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From brushed to FMI brushless AC motor

First-ever electromotors, dating back to the early nineteenth century, produced unidirectionally oriented moment of force utilizing mechanically engaged elements. As technological progress picked up pace, motion engineers have got rid of the mechanics, replacing it with solid-state components to deliver a more efficient technology—brushless AC motors and DC engines of the same no-brush type. So, what are the benefits of the modifications?

Brushed vs. brushless

To understand the basic distinction between the two technologies, it is essential to know their structural and operating particulars.

A mechanically commutated engine comprises a rotating wire-wound armature and a fixed-in-place ring with electromagnets or permanent magnets. In addition, the device includes a commutator, a shaft-mounted cylinder linked to the armature winding, and a couple of brushes, typically made of non-hard conductive substances (e.g., graphite).

As the armature is energized, a magnetic field is generated, making the rotor turn. The rotation continues in the same direction for half of the full circle, at which point current has to be reversed. This is the zero-torque point where brushes come into play, interfacing with the commutator to switch current. In this way, rotating motion goes on without interruptions, thus enabling the engine to produce torque continuously without changing it direction.

A brushless motor is identical to the previously described class in that both engines have moveable and stationary components. Namely, a rotor and a stator. However, brushless drives, in full concordance with the name, have no brushes to provide commutation. Instead, engines of the type are fitted out with semiconductor-based transistors.

Since mechanically engaged elements are no longer there to commutate, power is fed immediately to the stator coils, where it creates a magnetic field. Rotational movement results from the field interacting with the rotor magnets. Once the half-turn reference is reached, transistors do the job of reversing (or disconnecting) amperage, producing torque in the same direction.

What are the benefits?

Major improvements achieved through replacing the mechanics-based commutation with solid-state electronics are associated with the following characteristics:

  • size
  • efficiency
  • admissible speed range
  • safety
  • duration of life in service and maintenance costs
  • disturbances due to electromagnetic induction and loudness of operation
  • usage and regulation flexibility

The data below summarizes the distinctions between the two technologies based on the above specifications:

  • Using brushes to commutate
  1. Bulky because of the mechanical contact arrangement adding to both size and heft
  2. Rubbing of sliding contacts leads to substantial loss of capacity
  3. The admissible maximum RPM is limited due to the mechanics-based design
  4. Sliding contacts cause sparks; safe only in explosion-proof and non-flammable atmospheres
  5. Graphite commutating elements are easily worn out, contaminating the insides with dust
  6. Sliding contacts engaging with the commutating unit sound rather loud and generate considerable electronic noise
  7. Perform best at slow rotation rates, with torque degrading as a drive accelerates; simple velocity control
  • With brushless commutation
  1. Trimmed in weight and dimensionally after removal of the commutator assembly
  2. Notably high power-to-mass and torque-to-mass ratios
  3. Minimal rotor inertia enables motion at higher velocities
  4. Spark-free operation due to mechanical commutation replaced with electotransistors; safe to operate even in potentially explosive environments
  5. Reduced maintenance requirements and cost due to absence of wearable parts; enhanced durability with the operating resource tied only to bearing life
  6. Quiet operation and decreased electromagnetic interference
  7. Complex controlling logic, but extended capabilities to regulate motion, such as microstepping, holding torque delivery, etc.

From the comparison, brushless mechanisms look way better, fist of all, in terms of compactness and performance. However, mechanically commutated drives boast cheap price and simplicity, whereas the brushless type needs complex control arrangements and runs high risk of demagnetization at increased temperatures.

AC, DC and other configurations

Accounting for structural particulars, it is possible to subdivide brushless motors into outrunner engines and inrunner drives. In an actuator of the inrunner variety, a rotor ring fitted with magnets revolves inside a stator that fences the ring around. In outrunners, stator coils are fixed in place at the core of the assembly, whereas the rotor is on the outside and rotates.

As a rule, inrunner engines are smaller in diameter than the other type, but have a longer body. In terms of performance, outrunner drives deliver lower rotation rate per volt (Kv in specifications). Since Kv is in inverse relation to torque, outrunners generate larger moment of force quantities. Inrunners boasts better heat dissipation, demonstrating optimized efficiency as compared to the outrunner category.

According to the winding type classification, brushless motors incorporate coils of either Delta- or Y-configuration. In the Delta configuration, motor phases are wired together, with current supplied to the points of contact. In the Y-type winding (also known, as star), phases meet in the centre. In the delta configuration, torque delivery degrades as speed decreases, whereas the maximum velocity threshold is higher than with the Y-pattern wiring. Star windings are inferior in terms of maximum speed, but deliver considerable moment of force even at reduced RPM. The general assumption is that the the Y-type is more efficient.

Within the brushless permanent magnet category, it is also possible to distinguish DC and AC synchronous actuators. The engines have identical structural design: a rotor with ferrite or rare-earth magnets and a stator—no brush-and-сommutator assembly. Both brushless DC motors and AC drives are synchronous, meaning the magnetic fields, whether on the stator or the rotor, spin at identical frequencies and velocities. However, stators are wound in different patterns, entailing differing back electromotive force (EMF) profiles.

Technical details:

Back electromotive force is a phenomenon associated with electromagnetic induction, when specific voltage quantities flow across a conductor, opposing the change in applied current.

Rare-earth magnets are crystalline alloys containing rare-earth elements. Boasting excellent magnetic anisotropy, the magnets can deliver twice as much attraction force as compared to ferrite counterparts, thus creating a stronger magnetic field.

The stator of a brushless DC motor has winding of trapezoidal configuration producing a trapezoidally shaped back-EMF wave. Employing the sinusoidal winding pattern, AC actuators generate sine-shaped EMF waves, accordingly, which enables them to rotate smoothly while minimizing torque ripple. On the downside, the sinusoidal control relies on more complex algorithms than the trapezoidal one. Simultaneously, the trapezoidal configuration in a brushless DC motor is associated with substantial audible and electrical noise.

frameless motor (AC) brushless (48V)
FMI brushless AC motor (48V)

FMI brushless AC motors

  • The motors in the FMI series by Rozum Robotics boast proprietary design:
  • Brushless commutation minimizes friction-induced power losses, ensuring low-noise and spark-free operation and greater motor controllability
  • Durable and robust construction with a hollow shaft facilitating integration
  • Star winding allows for keeping up torque delivery even at slow rotation rates
  • With basic inrunner configuration and rare-earth magnets enables achieving ultra-high efficiency in compact dimensions  
  • Pertains to the AC synchronous subtype, providing smooth rotation without excessive cogging, noise, or electromagnetic interference
  • Temperature-sensing elements built into the rotor coils watch out for overheating to prevent demagnetization
  • Superior power-to-mass ratio, torque density, enhanced efficiency, and less trouble with maintenance—that’s what you get with an FMI brushless actuator.