Why a Brushless DC Motor Is More Efficient Than a Brushed One

High efficiency

A permanent magnet synchronous motor does not need rotor excitation the way an asynchronous motor does, so there is no copper loss and no iron loss on the rotor. Caroute puts the gain at 5% to 12% over an equivalent asynchronous motor at rated load.

The rotor core is built from silicon steel laminations and the magnets are NdFeB, which has low permeability and high resistance. That combination cuts eddy current loss and, just as importantly, keeps the magnet material from demagnetising as it heats.

A wide high-efficiency band

Peak efficiency is easy; holding it is not. At rated load, system efficiency stays above 80% across more than 70% of the motor's entire speed range.

On a boat that matters more than a peak figure, because a trolling motor spends its life at part throttle, not at the one speed where a datasheet was measured.

Power factor close to unity

With no rotor excitation the power factor sits close to 1. Less reactive power is drawn for the same shaft output.

Starting torque and overload

Mechanical and regulation characteristics resemble a DC shunt motor: high starting torque with low starting current, and a wide adjustment range with no starting winding of the kind a synchronous motor needs.

Maximum overload torque reaches four times rated torque. That is the reserve that gets a loaded hull moving against a current.

Power density

Against an asynchronous motor of the same volume and the same maximum operating speed, output rises by 30%. A smaller housing does the same work — which is why a Caroute 80 lb motor runs from a single 12 V battery and a 160 lb motor from 24 V.

Tolerance of a poor supply

Under closed-loop speed control, with the supply voltage varying from +10% to −15% and a 40 K ambient temperature spread, steady-state speed deviation stays within ±1% of the set speed across 0 to 100% of rated torque.

Battery voltage sags as a bank discharges. This is what stops the boat slowing as it does.

Stability under sudden load

The speed control system is self-controlled. A sudden load change does not produce oscillation or pull the motor out of step — the failure mode that a synchronous machine is prone to.

Simple structure, easy maintenance

No brushes and no commutator means no wear parts in the electrical path. Caroute states five times the service life of a conventional brushed electric motor and no maintenance requirement.

What It Means for Your Boat

Every one of these shows up as the same two things on the water: more range from the same battery, and fewer things that need replacing at the end of a season.