Article: One Cable, Three Wires: How Sensorless Feedback Eliminates the Encoder Cable
One Cable, Three Wires: How Sensorless Feedback Eliminates the Encoder Cable
One Cable, Three Wires.
Sensorless Feedback in the Pit.
Why a Roger sensorless brushless operator runs on three wires that carry both drive current and position information — and why that matters most where moisture, depth and difficult access make every additional cable a future warranty claim.
A brushless motor cannot be commutated mechanically — there are no brushes to do the job. Instead, the controller must know where the rotor is at every instant and energise the correct stator phases to keep it spinning. How the controller knows rotor position is one of the central engineering decisions in any brushless drive. Roger uses two approaches across the brushless range, both built into the motor itself, and neither requires the dedicated external encoder cabling that defines feedback in industrial brushless applications.
A Sensor, and a Cable to Go With It
In a typical industrial brushless drive, rotor position is reported by a dedicated sensor.
Each requires power, signal lines and, in higher-resolution applications, shielded routing back to the controller. Each represents additional cabling, additional terminations, and additional points of potential failure.
“In an underground gate operator pit, every additional conductor is a future warranty claim waiting for water to find it.”
The Motor Is Its Own Position Sensor
Roger's sensorless brushless operators infer rotor position from the back-EMF generated by the spinning permanent magnets themselves.
A brushless motor with permanent magnets on the rotor generates a voltage on the stator windings as the rotor spins past them — back-EMF. The magnitude and timing of that back-EMF directly indicates the rotor's position. During PWM operation, the controller actively drives two of the three motor phases at any given moment. The undriven phase is electrically silent except for the back-EMF the spinning rotor induces in it. The DSP samples this signal between switching events and reconstructs the rotor's angular position from it.
The same three conductors that carry drive current also carry position information. There is no additional cable, no additional termination, and no additional sensor to fail.
More Precision, Same Cable Count
For applications requiring higher position precision — typically heavier or higher-speed installations — Roger fits a contactless magnetic encoder integrated into the motor housing. It's the same encoder covered in Why Position Beats Time, and what matters here is where it lives.
The encoder is built into the motor assembly, and its feedback runs on a small number of conductors integrated into the motor cable.
There is no separate, externally routed encoder cable — even in sensored configuration.
Count the Cables Into the Box
The dominant long-term failure mode on underground gate operators is moisture ingress at electrical terminations. The pit environment cycles between wet and dry, accumulates contamination, and is rarely accessed for inspection between failures.
Every electrical conductor entering the pit is a potential ingress path. Every termination is a potential corrosion site. The total reliability of an underground installation is, to a first approximation, inversely proportional to the number of separate cables that have to enter the box.
Mains or transformer output
Motor drive
Encoder or Hall-sensor cable
Safety device cables
Mains or transformer output
Motor drive — also carries position
Safety device cables
No fourth cable for feedback
Cables that aren't there can't corrode, can't be cut by a careless excavation, and can't fail their seals over a decade in wet ground.
Four Outcomes in the Field
Particularly significant underground, where every termination is a future failure point.
Three motor wires terminate at the controller. There's no separate encoder cable to pull, terminate, label and test.
With fewer cables involved, fault localisation is straightforward. The controller's onboard diagnostics report directly on the three motor phases, and the position feedback is verified continuously against the drive signal.
The most common failure mode on competitor underground operators isn't the motor — it's the encoder cable corroding at its junction with the controller or the motor. Eliminating the cable eliminates the failure mode.
Year Ten, Not Year Three
Sensorless brushless control uses the motor's own electromagnetic signature as position feedback. The same three wires that drive the motor also tell the controller where the rotor is. In above-ground installations, that's a clean engineering simplification. In underground installations, it's the difference between an operator that runs for ten years and one that develops a feedback fault in year three.
Roger built sensorless control into the brushless platform because the architecture is fundamentally better, especially where access is hardest.
The Cable That Isn't There.
Underground installations are where good specification either pays for itself or doesn't. They're the hardest jobs to access, the slowest to diagnose, and the most expensive to return to — often with the gate out of service while the pit is pumped out and the lid is lifted.
Most reliability engineering adds something: a better seal, a heavier gland, another layer of protection. Sensorless feedback does the opposite. It takes a component out of the pit entirely, and a component that was never installed is the only kind guaranteed not to fail.
We don't simply resell a brand. We choose components we can stand behind technically, and we explain them properly so installers and specifiers can make informed decisions. We distribute engineering, not motors.
— The Edgesmith Team
Part 06 — The Service Interval That Disappeared. What wears out on a conventional operator, what Roger engineered out of the system, and what that means for service contracts and warranty.