akeeton said:
I've been looking at doing a scooter build for off-road purposes.
I'm modding/rebuilding an old busted up Mongoose scooter that was practically free. It has a 240 watt motor from Currie Technologies that I'd like to upgrade.
Narrowing down a motor has been... exhausting.
After pouring over the forums, it seems that the vast majority of people are using Turnigy motors. Are they rugged enough for off-road abuse? Given the fan-fare (and insane power output by comparison to the Currie), why would anyone choose not use those motors in their build?
RC brushless motors are basically hands-down a better deal for small electric vehicles than traditional DC brushed motors; you can get a 7-8kW peak, 3-4kW continuous brushless motor that's a bit smaller and lighter than the old 240W Currie; that 7kW peak motor used to go for around $100 on Hobbyking and now goes from around $175-200 through Alien Power Systems (or $120 shipped if you're willing to buy 10 or more of them from a factory owner on AliBaba).
As far as Turnigy motors in general go, they're decently rugged and have been used in off-road applications; the big question I have is how much power do you want for your price point? A Turnigy SK3 6374-149kV is good to about 3-4kW peak, and 2-2.5kW continuous (assuming a 36V or 48V electrical system; with a brushless motor in that power range running at 24V is just leaving power on the table). It's going to spin fast (149kV translates to a theoretical max speed of 5500RPM or so at the shaft at 36V, and about 7200RPM at the shaft at 48V), so you're going to want to gear it down 5:1 or so; at 48V a gearing of 5.5:1 gives you a max speed of 31mph on standard 8" pneumatic wheels, while at 36V, a gearing of 4.5:1 gives you a little over 28mph theoretical max speed.
Another really big thing to keep in mind: Turnigy-style brushless DC motors (despite having DC in the name) are basically miniaturized three-phase permanent magnet AC motors, which is why they can operate at far higher power ratings than brushed motors of the same size and weight; they use a fundamentally different commutation setup and they
will not play nicely with a brushed DC controller like the one the Mongoose comes with. A brushed DC controller is basically a throttle-controlled DC-DC converter; it takes a fixed DC voltage in from the battery and outputs a variable (the same or lower) DC voltage to the motor by varying the duty cycle of the internal semiconductors depending on the throttle input. A brushless DC motor requires that each of its three phases be switched on and off in a particular sequence in order to spin it, and the speed of the motor is varied through a combination of pulse width modulation (varying the duty cycle of the phase switched on) and varying the speed at which the phase switching sequence is executed. Making that work requires a decent amount of sophisticated power electronics, which you can find in the form of dedicated brushless ESCs (electronic speed controllers).
Now, depending on how you want to set up your scooter, you'll have a few different options when it comes to getting a controller. On the small, light, and fairly finicky side you have the Hobbyking brushless ESCs. They're typically about the size of a credit card, designed to interface with an RC throttle (which is very different from the 0-5V throttle the Mongoose most likely came with), a bit expensive, and more often than not severely overrated for vehicle applications. A Hobbyking "150A" RC airplane controller is going to be good for maybe half to a third of that in a vehicle, mostly because it's optimized for model airplanes. Model airplanes are very forgiving loads for a Hobbyking controller, mostly because the amount of torque a model airplane motor puts out (and thus the amount of current pushed through the controller) increases linearly with the speed of the propeller (and the velocity of the prop wash). A Hobbyking airplane controller typically sits right behind the motor, and thus is only pulling close to its rated maximum current while it's sitting in the middle of a high-speed stream of forced air. Vehicles don't typically offer anywhere near that degree of forced-air cooling, and typically demand full torque right at startup; trying to pull a full 150A (or probably much above 50-75A) out of a 150A RC airplane controller in a vehicle will probably blow it up.
On the larger, heavier, and much more durable side of things, you have the Kelly KBS miniature brushless motor controllers. They're the size of a small brick and weigh about two pounds, but they're fairly thoroughly optimized for use in small electric vehicles. They're designed to interface with a vehicle-standard 0-5V throttle (either a potentiometer, which is probably what the Mongoose came with, or a Hall throttle that works by swiping a magnet past an analog magnetic field sensor), they're quite physically robust, they're actually good to their nameplate ratings (a Kelly KBS48121X, which is rated for 130A peak and 55A continuous, will actually supply 130A for a short while until the controller gets hot and then throttle the current back down to 55A as needed. It will also supply 55A for nigh unto forever, and it's most likely not going to spontaneously blow up on you.) Kelly controllers also require Hall effect sensors mounted on the motor, which is a bit of a pain to troubleshoot at times and will run you a bit of extra money.
That said, a Kelly controller with Hall effect sensors will enable you to apply full throttle from a dead standstill the same way you could with your old brushed setup; this is generally less true for sensorless setups (especially Hobbyking controllers). A sensored controller can compute where the rotor is (and therefore which legs to switch initially) by reading the outputs of the Hall sensors regardless of how fast (or even if) the rotor is spinning. RC sensorless controllers typically sense the back EMF on the motor phases and use the back EMF waveform to determine when to switch the FETs. The problem with that is that a stationary motor emits no back EMF and so the controller can't actually tell where it is. Most RC sensorless setups get around this limitation by bumping the motor phases with small current pulses, then capturing the resulting back EMF and starting the commutation cycle in earnest; that works well for applications with viscous loads (airplane props, for example) or in cases where your drivetrain is geared down ridiculously far (where the apparent mass of the vehicle and thus the inertial portion of the vehicle load is minimal), but with low to medium gear ratios on a vehicle, the motor is loaded down enough that trying to bump the phases (especially if you slam the throttle wide open from standstill) will draw decent-sized current spikes. If you're lucky that only results in a lot of erratic rippling torque at very low RPM; if you're unlucky the motor will stall and make sad noises, and if you're having a really bad day one or more of the FETs in the controller will light on fire.
Finally, you're probably going to want to upgrade the batteries from the old 24V SLA system that came with the Mongoose. Here, you have
options. Again, because I'm a bit of an EV snob I would recommend trading up to lithium batteries of some sort; lithium batteries will pack the same energy as lead with better sustained current capacity in under a third of the weight and much less space. If you have a ton of money (or are existentially comfortable spending a ton of money on this conversion) you can assemble a pack yourself out of A123 26650 LiFePO4 cells. The A123 cells are incredibly high-power (capable of 120A constant current discharge
per battery), quite robust against a decent array of physical and electrical abuse, and fairly easy to keep in balance. It's also almost impossible to light them on fire by shorting them out, draining them down well below their LVC, or even overcharging them significantly; the first two of these things may not even do permanent damage to them. Unfortunately, they'll run you $8-$10 per cell (and so a 48V 10Ah A123 pack would cost you almost $500 plus the cost of a battery management system (BMS) or a balancing charger. A reasonably cheap BMS rated to 75-100A discharge will run you $50-$60 from various vendors in China, bringing your total battery cost for an A123 battery unit to $600-$650.
A more common, significantly cheaper (but somewhat more dangerous) solution for lithium batteries is (you guessed it) Hobbyking. You can get a number of 4-5Ah LiPo packs ranging in voltage from 4s (14.8V nominal) to 6s (22.2 V nominal) from Hobbyking; you're going to want to pick packs rated for 20-25C max discharge, where 1C is the current required to empty the pack in an hour (so a 20C 5Ah pack would theoretically be able to discharge at 100A without suffering ill effects). You can string Hobbyking LiPo packs together in series and parallel to produce a pack that comes out somewhere in the range of 48V and 10Ah nominal; 48V nominal can be approximated by setups on the order of 12s-14s LiPo (12s will run you a bit low, which will cost you top speed, and 14s is a bit high, and so a bit more costly but overall not a bad choice. 13s is probably a bad idea because your subpacks won't be identical and that can cause headaches). As with motor controllers, you're going to want to derate the battery discharge rate by 40-50% (and not actually approach it). If you want minimal voltage sag on a 10Ah Hobbyking LiPo pack, then you'll want to use cells on the order of 25-30C (giving you a theoretical max discharge rate around 250-300A and minimal sag somewhere around 75-100A). You can put together a 14s 10Ah LiPo pack from Turnigy 7s units for about $260 plus shipping and the cost of a BMS, which will probably come out to about $325-$350 total.
On the
really low end, 48V 10Ah of SLAs will run you about $120 plus shipping, or $150 or so total, with no BMS needed because lead-acid batteries are much less finicky about charging voltages and currents than lithium systems. Still, if you have the money I'd recommend going lithium.
In terms of power-to-weight ratio and reliability you're not really going to be able to beat a 63mm Hobbyking outrunner mated to a Kelly KBS controller and a 48 volt LiFePO4 battery system; the question then becomes how much money you're willing to spend and how much comfort you have (or are willing to spend a lot of time acquiring) with troubleshooting electromechanical systems. A Hobbyking SK3-6374 at 149kV will run you $80 plus shipping, and a 48-volt Kelly controller rated for 40A continuous (100A peak, which is more torque than you're probably going to be able to use on flat ground) will run you $120 plus shipping from Kelly. You can get Hall sensor boards and matching adapters for the 50mm, 59mm, and 63mm Turnigy brushless motors from Equals Zero Designs (full disclosure; that site is run by a good friend of mine named Charles Guan, and he has a
blog where he details most of his crazy EV projects. It's pretty good reading, and I recommend it if you're interested in doing this as a long-term hobby) for $40 or so. You'll want to add $80-100 to that for various auxiliary or drivetrain bits and bobs (sprockets, new chain, maybe a new throttle depending on how the old one interfaces with the controller), plus the cost of whatever battery system you choose to go with.
Converting that scooter to brushless power and replacing the batteries with 48V 10Ah of Turnigy LiPos will run you about $600 or so in total costs (with maybe a bit of overrun for shipping and unforeseen circumstances); converting the powertrain to brushless power and tacking on two more 12V 10Ah batteries (giving you 48V 10Ah in SLAs) will probably run you about $300 (but you'll want to budget around $350 to cover unforeseen cost controls). Either would be a pretty decent option; it all depends on how much you want to spend and how much time you're prepared to have this take. I've added a bunch of links at the bottom for you to take a look at; most of them are places where you can buy components, but I'm starting off with a link to one of Charles Guan's Instructables focused on selecting and sizing components for a brushless scooter power system:
Charles Guan's brushless scooter Instructable
Charles Guan's blog, including build logs of multiple scooters and other EVs
Hobbyking's full selection of "63mm" Turnigy outrunner motors (actually 59mm OD)
Alien Power Systems' full selection of brushless motors, including true 63mm OD motors
Kelly Controls, retailer of the Kelly controllers I mentioned above
Equals Zero Designs, retailers of
Hall effect sensor boards and
their matching adapters
A123 LiFePO4 cells (amazing if you can afford them, but extremely expensive)
Hobbyking's selection of LiPo batteries
