This is more of a follow up, describing where I got up to in my exploration of the subject matter. As such, it's turned into another epic read. Apologies for that but there was rather a lot of detail to cover.
Way back in the mists of time (Tue, 17 Jul 2018 18:35:11 +0000 to be precise), I created a thread with the subject : "Electric start for suitcase inverter generator by using its alternator as a BLDC motor?".
In fact that post is so old now, all bar my final follow up to the original thread is now expired in my news client (Pan). I've saved the content of that final follow up for reference (and posterity).
In essence, I was seeking advice on how best to create an add-on BLDC starter motor module to drive the 6 or 7 pole pair outrunner[1] three phase circa 300vac[2] alternator (used to generate (after fullwave rectification and smoothing) the circa 400vdc which powers the 50Hz
230vac sinewave inverter module's class D bridged amplifiers) as a dc brushless starter motor from a 12 or 24 volt starter battery pack[3].Briefly, it had occurred to me that the permanent magnet alternator used in my little 99 quid Parkside PGI 1200 B2 inverter genset could quite easily double up as a starter motor given a suitable BLDC motor controller (a modern electronic form of the classic "Dynastart"(tm) setup as used on some small marine engines around half a century back).
I was just trying to bring this idea up to date, hence my starting the original thread to gather opinions and ideas from a group who seemed more likely to have had some experience in the art of designing or using BLDC controllers to help me in my mission.
Rather encouragingly, after starting that thread, I came across a US patent application for exactly this idea (patent # US7180200B2) with a grant date of 2007-02-20 and a priority date of 2002-06-06 which rather validates my "Neat Idea" which I'd wanted to test for myself.
After getting over my surprise at seeing the unpatentable being patented, my next thought was, "If these guys (Richard T. Walter, Michael K. Forster and Shailesh P. Waikar) had been looking to expand the market for their employer's (Black and Decker Inc) rechargeable battery packs way back in 2002 and were granted an actual patent way back in 2007, why the Hell weren't we at least seeing electric start as a feature (optional or standard) of even the most commoditised of inverter gensets by now?
After all, the concept is such a no brainer enhancement of the utility of portable inverter gensets that the likes of Honda should have been incorporating this feature a decade ago and had that "patent" rendered null and void on the basis of a total lack of novelty (other than for the novelty that anyone would think that assembling a bunch of standard circuit modules together to sell more battery packs was an actual patentable idea).
Possibly Black and Decker Inc were just hedging their bets and looking to get a cross licence agreement with Honda to act as their official battery pack supplier should Honda ever decide to offer electric start options in their smaller portable inverter genset range. A lot of the existing components in the inverter module could pull double duty between
50/60Hz 230/120 vac inverter mode and 60/30 vdc driven BLDC switching mode with the most expensive addition being a 60 or 30 vdc converter module powered from a standard 12v SLA or a 3s or 4s LiPo battery pack along with an AS5047 magnetic position sensor bolted to the end of the genset's crankshaft to drive the PMG as a BLDC starter motor.Anyway, harking back to the original thread, I'd hoped to be able to achieve this goal without the complication of adding Hall effect sensors to the PMG, electing to rely upon sensorless operation of the BLDC controller alone. However, this idea was doomed to failure as I discovered when I finally got round to buying a cheap Chinese 50v 16A sensored/sensorless BLDC controller module with reversing option and analogue speed control input to experiment with.
The 16.8v (4 cell LiPo battery voltage) 20A rated R/C ESC module I had been gifted by my SiL to run my initial tests with (I'd made a simple servo tester from a 555 timer chip to drive the ESC module) couldn't get the generator to spin even with the spark plug removed, I could hear it's initialisation beeps and 'growls' as it attempted to drive the PMG but no visible motion came of its best efforts.
I guess the PMG was too much of a mismatch to the small propeller motor characteristics the ESC was designed to work with. The cheap Chinese BLDC module had better luck driving the PMG but only with the spark plug removed. Even using a 48v volt battery pack (51v) borrowed from my SmartUPS2000 which produced a nice and steady 480rpm at 400mA load on the
51v battery failed to make it rotate through the compression stroke with just my fingertip 'sealing' the spark plug hole.So, with Lasse Langwadt Christensen's advice about using three HALL elements rather than sensorless to overcome the issues of low speed and uneven mechanical loading from the engine, ringing in my ears, I concluded that only a sensored control that was rigidly locked to within a fraction of a degree of the shaft's rotation would be able to cope with the job being asked of the BLDC module.
module had been a free gift) on the cheap Chinese BLDC module was mitigated by the fact that it did at least possess the necessary Hall sensor inputs to allow me to go further with my experiments which left me pondering on the viability of adding the necessary Hall sensors to the PMG.
After searching the internet for more ideas on this, I came across the rather wonderful AS5047 magnetic position sensor which, with a suitable diametrically magnetised disk magnet on the end of a motor shaft, could recreate (after calibration) the effect of having three perfectly aligned Hall effect sensors configurable for motors with from one to seven pole pairs (as well as absolute position data) accurate to a fraction of a degree.
The claim made by AMS, the manufacturer, of "Superior accuracy and stability over the traditional three Hall sensor setup" seems to be well justified as its use in a powered skateboard adaptation to a VESC module demonstrated (just google AS5047 + VESC to track down the relevant youtube video) so this would seem to be the way to go forward on this project, especially if I can persuade AMS to supply me with a sample of the AS5047D-TS_EK_AB, Adapter Eval Kit (with chip and a 6mm dia magnet) for free. :-)
I've just checked AMS's website and it looks like only the chip itself, priced at $6.50 (damn Opera's VPN - I forgot to turn it off, I'm sure
option. The AS5047D-TS_EK_AB, Adapter Eval Kit (with chip and a 6mm dia magnet) only seems to be available at a single unit price of $15.75 - ISTR
Still, considering the undoubted performance improvement it will bring to my project, a modest doubling up of the BoM costs for a relatively trivial and the most doable of hardware modification possible, doesn't seem all that high a price to pay this close to the end of the day.
The fact that, without the compression loading (plug removed), I was able to get it cranking at a steady 480rpm using a 51v battery pack (just about spot on with my initial back EMF to speed ratio estimates) was a rather encouraging result. The only fly in the ointment so to speak, being that, as per Lasse Langwadt Christensen's suggestion, the project is proving to be a lot harder to implement than my initial optimism had suggested.
The irregular torque loading is proving to be far too demanding for a wishy washy sensorless control algorithm that's trying to guess when to commutate to the next step from zero crossing detections alone that require a minimum (and a not too wildly varying) speed for any success.
I'm going to try and solve the problem of mixing a 50v rated BLDC with a
400v PMG by using a couple of small 12vdc coiled 240vac 10A rated DPCO contact set with a 25ms release time spec relays to isolate those two component parts in a hopefully timely fashion. I'm guesstimating I'll have about 50 to 60ms grace before the voltage rises to a dangerous (to the BLDC module) level once the engine starts up.I also reckon I can speed up the relay release time by some 5 to 10ms by using an RC network with a high voltage switching transistor to give an operate impulse that will reduce the hold in current and allow the use of a high value snubber resistor to speed up the inductive discharge for a snappier decay of magnetic pull on the relays' armatures. There'll still be good old fashioned mechanical inertia to contend with but at least it won't be aggravated by any more electrical inertia than I can minimise with my drive circuit.
Furthermore, I can also switch a low value resistor across the output of the 50v dc-dc converter I plan on using in the finalised design to load down the PMG voltage being rectified by the body diodes in the 3ph Half bridge transistors in my 50 volt BLDC module. Initial tests using a 48v SLA battery pack won't need this refinement since the battery will be able to handle any such short lived charging events.
All of this extra complication with isolator relays could be neatly avoided as per Lasse's suggestion of using a high voltage 3ph half bridge driver module with a simple 800v 8A blocking diode between the driver module and the dc-dc converter's output but, unfortunately, such a solution doesn't come cheap. The 50v BLDC module and a pair of isolating relays is a pragmatic choice aimed at keeping the BoM costs to a minimum for what I hope will become a a project that will appeal to the more adventurous DIY enthusiast.
I'm sure that if the inverter genset manufacturers (including those who make commodity versions) see the light, they could easily integrate the required high voltage gubbins into the existing inverter modules for very little cost, leaving only the fitting of a AS5047 magnetic position sensor onto the end of the generator's crankshaft as the single most costly BoM item (with the 50 to 60 volt DC-DC converter integrated into the main inverter module being the second most costly item).
If the manufacturers modify the inverter modules, they can save the cost of the sensor module by offerng recoil start only economy versions to retain the cheaper end of the market whilst offering their customers the option to purchase an electric start upgrade kit[4] at a nice profit.
If they, the manufacturers, play their cards right, they might also be able to offer their customers a LiPo or LiIon battery pack upgrade to the 'self-dischargy-might-not-have-enough-charge-to-start-the-generator-next- time-you-need-it' SLA battery. :-)
[NOTES] [1] I have yet to determine the actual pole pair count. All I know at this stage is that a pole pair count of 6 or 7 is fairly typical of the class of alternator used by portable inverter petroleum (gasoline) powered generators. It's a fairly trivial (though a bit of a fiddly) exercise in "Pole Counting" which I haven't yet gotten round to doing. [2] The actual rms voltage required depends on the waveshape which might be more trapezoidal than sine in character (trapezoidal would be more suited to producing a DC voltage after full wave rectification with minimised ripple content). The important point being, regardless of wave shape, that the controller/inverter module lands up with a circa 400vdc rail to power its 50Hz 230vac inverter stage. [3] Even 24 volts may be insufficient. Testing with a 51 volt battery pack provided a speed of 480rpm with the spark plug removed using a cheap (12 quid delivered from China) 50v 16A rated sensorless/sensored BLDC module designed to drive electric scooter motors and the like, rated up to 360W. However, just blocking the spark plug hole with a fingertip was enough to stall the controller when relying on its sensorless control algorithm.I'm pretty confident that adding an AS5047 magnetic position sensor with suitable magnet attached to the end of the crankshaft of the engine/ alternator assembly to synthesise the required Hall sensor commutation signals (more accurately and reliably than the typical Hall sensor setup on sensored BLDC motors) will allow that cheap BLDC controller to pull the engine through the initial compression stroke (even if perhaps rather arthritically if it happens to be stopped in the wrong position) allowing it to reliably accelerate to a respectable cranking speed in time to crank through subsequent compressions with reasonable alacrity.
The AS5047 magnetic position sensor modification is the most practical way to upgrade the alternator to an accurately commutated hall effect sensored BLDC motor. I just need access to the pull starter end of the crankshaft rather than have to pull the flywheel/outrunner off the crankshaft in order to wedge three hall effect sensors into the stator assembly and just hope for the best with fingers crossed.
[4] The newer models of electric start gensets will, of necessity, include the mechanical modifications required to readily accommodate the sensor board and magnet. The cheaper pull start only variants will simply be lacking the sensor module, requiring that the customers who later opt for the upgrade merely have to bolt the module onto the end of the generator housing and plug a multi-core fly-lead into the waiting connector on the inverter module (and perhaps install a starter switch/ remote control sensor assembly to complete the upgrade).The manufacturer gets to both eat and have their cake - the potentially gone to waste unused components in the inverter module would be a very small cost investment against the very strong possibility of selling a highly profitable electric starter upgrade kit to their customers.