DC motor voltages used on battery lawn mowers; 40, 60 vs 80 affect blade torgue? - Page 1
Forum section: electronics / projects

author: videobruce
author_rank: Frequent Contributor
timestamp: April 25, 2019, 12:48:52 pm
content: Background on current battery operated lawn mowers;The current marketing hype seems to center around the voltage of the battery packs,notthe 'watt-hour' capability (which it should be). As with all battery operated devices, especially L-I, run time and torque (in this case) or lack of, are the biggest problems.The 'motor' used in these mowers rotation speeds appears to be similar with many; 2800 low, 3300 rpm high. The common battery voltages are 40, 60 & 80/82 volts. There are also 36 & 56v, but those are with limited models. They all are Lithium-Ion (no mention which of the six specific types they use of course).The question;Is a 80v system better for torque than a 40v system with the same watt/hr battery capability (40v 4 amp vs 80v 2 amp, both equate into a 160 watt/hr pack)??

author: DDunfield
author_rank: Regular Contributor
timestamp: April 25, 2019, 01:01:10 pm
content: You can design a motor to meet a certain torque spec. no matter what voltage it runs on (within reason).I'd expect the higher voltage motors to be slightly better in terms of efficiency, as IsquaredR losses in windings and wiring should be less.Dave(No actual experience with lawnmower motors specifically).

author: Siwastaja
author_rank: Super Contributor
timestamp: April 25, 2019, 01:09:27 pm
content: Completely irrelevant number; an internal implementation detail which doesn't matter much. Of course, as typical with marketing, meaningless numbers are given a lot of bullshit marketing weight.In a similar way, you can have a 12V 500W light bulb, or a 230V 15W light bulb.

author: Siwastaja
author_rank: Super Contributor
timestamp: April 25, 2019, 01:11:06 pm
content: Quote from: DDunfield on April 25, 2019, 01:01:10 pmNope, this is wrong - assuming same weight of copper in the windings, to get the same A*turns and torque, the I^2R loss is the same regardless of the design voltage...Quote... but this is true, assuming the same copper weight in wires. So, thicker cables must be used to compensate.

author: Zero999
author_rank: Super Contributor
timestamp: April 25, 2019, 01:42:40 pm
content: And guess what thinner wiring will be lighter, therefore boost efficiency, in a self-propelled lawnmower.

author: videobruce
author_rank: Frequent Contributor
timestamp: April 25, 2019, 02:42:48 pm
content: The reason for the initial question was, is a 80v motor better than a 60 or 40v for torque.A did run across a couple of comment that claimed a 80v motor is stronger (not the term used) than a 60 (or 40v). I kinda questioned that. This didn't come from importer marketing data, just a user.I understand current vs voltage, but 'line loss' in not a issue here, but without any specific internal specs on the actual motors (other than a couple of importers do state rpm's), I realizes there can be variances with motor designs.BTW, "Global Tool Works (Group)" manufactures Snapper & Greenworks and possible one other known name in the US.

author: james_s
author_rank: Super Contributor
timestamp: April 25, 2019, 03:46:31 pm
content: Higher voltage allows more power within reasonable constraints of current but it's really just marketing fluff. The only number that really matters to the user is the horsepower and they don't give you that.

author: Siwastaja
author_rank: Super Contributor
timestamp: April 25, 2019, 04:20:53 pm
content: System voltage choice is really a optimization thing, but there is no clear sweet spot in general; it's a fairly wide range, with multimodal sweet spots all over the place. Furthermore, these spots live with market situation, etc. It comes down to:* What MOSFETs/IGBTs are available at the time, at what prices (Vdsmax * Rdson / price, basically)* Motor dimensions discretize the winding choices: i.e., have 10 turns, or 11 - can't practically have 10.5.* Magnet wire availability and choice, can't have any arbitrary thickness, so number of paralleled strains discretizes as well...But the sweet area tends to be quite large, as can be seen from the fact that 500hp muscle EVs have been built as sub-100V systems, and OTOH small 20kW drivetrains designed with voltages over 700V.So the actual choice is far from trivial, but on the other hand, many different choices work almost equally well, it's small optimization.I'm assuming a lawn mower is around 1kW, maybe 2kW max. For this power, the battery-inverter-motor system can be sanely designed between about 30 to 150V, I'd say, with little difference over the resulting efficiency or performance.QuoteAssuming total wire length (battery to inverter, inverter to motor) of around 500mm, and 1kW power, between 40V (25A) and 80V (12.5A), the difference would be tens of grams.

author: james_s
author_rank: Super Contributor
timestamp: April 25, 2019, 04:27:10 pm
content: Things may have changed recently but in the cordless electric mowers I've dealt with there is no inverter or silicon at all, just a permanent magnet DC motor with carbon brushes. I have an older one that I converted to LiPo power replacing the old lead acid batteries.

author: videobruce
author_rank: Frequent Contributor
timestamp: April 26, 2019, 07:52:24 am
content: "May have changed"??Yes they have, at least 5, if not 10 years ago. Gone are old school carbon brushes. They all are brushless with electronic sensing speed control (hence the 2800/3300 RPM I stated). They all have a PCB for operation. I have had my carbon brush, cordless mower for over17 yearswhich is why I'm looking for a replacement.With what that has been posted here and reading elsewhere, maybe I should ask; is the any disadvantage of going with a lower voltage design with the same watt/hr rated pack equivalent?

author: james_s
author_rank: Super Contributor
timestamp: April 26, 2019, 02:56:51 pm
content: Mine is probably about that old, but it works so I haven't bothered to look st replacing it.Forget the voltage, really it's irrelevant to the end user. Read reviews on Amazon and select a mower that is well rated then shop around for the best price.

author: NiHaoMike
author_rank: Super Contributor
timestamp: April 26, 2019, 03:24:19 pm
content: What I'm surprised about is the lack of power assisted reel mowers, which are far more energy efficient. The actual power needed must be on the order of 100W or less given that fully manual ones exist.Quote from: Siwastaja on April 25, 2019, 04:20:53 pmI'm pretty sure Tesla uses 400V battery packs.Quote from: videobruce on April 26, 2019, 07:52:24 amWhat's the reason for replacing it?

author: Siwastaja
author_rank: Super Contributor
timestamp: April 26, 2019, 04:22:31 pm
content: Quote from: videobruce on April 26, 2019, 07:52:24 amFrom thedesign viewpoint, when within sane limits, no, but extremes do have disadvantages. For a battery lawn mower, 40, 60 and 80V are all clearly within sane range, and any difference will be negligible. 10V would be clearly problematic, I think, as would 400V.But because there is not much difference, nothing prevents them from resorting to "design-by-marketing" approach - since a good designer can build the thing at almost any voltage (within certain range and certain options), marketing can design the portfolio of voltages based on non-technical reasons, and the engineering provides. Simply put, need a 40V lawn mower? No problem, pay me and I'll design it for you. Need 100V? I'll do it. They'll perform the same. The 40V version may be even better or cheaper, I don't know without full schematic and BOM level design for both.But, from theconsumerviewpoint, there might be a clearcorrelation, that 80V mowers are more powerful, or provide more torque than 40V mowers; especially within one manufacturer. This works as long until someone gets the idea to build a very weak product with high voltage battery to get the marketing benefit. This actually happened to some extent on cordless drills; big players like BOSCH, Makita, etc. had (and still have) quite a good correlation of voltage vs. capability, but many cheap brands do use surprisingly high voltages to play the game.

author: videobruce
author_rank: Frequent Contributor
timestamp: April 26, 2019, 06:42:58 pm
content: Quote from: NiHaoMike on April 26, 2019, 03:24:19 pmAge,what appears to be shorter operating time even with a battery replacement,it's a bear to take inside each Fall since the batteries are not easily removed.For the last 12-15 or so years, the newer mowers all have removable battery packs since they aren't gel-cells anymore.

author: james_s
author_rank: Super Contributor
timestamp: April 26, 2019, 10:00:25 pm
content: I removed the two large gel cells from mine after they wore out and replaced it with a harness that I plug in either 2 or 4 3S 2200mAh LiPo packs which is more than enough to mow my small lawn. For me it was a great solution because I already had the LiPo packs and charger for my RC airplanes.I suspect the main reason drills were going higher and higher voltage is they were using sub-C NiCd/NiMH cells so the max power out of a cell was pretty much capped. Adding more cells was the only way to get more power and if you're adding cells you may as well put them in series and get more voltage rather than parallel which would require heavier wiring and controls.

author: flangefrog
author_rank: Newbie
timestamp: April 27, 2019, 07:01:36 am
content: Quote from: NiHaoMike on April 26, 2019, 03:24:19 pmHere's one I hacked together in a few hours. It was very very slow due to using an ebike hub but it worked. Took 200W.