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DIY Solar Powered Cordless Lawn Mower - Motor Build

12th June 2014

Who likes mowing the lawn? Not me!
It takes a good half an hour, and during the summer months, I have to do it twice a week.
The actual act of pushing the mower, I don't really mind, but I do mind having to flick the power lead out of the way when I perform a U-turn at the end of each pass.
I want a cordless mower!

Having trawled the local stores, I've been surprised at just how much a cordless mower costs. I could easily spend over £200 on what would be a fairly mediocre device.
And then there is the issue of replacement batteries, which are bespoke to each mower. Over the years, I could save time, but I'd be shelling out a lot of money on replacement batteries.

If you've read my website, you'll know that I've already got a solar system setup, and that that solar system generates a surplus of power during the summer months.
I've come to the conclusion that I should build my own 12 volt battery power lawn mower, charged up completely by the Sun!

There are a few things I'm going to need.
- A donor lawn mower. A petrol based rotary lawn mower seems to be the best thing to go for here. They are usually metal. They don't hover (flymo). They are solidly built. The platform (deck) lends itself to having the engine removed and a motor put in its place. I'm currently trawling Ebay and Gumtree for a suitably cheap lawn mower.
- A blade. Whenever I cut the lawn, I usually fill one bin bag full of grass, which ends up taking space in the bin. There is also the fact that each time I take grass away from the lawn, I'm effectively removing nutrients and not replacing them. For this reason, I've decided to go with a "mulching" blade as opposed to a standard cutting blade. The only drawback is mulching blades require more power.
- A motor. This is where it gets interesting. I reckon I'm going to need at least 1KW of motor power. This is in the region of 80amps when using 12 volts. Most DC motors in this power range are crazy expensive...I could easily spend over £100 just on the motor, alone. I'm not prepared to spend that. Read below on what my plans are for the motor.
Battery - I'm going to go with 12 volts, because that will easily integrate with my Solar setup. I already have a number of suitable batteries.

The Motor - Alternator conversion to Brushless DC motor AKA Hybrid Brushless

Requirements for a motor
- Must be cheap <£20
- Must run on 12 volts
- Must have an RPM of at least 2000 rpm
- Must be powerful >1KW (80amps)
- Must run for at least 30 mins with no sign of overheating

So I've been struggling to get hold of a really powerful DC motor that meets all my requirements, for cheap money(I want to spend less than £20 on the motor).

I was looking at scooter motors as a cheap DC motor, but they don't look to be very powerful. The only thing going for them is they are cheap and abundant.
I'd love to hear if someone has managed to create a DIY cordless lawn mower with a scooter motor.

I then did some reading on car alternator conversions. They have a lot going for them.
- Designed to run on 12v
- Can handle huge amounts of power
- They are easily gotten hold of
I've decided this is the way I'm going to go.

So I've purchased an alternator of Ebay for £8.50 (inc P&P) - crazy cheap!
This alternator is off a 1997 Vauxhall Astra Turbo Diesel.
The first thing I noticed was just how solidly built and heavy it is - this thing should be able to handle tonnes of power!

Alternator Brushless DC Motor Conversion

Alternator Brushless DC Motor Conversion
So my alternator arrived - I cracked it open to discover the alternator I bought had 4 poles on its windings.
This will never work with a standard 3 pole Brushless Electronic Speed Controller.
The thing with buying alternators, is it's nearly impossible to find out any information on its internal wiring with a simple google search. So the only way to find out, is to buy it and hope it's what you want. I could now either buy another alternator and hope it's a 3 pole winding or re-think my approach.
Even if I did find a 3 pole alternator, further reading on alternator conversions has taught me that I'd have to come up with a way to apply volts to the electromagnet. This is something I wasn't aware of before I went down this route...and I now think the electronics are becoming too complicated. I want a mower that I can just get out the shed, fire up, and just use. If it becomes complex and has to be fiddled with each time I want to use it, it'll be a failure.

Because of this reason, I've decided to ditch the alternator idea and instead purchase a large brushless motor.

DIY Mower Brushless Motor
My brushless motor has arrived...and it's a beast!
I used to do a lot of RC model plane flying, so I've seen a lot of brushless motors in my time...this thing is massive in comparison to the ones I used with my RC planes, and feels nice and weighty.

I paid £35 inc P&P
The Specs -
- C6354/16
- KV200, so 200 rpm per volt, which should equate to around 2400 rpm (12 x 200). This is comparable to your average rotary mower.
- 2210 watts, this is more than double the power of some 240v mowers, so should be more than adequate.
- 480 grams
- 63mm diameter
- 55mm length
- 10mm shaft diameter

This motor is an 'outrunner'.
For those of you unfamiliar with brushless motors - what this means is that unlike a standard DC motor where all the rotation occurs inside of the motor casing, with only the protruding shaft showing any external signs of rotation, an outrunner has the majority of the motor casing spinning.
So looking at the picture above, the red part at the top is stationary, and so would be the part of the casing that you would use to attach to the mower mount.
But the black part, spins.
This obviously has repercussions in terms of safety (tie back long hair) and could raise issues with mounting.

First thing I need to do is decide how I'm going to mount the blade to the motor. I really don't want the blade detaching at speed, as this could result in serious damage or injury.
I figure I might as well stick with the RC (radio controlled) side of things, as I know RC stuff is designed to handle the power and the RPMs that this motor will run at.
I've purchased a 10mm collet, which should arrive soon.
The only issue I foresee is the depth of the shaft. The motor has a 25mm shaft length. Depending on how I mount it, this might only leave about 15mm of the shaft sticking out the bottom of the mower, which wouldn't be very secure.

DIY Mower Brushless Motor Full On
To make a brushless motor work, you need two things...
1 - An ESC or Electronic Speed Controller. Because a Brushless motor has no commutator, it needs a method for switching the poles as it turns, and this is what the ESC does.
2 - A PWM or Pulse Width Modulation circuit. The ESC requires a signal to tell it how fast it should spin the motor. This signal is in the form of a PWM signal. The easiest and cheapest way to achieve this is with a Servo Tester.

The pic above is of a servo tester I purchased off Ebay for about £2.

It's very simple to use. You plug the ESC in and supply power to the ESC, which in turns powers up the Servo Tester.
And then as you turn the dial on the servo tester, the motor spins up, with RPMs controlled by the dial.

DIY Mower Brushless Motor Dead Mans Switch
With the servo tester up and running, I could quite easily take the build to the next stage and get it all bolted on to the mower frame...but!
We have to consider safety.
To switch on the mower, all you'd need do is turn the Servo Tester dial, but we are now lacking a "dead mans switch".
A dead mans switch is the spring loaded switch that is fitted to all mowers. Basically a switch that has to be forcibly pressed at all times and if something was to happen to the person mowing (becomes dead!), the spring loaded switch will spring to the off position, and the mower stops.

To keep things safe, I'd like to incorporate a dead mans switch into the build.
Considering we have to use the Servo Tester for the PWM signal, I've had to come up with a way to switch on the Servo Tester using a switch.
You may be wondering why I'm doing it with the servo tester as opposed to just switching off and on the entire mower - with a relay or somesuch.
This is because the ESC goes through a 3-4 second boot up each time it is powered up and it also requires the PWM signal to be in the low position before it'll work.
So this leaves us with the only option of switching off and on the PWM - leaving everything else on the mower turned on.
So to get things working -
1 - Leave the Servo Tester dial at its lowest speed.
2 - Solder your dead mans switch across the two solder points detailed by the green arrows in the above pic.

Now whenever you switch your dead mans switch, the mower powers up to its fastest speed.
By doing it this way, you do lose the ability to control the speed of the mower blade, but I couldn't see any reason why you would want to run it slower that its maximum speed.

You now have a fully working brushless motor setup, with dead mans switch.

DIY Mower Brushless Motor Wiring Setup
Here is the brushless motor wiring setup.
The ESC you can see in the Pic is a Turnigy Plush 80amp. I bought this a few years ago for around £25. The Turnigy series of ESCs are very reliable and I'd highly recommend.
The two crocodile clips go to your power source, eg, Battery.

Battery used in video is a lead acid 12v 4ah

This video is the first test of the motor fitted to the Mower deck.
I thought I would perform a quick test of current draw.
From the video, you can see the motor pulls about 40amps before shutting off.
I was creating load by squeezing the motor with quite a lot of force. Obviously not very scientific, but I reckon I was squeezing with similar force of a mower blade cutting long grass.
The shutting off isn't the motor stalling - it is actually the ESC shutting off because of its inbuilt battery protection circuit.
I suspect if I was to supply a better battery, one that could sustain a much higher current output, I'd probably see the amps go a lot higher because the ESC wouldn't prematurely turn off.

But anyway, as a first test, things are looking good.

Next - DIY Lawn Mower - The Build!



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