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I’m in the process of converting a Cub Cadet SLT1554 from gas to a hybrid-electric tractor using a Motenergy ME0909 PMDC Motor. See description/specifications below:

Motenergy ME9090 PMDC Motor 12-48V, 4 hp cont, 12.8 hp pk 4 Hp Brush-Type PM DC. 4000RPM at 48VDC

The Motenergy ME0909 is a Brush-Type, Permanent Magnet DC motor with very high efficiency. Capable of 4.8 KW continuous and 15 KW for 30 seconds. For voltages from 12 to 48 VDC input and 100 amps continuous (300 amps for 30 seconds). Designed for battery operated equipment. It makes a great replacement for the original Etek motor because it has the same bolt pattern and is actually lighter in weight.

Specifications

Power: 4 cont - 12.8 pk hp Voltage: 12-48 Volt rated Speed: 2150-4850 rpm Size: 6” OD, (w/o shaft) Shaft: 7/8” x 1-3/4”, 3/16” key Weight: 24 lbs.

Voltage Constant: 0.0107 Revolutions per volt: 93.45 RPM Torque Constant: 0.102 Nm/Amp Generated Current: 9.8A per Nm

If possible, I would like to operate the electric motor at 24 volts powered by two 12-volt deep cycle batteries in series.

At the same time in order to guarantee sustained power, I would like to have an onboard generator powered by an 8hp I/C horizontal shaft engine driving a 12 volt, 253 amp alternator recharging the batteries. I’m under the impression that I would need at least 2,984 watts of electricity to create 4hp, and 3,730 watts (5 hp) would be even better.

QUESTION: Is it possible to create some type of configuration so that a 12v alternator can charge two 12-volt batteries in series. Also, I’m concerned that the ME0909 ad speaks of only 100 amps for continuous service.

Would the battery not act as a “cushion” between the alternator and the electric motor?

Any assistance in this project will be greatly appreciated.

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  • \$\begingroup\$ Does an electronic speed controller come with the motor? What do you mean by "battery act as a cushion? \$\endgroup\$ – Charles Cowie Feb 9 '16 at 3:35
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    \$\begingroup\$ If it was me, I would forget the 8 HP motor and alternator and 24V operating voltage. I would use 4 batteries (48V) + a generator and commercially available 48V multi-stage inverter/battery charger. Then you can also charge by plugging into the wall if desired. And you can use the batteries to run a power tool for a minute or two if needed. Or start the generator and run for an extended period. \$\endgroup\$ – mkeith Feb 9 '16 at 6:34
  • \$\begingroup\$ mkeith, thank you for your reply. Your suggestion of using batteries only is the most recommended configuration as attested by the 4,388 electric vehicle projects listed in the website evalbum.com. I had hoped to utilize my 8hp I/C engine to extend the mowing time, if and when needed. \$\endgroup\$ – Colimotl Feb 9 '16 at 18:31
  • \$\begingroup\$ I was not suggesting to use batteries only. I was suggesting to use commercially available parts, including a 120V AC generator (or 240V depending on where you are). But if you already have the 8hp motor, I can see the temptation to put it to use. Maybe you can get a small AC generator (of the appropriate voltage for where you are) to connect to the 8 hp motor. And then use an inverter/charger for the battery bank. Just a thought. The generator, without any drive mechanism, is called a "generator head." You can google that. \$\endgroup\$ – mkeith Feb 10 '16 at 5:38
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If you want to charge a 24V battery from a 12V alternator, you will need a huge boost converter, which will be spectacularly expensive. It would be better to just buy an alternator designed for 24V systems, e.g. from a truck.

Yes, a battery can act as a short-term power source, which means that you can get a bit more power in the short term than the alternator can supply. But you still need to pay attention to the thermal limits on the motor: 100A continuous or 300A for 30s (and probably off or very low load for a few minutes after a high-power burst, so that it can cool down again).

For example (totally making up numbers here) if you calculate that your average load will be 60A, you could use an 80A alternator. It would supply the base load, and then when you want a 300A burst, you can pull that from the battery. While the electric motor is cooling after its burst (e.g. drawing only 20A), the alternator will charge the battery.

With a motor of this design, voltage = speed and current = torque; normally you use a PWM speed controller which acts in conjunction with the motor's internal inductance to behave like a buck converter. If you run it from 24V not 48V then you can put in only 24*100 = 2400W continuous (electrical) power; it will run half as fast on 24V (about 2400RPM) as it does on 48V (about 4800RPM) while providing the same torque (a function of the 100A limit). If you want to get the full 4.8kW (electrical) / 4hp (mechanical) power, you need to run it at 48V and 4800RPM.

Note that the motor is rated as 4.8kW electrical and 4hp (3kW) mechanical. That means its efficiency under full load is about 63%, which is terrible. If you want 3kW of mechanical power, you need to supply 4.8kW of electrical power.

Because your choice of supply voltage affects motor speed, it will affect your choice of gearbox ratios.

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  • \$\begingroup\$ William, thank you for your prompt and detailed response. I purchased the Cab Cadet with a seized engine. In order to test the hydrostatic transmission, I purchased a 5.5hp Honda vertical shaft engine and mounted it in place of the 27hp Kohler. The tractor moves faster than necessary, and I’m able to pull a 300-pound lawn roller up a slight incline without the engine laboring. I’m aware that driving the three mowing blades takes much more power. For details on my project, see: gas-electric-tractor.weebly.com See additional comments below. \$\endgroup\$ – Colimotl Feb 9 '16 at 18:34
  • \$\begingroup\$ My objective is to replicate the gas-electric project of two east European fellows with whom I’ve been corresponding. They used a 5.5 HP Honda combustion engine, 4Kw DC electric generator, 3.5Kw electric motor — no batteries — in construction of a yard tractor. Their project may be seen at: youtube.com/…. \$\endgroup\$ – Colimotl Feb 9 '16 at 18:35
  • \$\begingroup\$ sounds like a fun project - my dad has the exact same Cub Cadet with the v-twin Kohler in it! You might benefit from ditching the hydrostatic drive, because its very inefficient and an electric motor doesn't need one because it's controllable down to 0RPM. Keep in mind that at 24V, your electric motor has only 1.5hp mechanical output, but that's enough to move pretty dramatically. \$\endgroup\$ – William Brodie-Tyrrell Feb 9 '16 at 21:06
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The horsepower specifications are inconsistent with the kW specifications. Power specifications for electric motors usually refer to mechanical power delivered whether it is stated on horsepower or in kW. One horsepower is 0.746 kW.

If you operate the motor at 24 volts, you will only get half of the rated horsepower.

In order to charge the batteries with from a generator with a lower voltage rating, you need an electronic voltage boost converter.

You need to look at the recommended charging current for the batteries.

The 100 amp rating of the motor is consistent with the rated output power. 100 A X 48 V = 4800 W

Other than stating "very high efficiency" losses are being ignored.

Additional Comments

The 4 Hp specification seems to be from Robot MarketPlace. They seemed to have confused kW with horsepower. If you look at the "Power Curves" under the "Other Documentation" tab you can see that for operation at 48 volts and 100 amps, the motor produces about 87 lb-in torque and 4050 RPM. That is about 4.17 kW or 5.59 Hp. Since the input power is 4800 watts, the motor efficiency is about 86.9% at that point. You also need to consider controller efficiency, it should be in the neighborhood of 95%.

It seems to me that you should consider how you are going to use the tractor and what benefit you expect to gain from converting it to hybrid electric. Then make a block diagram of the system that you need and study the complete performance characteristics of the components. Make sure that you understand the performance of each component and know how to calculate the relevant parameters from the component specifications and performance requirements.

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  • \$\begingroup\$ Charles, thank you for your prompt and detailed response. I have been corresponding directly with Motenergy. See mechanical drawing and performance curves at: electricmotorsport.com/ev-parts/motors/brushed-motors/…. Also, thank you for your additional comment. Needless to say, I am an “enthusiast,” not an engineer. I now realize how impracticable was the idea of having a 12v alternator charge a 24v series of batteries. See additional comments below. \$\endgroup\$ – Colimotl Feb 9 '16 at 18:32
  • \$\begingroup\$ If I don’t go with four 12-volt batteries as suggestsd by mkeith, I would like to explore the possibility of using a (used) 48V, 180A alternator. My concern is that the manufacturer states that the ME9090 is “Designed for battery operated equipment.” Again, I am concerned about the amperage factor. Please note that I do not need full potential power, I need only a safe constant power. I have only 1/2 acre of grass that takes me approximately 30-45 minutes to cut with a lawn tractor. No steep hills. See additional comments below. \$\endgroup\$ – Colimotl Feb 9 '16 at 18:34
  • \$\begingroup\$ It would be good if you referred to ME0909 rather that 9090. I tripped over that at first and other interested viewers may also. What I can see without signing in at electricmotorsport is identical to RobotMarketPlace. I suspect the curves and drawings are the same also. The motor will overheat if you load it to more that 100 amps for very long. A 180 amp alternator will run the motor and charge the battery simultaneously except for brief times when you might overload the motor. The motor controller should have a current limit to keep the motor at 100 A or less. \$\endgroup\$ – Charles Cowie Feb 9 '16 at 19:13
  • \$\begingroup\$ One final entry: If I were to use a more powerful I/C engine driving a (used) 48V, 180A alternator, which in turn would power a ME1004 PMDC Motor, (24-48V, 10.75 hp cont, 21 hp peak), capable of 200 amps continuous, would I need any type of interface? (Controller?) My objective would be to operate the I/C engine at one optimum speed. Ground speed would be varied with the hydrostatic transmission. Could the battery be eliminated (perhaps with the exception of a small 12V battery for the electric start)? \$\endgroup\$ – Colimotl Feb 11 '16 at 1:05

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