The electric motor stands as a cornerstone of modern engineering. It powers everything from household appliances to industrial machinery and now even the personal car. At its core, a standard electric motor comprises several key components working in harmony. A rotor sits within a stator usually surrounded by an enclosure or frame. Differential energisation of electrical coils within the motor facilitate a changing magnetic field which induces the rotor to turn. There is usually some way to mount the motor, as well as a way to wire it to a power source. Together, these elements form the fundamental structure of the electric motor, converting electrical energy into mechanical motion with remarkable efficacy. In this review, we look at some of the different types of electric motor in use today. These different devices are distinguished by their internal workings which usually also determines their suitability for a particular application.

More...

Electric motor diagram

Key components of an electric motor

To begin with, all electric motors group into being either AC or DC motors. Direct current powers DC motors, while AC motors require alternating current. In their simplest forms, AC and DC motors just have the essential hardware to make them run. Importantly, these basic AC and DC motors are usually the electric motor types being referred to when talking about  'AC motors' or ' DC motors', even though technically all electric motors run on either AC or DC.

The components that make up these bare-bones AC and DC motors include permanent magnets or a ferromagnetic core and the electrical copper coil windings. Sometimes control circuits, often integrated into motor drivers, manage the sequencing and timing of coil energisation. These basic electric motor types, however, do not have any control or feedback mechanisms. Therefore, they cannot monitor and control the speed and position of the rotor. Such elementary AC and DC motors are mostly used In situations where a motor just needs to spin and where variable speed and rotor position are inconsequential.

Brushless

Traditionally, electric motors have permanent magnets as part of the stator while the copper windings are part of the rotor (see the brushed motor diagram below). Electrifying the copper windings in the rotor creates the magnetic flux needed for it to rotate. However, as the rotor turns, there has to be a special mechanism to maintain that transfer of electrical energy to the rotor coils, since static wiring to a rotating core is obviously impractical. That special mechanism turns out to be a commutator and a couple of conductive contacts under tension. The commutator is a rotary electrical switch which periodically reverses the current flowing in the rotor coils. Spring-tensioned carbon brushes pressing against the commutator facilitate the flow of electricity into the rotor coils. Since this type of electric motor requires carbon brushes to run, it is often referred to as 'brushed'.

Diagram of the brushed electric motor type

Simplified diagram of a brushed motor

Diagram of the brushless electric motor type

Simplified diagram of a brushless motor

Today, modern electric motors are increasingly of the brushless type. In brushless motors, the copper windings have been moved from the rotor to the stator, and the permanent magnet is now part of the rotor instead of the stator. This arrangement eliminates the need for delivering electrical energy to a rotating central shaft. Consequently, one of the immediate benefits of this design is that it does away with the need for a commutator and carbon brushes completely. Accordingly, the lack of carbon brushes in electric motors of this type has seen them designated as 'brushless'.

Brushless Benefits

In practice, brushless electric motor types have a number of advantages over brushed ones. These are outlined below:

  • Brushless electric motors are usually more compact than brushed equivalents, allowing the devices they operate within to be smaller overall.
  • Brushless motors have reduced power loss due to friction, making them more efficient than brushed ones.
  • Because brushless motors experience very little friction when they rotate, they generate less internal heat. As a result, brushless motors can run continuously for longer periods without overheating.
  • The lower internal friction of brushless electric motors also makes them quieter than equivalent brushed ones.
  • As already indicated above, there is no need to replace the carbon brushes in a brushless motor. In brushed ones, the carbon brushes normally wear down with long-term use.
  • Brushless motors are able to go at higher speeds and generate higher torques than equivalent brushed devices.

There is, however, one disadvantage of brushless electric motors and that is they are generally more expensive than the equivalent brushed devices.

Servo Motors

The servo motor mechanism

As alluded to earlier, AC and DC motors do not receive any feedback on what the rotor is actually doing. Such a configuration is referred to as open-loop. However, in some applications, it is important to know how much the rotor has turned and the speed at which it is doing so. There are a couple of different ways to accomplish this, one of which is to use a servo motor, while another is to use a stepper motor (discussed in the next section).

Servo motors are essentially AC or DC motors that operate within a closed-loop configuration. That is, they have an additional encoder component that keeps the motor electronics updated as to the movements of the rotor. With the information provided by the encoder, the motor driver electronics are able to monitor actual rotor position and speed, and to make adjustments when necessary.

Stepper Motors

As with servo motors, stepper motors also try to keep track of rotor position. However, unlike servo motors, they do this indirectly by enumerating the degree to which the rotor should have turned rather than the actual rotation. To this end, the stepper motor relies on knowing the number of steps the rotor must move through to position it. 

Although all electric motors operate by changing magnetic fields in a series of steps, stepper motors take this much further by incorporating a lot more incremental steps. In practice, stepper motors have many more magnetic poles than non-stepper motors. This allows them to move the rotor much more precisely than an AC or DC motor. For instance, hybrid stepper motors often have step sizes of 1.8 degrees (200 steps per revolution) or 0.9 degrees (400 steps per revolution).

Notably, stepper motors can also be AC or DC-powered, however, the vast majority of them run on DC. In addition, all stepper motors have a brushless configuration with the magnet or ferromagnetic core as part of the rotor and the copper windings on the stator. Finally, as alluded to earlier, most stepper motors have an open-loop configuration, meaning they lack the feedback mechanisms to monitor the true speed and position of the rotor. It is important to note, however, that variants of the stepper motor exist which do have an encoder and operate within a closed loop. In some specialised applications, the use of such stepper motor-encoder combinations can be beneficial or even necessary.

Stepper Electric Motor Types

Importantly, stepper motors come in 3 main variations:

  1. 1
    Permanent magnet stepper motors
  2. 2
    Variable reluctance stepper motors
  3. 3
    Hybrid stepper motors

Permanent Magnet Stepper Motors

A permanent magnet stepper motor has multiple permanent magnets incorporated into the rotor. Just like other electric motors, energising the coils in the stator induces a magnetic field that interacts with those of the rotor's permanent magnets. Coordinated energisation of the coils induces the rotor to turn, while the multiple magnetic poles allow it to rotate in precise steps.

Permanent magnet stepper motor diagram

Simplified diagram of a permanent magnet stepper motor

Variable Reluctance (Switched Reluctance) Stepper Motors

Variable reluctance stepper motor diagram

Simplified diagram of a variable reluctance stepper motor

With variable reluctance (also known as switched reluctance) stepper motors, there are no permanent magnets. Instead, a rotor made of ferromagnetic material responds to the magnetic field induced by stator coil energisation. The ferromagnetic core has protrusions or teeth, some of which align with the energised stator coils. As with other stepper motors, the sequence of stator coil activation induces the rotor to rotate. However, unlike permanent magnet electric motors, the rotation is not due to the interaction of distinct magnetic fields.

variable reluctance in action

Variable reluctance in action (Carmelo 1955, CC BY-SA 4.0, via Wikimedia Commons)

Instead, the rotor in a variable reluctance motor moves to minimise the magnetic reluctance of the induced magnetic field flux (ie. the flow of magnetic energy) generated by the stator coils. 

The concept of magnetic reluctance can be thought of as being similar to electrical resistance but applied to magnetic field flux. Since magnetic field flux will always want to flow along a path of least resistance, it will always try to minimise magnetic reluctance. Therefore, the ferromagnetic rotor will have a tendency to rotate to align some of its teeth relative to the stator magnetic field. In this way, the magnetic reluctance of the field moves to a minimum.  

Once again, as with other electric motors, changing which stator coils are energised in a precise way forces the rotor to keep realigning itself thereby making it spin.

Hybrid Stepper Motors

As you might expect from the name, hybrid stepper motors are a cross between permanent magnet stepper motors and variable reluctance ones. They incorporate the useful features of both to create a more efficient stepper motor. Consequently, hybrid stepper motors are much more prevalent than either permanent magnet or variable reluctance devices.

Hybrid stepper motor diagram

Simplified diagram of a hybrid stepper motor

From their permanent magnet siblings, hybrid stepper motors have adopted the permanent magnet as an integral part of their rotor. However, unlike permanent magnet stepper motors, which have their magnetic poles arranged radially, the hybrid motor has an axially oriented magnet. This makes each end of the rotor opposing poles of the magnet. 

In addition, surrounding the magnet is a toothed ferromagnetic layer. Like variable reluctance motors, some of its teeth will align with those of the energised stator coils minimising magnetic reluctance. However, unlike a variable reluctance motor, the teeth will also have their own magnetic poles induced by the underlying permanent magnet. In this way, the hybrid stepper motor combines the torque-inducing capabilities of both variable reluctance and permanent magnet motors into a single device. 

Importantly, since the magnet is oriented along the rotor axis, the teeth at each end of the rotor will take on opposite poles of the magnet. Furthermore, the teeth at one end of the rotor will also be radially offset from those at the other end. Therefore, when looking down the length of the rotor, the magnetic poles on the teeth will appear to alternate (see the simplified diagram of a hybrid motor above). This alternating magnetic pole arrangement allows the rotor magnetic fields to interact with those of the energised stator coils allowing it to move in precise steps.

In reality, hybrid stepper motors have many more teeth than shown in the simplified diagram above. In fact, hybrid stepper motors appear more like the diagrams below. Each rotor segment or rotor cup, representing opposite poles of the underlying magnet, has a large number of teeth. The stator protrusions on which the copper windings sit, also have teeth. These align with those of the rotor when the appropriate stator coils are energised. In this way, the hybrid stepper motor can turn with far more steps than a permanent magnet stepper motor. Furthermore, the combined effect of magnetised teeth allows for a far more accurate and efficient stepper motor than one that works solely by variable reluctance.

Typical hybrid stepper motor construction

The components of a hybrid stepper motor (image credit: Orientalmotor)

Hybrid stepper motor turning mechanism

The high rotor / stator tooth count facilitates the precise stepping motion of the hybrid stepper motor (image credit: Orientalmotor)

Electric Motor Variants

Direct Drive

A direct-drive electric motor is a type of motor where the rotor directly connects to the load without any intermediate mechanisms such as gears or belts. Direct-drive motors have a number of advantages over non-direct-drive ones. These are:

  • They are more efficient
  • They require less maintenance
  • They operate more smoothly
  • They do away with the need for other components such as gears
  • They can generate a lot of torque
  • They have high position accuracy
A selection of direct drive electric motor types of direct drive motors

(Image credit: Allied Motion)

Direct-drive motors are commonly used in applications where precise control and high torque are required. This includes electric vehicles, industrial machinery, and domestic appliances. In addition, direct-drive motors come in two flavours: frameless and housed.

Frameless

Not all electric motor types have an enclosure. As with framed motors, frameless ones still have a rotor and a stator. However, frameless motors lack the frame or housing that contain these essential components. Since practically all frameless motors are direct-drive motors, they also lack an output shaft to drive the load. Instead, frameless motors are usually directly integrated into the enclosure and the driving mechanism of the final end product. In addition to the advantages of being a direct-drive motor, the lack of a frame also maximises the design flexibility with which it can be incorporated into its parent device.

Housed Direct Drive

In addition to frameless, direct-drive electric motor types can also come with their own housing. This subtype of direct-drive motor usually also includes additional components such as encoders and other sensors within the housing. However, unsurprisingly, design flexibility is negatively impacted as the frame specifications of housed motors also have to be considered when incorporating them into end products.

Housed direct drive electric motor types

(Image credit: Allied Motion)

Slotless

Finally, the last type of electric motor discussed here is the slotless variety. As the name implies, slotless electric motors have no internal 'slots' due to the complete absence of stator teeth. This means the copper coils that normally wind around the stator teeth have to be wound in a special way on the inner surface of the stator.

Slotless electric motors offer the following advantages over regular (slotted) electric motors:

  • They have a lower vibration
  • They generate lower amounts of noise
  • Slotless motors are also more efficient
  • They can have higher speeds
  • Slotless motors do not heat up as much when operating
  • They exhibit lower cogging (see next)
Diagram of the slotless electric motor type

Simplified diagram of a slotless electric motor

Cogging

Cogging refers to the jerky rotation that can occur in regular (slotted) electric motor types, particularly at lower speeds. This phenomenon arises from the interaction between the magnetised rotor and the teeth of the stator.

Like variable reluctance motors, a rotor will always move towards a state of least magnetic reluctance. This occurs each time a magnetic pole on the rotor aligns with an energised stator tooth. As the rotor spins, however, the aligned rotor pole will inevitably pass the location of least magnetic reluctance towards a position opposite the stator slot or inter-tooth zone. As this occurs, there will be a brief moment where the magnetic reluctance will be increasing. This acts to pull the magnetised pole on the rotor back towards the active stator tooth it has just passed. Repeated occurrences at each energised stator tooth ultimately causes vibration and noise in the electric motor. This is known as cogging. Cogging also results in losses in operational efficiency and the generation of torque ripple. Both of these phenomena can be particularly disruptive in some electric motor applications.

Cogging - minimum magnetic reluctance

When the rotor's magnetic pole is directly opposite a stator tooth, magnetic reluctance is at its lowest.

Cogging - increasing magnetic reluctance

Once the rotor's magnetic pole passes the stator tooth, magnetic reluctance briefly increases which resists continued rotation. Repeated across many teeth leads to the phenomenon of cogging.

Slotless electric motors have no stator teeth, therefore they are less susceptible to cogging. For some sensitive applications (such as medical instruments), especially where noise and vibration have to be kept to a minimum, the slotless electric motor is essential. 

Conclusion

The realm of electric motors encompasses a diverse array of designs, each tailored to specific applications and performance requirements. From the fundamental AC and DC motors to the more advanced servo and stepper motors, each type offers distinct advantages and capabilities. The evolution of technology has led to innovations such as brushless, direct drive and slotless motors, pushing the boundaries of efficiency and precision. As we continue to explore new frontiers and push the boundaries of what is possible, electric motors remain a cornerstone of innovation and progress in the modern era.