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Discussion Starter #1 (Edited)
I've been saving a Toro Z-Master 72" mower for a few years (the ICE quit), mulling over the idea of converting to electric.

For starters:
- A new engine costs about $2,000 so that's my working budget.

- Engine is a Kohler Command Pro 725cc 25hp 39.2lb/ft of torque, horizontal output shaft that has a fore and aft output, the fore driving the propulsion, the aft the mower deck.

My plan (unless someone has experience with it not working) original is to keep things simple and mount two A/C motors in place of the gasoline engine, one facing forward and one facing aft, and run them at full 3600 RPM (The working speed of the ICE engine) from an inverter. Now I'm considering running two DC motors, one less part in the system (the inverter).

This would keep most of the OEM belts and pulleys, and I could use the OEM control arm, parking brake, master switch to turn off and on the drive motor, and the OEM PTO switch to turn the mower deck off and on (probably via relay).

I'm looking for advice on how to calculate appropriate replacement electric motor(s), inverter, and enough battery to run for 2 hours.

3:5 rule says I need 15hp of electric motor, but 2 x 7.5hp electric motors alone put the project out of price range.
 

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to keep things simple and mount two A/C motors in place of the gasoline engine, one facing forward and one facing aft, and run them at full 3600 RPM from an inverter.
I'd argue that that is not very simple.

Inverters are easy-ish and cheap-ish to buy, but not at the 7-15hp range.

However...

No way that you need 7.5hp to push a lawnmower forward. With the gearing it's on and the speed it goes, I can't see that taking even 1 hp. 1hp is enough to propel a motorbike 35mph. You could replace that shaft with a small DC motor. Something like a treadmill motor will be 2.25 hp, and you'd get a speed controller out of a scrap treadmill too. You can get those for nearly free if you're the DIY type.

The blades I don't know. I'd sure like to see someone do this and then measure the actual power draw required by the blades when cutting at a familiar rate. I suspect 95% of the engine power goes to the blades.

What I think would be a good match there is a smaller (5-7" maybe) pump motor from a forklift. You could probably get one for free-$100. That'll be in the right ballpark for power. And it'll have ungodly torque for when you hit some heavier grass. But, it's not actually good at regulating speed. A permanent magnet motor would be a good fit, but, I don't know offhand where you'd get one unless buying new.

An electric GoKart kit is probably your best bet. Permanent magnet ones are 2/3 the price of brushless ones.

https://www.electricmotorsport.com/...708-pmdc-motor-24-48v-8-hp-cont-15-hp-pk.html -- New, $450, motor only.

https://www.electricmotorsport.com/...rottle-contactor-wire-kit-and-fuse-block.html -- Same as above, kit including controller for $1200, (no batteries).

"Enough battery to run 2 hours" is a really difficult thing to figure out because it's so massively variable based on what you're cutting and how aggressively. You can spin the blades up and drive around at lawnmowing speeds all day on a fairly small battery pack, but, cutting grass is what's going to affect that. You'd need some comparable metrics.

For this project I'd almost say just go with lead-acid batteries, cheap charger, and good enough. Else, Nissan Leaf cells/bms are probably where you'll spend the rest of your money.

If you could get a smaller form factor cheaper, you could start out with a cheap battery pack and see how far it gets you, then add capacity later.

You could try temporarily starting with some spare/borrowed lead-acid batteries (just automotive starter batteries, 2-4 or whatever voltage you're targeting, it's not good to deep discharge them but, meh, do it once for science), set your watch and see how long they last or what percentage of the job they do, then put them back in the vehicles they belong and use that to calculate what amount of lithium to buy.
 

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Discussion Starter #3 (Edited)
I'd argue that that is not very simple.

Inverters are easy-ish and cheap-ish to buy, but not at the 7-15hp range.

However...

No way that you need 7.5hp to push a lawnmower forward. With the gearing it's on and the speed it goes, I can't see that taking even 1 hp. 1hp is enough to propel a motorbike 35mph. You could replace that shaft with a small DC motor. Something like a treadmill motor will be 2.25 hp, and you'd get a speed controller out of a scrap treadmill too. You can get those for nearly free if you're the DIY type.

The blades I don't know. I'd sure like to see someone do this and then measure the actual power draw required by the blades when cutting at a familiar rate. I suspect 95% of the engine power goes to the blades.

What I think would be a good match there is a smaller (5-7" maybe) pump motor from a forklift. You could probably get one for free-$100. That'll be in the right ballpark for power. And it'll have ungodly torque for when you hit some heavier grass. But, it's not actually good at regulating speed. A permanent magnet motor would be a good fit, but, I don't know offhand where you'd get one unless buying new.

An electric GoKart kit is probably your best bet. Permanent magnet ones are 2/3 the price of brushless ones.

https://www.electricmotorsport.com/...708-pmdc-motor-24-48v-8-hp-cont-15-hp-pk.html -- New, $450, motor only.

https://www.electricmotorsport.com/...rottle-contactor-wire-kit-and-fuse-block.html -- Same as above, kit including controller for $1200, (no batteries).

"Enough battery to run 2 hours" is a really difficult thing to figure out because it's so massively variable based on what you're cutting and how aggressively. You can spin the blades up and drive around at lawnmowing speeds all day on a fairly small battery pack, but, cutting grass is what's going to affect that. You'd need some comparable metrics.

For this project I'd almost say just go with lead-acid batteries, cheap charger, and good enough. Else, Nissan Leaf cells/bms are probably where you'll spend the rest of your money.

If you could get a smaller form factor cheaper, you could start out with a cheap battery pack and see how far it gets you, then add capacity later.

You could try temporarily starting with some spare/borrowed lead-acid batteries (just automotive starter batteries, 2-4 or whatever voltage you're targeting, it's not good to deep discharge them but, meh, do it once for science), set your watch and see how long they last or what percentage of the job they do, then put them back in the vehicles they belong and use that to calculate what amount of lithium to buy.
Thank you for the input! I've been thinking that two 7.5hp motors seem like overkill, but no experience with the conversion made me second guess it.

I'd mention that this is a BIG commercial mower, mows 6' in a single pass, all the videos and research I've seen uses much smaller mowers.

That said, my intuition was the opposite of yours, I was figuring that machine propulsion would use most of the engine's power and the blades less. I'm basing this on anecdotal experience, turn the blades on and the gas engine lags, but once they're up to speed they act as a flywheel (three 24.5" blades). Not much slows them down, but if something does, it usually stops them completely, the belts slip, and I end up having to shut it down and clear out whatever it is.

Whichever the case, when I do get it together I will certainly hook up an ampmeter to both to see what the current draw is, so others can benefit from the experience, but it will also give me good data if I go with dissimilar sized motors and need to swap them, get something bigger, etc.

More about simplicity... I'm thinking constant speed motors (no controller) because mower speed and turning is controlled with the hydraulics. On the gasoline engine the only time the throttle is used is when you start it up or shut it down, (or if I get off it to pick up something like a stick that I don't want to mow over) the rest of the time it's throttled up to 100%.

From an electrical standpoint it would make sense to replace those hydraulic motors with electric, and have a controller for each. However mechanically it would make the project a non-starter because I would have to rework the wheel bearings, etc.

From a mechanical standpoint however, two 3,600 RPM electric motors are a possibility, it's just pulling out the gas engine, puling off the fuel tanks, mounting the electric motors where the gas engine was and the batteries where the fuel tanks were (within the realm of my welding skills).

Thanks for the links too! A lifetime ago I used to work on Raymond Forklifts, and one of the ideas that I'd considered was just picking up a couple of DC motors and running (fused) directly to the batteries with a contactor to turn off and on. The old SCR trucks used to do this, the SCR control system would control the slow speeds, but when you called for top speed a contactor would pull in and run current directly from the battery to the motor (or in the case of the lift pump it was just a contactor that pulled in to turn it on and off, no speed control). I abandoned THAT idea because I thought it would be cost prohibitive... when I worked on forklifts SCR was "old tech", transistor controlled DC was king, and nobody even thought of AC on a forklift.

I'll have to revisit it, two 24V motors, contactors, and 12V deep cycle batteries in series sure would simplify the electrical portion to get me to "proof of concept".


Thank you again!
 

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The mower manufacturer didn't pay for a 25 horsepower motor because they like to throw away money - that size is needed. It may never run at 25 horsepower: it is routine for properly-designed equipment to run the motor at lower speed for better efficiency, less noise, and longer life. Honda used to recommend designing for 3,000 rpm when using its motors, which had their peak power output at 3,600 rpm. So an electric direct replacement may only need to have 15 to 20 horsepower (11 to 15 kW) output... but a couple of kilowatts won't do it.

If you are going to run two motors both at constant speed, and the same speed as each other, I see no reason to use two separate motors. Just use one with a double-ended shaft, directly replacing the engine. That way, it doesn't matter whether the power is needed to drive the wheels or to turn the blades, and you can't get the relative sizing of the motors wrong.

One of the other recent (active within the last few months) mower conversion threads here had a substantial discussion of power and energy requirements, so there's no need to start from scratch. Two hours of mowing a six-foot-wide swath of grass will take a lot of battery.

... I'm thinking constant speed motors (no controller) because mower speed and turning is controlled with the hydraulics. On the gasoline engine the only time the throttle is used is when you start it up or shut it down, (or if I get off it to pick up something like a stick that I don't want to mow over) the rest of the time it's throttled up to 100%.
I think you're confusing "constant speed" with "uncontrolled" - they're not the same thing at all.

While you rarely move the power lever of the gas-engine machine, that's not the throttle. There is a governor on the engine, and it is always adjusting the throttle to keep the engine speed roughly constant despite changing load. If you push the lever to 100% and just sit there with the machine not moving and the blades not engaged, the engine runs at the highest designed speed, but the throttle is nowhere near fully open; as you add load by engaging the blades or pushing the drive levers the governor opens the throttle to add power to maintain that speed.

Imagine in a car with a manual transmission if you just blocked the throttle all the way open, and raised the clutch pedal to engage the clutch to start moving, then controlled the car's speed entirely by slipping the clutch, with the engine speed coming down under load and going back up when you slipped the clutch more to slow down. You wouldn't do that because it would be insane, and no vehicle works that way - without control.

If you replace the engine with an electric motor and just connect the motor to the battery - without a controller - you will get something more like the full-throttle car example than a properly working governed engine. When there is no load the motor will spin at the highest speed allowed by the voltage, which is probably too fast. When you add load it will slow down to speed dependent on the motor characteristics, load, and available battery voltage. In the mower, you might have 5,000 rpm when there is no load, but only 3,000 rpm while driving with the blades running... and as you changed driving speed the blades would change speed because the motor would change speed in response to the changing load. There's a reason that the old forklift used an SCR controller, even if it bypassed it when maximum power was needed. I think in Matt's suggestion he is thinking that a permanent magnet brushed DC motor would work adequately in this configuration, because their available power depends so heavily on load that they settle in to a reasonable constant speed.

There are machines which use a direct-to-battery uncontrolled brushed motor, but they have a predictable load - not like a mower - so their speed is predictable. They are also usually tolerate of a changing running speed. There are lots of electric motors without controllers in industrial situations, but they're induction motors running from AC power, so the AC power generating stations are really the controller, producing a constant frequency AC power.

You can take the AC power approach with a mower conversion, but that just means you use an AC controller/inverter set to run an AC motor (induction or synchronous) at a constant speed. That's what you're getting if you do what you originally described and "run them at full 3600 RPM from an inverter.".
 

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a couple of kilowatts won't do it.
If that refers to my comments, then I said a couple kW is probably fine for traction. The bulk of the power needed should be for the blades (the other 23 hp).

...

If it's hydraulic you might just consider stealing the hydraulic pump off a forklift, replacing the pump entirely, and running with that. It might be powerful enough. Duty cycle might be a bit of an issue so, I'd add some fan cooling.

No speed control and just using a contactor would save you a lot of money.
 

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Discussion Starter #6
If you are going to run two motors both at constant speed, and the same speed as each other, I see no reason to use two separate motors. Just use one with a double-ended shaft, directly replacing the engine. That way, it doesn't matter whether the power is needed to drive the wheels or to turn the blades, and you can't get the relative sizing of the motors wrong.
Turning off and on the blades. The ICE setup has a clutch to engage/disengage the blades, 12V electromagnetic. I guess I could keep that, but from what I've seen electric motor output shafts are usually a different size than the ICE output shafts, so going to two motors gets me back to the simplicity of just finding a v-belt pulley that fits the output shaft. Other than that, I'm expecting to not get the right sized motor(s) the first time around, but if I can get it sort of working, and leave myself options for incremental changes (e.g. let's try a larger/smaller diameter pulley, switch out the drive motor for the blade motor, get a bigger blade motor, etc.) the chances increase that I'll eventually have something that works well and I can put together a parts list for someone else (I wish there was one for me :D ).

I think you're confusing "constant speed" with "uncontrolled" - they're not the same thing at all.
Good point. I'll have to give that some more thought. Uncontrolled might still work for the blades, constant speed, nearly constant load (my thoughts are relating to the lift motor on a forklift, sometimes you're just picking up the forks empty, sometimes you're picking up a 4,000lb pallet, in both cases you move the handle, contactor engages, turns the pump and you can actually calculate how heavy the load you're picking up is by the amp draw.)

Talking through it that way... it might still work for the drive too, again, more thought, but thanks for mentioning it, gives me something to mull over.

If it's hydraulic you might just consider stealing the hydraulic pump off a forklift, replacing the pump entirely, and running with that. It might be powerful enough.
I wish! I'm probably going to fumble the explanation a bit, but the way these work (at least on the mower and also on my skid steer) is there's a hydraulic pump/motor combination for each side:
- The motor is really simple, hydraulic fluid fluid flows in, motor turns in relationship to the pressure/volume.
- The pump is where all the action happens, move the lever forward, it moves a plate that changes the stroke of pistons that rotate around a shaft.

Here's an OK animation
watch for the lever on the left side changing.


SO! Anyone have a link to a place to get a couple of good used 24V forklift motors?

I'm thinking 24V because then I can stick a 12V deep cycle battery on each side of the machine (so I'm not listing to one side) and wire them in series, if I need more cutting time I can add batteries in pairs until I get to where I want to be (If I knew how to calculate need better I might be inclined to go 48V and 4 batteries, but at some point battery weight is going to become an issue).

Anyone have an old SCR Raymond Pacer sitting around? I know those motors would have more power than I'd ever need... probably more than is safe (visions of a "Tim the tool man Taylor" come to mind).
 

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If that refers to my comments, then I said a couple kW is probably fine for traction. The bulk of the power needed should be for the blades (the other 23 hp).
I agree that the majority of the power is likely to drive the mower blades; my comment about required power was in response to the (repeated) idea that 15 kW (total) of motors would be "overkill".
 
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