with circuit specific features, but they all allow the use of N-Channel devices for both the low-side and high-side of any given half-bridge in a THB)....

Method 1 & 2 are well documented.....it is important only to note that "Sensorless" designs require some compromises....specifically they require a trapezoidal wave-form to allow a "quiet period" in every cycle on each leg for accurate zero-point-crossing sensing....the slight "loss in power" associated with this "quiet period" is typically compensated for by slightly larger current during the active portions of the cycle....From a physics point-of-view this may at first seem a bit troubling,but in practice the method has proven very reliable, and the nuances of "why" it works are, again, well documented....the fact that most credible firmware used to drive these motors Sensorlessly has built-in methods of dealing with "missed commutation signals" is actually what led me to the third drive method...

Method 3....synchronous operation....in the form of an induction motor was the first type of three-phase motor...originally designed/invented by Tesla....and synchronous 3-phase motors are still the primary industrial workhorses....A typical 3-pahse synchronous motor is designed to operate at a fixed RPM determined by the pole count and the line frequency of the grid....the rotor is "locked" to this rpm and theoretically in a no-load condition only draws enough current to overcome friction and iron/copper losses....as the load increases it draws more current in an effort to maintain the rpm dictated by the line frequency....if the load suddenly decreases the motor actually "outputs power" for a brief period of time....one way of looking at the reason this works is "conservation of momentum".....but it isn't any given motor's momentum that is being "conserved"....the "grid" is comprised of multiple large generators with massive rotors driven by water or steam and "synchronized" with each other by "oscillating" between generating electricity and being "driven" by electricity....assuming a large enough collection of generators are inter-connected....nearly 100% of the mechanical energy input into any particular generator will be "added in phase to the grid"....having large synchronous 3-phase motors connected to the grid actually increases the grid's stability by increasing the rotational momentum....In practice the field windings in commercial generators are varied in response to demand and the mechanical energy input into any given generator is also varied in response to demand.....

Sooo, what do Tesla designed synchronous 3-phase induction motors have to do with modern BLDC motors? A modern BLDC motor uses very powerful rare-earth magnets in place of more traditional rotor windings or "rotor cores"....the use of magnets in the rotor can vastly increase the field strength between the rotor and the stator windings thus increasing the potential power to size ratio while also greatly diminishing the frequency dependance of the motor....In traditional synchronous 3-phase motor design variation from the name-plate line frequency rapidly decreases motor efficiency and output power.....while BLDC motor performance is still bounded by frequency,the boundaries are considerably larger....typical BLDC motors will operate from 30hz to 500hz or more......with output power typically being fairly constant over more than half of that frequency range.....

....And it turns out BLDC motors can be driven synchronously. Way back when I started this thread I was told by several "experts" that BLDC motors simply could NOT be driven synchronously....but no one could tell me 'why'....so in my investigation into BLDC motors, I decided I would also explore various ways to drive them...including to test the theory that BLDC motors could in-fact be driven synchronously.....And the "common" hobby-class, off-the-shelf BLDC motors I have tested 100%, absolutely can, with zero rotor position information used in the drive feed-back loop, be operated synchronously with relatively high reliability.....Of course, not with out caveats....For reliable synchronous operation the drive circuit has to be capable of large currents at start-up....as much as 5 to 50 times the "no-load current" @ nominal rpm (once that rpm has been reached). "Tuning" of the drive wave-form for a particular motor improves the overall efficiency....current feedback and a response algorithm are essential to maximizing efficiency // preventing "locked rotor" failures // responding to variable loads....But all of these caveats are present to some degree or another in both of the other a fore mentioned drive methods. Each method has advantages....the "Sensored" method has the highest reliability factor and the associate highest cost.....the "Sensorless" method is a mature, relatively reliable approach with low cost being a key design parameter.....the Synchronous approach is 'new' (as applied to hobby-class BLDC motors) and from my testing appears equal in reliability to the Sensorless method, indistinguishable from a cost-of-circuit-components perspective (the fact that mass-market sensorless drives are dirt cheap and readily available certainly gives them a cost/time advantage over a one-off//low-volume DIY synchronous driver build....and there will likely always be a much larger market for the "sensorless" drivers than any type of synchronous driver) , with the primary advantage to the synchronous approach being determinate control of rpm, and superior ability to provide variable power at any given rpm....And it is these two characteristics of synchronous drive that made me want to investigate it....so to that end, I am content with my findings, but not likely to pursue actually building a spindle//spindle driver...

While I can see a few "special cases" where DIYing a spindle from a hobby-class BLDC might be justified (geared head for really high rpm applications like PCB milling or low rpm applications like lathes, mills, drills, etc), the vast majority of DIY CNC builds are routers; I think the existing market of available BLDC spindles designed for exactly this demand is generally the best/lowest cost VFD solution. Another area I thought might be a "viable use of hobby-class BLDC motors in the DIY CNC hobby" was "servos" to replace steppers....In most modern industrial CNC machine designs servos have replaced steppers to increase feed rates, precision and provide relatively constant torque over a wide rpm range.....when I began investigating DIY servos from hobby-class BLDC motors the price/availability of production servos were simply beyond my budget...(and most DIY budgets, lol)...if anyone outside of industry could even find a supplier willing to sell them.... this made a DIY servo intriguing to me ....but the price/availability of servos has fallen/increased dramatically.....to the point that I no longer think dedicating the time to developing a DIY approach is worthwhile.

So, in short, I have spent a fair amount of timer over the last 4 years investigating driving hobby-class BLDC motors.... if anyone is interested in the details of the circuits or the firmware, I am more than willing to share my findings..... Please do not view this as an offer to provide a step-by-step set of instructions on how to build a DIY spindle from hobby-class BLDC motors.....as stated, I no longer think this is a cost effective pursuit given the current pricing of off-the-shelf router-spindles and I have certainly NOT designed/built an actual spindle, spindle driver or written the firmware requisite for a fully functional CNC spindle....I have simply designed and built a versatile BLDC motor testing platform and written ad-hoc firmware to test various theories....there would be a LOT of work designing/debugging/testing to transition my testing platform / firmware into any sort of functional spindle/servo driver....not to mention the mechanical design of the spindle/servo itself....But if anyone is determined to DIY a BLDC spindle, my findings could at least help lighten the load....

Fish