Grizzly G0720R Conversion from BLDC to 3-Phase + VFD

ycroosh

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Good part of the day,
Right before Christmas the main power supply board on my Grizzly G0720R milling machine let out some of it's magic smoke. I was ultimately able to fix the board, but the design doesn't make me feel warm and fuzzy. The motor seems to be pretty well made, but the controller is pretty crude. Besides having no back-EMF protection to speak of, it's not driving the motor that well either. As far as I can see on the 'scope, the control logic is pretty basic. I.e. the controller is not doing any timing advancing, etc. It's basically turning the transistors on and off after the encoder pulses. This results in pretty rough-running motor, which (now that I know to notice it) is annoying me. On top of that, I don't trust that it won't just grenade again at the worst possible moment.
Incidentally, I have a brand new 3PH 1HP Baldor motor and a matching Poweflex40 VFD that would fit the machine nicely if mounted upside-down. On paper this would be a downgrade: G0720's spec sheet claims that the motor is rated at 220V/2HP (although I don't believe that number, even when adjusted for Chinese-sized horses). On the other hand, my thinking is that with 1HP 3PH motor with 2-step V-Belt pulleys and VFD would probably give me more usable RPM range. I.e. more torque at low speed when in "low gear" and more top speed in the "high gear". Currently max RPM is 1600, which is too slow for me, since I tend to use 1/2" and smaller end mills, all the way down to 1/16". Moreover, my last mill was rated at 600W (about 3/4HP) and I don't remember feeling that it was underpowered.
All that said, I have a few questions:
1. Given different torque curves of 3PH vs. BLDC motors, would 1HP 3PH motor be underpowered for this size of machine?
2. I noticed that a lot of SIEG machines use toothed belts. Is that warranted at such low HP, or can I get away with either a V-Belt or a "serpentine" belt (I don't recall what the correct term for that is)?
3. Is there any danger in increasing spindle RPM to 3000-ish RPM? I.e. are there any concerns with the bearings that come with this machine, etc.?

I'd appreciate you input
Thank you
Yuriy
 
More some macro level comments as opposed to the details. I previously had looked at this mill and ended up with a BF-30 which has a 3 Hp BLDC motor, 3 speed gearbox and and a usable speed range of around 100-3000 RPM. The 1600 RPM confines you to pretty low speed work. I could stall the 3 Hp BLDC motor on a very deep cuts with a larger endmill, but otherwise it could put out a nice pile of chips. Given that your mill has a 2 HP BLDC, I wouldn't want to go any lower than that, in particular with the wider speed range you are considering. Convention with a VFD conversion on this size machine, one would go with something along the lines of a Marathon BlackMax TENV 2-3 Hp motor. These give constant torque down to ~15 RPM and top out at around 6000 RPM with full Hp. You will not have the low speed cooling issue of a TEFC, and much wider operating window. You still would need two belt speeds. A standard Baldor motor may have a constant torque range down to around 500 RPM, and Hp will start to decline above ~100Hz. In CNC conversions of similar sized bench mills I typically see use of these inverter/vector motors in the 2-3Hp range. There is a larger weight and motor size factor relative to a BLDC. I think there will also be a significant improvement in going to 230VAC volt. It will probably take some extensive modifications to fit a larger motor.

There is a concern about the reliability of the machine specific control boards for these BLDC drives, they seem a common point of failure and the cost of replacement is steep. I knew other owners with the BF-30 that had a control board failure and ended up with a VFD drive. Also reported as a point of failure of some of the Grizzly machine owners in their reviews.

I have a knee mill with a 3 Hp VFD drive, they use a single V belt drive. Most of the belt drives I see in CNC conversions seems to be flat or ribbed belts. Do not see the need for a cogged drive belt in this setting. There are limitations to pulley size and Hp rating depending on the type and size of belt. Example is in this post: https://www.hobby-machinist.com/threads/pm932-cnc-build.21442/page-4#post-285150

Can't really comment on the spindle bearings, on my BF-30 they would get quite hot when pushing 3000 RPM for any length of time. Better spindle bearings are readily available. These are all pretty typical changes made when converting these to CNC. I would review some of those threads.

I think you would be better off selling your mill w/o modifications and possibly looking at something along the lines of the PM-833TV which has a 50-3200 RPM speed range.
 
Thank you. This is very helpful. Just the info I was hoping to get. I didn't think to look at CNC conversion threads. Makes sense...
I'd love to upgrade to the PM-833TV mill (have been eyeballing it for some time) but the resale on G0720 is likely not great, so I'd be paying 2K$+ out of pocket for the difference. I can probably talk my better half into this, but I really want to upgrade my 30-year old Jet 1024 lathe first.
Mechanically my mill is pretty decent. If I spend $600-$700 on the new inverter duty motor (my VFD is actually rated for 2HP already) I'll still be ahead, at least when it comes to the price and in reality the I can probably get at least part of that back by selling the BLDC motor and controller.
In my case mounting a flange-mounted motor would not be that big of a deal.
The cover on SIEG SX4 is thick cast iron and there is over 2" of usable vertical space. I'd need to design some sort of belt tensioning system and cut some access openings. Pulleys I can either make from scratch or modify something from McMaster. Electronics I can deal with pretty easily. In other words, I think this is a manageable project for a few weekends.

Thank you
Yuriy
 
Quick update on my G0720:
I ended up finding a used 2HP Marathon BlackMax motor for a very reasonable price on eBay. It's a bit big but should fit if mounted upside down (sort of how PM-833TV has its motor mounted). Once it arrives I will mill an opening in the top cover, make some sort of adjustable mounting plate, etc.
That said, the repaired board seems to be working very well so far. I ended up replacing all of the output transistors and the gate drivers (for a good measure). I've posted the troubleshooting steps on my blog if anyone is interested, or ends up in the same situation as me.
Interestingly, I think my BLDC motor didn't seem to be timed correctly, so the rotation was not smooth (it was basically rotating in uneven pulsing motion). I connected my scope to one of the motor leads and the corresponding Hall effect sensor leads. The mounting holes on the encoder give it a bit of wiggle room, so I was able to tweak the timing by a small amount. This appears to have made the rotation a bit smoother.
 
I knew at some point someone else would have comments on the drive electronics of the G0720R. Have replaced the drive electronics on a machine at work twice. As you suggested it is a very interesting drive and I share your concern over whether it can make 2HP (it can't for very long) and it has a very "slushy" speed control, varying quite a bit under load. If it were my garage machine, I would do what you are doing.
I expect your bearings to get warm at 2000+. A single V belt should be OK, in fact a little slippage under overload might be desirable as it might avoid some damage.
I think you are on the right track. Looking forward to a picture when installed.
 
A few quick updates:
First of all, I posted the repair instruction earlier today to my blog: Grizzly G0720 Motor Power Supply Repair. Both of my readers already read it, so hopefully, it might be helpful to someone here as well ;)
As far as the 3-Phase conversion goes, I went ahead and ordered a 2HP Marathon BlackMax motor. Found one on eBay that looks barely used for a bit over $200, including an encoder that I can put to good use elsewhere (I can't have too many rotary encoders). That thins is a beast, though. Weight-vise it clocked at 73 pounds, so I think I need to add an air strut to the head or I will probably either burn the Z motor or wear out the lead nut. I'd be curious to hear what y'all think.
Once I get on with the conversion I will post the pictures here (right now I'm up to my eyeballs busy at work and with a new 9-weel old puppy).
Thank you in advance
Yuriy
 
The BlackMax is actually the lighter version, at least the later models the out fins are aluminum. The BlueMax (marine rated version) which is the same internals but all cast iron. What I have seen in the past to offset the head weight is to use either a small chain with sprocket or cable with pulleys going over the head and a counter balance on the back of the mill. I have seen gas rods, but may be a problem depending on the travel.
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