The range of models that define frameless motors adhere to all shapes and sizes towards the provision of both rotary-defined and linearly-defined motions. Their existence under the permanent magnet brushless motors characterizes them with high efficiency and high torque density. Based on the routine definition, they can be Servo Motors, Synchronous Permanent Magnet Motors, Brushless AC Motors, or Brushless DC Motors. The article takes a look at these motors on a slotted and slotless basis.
The Functional Challenge
Force (Linear) or Torque (Rotary) determines these machines’ key outputs. The numerous applications under tracking, photonics, metrology, scanning, and imaging face the primary challenge of working under a smooth and predictable force or torque. The working mechanism of the Traditional Permanent Magnet Brushless Motors is through exhibiting a cogging torque. It entails the interaction between the stator teeth slots and the permanent magnet as a result of a torque disturbance. The term cogging refers to an angular cyclical torque that results in a torque ripple and subsequent velocity ripple. Thus the motor control becomes a victim of a non-linear element.
This challenge has led to years of implementing various methodologies to cope with cogging. They include attributing the motor controller with electrical compensation, special mechanical modifications, skewing magnets, and skewing lamination. However, the success rate of such methodologies is of a marginal view considering the smooth-motioned applications that deal with light loads.
The Solution Criterion
The design of slotless motors optimizes a smooth operational mechanism. Minimal non-linear effects become evident due to it creating a predictable torque output. It is referenced as a slotless motor when dealing with rotary (torque) and air core motors when working under a linear (force) mechanism. The slotless motor air gap design only considers copper phase coils. The proper installation of these coils creates the needed torque or force after interacting with the permanent magnetic flux. Eliminating the discontinuity in the iron teeth from the motor-air-gap also gets rid of the cogging torque. The direct-drive precision systems benefit from the effectiveness of this slotless technology. It is possible due to the absence of uncontrollable motor torque disturbances as the phase current determines all the torque functions.
Motor Topology
The permanent magnet brushless motor functional design offers two critical internal parts. They are the electromagnet assembly and the permanent magnet assembly, respectively. When the technical operation is linear, the internal components are the forcer and the magnet track. In a functional rotary form, the reference of the internal parts is as stator assembly and rotor assembly.
The makeup of the rotor/magnet track includes a magnetic steel component. It is internally buried or affixed with permanent magnets. The composition of the stator/forcer comprises of a multi-phase electromagnet. A typical construction implements three phases. Thus let us consider the comparison and contraction of the slotless and slotted rotary motors on a technical note.
The Construction of the Rotor and Stator
1. Permanent magnet Rotor Assembly
A magnet attached steel ring or shaft-like form defines a typical rotor assembly. A single piece or mechanically discrete ring characterizes the magnet. The magnetization of the individual magnetic fields is also evident. The rotor assembly’s permanent magnets directly relate to the number of poles. The poles’ view can be in pole pairs like the north-south pole pair count. The mechanical speed and the electrical frequency transfer ratio determines the pole count. A typical functional scenario will see the lower pole count motors outperform the higher pole count motors in terms of mechanical speed. The higher pole count motors are, however, on the quest to achieve higher rates through the implementation of modern high bandwidth electronics.
2. Electromagnetic Stator Assembly
Several electromagnetic phases are a requirement needed to facilitate the rotation of a motor. The brushless permanent magnet motor operates on three typical phases. A motor controller outputs current that energizes these electromagnetic phases. The monitoring of the rotor position is through the use of encoder feedback by the motor controller. It thus leads to the creation of the required current vector needed to generate torque at the stator phase. The successful establishment of the torque will pave the way for its speed and position manipulation and thus sufficient for any given motion control application.
Radially protruding teeth on soft iron laminations constitute the stator assembly. Slots are the spaces in-between the teeth and allow the insertion of the electromagnetic coil wire. Thus a slotted motor is the best term to describe this motor.
2.1 Slotted Motor
The traditional design makes use of teeth to ensure the rotor magnets get the focused electromagnetic flux. It also contributes to the reduction of the magnetic circuit’s overall air gap. Multiple teeth exist in a single phase. Easy-to-manufacture, efficiency, motor constant, and right toque-output balance are factors behind the slotted motors predominating the motor topology. For any availed motor-size, the slotted motors will yield a resultant motor constant (Km) of the highest value. The motors are also responsible for the lowest attainable inertia by providing high acceleration and high-efficiency rates.
2.2 Slotless Motor
An angle sinusoidal torque output without the presence of higher harmonic distortions describes a perfect permanent magnet brushless motor. In terms of approximation, the slotless motor is nearest to this goal. They are without corresponding slots or stator teeth. The motor operation requires an electromagnetic phase relationship. Its formation is through the spatial orientation of phase coils. Energized coils will create electromagnetic fields like the ones under a slotted motor. However, the resultant output will be a sinusoidal torque-versus-angle-curve.
The absence of corresponding slots and teeth cripples the existence of cogging torque. The lack of stator teeth in slotless motor design allows the presence of a large magnetic-air-gap between the stator and the rotor of the motor. Thus regardless of the motor size, there will be a corresponding lower torque output from a resulting lower flux density. The slotless technology is a preference for a critically-defined operation. However, the critical requirement of a continuous torque solution will demand the applicability of the slotted motors.
3. Torque Versus Angle Curve
Torque depicts a rotary motor’s key output, which applies to both the current and position functions. The torque-versus-angle-curve methodology is commonly in use in the analysis of this phenomenon. It determines the motor torque output with the inclusion of the cogging torque. It thus gives the closest analytical and verifiable figure that predicts an application’s motor performance. To put the methodology to the test, you will have to energize a motor phase and, at the same time, do a manual rotation of the rotor. Also, take the measurements of the torque transducer’s created torque.
A general sinusoidal shaped torque-versus-angle-curve profile applies to all the brushless permanent magnet motors. Some harmonics usually also refer to it. Harmonic distortions of significant proportions can also result from the contributions of cogging torque. Thus the velocity ripple is impacted by the motor operation resulting from a torque ripple distortion.
Conclusively, the operation of a Servo Control System should consider slotless motors if it requires good Kt (Torque Constant) linearity and smooth operations. It should, however, settle for the slotted motors if it requires high acceleration and high torque density. The difference in motor designs will impact how a cogging toque affects motor performance. However, stator laminations and magnet skewing are some steps that will minimize the cogging torque impact.
These two mentioned technologies come with large-through-holes and are useful in the design of applications that warrant a low profile direct drive.
