Slotted and Slotless Motors – A Comparison

Permanent magnet brushless motors are available in all sizes and shapes providing both linear and rotary motion. They are known for high efficiency and high torque density. They are also regularly called Brushless DC motors, Synchronous Permanent Magnet motors, Brushless AC motors, or Servo Motors. The important output of these machines is torque (rotary) or force (linear).

Smooth and predictable torque/force while moving is the key challenge for numerous applications in imaging, metrology, photonics, scanning, and tracking. Traditional permanent magnet brushless motors display cogging torque, a torque disturbance from the interaction between the permanent magnet and the slots between stator teeth. Cogging is cyclical torque with angle that develops torque ripple, (and corresponding velocity ripple), adding a non-linear element to the control of the motor.

Various techniques to lessen cogging have been used for years, they include; skewing magnets, skewing laminations, special mechanical modifications, and electrical compensation in the motor controller. In the majority of applications that require smooth motion with light loads, the success of these techniques is marginal.

The Solution

Slotless motors are engineered to enhance smoothness and produce predictable torque output with marginal non-linear effects. Typically referred to as slotless motors when rotary and air core motors when linear, slotless motor designs place only copper phase coils in the air gap of the motor. These coils, when positioned properly, interact with the permanent magnetic flux to form force or torque. Cogging torque is eliminated because the discontinuous iron teeth are removed from the motor air gap. Slotless technology is particularly effective with direct drive precision systems as all torque is a function of phase current and there are no undesirable or uncontrolled torque disturbances from the motor.

Motor Topology

There are two critical internal parts to a permanent magnet brushless motor – the permanent magnet assembly and the electromagnet assembly. In the rotary form, these two parts are known as the stator assembly and the rotor assembly. In linear form, they are the magnet track and forcer. The rotor/magnet track comprises of a magnetic steel component with permanent magnets affixed or buried internally. The stator/forcer is a multi-phase electromagnet with three phases as the most common construction. The following article will focus on comparing and contrasting slotted and slotless rotary motors. The same principals also apply to air core linear motors and actuators.

Rotor and Stator Construction

Permanent Magnet Rotor Assembly

The rotor assembly usually takes the form of a steel ring or shaft with magnets attached. The magnets can be mechanically discrete or a single piece ring with the separate magnetic fields magnetized into it. The number of poles is directly related to the number of permanent magnets on the rotor assembly. Poles are at times defined as pole pairs, i.e. number of north-south pole pairs. The pole count is the transfer ratio between mechanical speed and electrical frequency. Usually, lower pole count motors operate mechanically faster and higher pole count motors function slower, but advanced high bandwidth electronics are progressively capable of attaining high speeds from motors with high pole counts. Figure 1a illustrates an example of a rotor with mechanically discrete magnets from a Celera Motion Omni™ Series motor.

A Celera Motion Omni™ Series direct drive low profile rotor

Figure 1a. A Celera Motion Omni™ Series direct drive low profile rotor. The magnets are bonded to the surface of the rotor hub ring. This example has 32 poles and the magnets are shaped on the surface to minimize cogging torque.

Electromagnetic Stator Assembly

For a motor to rotate, many electromagnetic phases are required. There are normally three phases in the brushless permanent magnet motor. These electromagnetic phases are energized through existing outputs from a motor controller. The motor controller usually uses encoder feedback to monitor the position of the rotor and develops the precise current vector in the stator phase set to produce torque. Once torque is developed, it can be regulated together with speed and position to manage any motion control application.

The stator assembly usually has soft iron laminations with radially protruding teeth. The spaces between these teeth known as slots allow electromagnetic coil wire to be inserted. This motor type is called slotted motor. Figure 1b illustrates an example of a Celera Motion Omni™ Series motor.

Electromagnetic Stator Assembly

Figure 1b. A common direct drive frameless style motor with high pole count and ring type low profile construction. The teeth and slots of the stator are clearly visible in this example. The gray ring in the center of the motor is the rotor made of magnetic material with 36 magnetically skewed poles magnetized into it. Note the pictorial approximation of these poles. The skewed pattern helps to minimize cogging torque by axially altering the magnetic field and compensates for the fundamental frequency of the cogging torque.

Slotted Stator

Traditional slotted stator designs use teeth to focus electromagnetic flux towards the rotor magnets and decrease the total air gap in the magnetic circuit. There are typically multiple teeth per phase. Slotted motors are the predominant motor topology as they provide a good balance between torque output, efficiency, motor constant, and manufacturability. Slotted motors usually yield the maximum motor constant (torque/watt1/2) for a given motor size. They also offer high efficiency and high acceleration rates with the lowest inertia.

As mentioned above, the spaces between stator teeth allow for electromagnetic phase wire to be inserted. The slots are the main cause of cogging torque as they create a discontinuous permeability as the magnets travel past each slot. It is standard practice to skew or stagger stator teeth or rotor magnets to reduce the fundamental frequency of the cogging torque. Figure 2 below illustrates a Celera Motion Omni™ Series stator with skewed teeth.

Slotted Stator

Figure 2. The teeth and slots of the stator are clearly visible in this example. The skewed pattern helps to minimize cogging torque by skew that compensates for the fundamental frequency of the cogging torque.

Cogging torque in slotted motors usually ranges from 1 to 5% of a motor’s rated torque based on the design of the motor. In applications with heavier payloads, the cogging torque is small compared to the driving torque and has a minor impact on system performance. However, in applications with lighter payloads or where smooth motion is a critical necessity, the cogging torque normally results in velocity ripple, which can have unfavorable effects on performance.

Cogging torque frequency is a function of the motor’s slot count and pole count. The important frequency of this cogging is the least common multiple between the pole and slot count. However, because of manufacturing variability and 3D effects, there are also lower and higher harmonic attributes to the cogging torque profile with angle.

Slotted stators also experience magnetic saturation with growing current. This is also denoted as torque constant (Kt) linearity. To improve the motor size and output, it is common for the iron to be near magnetic saturation at the continuous thermal limit of the motor. In certain motors, as much as 10% Kt linearity error is embedded into the continuous output rating.

Slotless Stator

The ideal permanent magnet brushless motor has sinusoidal torque output with angle without any higher harmonic distortion. The slotless motor is the closest approximation to this goal. The slotless stator does not have stator teeth or their corresponding slots. Phase coils are spatially oriented around the stator to create the electromagnetic phase relationship needed for motor operation. When energized, the coils form an electromagnetic field similar to the slotted motor, but result in a torque versus angle curve that is sinusoidal. There is zero cogging torque as there are no teeth with corresponding slots.

Figure 3a illustrates a Celera Motion Agility™ Series slotless motor with a rotor built with mechanically discrete magnets. Figure 3b illustrates a Celera Motion Agility™ Series slotless motor with a rotor engineered with a single piece ring magnet.

Slotless Stator

Figure 3a. The rotor in the center is made with 8 discrete magnets and has eight poles. Slotless stators have a very thin radial cross-section allowing for larger rotor diameter.

Slotless Stator

Figure 3b. The rotor in the center is a single piece ring magnet with 12 poles

With a slotless motor, all torque is a function of current applied to the winding. This streamlines the servo control system and allows for smoother running. The motor also has considerably better Kt linearity over its slotted counterpart.

One consideration with slotless designs is the large magnetic air gap between the rotor and stator because of the removal of stator teeth. This results in lower flux density and correspondingly lower torque output for a particular size motor. The torque output of a slotless design is usually 70-75% of an equivalently sized slotted motor and Celera motion can improve many Agility Series motor designs to attain up to 85%. If smoothness is critical, the slotless technology is favored, but slotted motors are probably a better solution if continuous torque is the most critical need.

Torque Versus Angle Curve

The central output of a rotary motor is torque, which is a function of both position and current. The most common method used to test this phenomenon is a torque versus angle curve. The torque versus angle curve represents motor torque output, including cogging torque, and is the closest figure of merit to forecast how the motor will perform in the application. Torque versus angle can be measured by energizing a motor phase while manually rotating the rotor and measuring the torque produced with a torque transducer.

All brushless permanent magnet motors have a torque versus angle profile that is commonly sinusoidal in shape. It typically contains several harmonics. Cogging torque is one of the contributors that can result in substantial harmonic distortion. This distortion causes torque ripple as the motor is operating and will affect velocity ripple.

Figures 4 and 5 show how cogging torque is the main difference between slotless and slotted motor technologies. Figure 4 plainly illustrates that when a slotted motor is not working at its full rated torque, the cogging is a relative percentage of the output and torque ripple is significantly higher. It is easy to see in Figure 5 that there is zero cogging torque in the torque versus angle curve.

Torque Versus Angle Curve

Figure 4. The purple curve is the theoretical sinusoidal torque versus angle. The green curve is the compromised result of cogging torque. The red curve is cogging torque and the blue curve is resulting torque from all three phases operating together. All values have been normalized to 1 for ease of display. This example has cogging torque at 5% of the motors rated torque.

Torque Versus Angle Curve

Figure 5. The absence of cogging torque allows the motor to produce a constant torque vector as it rotates or moves. All torque is directly related to the current supplied to the winding. Torque output is linear with changes in current and motion much more controllable. The purple line above is superimposed on the green line, so both curves are overlapping.

Summary and Conclusions

At Celera Motion, the product offering includes both slotless motors and slotted motors. Slotted motors are good for high acceleration and high torque density, while slotless motors are optimum for good Kt linearity and smooth operation when working in a servo control system.

Cogging torque will differ significantly with different motor designs and steps are usually taken to reduce its impact, like skewing the magnet or stator laminations. Both technologies provide large through holes and can be engineered for low profile direct drive applications. The main performance characteristics of each motor type are summarized in Table 1.

Table 1. Summary of key performance characteristics of slotted and slotless motors.

Parameter Slotted Slotless
Smoothest Motion (Lowest Velocity Ripple)
Highest Torque Constant
Torque Constant (Kt) Linearity
Largest Through Hole
Highest Acceleration Rates

Slotless technology is provided in several product lines from Celera Motion, including Agility™ Series frameless rotary motors, Infinity™ Series arc-shaped frameless motors, Javelin™ Series frameless linear motors, and AgilityRH™ Series rotary actuators.

Celera Motion

This information has been sourced, reviewed and adapted from materials provided by Celera Motion.

For more information on this source, please visit Celera Motion.


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