What’s the difference between BLDC and synchronous AC motors?

The short answer is: brushless DC (BLDC) and synchronous AC motors are similar in construction and operation. Some manufacturers and experts even group them together as similar technologies, in the category of  “permanent magnet synchronous motors.” Their key difference, however, lies in the stator coil windings and resulting back-EMF waveform of each motor. This gives them distinct performance characteristics and dictates separate drive techniques for each.

Similarities in construction

Despite the specificities of their names, BLDC and synchronous AC motors are both brushless, and both run at synchronous speeds. Brushless means that they rely on electronics (typically Hall-effect sensors), rather than mechanical brushes, to control current to the windings. And synchronous means that their rotor and stator magnetic fields rotate at the same frequency, or with synchronous speed

Both BLDC and synchronous AC motors have permanent magnets (typically four or more) mounted to the rotor. The rotor magnets can be either ferrite, which are less expensive but have a relatively low flux density, or rare earth alloy (such as Neodymium), which have a higher flux density but are cost-prohibitive for some applications. The stator is made of steel laminations, with windings (typically three) placed in slots cut axially in the laminations.

The rotor permanent magnets create a rotor flux, and current supplied to the stator windings creates electromagnetic poles. When the rotor position is such that a N pole of the rotor is near a N pole of the stator, the poles repel each other and torque is produced.

Differences in operation and performance

In BLDC motors, the stator coils are wound trapezoidally, and the back-EMF produced has a trapezoidal wave form. Because of their trapezoidal waveform, direct current is required in order to get the best performance form BLDC motors. In contrast, synchronous AC motors are wound sinusoidally and produce a sinusoidal back-EMF, so they require sinusoidal drive current in order to achieve the best performance.

The type of drive current also has an effect on the amount of noise that the motor produces. The trapezoidal drive current used by BLDC motors tends to produce a greater amount of audible and electrical noise in comparison to sinusoidally driven synchronous AC motors.

Commutation, which is the act of switching the motor phase currents to drive the appropriate stator coil, is determined by the rotor position. In BLDC motors, the rotor position is typically monitored by three Hall-effect sensors and commutation is achieved in six steps, or every 60 electrical degrees. Because the commutation is not continuous, a torque ripple is produced at each phase commutation (every 60 degrees).

Synchronous AC motors benefit from continuous monitoring of the rotor position via a single Hall-effect sensor or a rotary encoder, in conjunction with control logic. Because their commutation is continuous, synchronous AC motors are able to operate without the torque ripple found in BLDC motors. Sinusoidal commutation, however, requires more complex control algorithms than trapezoidal commutation.

While their construction is virtually identical, the difference in drive current and back-EMF between BLDC and synchronous AC motors is a significant distinction. Applying the appropriate drive current and control is an important factor in their operation and performance.