Although, by the time you add brushless controllers for the amps and voltage for the bigger brushless ones, you could probably do conversion with an S28-150 Magmotor (http://www.robotmarketplace.com/products/MAG-S28-150.html), two car batteries and 40v 100amp SSR from jaycar, for cheaper then the brushless setup. _________________ https://www.halfdonethings.com/
Sat Jan 15, 2011 12:26 am
bytraper
Joined: 31 Oct 2005
Posts: 195
Hmm, Thanks man, I'll have to look into the controller side more and see whats available.
Sat Jan 15, 2011 9:27 am
Nick Experienced Roboteer
Joined: 16 Jun 2004
Posts: 11802
Location: Sydney, NSW
You will need much more than 600W for a lawn mower, that first motor Aaron suggested is more like it. The S28 Mag motor will get too hot after running for long enough to mow a lawn - an Ampflow A40-300 Motor would be better and more in the RPM range you want. You also have to think about the motor bearings; even the A40 motor might not like the shock of hitting concrete paths and small branches.
Sat Jan 15, 2011 9:55 am
bytraper
Joined: 31 Oct 2005
Posts: 195
Its not going directly onto the shaft, its going to a pulley above deck. The 3000 rpm is about the max rpm required and the torque needs to be around 500w, the blades don't need to spin ultra fast, plus also need a little efficiency. Don't want to start it up, walk out and have the thing go flat!
Sat Jan 15, 2011 5:05 pm
Nick Experienced Roboteer
Joined: 16 Jun 2004
Posts: 11802
Location: Sydney, NSW
Where are you getting the 500 watts from? Most mower petrol engines are several HP which works out to around 1500 watts. That is really the entry level and if you over-spec the motor and controller / switch, then they will run cooler and last longer.
What sort of batteries are you planning to use? Car batteries sound like a lot of weight to be pushing around.
Sat Jan 15, 2011 6:33 pm
Daniel Experienced Roboteer
Joined: 30 Aug 2005
Posts: 2729
Location: Gold Coast
yeah, 0.8hp does sound very very small for a lawn mower
Sat Jan 15, 2011 7:03 pm
Valen Experienced Roboteer
Joined: 07 Jul 2004
Posts: 4436
Location: Sydney
existing electric battery lawn mowers use 500W motors and 2x 10Ah SLA's
dad has one, it mostly does the job.
Though its been 2 years and the batteries are on the way out so I'll replace it with a lipo pack.
its ok provided the lawn isn't long or wet _________________ Mechanical engineers build weapons, civil engineers build targets
Sat Jan 15, 2011 8:57 pm
bytraper
Joined: 31 Oct 2005
Posts: 195
That sounds like exactly what Im trying to copy. The mower I saw had a 500w motor and it was decent.
Thats what I'm trying to find, I can get one from overseas no problem, but I don't want to buy 100 of them, just one.
I'd even be prepared to go to brushed, it would make the actual controller easier to make.
Sun Jan 16, 2011 2:15 pm
Don Experienced Roboteer
Joined: 21 Feb 2010
Posts: 355
Location: Gladstone, Queesland
why are u trying to use a electric motor for a lawn mower?
Sun Jan 16, 2011 2:44 pm
bytraper
Joined: 31 Oct 2005
Posts: 195
To save the trees by killing grass..
Nick, how did you work out several horsepower = 1500w ? 1500w is about 2hp!
Most mowers have a minimum of a 3.75HP motor on them, while the commercial or slasher type mowers use a 6.5-7HP motors. But for a well cared for lawn a 500w motor is perfect, as long as what valen said stays true, the grass isn't too long or wet the 500w motor will do a decent job while saving the world.
I get a lot of enquiries for them, and since I started building my own, I've decided initially to copy an existing one, then build a controller with an pwm and adjustable frequency to optimise the battery life (or go brushless).
Sun Jan 16, 2011 4:02 pm
Nick Experienced Roboteer
Joined: 16 Jun 2004
Posts: 11802
Location: Sydney, NSW
"Nick, how did you work out several horsepower = 1500w ? 1500w is about 2hp!"
several = 2 (or maybe 3) I just used an on-line power converter.
If you really want around 500 watts, then larger scooter motors might be the best choice. Oatley has a typical range or you can get fancier ones from US electric vehicle sites.
I like the motor, however the efficiency is pretty low for a brushless... like you said, maybe better to go brushed. The controllers they sell are all Keyan (Chinese made).
I'll start the search for a brushed motor (much easier to control anyway) and much cheaper too.
Not interested in reselling though, this is really just helping out one of my friends to build an electric mower.
Thanks nick!
Sun Jan 16, 2011 5:30 pm
Valen Experienced Roboteer
Joined: 07 Jul 2004
Posts: 4436
Location: Sydney
I'd put one of the big brushless motors on it from HK, with a V belt drive and a servo exerciser to run the controller. Put the controller into "governed" mode and it will maintain a set speed. It'll change power output to maintain it which is what you want.
V belt will protect the motor from impacts to the blades, and let you put the motor into the sweet spot.
Use lipoly batteries, ~5Ah should do, they will have much more oomph than SLA's and be worth at least 10, more like 20Ah worth of SLA and weigh 500 grams rather than 5 kg. _________________ Mechanical engineers build weapons, civil engineers build targets
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