Apparently the braking resistor can help with the transients from the back EMF, but not in the current stable firmware. In the next release it will actively monitor the bus voltage and if it starts to spike it will send the current through the breaking resistor. In the mean time I think I am going to get a nice large 60V cap to throw across the ODrive’s power supply terminals. I have also decreased the acceleration / deceleration from 4000 Ve/s to 1000 Ve/S which seems to help a ton.
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BLDC would be smaller, more efficient, and lighter. @Vince.Fab has pretty much torture tested his AC Makita trim router, so their BLDC version is likely up to the task as well (provided it doesn’t overheat). It has collets that support 1/4" and 3/8" (ER11 and ER16 compatible) endmills. ER11 and ER16 collets are generally only available on HF spindles that are overkill for the application. But the HF spindles could be a lot quieter, at least until you start cutting with them.
You might even want to consider using a brushless trim router with its built in speed control and battery.
If BLDC motors are so wonderful, why do most commercial mills use AC motors? There must be some tradeoff here. Is it cost? Or do the advantages of BLDC go away at some point?
The big AC motors are cheaper to manufacture because they don’t need expensive rare earth magnets, they are also ‘self regulating’ with respect to drive current and speed, this gets even better when driven with Field Oriented Control VFDs. The VFDs are also dirt cheap now thanks to some advances in power electronic components.
If you’re not really worried about weight or some inefficiency, you want the cheapest, simplest, electronics possible between your three phase power and your motor and you want high power outputs at relatively low speeds then the good ol’ squirrel cage induction motor is still a decent proposition.
Brushless has taken over for many things, where high speed or low weight are requirements and continues to get better as the drive electronics improve but there’s tradeoffs, such as needing active speed & position feedback from sensors to properly ramp current with applied torque which you just get “for free” with induction motors.
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So I found a page by Tesla of all places that compares them a bit and largely backs up what you’re saying. I think the important things are:
With DC brushless, as machine size grows, the magnetic losses increase proportionately and part load efficiency drops. With induction, as machine size grows, losses do not necessarily grow. Thus, induction drives may be the favored approach where high-performance is desired; peak efficiency will be a little less than with DC brushless, but average efficiency may actually be better.
Permanent magnets are expensive – something like $50 per kilogram. Permanent magnet (PM) rotors are also difficult to handle due to very large forces that come into play when anything ferromagnetic gets close to them. This means that induction motors will likely retain a cost advantage over PM machines.
I think I need to look at actual products and compare them, rather than trying to generalize.
Yep, that was a good post, shame that engineer didn’t continue because they clearly know a thing or two about motors.
In a vehicle size and weight really matter, also, the ability to recover energy from the motor is much more important than in stationary systems (of normal sizes, get into the MW range and this changes).
Changes in controls technology and, critically, the power handling capacity and cost of the power electronic components are substantially changing which motor types are considered optimal in which applications. As is commonly said in the battery world, the next great technology is already here, it just needs $10,000,000,000 in R&D and production scale to equal what we currently have.
This is very much true in the world of the induction motor as smart controls electronics in what is bundled under “VFD” now actively vary the currents induced into the rotor winding to manage the motor efficiency at lower output loads which has given them a new lease of life for installed applications such as fans, fluid pumping, conveyor systems etc.
It’s interesting to look at areas like wind power generation for how they are changing over from old-school, looks like a power turbine in a power station to all electronic systems.
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So Tesla makes an Edison automobile 
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So I upgraded my ODrive to the latest release candidate firmware. It actually simplified my code and seems to be working much better. Only problem now is my RPM seems to max out at 28k. Pretty sure I am running into a “bandwidth” issue and need to tune the ODrive which I still have not done. Keep in mind I have no idea what this “bandwidth” actually is. lol.
In other news my code is working properly. There are a few edge cases I need to figure out. A fun one is that if I plug the arduino into my computers USB port before turning the ODrive on, the ODrive’s microcontroller gets 5V through the arduino and turns on. The 24V bus however is at 0 so the ODrive goes into the undervoltage error state. I need a way to power the arduino from the ODrive in the event I am not using the USB Serial data, but not back feed the power from the USB into the ODrive when I am using the USB Serial data.
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I was only thinking of combining it with the nomad Spindle because the bearings would die a horrible death on their own. It’s also easy to source a higher KV version to hit the 20k RPM sweet spot with ease.
I have 2 6s 12ah lipos I use on my RC lawn mower I built with my friends a few years ago. So I could run for a long time on a single charge with those if I stole my Castle creations Momba Monster Max ESC out of my Emaxx.
Now I might look around to find a spindle cartridge I can make fit my Shapeoko as it might be worth a shot being all I need to buy is a outrunner and pully / belt and machine a mount.
Are you getting enough voltage to the motor?
The vbus voltage is right at 24v. Makita runs this thing at 18v - 20v and is able to reach 30k rpm so the kV rating should be high enough for me to do that on 24v.
My best guess is the sensorless estimator is not able to energize the coils at the right frequency due to some sort of data constraints. I think this is what they mean by bandwidth. I am going to bug the ODrive developer tonight.
And @Vince.Fab is apparently getting >35,000 RPM with his 27.2 V bus (albeit with no fan and apparently without OFC).
Yep and with FOC it should be able to achieve a higher top speed.
In other news, it looks like the version 4 of the ODrive will come in a single channel variant which will reduce the overall cost.
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Alright guys, it’s time to buy another power supply.
24v20a is pretty good for most applications but I think it’s time to push the envelope a bit. Seems like 1000w would do it right? 24v42a
Any opinions on trying to find something around $100?
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Personally I was looking at one of the 24V versions of this:
https://www.meanwell.com/webapp/product/search.aspx?prod=SE-1000
or one of these:
https://www.meanwell.com/webapp/product/search.aspx?prod=PSPA-1000
but they are a bit more than $100 dollars.
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Wouldn’t it be better to move to a higher voltage?
I would HIGHLY recommend putting a temperature sensor on that motor though.
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You shouldn’t be limited by a power supply that size.
Connect 2 big 12V lead acid/other batteries in series and use your current 27V power supply across the string keeping them charged? You should put a diode in series with the power supply to protect it from the battery and motor back EMF voltages.