Brushless DC (BLDC) motors have become the prominent choice across electric vehicles, drones, industrial automation, HVAC systems and consumer appliances because of their high power density, long service life and controllability. 

Related Electrical Calculators

Unlike brushed DC motors, BLDC motors use electronic commutation instead of mechanical brushes which eliminates brush wear, reduces electrical noise and allows significantly higher speeds. 

However the true value of a BLDC motor in any application is ultimately measured by one number is its efficiency. 

Efficiency determines how much of the electrical energy supplied to the motor is converted into usable mechanical work at the shaft & how much is wasted as heat.

Understanding where these losses occur and how they change with operating conditions is essential for designers who want to extend battery life, reduce thermal stress and select the right operating point for a given load.

BLDC Working Principle

A BLDC motor consists of a permanent-magnet rotor and a wound stator, typically with three phases. 

An electronic controller energizes two of the three phases at any instant in a sequence determined by rotor position feedback (commonly from Hall-effect sensors), producing a rotating magnetic field that the rotor follows. This is known as trapezoidal or 120-degree conduction. 

Because two phases are always connected in series across the DC bus, the fundamental voltage balance for a BLDC motor at steady state can be written as follows:

V = E_b + 2 x I x R

Where

V – DC bus voltage applied by the controller

E_b – Back electromotive force (back-EMF) generated by the spinning rotor

I – Phase current and 

R – Resistance of a single phase winding. 

The back-EMF is directly proportional to motor speed through the back-EMF constant, Ke, so as the motor accelerates, Eb rises and the current the motor draws for a given voltage falls. 

This relationship is the starting point for any efficiency analysis because current and speed cannot be chosen independently; they are linked through the motor’s electrical parameters.

BLDC Motor Losses 

The difference between the electrical power delivered to the motor terminals and the mechanical power delivered at the shaft is dissipated as loss, almost entirely in the form of heat. 

In a BLDC motor these losses fall into three broad categories.

Copper (Resistive) Losses

Copper loss, also called I²R loss, arises from current flowing through the resistive windings. Because two phases conduct in series in a 120-degree trapezoidal drive, the total copper loss is:

P_cu = 2 x I² x R

Copper loss grows with the square of current, which means it dominates at high load and is relatively small at light load. It is also the loss most directly affected by winding design, wire gauge, and operating temperature, since resistance itself increases as the windings heat up.

Iron (Core) Losses

Iron losses occur in the laminated stator core due to hysteresis and eddy currents induced by the alternating magnetic field. These losses increase with electrical frequency, and therefore with motor speed, following an approximately power-law relationship. A practical empirical model widely used for estimation is:

P_fe = K_fe x (N / 1000)^1.6

Where 

K_fe – Core-loss coefficient specific to the lamination material and geometry, referenced to a speed of 1000 RPM 

N – Motor speed in RPM. 

Unlike copper loss, iron loss is largely independent of load current and is instead governed by speed making it the dominant loss source at high speed and light load.

Mechanical Losses

Mechanical losses include bearing friction and windage (aerodynamic drag on the rotor and any cooling fan). 

Windage losses in particular scale roughly with the square of speed, since aerodynamic drag power increases with the cube of velocity but torque loss increases with the square in most practical motor geometries. A common approximation is:

P_mech = K_mech x (N / 1000)²

Mechanical losses are usually the smallest of the three categories at moderate speeds but become significant in high speed motors such as those used in drones or turbochargers.

Calculating Overall Efficiency

Once the individual losses are known, the input electrical power, output mechanical power, and overall efficiency can be determined as follows.

P_in = V x I

P_out = P_in – P_cu – P_fe – P_mech

η = (P_out / P_in) x 100%

This breakdown allows an engineer to see at a glance which loss mechanism is limiting efficiency at a given operating point and to target design or control changes accordingly. 

Example: If copper loss dominates, reducing winding resistance or lowering current for the same torque (via a higher pole-pair or better magnet grade) will have the greatest impact. 

If iron loss dominates, reducing operating speed (or) switching to thinner laminations or lower-loss core material is more effective.

Efficiency Curve & Optimal Operating Point

A defining characteristic of BLDC motor efficiency is that it is not maximized at either extreme of the operating range. 

At very low current and low speed, output power is small, so even modest fixed losses (iron and mechanical) consume a large fraction of the input, driving efficiency down.

At very high current, copper loss grows with the square of current and quickly overwhelms the useful output. 

Between these two extremes lies a peak efficiency point, where the marginal increase in copper loss is exactly balanced by the marginal gain in output power. 

This is the reason for efficiency plotted against current (or equivalently against speed, since the two are coupled through the voltage balance equation) forms a characteristic hump-shaped curve rather than a monotonic one.

Locating this peak analytically requires differentiating the efficiency function with respect to current and solving for the stationary point, which becomes algebraically complex once the non-integer exponents of iron and mechanical loss are included.

In practice, a numerical sweep across the full current range from near zero up to the stall current limit of V divided by 2R is a fast and robust way to identify the exact current and corresponding speed that yield maximum efficiency for a given set of motor parameters.

Practical Factors affecting Practical Efficiency

• Winding temperature: resistance rises with temperature, increasing copper loss during sustained heavy loads.
• Switching losses in the controller: MOSFET or IGBT switching and conduction losses are external to the motor but reduce overall drivetrain efficiency.
• PWM ripple current: high frequency ripple superimposed on the average current adds additional copper & core loss not captured by simplified DC models.
• Magnet grade and demagnetization: weaker (or) degraded magnets reduce the back EMF constant that is requiring more current for the same torque.
• Cooling: Better thermal management indirectly improves efficiency by keeping the winding resistance lower.

Conclusion

BLDC motor efficiency is governed by the interplay of copper, iron, and mechanical losses, each of which responds differently to current and speed. 

Copper loss rises with the square of current, iron loss rises with a power-law function of speed, and mechanical loss rises roughly with the square of speed. 

Since, current and speed are linked through the fundamental voltage balance of a trapezoidally driven BLDC motor, there exists a single, well-defined combination of current and speed that yields the highest possible efficiency for a given motor and supply voltage. 

Identifying this point whether analytically or through numerical optimization allows designers and operators to size systems correctly, extend battery runtime and reduce thermal stress making efficiency analysis one of the most valuable tools in BLDC motor selection and control design.