If you've been busily clicking around Hobbyking since I last linked the site, you may have noticed that there are literally hundreds
of choices to pick from in the speed controller category. Almost all of them seem like
they should work for vehicle control. And this is true for the most part - they will start and run a motor, but some have characteristics which make them less suitable, and all of them
are overrated. Literally. So before I start picking favorites, here's a section on...General Guidelines for Picking Generic ESCsHigh voltage specification and Battery Elimination Circuit
The cutoff for when a controller is considered "High Voltage" seems to be around 6S lithium batteries, or around 24 volts. Below this voltage, controllers often come with linear
regulated power supplies built into them which can power (in their intended application) a receiver and servo motors, called a BEC. This is handy for when you need to convert a throttle signal (more on that later), but the logic power supply is often not enough for things like lights. However, linear supplies work by burning off excess voltage - therefore they tend to run hot and the power dissipation is unacceptable for voltages much above 20 volts for most 5 and 3 volt regulators. Therefore, some controllers will advertise a "SBEC" or "UBEC", both designations for a switching regulator built-in. These types are usually more expensive for the same amp count.
Controllers designed for use at above 6S ltihium cells (8S-12S) are usually advertised specifically
as High Voltage. These usually do not
come with a BEC and the signal wire only exists to take inputs.
It is generally not recommended to operate a R/C controller anywhere near
its maximum rated voltage. Due to cost cutting, components with breakdown and failure voltages just barely above the advertise threshold are often used - 25 volt capacitors and 30 volt power semiductors in a controller rated for 6S (22.2v nominal, 25v peak) are common. Power surges resulting from motor speed change can exceed those limits and cause rapid (fiery, smoky) controller failure. For EV systems, a HV type controller is most likely necessary unless you are intending to run an 18 volt system (5S lithium batteries or 15 nickel cells), which is actually just fine and workable - but less power than I would like.
Speaking of "rated", it's important that you...Derate everything. No, seriously, everything .
A controller the size of a credit card will not actually flow 100 amps all day at 36 volts. The specifications given for aircraft controllers are generally a continuous operating current at maximum temperature of the components assuming a constant high speed airflow is present
and there are no surges
. In aircraft, this is expected since propellers and fans create high speed exhaust streams, and throttle is generally constant and unvarying during cruise. In a vehicle... not so much. In fact, the opposite is true. You might be tempted to stuff the controller into the frame, this eliminating the airflow cooling potential. And a vehicle is pretty much entirely a surge load. Most industrial controllers achieve their ratings using a continuous thermal equilibrium method too, but with no assumptions of cooling. Surge ratings are often a function of maximum acceptable semiconductor or circuit board trace temperature rise, or possibly a maximum voltage ripple specification. Things aren't that strict in the modelling world.
Given no airflow (convection-only cooling), it's wise to derate the "continuous" amps by at least 3 to 5 times and the maximum current, usually given as no more than a few amps above the "continuous" rating, by 25% to 50% (i.e. reducing 100 amps to 75 or 50). If you can build in active cooling, such as a fan or waterblock, you are much better off. However, I'd still derate continuous amp draw by 40% or more (i.e. a 100A controller is a 50-60A controller).
Peak demand on acceleration can of course exceed this maximum, and the more frequent the excursion the closer to the lower end of the derate is necessary. Frequent start-stop driving in a city environment would warrant more derating, cruising along a trail or roadside less. It depends on the application, but current capacity must
be derated as part of design.Soft-Start and other special features are useless
Many ESCs come advertised with a 'soft start' feature. What that does is reduce the amount of current the controller delivers to the motor upon startup, such that your airplane doesn't start spinning instead of the propeller. However, we're not driving propellers with this, and therefore all the soft start does is make the vehicle more likely to cog and rattle on startup. Lower current means lower force to overcome inertia. You must check the calibration settings to make sure it is turned off.
Another common feature is speed governor mode, commonly used for R/C helicopters which keep the rotor speed constant but vary thrust by changing the blade pitch. Speed Governer mode sets an internal control loop that links motor speed with throttle position. While this may sound like what an ESC should be doing
in the first place, the default most is in fact open loop
- if you set a certain throttle, and then grab the motor and apply load, the controller does not try to compensate for the extra load, and the motor slows down. With governor mode on, the controller actively ramps up the power applied to the motor to try and keep the speed constant.
I imagine this would function kind of like eternal cruise control on a vehicle, but I have never experimented with Governor mode on my own. Sudden changes in speed command with a closed loop control are liable to cause current surges no matter if the command difference is large or small, in my opinion, and it should be avoided.Timing is important
While the technical explanation is mostly out of the scope of this Instructable, you will also see things like 7.5, 15, 22.5, 30 degree, etc. adjustable timing on the controllers. This part is
important: timing refers to how soon the controller switches to the next motor state in relation to the magnet rotor position. This affects the current draw and torque production of the motor at high speeds. The simple technical reason is that winding inductance
causes slow current switching in the motor coils, and at high speeds, the stator magnetic field generated by the windings falls behind the magnet rotor position, causing reduced torque and wasted power since all of it isn't going to producing torque any more.
For outrunner motors especially, high timing is crucial. The massive iron cores in most outrunners have lots of inductance, and on most product pages they will even warn you to use "high timing" for outrunners.
By the way, adjusting this timing advance on-the-fly using current and rotor position feedback such that the motor always produces maximum torque is called field oriented control
or vector drive
. If it looks alike, it's probably alike.
Most "cheap" Chinese ESCs descend from a few original bloodlines and are knockoffs of eachother. It's some times amusing to watch emergent strains that combine features of other controllers into one, and Hobbyking is a veritable tropical rainforest of a controller ecosystem. Since most of these controllers are made by a few houses and just colored differently depending on the contractor or distributor, one behaves much like another for non-HV types. The difference between one LV 80 amp controller and another on Hobbyking are probably minimal to literally none. HV designs are more varied, however, so this guideline doesn't really apply for them.Controller Examples
So now I will introduce some of the common aeromodelling controllers I have seen or have personally used in vehicle builds. Most manufacturers and distributors will explicitly warn you against using their controllers on vehicles, since as mentioned before, vehicle loads and propeller loads are vastly different and the software written for one is suboptimal for use on the other. So I guess I should really say that you should not use airplane controllers on any kind of passenger-carrying ground vehicle.
Now then, onto my personal favorite for these kind of things, brought to you once more by Hobbking:Turnigy Sentilon 100A HV
Pictured in the lead image for this page, this controller has some of the best low-speed stability (lack of cogging) I have seen. It seems to be a Hobbyking exclusive version of a HobbyWing (watch those letters) OEM design
, which seems to be a possibly grey market evolution of the Jeti Spin Opto 99
. For an airplane controller, it seems to be rather primitive and limited in configuration options; but this is what makes it well-suited to being a vehicle controller. This is the controller I have run in melon scooter and several of my hub motor scooters in the past.
This thing can actually sustain 90 to 100 amps in strong airflow (having been tested on a ducted fan experiment of mine), but in vehicle usage with convection cooling I would try to make cruise current no more than 30 amps and keep peak currents under 100 amps. The second image shows what happens when you try to pass 140 amps through it for 5 seconds. However, otherwise, at a dollar per amp, it's quite economical for powering vehicles in the 1 to 2kW range.Castle Creations Phoenix HV 110
Castle is a long-time manufacturer of model controllers whose products have also seen extensive usage in small EV conversion. The Phoenix series has the reputation within the electric bicycle community of starting and running just about anything
- meaning its estimation algorithm is robust and can handle faulty or poorly wound motors. I haven't personally used the Phoenix line since they are much more expensive, but Jerome's scooter
uses a Phoenix 60 with a 50mm type outrunner. Being a reputable brand in the R/C community, I would expect their ratings to be closer to the truth.Larger Sensorless Controllers
There are indeed much larger sensorless controllers, up to 200 and 300 amps capacity. However, while I will list them, I will not
recommend them - this amount of power should be handled by a real vehicle controller, which can pay attention to how much current it is passing (R/C controllers do not actually measure and control current draw), and have more robust designs, more failsafes, and vehicle-related features. The "legit" brands are getting close to, or exceeding, the cost of a vehicle controller for the same number of amps anyway!
The Turnigy Monster 2000
seems to have had a first-generation spontaneous combustion problem, but more recent reviews are positive.
The JETI Spin 200 and 300
are some of the best model ESCs available, and are ungodly expensive as a result.
If you want even more extravagance, Schulze is like the Armani Exchange or something of the R/C world, and they have 300 to 400 amp designs
. Their controllers also feature options like full datalogging and sensored commutation
(but if you want sensors, there are better options... read on!)Update November 2013
: I've added some more controllers to this list; ones that I've used, have, examined, or otherwise have messed with.Trackstar 200A 8S 1/5 Scale
This humongous thing is the subject of a full teardown on my website. It has ultra-low resistance, a ground-vehicle optimized starting routine, and active fan cooling. Based on the semiconductor count and thermal design, I think this can push 70-80 amps in a vehicle application, and definitely nameplate for acceleration. I'll have more to say about this once I have it on one of my own devices, but for 50 cents per (nominal) amp with actual robust construction, it's most likely legit.dLux 160A HV
This line from Hobbyking seems to be a derivative of the Castle series below. They have a double sided heat sink (and the FETs on both sides to use them). I paired one of these with a 50-60 type outrunner, and two of that combo push around my go-kart BurnoutChibi
with no problems. Based on examining their power devices and board layout, I think these are suitable for 25-30 amp continuous current draws in a vehicle application, with short bursts up to nameplate. There are "shorter" versions of it with 120 and 80 amp nameplate ratings, but I'd say the overhead is worthwhile since this is an air ESC and the starting algorithm is basic.Generic "200A" Suppo-type controllers
These are marketed under various brand names with slight variations - I took apart no less than 4 of them in the same post
that I investigate the Trackstar 200A. I both love and hate these, because there's some things about them that are nice but most things about them are disappointing. The nice things:
- They are extremely simple and basic programming and intelligence-wise
- The number of devices in parallel make for good burst and acceleration current overhead, from sheer thermal mass
The horrible things:
- The three stacked layers of FETs mean that there is extremely poor cooling no matter how hard you fan it - only the top layer receives significant cooling. You'd need to unshell the controller and fan it directly, and even then the center "core" is likely to remain hot.
- The heat sink often is installed incorrectly and might cause phase-to-phase shorts as they contact the output wires
- They're low voltage, relying on paralleling many shady or off-brand semiconductors to achieve the current rating, and the switching time is long, so they switch inefficiently.
However, I think these things can be reasonable for a scooter type application, powering a single 50mm class motor, at no more than 28 volts (7S lithium polymer or 8S A123/ LiFePO4), between 20-30 cruise amps and brief excursions up to 100 amps. They're about the cheapest way to get a ton of amperage overhead for acceleration.Types to watch out for
I only have two data points as of right now based on personal experience for controllers to absolutely avoid: The "K-force" series from Hobbyking and the "Super Brain" are all very smart (read: specialized) controllers which implicitly perform soft-starting and locked rotor detection. They will treat almost all low speed loading as a stall condition and shut down. As outlined above, controllers that are too smart aren't well suited to running vehicles.