Brushless Motors: Sensored vs. Sensorless

There are all sorts of motors that qualify as brushless motors, but a significant point of difference between brushless motor designs used in RC truck applications is sensoreless vs. sensored operation.

Explaining the difference is best done by defining what sensored means. All brushless motors used in RC applications have three main wires, but sensored motors have an addition multi-wire loom that connects the “brains” of the speed control to the motor. On the motor end of the loom are sensors that tell the speed control a number of things, but most importantly the exact position of the rotor within the motor through three Hall Effect sensors.

Sensorless motors simply do not have sensors inside the motor—hence the name sensorless. They determine the likely position of the rotor based on feedback called back-EMF. EMF is short for Electromotive Force. Back-EMF also known as counter-EMF is essentially the resistance or voltage pushing back against the current flowing to the motor.

Logical question: why do you need sensors? Sensors are not required as is evident by the fact that sensorless motors work, but sensors do perform a valuable function. When the speed control knows the rotors specific location, it can better control throttle feel. This is noticed in a smooth controllable throttle feel and a truck that doesn’t hesitate at low speeds. Sensored motor manufacturers also state sensored designs have more startup torque or power off the line. The sensors can also tell the speed control if there is anything wrong with the motor. The speed control can use this information to shutdown the motor and avoid damage.

The natural follow up question: if sensors are so beneficial, why don’t all motors have them? One of the most noticeable downsides of sensored motors is that the sensor loom can break or come loose. Many speed controls will cease to power the motor if the sensored are not detected. Another downside is cost. More parts simply mean higher costs. In contrast, it is said sensorless motors are more efficient at high speeds.

 

Cogging
Cogging is a noticeable hesitation at low speeds, often when starting from a stop. Technically, this is the incorrect term and engineers use the word cogging differently, but the point is brushless motors can exhibit a very noticeable stutter at low speeds. Speed control manufacturers that favor sensorless designs go to great lengths with their programming to eliminate any stutter.

Tips for Reducing Cogging
> Install a smaller pinion gear
> Upgrade to high quality, higher C-rating batteries
> Use higher voltage batteries

Hybrid Controllers
In addition to strictly sensorless and strictly sensored controllers, there are two types of hybrid brushless controls. One type, such as the Traxxas Velineon VXL-3s, will automatically switch to sensored mode when a sensor loom is installed and sensors are detected. Another type of hybrid, such as Tekin RX8, switches from sensored at startup and low speed speeds to sensorless at high speeds.

 

Recommended Applications
For general bashing, you can use either sensored or sensorless. For rock crawling, a sensored system is mandatory. For racing, sensored or a hybrid controller that switches from sensored to sensorless is recommended.

 

Find out more information about brushless motors and Kv vs. turns here.

 

Special thanks to Novak for image of sensored endbell

 

Links

Novak

Traxxas

Tekin

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Filed Under: FeaturedGeneral RCTech

Matt Higgins About the Author: Matt has over 25 years of experience in RC and has worked professionally in media for over a decade. Matt enjoys everything from racing to rock crawling to bashing, and he believes RC should be all about having fun. Matt is as at home covering a world championship in an exotic country as he is showing a new hobbyist how to set gear mesh. His desire to share the hobby with as many people as possible inspired him to create RC Truck Stop and RCTruckStop.com.

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  1. Gene says:

    This is very useful. Keep up the good work.

  2. Martin Fisher says:

    Very helpful info. Thank you. Brushless motors are still new ot me

  3. Rob M says:

    This was very helpful as I am goin from brushed to brushless on 2 or my elec. rc cars

  4. SC10NOOB says:

    This was a huge help. I want to upgrade to sensored now and race this year

  5. Myrum Abel says:

    I run a sct late model class at our local track that everyone runs the same motor witch is a traxxas vileneon…I am running a mamba max pro esc and even tried my tekin rs esc…neither of them is getting any speed…some of the guys running same motor is hauling butt…I have done all I can think of…it’s like I’m only getting half power…I have changed batteries…radios…please help

    • Matt Higgins Matt Higgins says:

      If you have changed all of that, I assume you have changed the motor too (make sure you don’t have bum motor). Try this: Set your radio all the default settings and make sure your radio’s end points for throttle are set to the maximum and then recalibrate/setup the speed control to the radio per the instructions. Also, make sure you talk to the race director or track official about boost or timing. If boost or timing advance is allowed (non “blink” mode) and those guys are using boost that would/could explain the speed difference. Last, try some big changes in gearing to see how your setup reacts–just watch motor temps with each change. Check out this article on gearing.

      • Frank says:

        Hey there…. My name is frank. I just picked up a viper sensorless
        Brushless combo. Guys at hobby store swore it was good for the track.
        And the cogging is awful. Just ran for first time today. I’m kinda disappointed
        I put it in sc10 4×4. Is it just the wrong motor ?

        • Matt Higgins Matt Higgins says:

          You got steered wrong–in my opinion. I do not suggest sensorless for track use at all. Viper is a great brand for racing, but most of their systems are sensored and thus a”good for the track.”

          • Frank says:

            It’s seems that way….. Is it completely out of question to use it at track?
            I’m kinda pissed cuz we all race together and they own the shop I go to
            Is it dangerous or damaging to car? Figure you can trust
            The owner of a hobby shop

          • Matt Higgins Matt Higgins says:

            The cogging won’t hurt anything, but it’s irritating and ruins slow speed throttle feel. Experiment with gearing to see if you can diminish the cogging. Good luck. Since Viper is predominantly a racing company, they probably thought it would be good for the track.

          • Frank says:

            So throw some rally tires on it and make it a bashed then? Haha
            Thank you for the help. I realized today that ya can’t trust em all

  6. Tom_Banks says:

    Now i get it. thanks for explaining the differences. i just kept hearing sensorless was faster and therfor better

  7. Rick says:

    Matt,
    I have a neu castle 1410 that I am using in a non-traditional way. I am using it to drive an auger system in a model grain bin I built for farm shows. Can I use a regulated AC to DC converter to run the unit? I have a smaller motor that this works fine on but that one does not have enough torque to drive my system. Can I use a speed controller(i have one for my smaller motor) to drive this neu castle motor? I am not sure of the specs I would need(voltage, amps) but I was hoping you might be able to assist me so that I do not have to spend $200-$300.

    Thank you,
    Rick

    • Matt Higgins Matt Higgins says:

      As you know, Castle Creations works with Neu to make that motor, so your best source of info would be the tech folks at Castle Creations. They do list that motor as intended for use with 2S or 7.4 volts with a maximum of 3S or 11.1V. Castle Creations recommends its Sidewinder SV3 speed control and Mamba Max Pro–$120 and $127, respectively. I’d go with the Mamba Max Pro, but run it off 2S.

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