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 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
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.
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
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.