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Electric Riding Mower Conversion

An attempt to convert a Husqvarna GT2254 with 54" deck to electric power.

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My objective here is to organize my thoughts and data regarding electric lawn mower conversion, as I research implementation and operation into the process. Hopefully this can assist others in doing their own work in a similar vein.

My father has put me on the task of converting an old Husqvarna GT2254 riding mower to electric power.  Mostly so he doesn't lose the zero-turn in the pond behind his house. The yard is fairly big, and even with a 54" deck, requires at least 3 hours to mow normally.  The zero-turn will be used for most of the mowing, but has trouble around the pond, so he wants this one to do that job.

Yesterday, I started on my research.  While there are kits available, sorting through all the data to get the quotes seems daunting, and the popular option from Canada www.electrictractor.net is not designed for such a large mower deck.  Other options were running $1572 US, without battery packs, or required knowing things about the mower that the manufacturer and their suppliers just do not provide, or still have on record.

That said, here is what I can piece together, from available information.

1.) If your mower has a hydrostatic system, you may not need a motor controller, as the hydrostatic system provides all of the gearing and conversion from the motor to propulsion and mower deck.

2.) If you do NOT have a hydrostatic system, you may need a motor controller to enable motor speed and torque controls.

3.) A 12 or 24 Volt DC system will have a harder time keeping up than a 36VDC or even 48VDC system.  So it would likely be wisest to have a 48VDC system to keep current requirements down, and motor power up.

4.) Resources for DC motors of this type are scarce if you're used to looking for industrial motors.  Try looking for Golf Cart motors instead.

5.) Know your motor shaft design.  A keyed circular shaft is easier to find than a toothed one.  If your drive train uses pulleys, you may need to get new ones to convert your machine.

6.) A project that I found VERY useful and full of helpful information can be found at the following URL: https://ecorenovator.org/introducing-ecorider-riding-mower-electric-conversion-project/

7.) From what I gathered, the standard mower should operate at a 48VDC minimum, and be expected to have a constant load of 65A to 100A DC depending on what you're doing with the mower.

8.) You will also want to do some maintenance on all of your bearings to minimize friction.  Be it greasing or replacements, these optimizations and cleanings should decrease motor load.

9.) Horsepower: Gasoline engines do not produce torque as quickly as electric motors.  As such, they tend to be slightly oversized for the job.  At the same time, the tendency is an approximate 2:1 ratio in rating (Gas:electric) according to practicalmachinist.com, but doesn't account for the difference in RPM, just torque.  Which means that a gearbox might alter this.  -- more research needed --

10.) Batteries: Lead acid is cheaper than Lithium chemistries, but heavier and has a shorter life.  And then to do things properly, you have to match the batteries, which is not my expertise.  I'll have to do a LOT of research here, and make sure the battery packs are designed to minimize issues from this source, for safety reasons!

11.) Life per charge.  I would like to have the charge last somewhere between 45 minutes to an hour, with the mower deck running. This should provide plenty of time to mow around the pond, or pull a trailer full of wood around, if I'm lucky.

12.) Battery discharge rates may be important to discover information about before I purchase the cells.  Not to mention charging methods.  I will likely have to implement some sort of voltage monitor to verify all cells are discharging at the same rate though.

Where to go from here:

1.) I need to figure out what RPM the engine on the mower has, as well as the properties of the transaxle, and weight of the chassis.  These may provide significant information regarding what kind of motor I need.

2.) Kellycontroller.com seemed to be a really good option for finding motors and motor...

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  • Shaft Torque and Manufacturer Data

    FrictorOTS08/06/2020 at 18:42 0 comments

    Good news for me: Briggs and Stratton will provide torque curves and related data if you ask them.
    So for the 40H777-0241-E1 engine used on the GT2254 riding mower from 2005, I received the following table:

    Model Series40H7
    Displacement656
    Oil Capacity64 oz (1.9 L)
    Valve ConfigurationOHV
    Flywheel Torque130 lb/ft (176.0 Nm)
    Governor Arm Torque70 lb/in (8.0 Nm)
    Carb Mount6.5 lb/in (7.5 Nm)
    Plug Gap0.03 in (0.75 mm)
    Intake Valve0.004- 0.006 in (.10 -.15 mm)
    Exhaust Valve.004 - .006 in (.10 - .15 mm)
    Idle Speed1750 RPM
    Air Gap.008 - .012 in (.20 to .30 mm)
    Flywheel Puller Part19203
    Flywheel Holder Part19433
    Connecting Rod Torque100 lb/in (11.5 Nm)
    Sump Torque200 lb/in (22.5 Nm)
    Head Torque220 lb/in (25.0 Nm)

    Husqvarna also responded for a request regarding the load ratings of the lawn mower in question, stating that it was rated for 846 lbs of No Load Weight.  This seems illogical to me, as I highly doubt the mower weighs this much, even with the deck installed, and somebody much heavier than myself riding on it.  Full Load data was not provided, and when asked for clarification, I was told that the 846 lbs load was specifically No-Load.

    HP of a motor is measured at full RPM, but Torque ratings are taken at 3060 RPM according to a gross power chart that came with the table above.  And do not include losses from installing the air cleaner or exhaust systems.  This makes things slightly trickier, but I'll worry about that loss later.

    Horsepower = Torque * RPM / 5252

    (Source page 1, column 2) 

    This source also gives Mechanical HP = 746 W = Electric HP. 

    Torque is defined by science as kg * m^2 / s^2 in units.  Or mass * distance^2 / time^2.

    The ratio between the drive shaft torque and the flywheel torque is simple to calculate if I treat the flywheel and shaft as one assembly.  This causes the mass parts of the Torque definition to cancel itself out, and since they're both acting within the same timespan, the time variables cancel out.  Leaving a ratio of two circumferences.  The diameter of the shaft is listed at 1.125 inches and measures within tolerance of that value.  The diameter of the flywheel, I am taking from measurements as well, and comes out to 9.5 inches (approx.).  Thus:

    T_flywheel / T_shaft = ( pi() * 9.5 )^2 / ( pi() * 1.125 )^2 = C_flywheel^2 / C_shaft^2

     ~Remember, diameter is 2x radius, and circumference (C) is 2 * pi * r .~

    or T_flywheel / T_shaft = 890.732 / 49.965 (within about a significant figure of tolerance)

    Thus:  T_flywheel / T_shaft = 176.0 Nm / T_shaft = 890.732 / 49.965

    T_flywheel * C_shaft^2 = T_shaft * C_flywheel^2

    T_flywheel * C_shaft^2 / C_flywheel^2 = T_shaft

    176.0 * 49.965 / 890.732 = T_shaft

    8793.8 / 890.7 = 9.9 Nm within one sig fig of tolerance.  (slightly less if ignoring significant figure truncations)

    This is what I have for today's update.  I need to do some research into the motors and such before I continue further.

  • Mower Specific Data

    FrictorOTS06/16/2020 at 15:21 0 comments

    So this update is specific to the lawn mower in question.  Exact details will vary with mower system and is mostly so I can collect the data in an organized fashion in one place.

    1.) Make and Model:  Husqvarna GT2254, 2005, with 54" deck.  Electrolux 96025000201 model.  Catalog No. L0GT2254

    2.) Weight values: Not specified in any manual or advertising.

    3.) Drive wheel diameter and location: Rear wheel drive, 23" diameter. Double-wall 10 PSI, 23 x 10.50 - 12.  Estimated 880 lb load

    4.) Non-drive wheel radius and location (Radius used by DD Motor Systems in their online form for quotes): Front tires/steering, 8" radius.  Double-wall, 10 PSI,  16 x 6.50 - 8. Max 415 lb load.

    5.) Engine: Briggs and Stratton, 22 HP ELS656.  Model number (as listed on engine) is 40H777-0241-E1.

    6.) Mechanical drive train:  Pulleys to integrated trans-axle.

    7.) Motor shaft: 1 1/8 diameter, 4 5/16" long, with a 7/16 x 20 bolt threaded into the shaft to hold on the clutch.  And a 1/4" key slot about 4" long (yep, even into the curve of the shaft up to the motor)

    8.) Drive train pulley: 4" idler pulley, v-groove, metal, with no key slot, rather the inner wall of the pulley is punched to make a pseudo-key.  (Imaginative, if garbage design).

    9.) Trans-axle Pulley: 8.5" diameter, metal, v-groove.  (The Trans-axle does all the gearing changes for the forward and reverse and speed of the mower.)

    10.) Electric Clutch for mower deck: Fully functional, I'd like to avoid changing this if at all possible, and am leaving it out of the equations, on the assumption that I won't need to at this point.

    11.) Drive train to trans-axle gives a pulley ratio of 4:8.5, or just a bit more than 1:2.

    12.) Maximum no-load speed (assumed to be max forward velocity as per manual): 5.4 MPH

    13.) Minimum no-load speed (assumed to be maximum speed as listed for lowest gear in the manual): 0.7 MPH

    14.) No maximum load given, nor max load speed.  I expect the velocity should be equal to unloaded speed.

    15.) Velocity on any given grade aside from 0% is unlisted.  I would assume 5% grade would not alter maximum or minimum velocities. (this may be ignorance)

    16.) Engine RPM: Briggs and Stratton does NOT list this, nor does it show on any of the engine's markings.  I can therefore only estimate based on other engine RPMs of similar type.  This puts RPM in the 3100 to 3600 RPM range, and I will guestimate at the higher end, for the purposes of this project.

    17.) Range: I would like the mower to run for about 1 hour on a charge, if possible.

    18.) Battery voltage: 48VDC ideal.  (NEC does not provide guidance on this, at least as of 2008.  This has hopefully changed with the re-introduction of electric vehicles.)

    19.) Battery Type: Lead-acid deep discharge/cycle type.  Short overall life, heavy, and bulky, but cheap to procure and replace if something happens.  

    20.) Expected load current, based on readings listed in my description of this project, come out at 120A max, with some safety built in.

    21.) Thus ampere-hours of the battery pack should come in around 100Ah to 120Ah.

    22.) Duty Cycle: 30 minutes to 1 hour of operation on a charge. So 50 to 100% of capacity.

    Most of what I've listed here are based on the estimate form given by D&D Motor Systems on their website.  And may need expansion or alteration to fully describe what I  need later.

    06/16/2020 FrictorOTS

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