Electric motor types include AC, DC, brushed, brushless, stepper, servo, universal, hysteresis, synchronous, asynchronous, compound, single phase, three-phase, linear and inductor motors.
Factors used to classify the types of electric motors include current source, structural design, rotational speed, mode of operation, and purpose.
This article discusses the electric motor types, and their classification criteria, according to the outline below;
An Overview of the Electric Motor Types
The two main types of electric motors are AC and DC motors.
These are the main types because the type of current which is supplied to the electric motor determines how it will operate, as well as its optimal design and other factors.
AC electric motor depends on an alternating current supply to operate, while DC motor uses direct current.
Types of electric motors are discussed below, based on the factors used in their classification;
-Types of Electric Motors based on Current Source
1). AC Electric Motor as one of the Electric Motor Types
The AC electric motor, converts an alternating current to mechanical energy.
AC generators may come in various types as well, due to differences based on other classification factors like design and operation.
However, the two basic (sub) types of electric motors under this category are asynchronous and synchronous AC motors. These two are differentiated based on the relative rotation rates of the stator and rotor.
The AC electric motor is more widely used than DC motors. This is because AC is a more practical and versatile form of current electricity, giving the AC motor a wider range of application.
2). DC Electric Motor
A DC electric motor, converts a direct current to mechanical energy, in the form of rotary motion, through the effect of electromagnetism.
Three main subtypes within this category are the PMDC, shunt, series and compound wound motors.
In terms of design and structure, the DC motor is simpler than its AC counterpart.
Because of the unidirectional flow of DC electricity, DC motors do not operate in phases. Their output can also be controlled by varying the amount of voltage being supplied.
-Types of Electric Motors based on Structural Design
3). Brushed Electric Motor
Brushed electric motors are equipped with ‘brushes’ which are moveable connection terminals that supply electric current to the conductor coil.
Brushes are used alongside commutators, to transfer current from the stationary source to the rotating coil . They may occur in both AC and DC motors.
While they have a simple design, brushed electric motors require significant levels of maintenance and are susceptible to connectivity problems.
4). Brushless Electric Motor as one of the Electric Motor Types
Brushless motors are one of the most efficient types of electric motors.
In the absence of brushes and commutators, they depend on electromagnetic induction to sustain rotor movement and mechanical energy conversion. This is called electronic commutation .
Brushless motors require effective permanent magnets to operate. They are designed to be generally smaller and more efficient than the brushed electric motor.
-Types of Electric Motors based on Mode of Operation
5). Synchronous Electric Motor as one of the Electric Motor Types
Synchronous motor can be considered one of the types of electric motors which depend solely on alternating current .
The synchronous motor operates under stable conditions of electromechanical conversion. This results in constant rotation speed, which can only vary with changes in frequency of electricity supply .
Because the stability of rotation, synchronous motors are used in applications that require precision and consistency.
The constant electromechanical speed of synchronous motors is often produced through DC excitation, whereby a direct current is supplied to create the magnetic field .
Areas of application of this type of electric motor include automation systems, where accuracy and consistency are required.
Other types of electric motors can be identified under the synchronous category. These synchronous motors differ from each other in terms of other characteristics such as phase of operation and excitation mode.
6). Asynchronous Electric Motor
Asynchronous motor is one of the AC-driven types of electric motors.
It converts alternating current to mechanical energy in the form of rotary motion, which does not occur at a stable or consistent speed.
Unlike the synchronous electric motor, asynchronous motors do not have a DC excitation system.
Based on structural design, some secondary types of electric motors can be distinguished under the asynchronous category. These include the commutator-based and induction motors.
7). Single-Phase Electric Motor
The single-phase electric motor operates using single-phase AC supply.
This type of current does not typically produce a rotating magnetic field, but rather produces a half-cycle, pulsating magnetic field that oscillates between 0 and 180°C.
Because the field produced is not rotary, most single-phase electric motors are not self-starting . Rather, they depend on an external starting mechanism which provides the torque needed to initiate rotary motion.
The use of single-phase current implies that this type of electric motor has a relatively simple mode of operation.
Speed of electromechanical conversion may be controlled by changing the frequency of current supply. Single-phase electric motors are also suitable for relatively simple applications, like electric clocks.
8). Three-Phase Electric Motor as one of the Electric Motor Types
The three-phase electric motor runs using alternating current which flows simultaneously in three phases.
Because of the multiphase electricity supply, this is one of the types of electric motor which delivers a stable, constant electromechanical rotation over various loads.
Unlike most single-phase motors, the three-phase motor is self-starting, and is used in various industrial processing that require consistency and precision.
Various types of electric motors can be classified under the three-phase category, based on differences in structural design and operation. These include asynchronous, synchronous, and three-phase induction motors.
9). Unexcited Electric Motor
The unexcited electric motor represents a broad group including various types of electric motors (that differ in design and/or operation) which are synchronous, but do not depend on DC excitation.
Unexcited motors are designed to depend solely on the interaction between the oscillating electromagnetic flux produced by current in the conductor, and the revolving flux created by the field windings or permanent magnet.
Some types of electric motors which can be classified under this category include universal, repulsion and reluctance motors.
10). Separately-Excited Electric Motor as one of the Electric Motor Types
In a separately-excited electric motor, the armature of the rotor, receives a separate DC supply from the field windings of the stator .
This mode of operation implies that the electromagnetic field from the windings can be easily controlled by altering the DC supply.
Because of their high performance and flexible speed-control, separately excited motors are used across a wide range of industries .
11). PMDC Electric Motor
The permanent magnet DC motor, depends on a permanent magnet to create the magnetic field which will interact with the flowing charges in the armature.
Similarity exists between the working principle of the PMDC motor, and that of conventional electricity generation processes. Rotary motion is produced when the permanent magnetic field interacts with the flowing charges in the conductor.
The permanent magnet is itself a part of the stator assembly, replacing the field winding.
Because of the presence of a permanent magnetic field, PMDC electric motors generally do not require any excitation from a DC source . This increases the efficiency and performance compared to other types of electric motors.
The absence of field windings also enables the PMDC motor to be generally smaller than other types.
However, efficiency may reduce as the strength of the permanent magnetic field decreases. PMDC motors are used for applications that require stability and speed control.
12). DC Shunt Electric Motor as one of the Electric Motor Types
A DC shunt motor is a type of electric motor in which the field windings and the armature windings are connected in parallel, to each other .
This type of connection is known as a shunt, and acts as a link or pathway through which current may flow from the field windings to the armature.
DC shunt motors are self-excited, since the current used to create the revolving electromagnetic flux, is circulated within the motor itself.
Because the same voltage flows across the field and armature windings, torque may decrease as speed of rotation increases, in a DC shunt motor. This effect is due to resistance set up by the armature, to voltage from the field windings.
However, the DC shunt motor offers good regulation and a constant electromagnetic flux. Torque can also be increased by increasing the supply current to the field windings.
13). DC Series Electric Motor as one of the Electric Motor Types
The DC series motor is also self-excited, and very similar to the DC shunt motor.
They only differ in terms of the electrical circuit configuration of the field windings and armature.
Whereas the two windings (field, armature) in a shunt motor are connected in parallel, these components share a series connection in the DC series motor .
An advantage of the series connection is that it allows both the field and armature windings to operate at their full capacities, since there is no notable resistance or voltage drop between them.
As a result, the DC series motor produces a large initial torque. This capability is useful in systems where a large starter toque is required, such as automobiles.
The strength of magnetic flux from the field windings may however reduce as the motor operates, due to an opposing electromotive force (EMF) from the rotating armature. This implies that the DC series motor may not be ideal for applications where consistency and stability are needed.
14). DC Compound Electric Motor
The DC compound motor can be described as a hybrid device, which possesses characteristics of two different types of electric motors.
DC compound motor combines the design/configuration attributes of both shunt and series DC motors .
There are usually two field-armature winding assemblies in this type of electric motor. One of the two assemblies shares a series connection (between the armature and field windings). while the other shares a parallel (shunt) connection.
Based on minute details of configuration and operation, DC compound motors can be classified into subcategories that include short shunt, long shunt, cumulative and differential motors.
In long shunt DC compound motors, the shunt field winding shares parallel connection with the shunt armature winding and the series field winding. The short shunt DC compound motor features a parallel connection only between the shunt field and armature windings, while series connection occurs between the series field and armature windings as well.
In the cumulative DC compound motor, the magnetic fluxes from the field and armature windings do not oppose each other. Rather, they work cumulatively to produce a strong total flux with high torque output.
On the contrary, the differential DC compound motor produces opposing fluxes from its field and armature windings. This causes an inverse relationship with load current, such that the total flux of the motor decreases as the load current increases.
Differential compound motors are not generally used for practical applications because they are prone to generating excessive electromotive force.
15). Linear Electric Motor as one of the Electric Motor Types
A linear motor differs from other types of electric motors based on its winding configurations.
In linear motors, the field (stator) and armature (rotor) windings are unrolled. This implies that the conductors are extended rather than wound along the length of the device.
As a result of this configuration, the linear electric motor produces an electromotive force that acts in a linear (rather than rotational or circular) direction. There is therefore no rotary torque, as this is replaced by the linear force.
Another way to describe the working principle, and output of a linear motor, is as a conversion of current electricity to translational motion .
To achieve its mode of operation, the linear electric motor has a flat cross-sectional design. In many cases this includes a flat permanent magnet which creates the magnetic field that interacts with the electromagnetic force generated by the unrolled stator (field) winding.
When these two force fields interact, a linear-trending electromechanical force is produced, which makes the armature to move in a linear (forward or backward) direction, rather than a rotary direction.
Both the linear electromechanical force and the speed of armature translational motion, can be controlled in a linear motor, by altering the amount and frequency of supply current respectively.
16). AC Series Electric Motor
AC series motor is an electric motor that runs on alternating current and has its field and armature windings connected in series to each other.
As a result of similarity in design to the DC series motor, AC series motor is also known as the ‘modified DC series motor’.
Based on the mode of current flow between the field and armature windings, DC series motors can be categorized as compensated and uncompensated.
In Uncompensated DC series motors, current flow reversals may occur between the field and armature windings. This can be referred to as the ‘transformer effect’ and occurs because of the opposing directions of current flow in the two windings.
Uncompensated DC series motors have a winding which is connected in series between the field and armature windings. This component eliminates the transformer effect by rectifying the current-reversal effect that occurs as electricity flows from the field winding to the armature.
17). Stepper Electric Motor as one of the Electric Motor Types
The stepper electric motor produces rotary motion in a stepwise manner .
Rather than produce electromechanical energy in the form of constant rotation, this type of electric motor generates electromechanical rotation which makes one complete revolution in a series of multiple steps.
To achieve this, the rotor is placed under the influence of an external magnetic field. This field causes the armature windings to move in pulses or phases, due to stepwise alignment of the charged conductor with the field.
Stepper motors are often brushless and may require minimal maintenance. They are relatively precise, and their precision depends on the number of steps in each complete revolution. Areas of application include plotters, generators, process control tools, and other electronic systems.
18). Servo Electric Motor
The servo motor, or servomotor, is a type of electric motor that is designed to control the angular direction and velocity of rotation.
By controlling rotation, the servo motor achieves a high degree of precision, which is important for some applications like robotics.
The servomotor is fairly complex in its design and operation, compared to other types of electric motors. It is usually a system comprising of gears and controllers to coordinate the generation and implementation of mechanical energy in the form of rotary motion.
Also, servo motors can be driven by either AC or DC currents. They may also generate rotary or linear motion, depending on their design.
Components of a servomotor include sensor, feedback system, controller and gear assembly.
The feedback system, senor and controller help to achieve precision with regards to the desired angle of rotation, while the gear assembly is used to control torque and rotational speed.
Unlike other motors that mostly rotate 360°, the servo motor achieves a maximum rotation of 180°, which occurs in two segments of 90° each.
19). Hysteresis Electric Motor
The hysteresis motor is an electric motor which functions based on residual magnetism hysteresis losses.
It can be categorized under synchronous types of electric motors, and may operate using 1-phase or 3-phase current supply.
In hysteresis motors, the rotor is usually made from ferromagnetic materials with high residual magnetic properties that permit hysteresis losses to occur, like steel.
Alternatively, the rotor may comprise of an outer magnetic ring and an inner non-magnetic core . It is also usually cylindrical and may be connected to a shaft that has no magnetic properties.
Hysteresis motors do not use DC excitation. They rather produce electromechanical force by induction, as the current flowing through the armature (rotor) windings generate magnetic field which interacts with the field generated by the field (stator) windings.
After the electromechanical force has been produced and rotation begins, an eddy current flow is induced in the rotating field (stator) windings. This current generates an electromagnetic torque which interacts with the residual magnetism due to hysteresis losses, in the rotor windings.
As a result of this interaction between both magnetic fields, the rotor and stator may then rotate synchronously.
Compared to other types of electric motors, the hysteresis motor has a simple and efficient design, which usually excludes brushes and rotor windings. However, the amount of torque produced by this motor is relatively small.
20). Repulsion Electric Motor as one of the Electric Motor Types
The repulsion motor is a type of electric motor which operates based on the repulsion of like poles.
It can be classified as one of the types of electric motors that depend on alternating current, and also usually operates using single-phase AC.
In a repulsion motor, the current supply is connected to either the field or armature windings. Commutators and brushes are also present, and act as terminals to support the transmission of current to the armature.
Between the armature and field windings, there is usually no direct connection in repulsion motors. Rather, the rotor achieves motion through electromagnetic induction.
Movement of the rotor creates an electromagnetic field that is synchronous with the magnetic field produced by the stator (or field windings). Repulsion of the like poles of these two fields, produces a torque which provides a propelling force for the rotor.
A rotation angle of 20° is usually applied to the stator, in order to induce continuous rotation of the rotor. The speed of rotation and the intensity of repulsion can be adjusted by altering the angle of rotation of the stator.
-Types of Electric Motors based on Purpose of Application
21). Reluctance Electric Motor
The reluctance electric motor works based on the generation of torque from magnetic reluctance.
Based on its mode of operation, reluctance motor can be categorized as one of the types of electric motors that work with synchronous, 1-phase current.
In a reluctance motor, the rotor typically occurs as a ferromagnetic material with no windings or permanent magnet. These sub-components are absent because the rotor depends on inequalities in magnetic flux intensity to achieve rotary motion.
When an electromagnetic field is induced in the stator of the reluctance motor, this field influences the rotor which is made from ferromagnetic material.
As a result, the rotor is forced to align with the field, at the region of least reluctance or repulsion. Consequently, the rotor achieved rotational motion in a synchronous manner with respect to the electromagnetic field.
22). Universal Electric Motor as one of the Electric Motor Types
The universal motor is a type of electric motor that is designed to generate variable-speed rotation and high operational torque .
They may use single-phase AC or DC, and may occur as brushed motors in which a series connection links the field and armature windings. Series connection implies that both windings conduct current-flow in the same direction.
Universal motors are ideal for high-speed operations and have the advantage of being relatively inexpensive.
Comparing Electric Motor Types : DC and AC Motors Comparison
|DC Electric Motor||AC Electric Motor|
|Converts DC to mechanical energy||Converts AC to mechanical energy|
|Good for speed control||Good for high-speed and torque output|
|Operates in single-phase||May operate in single-phase or three-phase|
|Is self-starting||Usually requires external starting mechanism|
|Requires commutation to convert DC input to AC||May not require commutation since input is AC|
|Power is supplied by DC sources like batteries||Power is supplied by AC sources like rectifier and generator|
|Brushes are usually present||Brushes may be absent|
|Torque is usually stable across different rotation speeds||Torque may vary, and decreases with increase in speed|
|Permanent magnet provides electromagnetic influence||Electromagnetic influence is provided by field windings|
|Usually heavier due to presence of permanent magnet||Lighter than DC motor|
|Relatively high efficiency||Relatively low efficiency due to unstable electromagnetic induction|
|Used in applications requiring speed control, such as conveyor belt systems||Used in applications requiring high speed and performance, such as water pumping systems|
Types of electric motors are;
- AC Electric Motor
- DC Electric Motor
- Brushed Electric Motor
- Brushless Electric Motor
- Synchronous Electric Motor
- Asynchronous Electric Motor
- Single-Phase Electric Motor
- Three-Phase Electric Motor
- Unexcited Electric Motor
- Separately-Excited Electric Motor
- PMDC Electric Motor
- DC Shunt Electric Motor
- DC Series Electric Motor
- DC Compound Electric Motor
- Linear Electric Motor
- AC Series Electric Motor
- Stepper Electric Motor
- Servo Electric Motor
- Hysteresis Electric Motor
- Repulsion Electric Motor
- Reluctance Electric Motor
- Universal Electric Motor
These types each have distinctive characteristics, and are differentiated based on their current source, design, mode of operation, and application.
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