Types of Electric Motors - Classification of AC, DC & Special Motors
Classification of Different Types of Electrical Motors
An Electrical Motor is a machine that converts electrical energy into mechanical energy. It is used for generating torque to lift loads, move objects & various other mechanical work. In the following article, we will discuss the different types of electric motors such as AC, DC and special types of motors etc.
An electrical motor is mainly classified into three types.
AC Motors
DC Motors
Special Motors
AC Motor
The AC electric motor converts AC (Alternating Current) electrical energy into mechanical energy. These electric motors are powered using a single-phase or three-phase alternating current. The basic working principle of AC motor is the rotating magnetic field (RMF) generated by the stator winding when an alternating current is passed through it. The rotor (having its own magnetic field) follows the RMF & starts rotation.
The AC motors are further classified into two types.
Synchronous Motor
As its name suggests, such AC motor has a constant speed called synchronous speed that only depends on the frequency of the supply current. The speed of such electric motors only varies with variation in supply frequency & remains constant upon varying loads. It is used for constant speed of application & precision control.
A synchronous motor has the same stator design as an asynchronous motor & it generates a rotating magnetic field when supplied with input alternating current. While the rotor design may vary i.e. it uses a separate DC excitation to generate its own magnetic field.
Excited Synchronous Motor
Such synchronous motor requires DC excitation. The DC excitation means that the rotor has a separate DC supply to generate its own magnetic flux. This flux reacts with the revolving flux of the stator to generate rotation. The utilize wire wound rotor with commutator & brushes assembly to supply current to the rotor's windings.
Single Phase Synchronous Motor
Such synchronous motor runs on single-phase AC supply. To be precise, it actually uses two-phase, the second one being derived from the first phase. The reason for using two phases is that a single phase cannot generate a rotating magnetic field. Such motor can start in either direction i.e. its direction is unidentified, which is why there is an extra starting arrangement used for giving it a direction.
The speed of such a motor only depends on the supply frequency. They are used in recording instruments, electric wall clocks.
Three Phase Synchronous Motor
These synchronous motor runs on three-phase power supply. The benefit of a three-phase alternating current is that it generates a rotating magnetic field in the stator while the arrangement of the phases decides the direction of the rotation. This motor does not need any special starting mechanism to decide its direction. However, the rotor still needs an extra DC source for excitation.
They are used in industries for applications that require constant speed over a range of loads & require precise positioning in robotics.
Unexcited Synchronous Motor
Such synchronous motor that does not require DC excitation i.e. the rotor does not require separate DC supply for generating magnetic flux. They utilize squirrel cage rotors such as the one used in induction motor.
Reluctance Motor
It is a single-phase synchronous motor that works on the principle of generating torque based on the magnetic reluctance. There are two types of stator windings i.e. main windings & auxiliary windings. The auxiliary windings in used for starting the motor. It has a squirrel cage type rotor (with no windings) just like in an induction motor made of ferromagnetic material.
The motor starts like an actual single phase induction motor using the auxiliary winding. Once the motor reach near synchronous speed, the auxiliary winding is disconnected & the rotor is locked in synchronism due to the rotor's ferromagnetic nature trying to keep itself in the less reluctance position inside the rotating magnetic field.
Motor Hysteresis
Such type of synchronous motor works on the principle of hysteresis loss or residual magnetism occurring in the rotor. Such electric motors are operated using single-phase as well as three-phase AC supply. in a single-phase hysteresis motor, there is an auxiliary winding beside the main winding as in a reluctance motor. The rotor which is in a cylindrical shape is made of ferromagnetic material with high magnetic retentivity or hysteresis loss such as hardened steel. The rotor is supported by a non-magnetic shaft.
The motor starts as an induction motor. The stator's rotating magnetic field induces eddy current in the rotor. The eddy current generates torque along with the hysteresis torque due to the high hysteresis loss property of the rotor's material. Due to the eddy's current torque, the motor behaved as an induction motor.
Once the reaches the near synchronous speed, the stator's rotating magnetic field pulls the rotor in synchronism. The rotor ferromagnetic nature produces opposite magnetic poles due to the stator's RMF & it starts behaving as a permanent magnet. At such speed, there is no relative motion between stator & rotor. So there is no induction. Therefore, there is no eddy current or eddy current torque. The torque produced by the motor at synchronous speed due to hysteresis, which is why it is named Hysteresis motor.
The main advantage of a hysteresis motor is that it is brushless and there are no windings inside the rotor. It does not generate any noise and it operates quietly.
Disadvantages
It generates very low torque
If the load torque increases a certain limit, its speed drops thus it no longer operates as a synchronous motor
It has less efficiency
It is only available in small sizes.
It is used in record players that require constant speed for recording & playback features. Also electric clock requires constant speed etc.
Asynchronous Motor
The type of AC motor that never runs at synchronous speed is called asynchronous speed. Its rotor speed is always less than the synchronous speed. It does not require separate rotor excitation.
Induction Motor
The induction motor is a type of AC asynchronous motor that works on the principle of electromagnetic induction between the stator & the rotor. The revolving magnetic flux induces a current in the rotor due to electromagnetic induction which produces torque in the rotor. It is the most used electrical motor in industries.
It is mainly divided into two types based on the construction of its rotor.
Squirrel cage induction motor
The rotor of such an induction motor resembles a squirrel cage. It is made of copper bars connected at both ends using a conductive ring to make a closed-loop circuit. There is no electrical connection to the rotor.
The stator's varying magnetic field induces a current in the rotor's bars. The induced current generates its own magnetic field in the rotor which interacts with the stator's revolving magnetic field & tries to eliminate it by revolving with it in the same direction.
Slip Ring or Wound Rotor Induction Motor
A slip ring or wound rotor induction motor is another type of induction motor where the rotor is made of windings that are connected with the slip rings. The slip rings are used to connect the windings to external resistors for controlling the rotor current & hence allowing the control of speed/torque characteristics.
It has the same operation principle as squirrel cage induction motor except for the induced current in the rotor can be controlled using the external resistors. The external resistance also helps to increase the rotor resistance during the startup of the motor to reduce the high inrush current. It also increases the starting torque to left high inertia loads.
The downside of slip rings is that it constantly slides with the brushes that require costly maintenance due to mechanical wear and tear. The construction is complex and it is more expensive than a squirrel cage motor.
Capacitor Start Induction Run Motor
It is a single-phase induction motor that utilizes a capacitor in series with its auxiliary winding to generate extra torque during its startup. Its name clearly suggests that the capacitor is only used for starting the motor & it is disconnected once the motor reaches near synchronous speed using a centrifugal switch.
It has two stator windings called main windings & auxiliary windings. The auxiliary winding is connected in series with the capacitor using a centrifugal switch. When the motor starts, the current flows through both windings, generating high starting torque. Once the motor reaches 70-80% full speed, the centrifugal switch disconnects the supply to the auxiliary windings. The motor resume operating on the main winding.
Capacitor Start and Capacitor Run Motor
It is also a single phase induction motor but it utilizes two capacitors in its operation. The two capacitors are the start capacitor & run capacitor. The starting capacitor is used for only starting the capacitor to offer extra high starting torque while the running capacitor is used continuously for normal operation to start the motor. The starting capacitor is connected & disconnected using a centrifugal switch.
When the motor starts, both capacitors are connected offering high starting torque to the rotor. As the rotor's speed pickup, the switch disconnects the starting capacitor. Such motor uses both main & auxiliary winding continuously which is why its operation is smoother than the motor running on main windings only such as capacitor motors run.
Commutator Motor
This is a type of AC motor that utilizes commutator & brush assembly to supply power to its rotor. Such electric motors have wound-type rotors.
AC Series Motor
As we know, electric motors have two types of windings i.e. the stator windings known as the field windings & the rotor windings or armature windings.
When these both windings are connected in series it is known as series wound motor. It is also known as a universal motor due to its ability to run on both AC & DC supply as well.
The field windings carry the same amount of current as the rotor windings. The brushes that supply current to the armature winding through the commutator shorts the armature windings & acts as a shorted transformer. The brushes generate arcs that decrease with the increase in speed.
AC Compensated Series Motor
It is a modified form of AC series motor where an additional winding known as compensating winding is added in series with the existing field & armature windings to eliminate the transformer effect that happens in an uncompensated series motor.
Repulsion Motor
Repulsion motor is also an AC single phase motor where the AC input is only applied to the field or stator windings. The armature windings are connected to the commutator. The armature windings are shortened by using a pair of shorted brushes. There is no electrical connection between the field windings & armature windings. The rotor current is generated through induction.
The brushes are configured in such a way that it can be moved to change its angle with respect to an imaginary stator axis. The motor can be stopped, start & reversed by changing the angle of the brushes & also vary the speed of the motor.
As the rotor is shorted using the brushes to form a loop, a current is induced when alternating current flows in the field winding. This induced current flowing in the rotor's windings generates its own magnetic field. The direction of the magnetic field depends on the brushes angle. This magnetic field interacts with the stator's field & rotor reacts accordingly. For rotation, the brushes are rotated slightly at 20° in either direction to rotate the motor in that direction. Placing the brushes at 90° or 180° or 0° would stop the motor. Varying the angle increases or decreases the repulsion between the stator & rotor's magnetic field & the rotor's speed varies.
The starting torque can also be controlled by varying the angle of brushes providing maximum starting torque at 45°. This motor was used for traction due to its superior speed regulation but it has been superseded by other traction motors.
Types of Electric Motors - Classification of AC, DC & Special Motors
Classification of Different Types of Electrical Motors
An Electrical Motor is a machine that converts electrical energy into mechanical energy. It is used for generating torque to lift loads, move objects & various other mechanical work. In the following article, we will discuss the different types of electric motors such as AC, DC and special types of motors etc.
An electrical motor is mainly classified into three types.
AC Motors
DC Motors
Special Motors
AC Motor
The AC electric motor converts AC (Alternating Current) electrical energy into mechanical energy. These electric motors are powered using a single-phase or three-phase alternating current. The basic working principle of AC motor is the rotating magnetic field (RMF) generated by the stator winding when an alternating current is passed through it. The rotor (having its own magnetic field) follows the RMF & starts rotation.
The AC motors are further classified into two types.
Synchronous Motor
As its name suggests, such AC motor has a constant speed called synchronous speed that only depends on the frequency of the supply current. The speed of such electric motors only varies with variation in supply frequency & remains constant upon varying loads. It is used for constant speed of application & precision control.
A synchronous motor has the same stator design as an asynchronous motor & it generates a rotating magnetic field when supplied with input alternating current. While the rotor design may vary i.e. it uses a separate DC excitation to generate its own magnetic field.
Excited Synchronous Motor
Such synchronous motor requires DC excitation. The DC excitation means that the rotor has a separate DC supply to generate its own magnetic flux. This flux reacts with the revolving flux of the stator to generate rotation. The utilize wire wound rotor with commutator & brushes assembly to supply current to the rotor's windings.
Single Phase Synchronous Motor
Such synchronous motor runs on single-phase AC supply. To be precise, it actually uses two-phase, the second one being derived from the first phase. The reason for using two phases is that a single phase cannot generate a rotating magnetic field. Such motor can start in either direction i.e. its direction is unidentified, which is why there is an extra starting arrangement used for giving it a direction.
The speed of such a motor only depends on the supply frequency. They are used in recording instruments, electric wall clocks.
Three Phase Synchronous Motor
These synchronous motor runs on three-phase power supply. The benefit of a three-phase alternating current is that it generates a rotating magnetic field in the stator while the arrangement of the phases decides the direction of the rotation. This motor does not need any special starting mechanism to decide its direction. However, the rotor still needs an extra DC source for excitation.
They are used in industries for applications that require constant speed over a range of loads & require precise positioning in robotics.
Unexcited Synchronous Motor
Such synchronous motor that does not require DC excitation i.e. the rotor does not require separate DC supply for generating magnetic flux. They utilize squirrel cage rotors such as the one used in induction motor.
Reluctance Motor
It is a single-phase synchronous motor that works on the principle of generating torque based on the magnetic reluctance. There are two types of stator windings i.e. main windings & auxiliary windings. The auxiliary windings in used for starting the motor. It has a squirrel cage type rotor (with no windings) just like in an induction motor made of ferromagnetic material.
The motor starts like an actual single phase induction motor using the auxiliary winding. Once the motor reach near synchronous speed, the auxiliary winding is disconnected & the rotor is locked in synchronism due to the rotor's ferromagnetic nature trying to keep itself in the less reluctance position inside the rotating magnetic field.
Motor Hysteresis
Such type of synchronous motor works on the principle of hysteresis loss or residual magnetism occurring in the rotor. Such electric motors are operated using single-phase as well as three-phase AC supply. in a single-phase hysteresis motor, there is an auxiliary winding beside the main winding as in a reluctance motor. The rotor which is in a cylindrical shape is made of ferromagnetic material with high magnetic retentivity or hysteresis loss such as hardened steel. The rotor is supported by a non-magnetic shaft.
The motor starts as an induction motor. The stator's rotating magnetic field induces eddy current in the rotor. The eddy current generates torque along with the hysteresis torque due to the high hysteresis loss property of the rotor's material. Due to the eddy's current torque, the motor behaved as an induction motor.
Once the reaches the near synchronous speed, the stator's rotating magnetic field pulls the rotor in synchronism. The rotor ferromagnetic nature produces opposite magnetic poles due to the stator's RMF & it starts behaving as a permanent magnet. At such speed, there is no relative motion between stator & rotor. So there is no induction. Therefore, there is no eddy current or eddy current torque. The torque produced by the motor at synchronous speed due to hysteresis, which is why it is named Hysteresis motor.
The main advantage of a hysteresis motor is that it is brushless and there are no windings inside the rotor. It does not generate any noise and it operates quietly.
Disadvantages
It generates very low torque
If the load torque increases a certain limit, its speed drops thus it no longer operates as a synchronous motor
It has less efficiency
It is only available in small sizes.
It is used in record players that require constant speed for recording & playback features. Also electric clock requires constant speed etc.
Asynchronous Motor
The type of AC motor that never runs at synchronous speed is called asynchronous speed. Its rotor speed is always less than the synchronous speed. It does not require separate rotor excitation.
Induction Motor
The induction motor is a type of AC asynchronous motor that works on the principle of electromagnetic induction between the stator & the rotor. The revolving magnetic flux induces a current in the rotor due to electromagnetic induction which produces torque in the rotor. It is the most used electrical motor in industries.
It is mainly divided into two types based on the construction of its rotor.
Squirrel cage induction motor
The rotor of such an induction motor resembles a squirrel cage. It is made of copper bars connected at both ends using a conductive ring to make a closed-loop circuit. There is no electrical connection to the rotor.
The stator's varying magnetic field induces a current in the rotor's bars. The induced current generates its own magnetic field in the rotor which interacts with the stator's revolving magnetic field & tries to eliminate it by revolving with it in the same direction.
Slip Ring or Wound Rotor Induction Motor
A slip ring or wound rotor induction motor is another type of induction motor where the rotor is made of windings that are connected with the slip rings. The slip rings are used to connect the windings to external resistors for controlling the rotor current & hence allowing the control of speed/torque characteristics.
It has the same operation principle as squirrel cage induction motor except for the induced current in the rotor can be controlled using the external resistors. The external resistance also helps to increase the rotor resistance during the startup of the motor to reduce the high inrush current. It also increases the starting torque to left high inertia loads.
The downside of slip rings is that it constantly slides with the brushes that require costly maintenance due to mechanical wear and tear. The construction is complex and it is more expensive than a squirrel cage motor.
Capacitor Start Induction Run Motor
It is a single-phase induction motor that utilizes a capacitor in series with its auxiliary winding to generate extra torque during its startup. Its name clearly suggests that the capacitor is only used for starting the motor & it is disconnected once the motor reaches near synchronous speed using a centrifugal switch.
It has two stator windings called main windings & auxiliary windings. The auxiliary winding is connected in series with the capacitor using a centrifugal switch. When the motor starts, the current flows through both windings, generating high starting torque. Once the motor reaches 70-80% full speed, the centrifugal switch disconnects the supply to the auxiliary windings. The motor resume operating on the main winding.
Capacitor Start and Capacitor Run Motor
It is also a single phase induction motor but it utilizes two capacitors in its operation. The two capacitors are the start capacitor & run capacitor. The starting capacitor is used for only starting the capacitor to offer extra high starting torque while the running capacitor is used continuously for normal operation to start the motor. The starting capacitor is connected & disconnected using a centrifugal switch.
When the motor starts, both capacitors are connected offering high starting torque to the rotor. As the rotor's speed pickup, the switch disconnects the starting capacitor. Such motor uses both main & auxiliary winding continuously which is why its operation is smoother than the motor running on main windings only such as capacitor motors run.
Commutator Motor
This is a type of AC motor that utilizes commutator & brush assembly to supply power to its rotor. Such electric motors have wound-type rotors.
AC Series Motor
As we know, electric motors have two types of windings i.e. the stator windings known as the field windings & the rotor windings or armature windings.
When these both windings are connected in series it is known as series wound motor. It is also known as a universal motor due to its ability to run on both AC & DC supply as well.
The field windings carry the same amount of current as the rotor windings. The brushes that supply current to the armature winding through the commutator shorts the armature windings & acts as a shorted transformer. The brushes generate arcs that decrease with the increase in speed.
AC Compensated Series Motor
It is a modified form of AC series motor where an additional winding known as compensating winding is added in series with the existing field & armature windings to eliminate the transformer effect that happens in an uncompensated series motor.
Repulsion Motor
Repulsion motor is also an AC single phase motor where the AC input is only applied to the field or stator windings. The armature windings are connected to the commutator. The armature windings are shortened by using a pair of shorted brushes. There is no electrical connection between the field windings & armature windings. The rotor current is generated through induction.
The brushes are configured in such a way that it can be moved to change its angle with respect to an imaginary stator axis. The motor can be stopped, start & reversed by changing the angle of the brushes & also vary the speed of the motor.
As the rotor is shorted using the brushes to form a loop, a current is induced when alternating current flows in the field winding. This induced current flowing in the rotor's windings generates its own magnetic field. The direction of the magnetic field depends on the brushes angle. This magnetic field interacts with the stator's field & rotor reacts accordingly. For rotation, the brushes are rotated slightly at 20° in either direction to rotate the motor in that direction. Placing the brushes at 90° or 180° or 0° would stop the motor. Varying the angle increases or decreases the repulsion between the stator & rotor's magnetic field & the rotor's speed varies.
The starting torque can also be controlled by varying the angle of brushes providing maximum starting torque at 45°. This motor was used for traction due to its superior speed regulation but it has been superseded by other traction motors.
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