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Defined Types of Motor part 1


Classification of Motors

When it comes to motors, there is a wide array of types available, each serving specific purposes. Proper classification is essential for various reasons, including assisting in selecting the right motor for a particular application, explaining concepts clearly to others, or using industry-specific terminology effectively. The classification of motors typically involves four main methods:

1. Categorizing them based on the conversion principle they operate on, such as electromagnetic, electrostatic, or ultrasonic motors.

2. Sorting them according to the type of power supply they require, which could be DC, single-phase AC, or three-phase AC.

3. Grouping them based on their rotation mode, considering factors like speed control mechanisms or reverse rotation capabilities.

4. Lastly, classifying motors based on their structure, which involves understanding the combination of rotating and static components that make up the motor's design.


  • AC Motor
    • 1. Synchronous Motor
      • - Excited Synchronous Motor
        • * Single Phase Synchronous Motor
        • * Three Phase Synchronous Motor
      • - Unexcited Synchronous Motor
        • * Reluctance Motor
        • *Hysteresis Motor
    • 2. Asynchronous Motor
      • -Induction Motor
        • *Squirrel Cage Induction Motor
        • *Slip Ring or Wound Rotor Induction Motor
        • *Capacitor Start Induction Run Motor
        • *Capacitor Start and Capacitor Run Motor
      • -Commutator Motor
        • *AC Series Motor
        • *AC Compensated Series Motor
        • *Repulsion Motor
        • *Repulsion-Start Induction-Run Motor
  • DC Motor
    • 1.Brushed DC Motor
      • -Separately Excited DC Motor
      • -Self-Excited DC Motor
        • *Series Wound DC Motor
        • *Shunt Wound DC Motor
        • *Compound Wound DC Motor
      • -Permanent Magnet DC Motor (PMDC)
    • 2.Brushless DC Motor
    • 3.Coreless or Ironless DC Motors
  • Special Motors
    • -Servo Motors
    • -Direct Drive
    • -Linear Motors
    • -Stepper Motor
    • -Universal Motor


An Electrical Motor is a machine that converts electrical energy into mechanical energy. It is extensively utilized for generating torque to lift heavy loads, move objects efficiently, and perform various other mechanical tasks. In the subsequent article, we will delve into the distinctive characteristics and applications of various types of electric motors, including AC motors, DC motors, and specialized motor variants.

An electrical motor is mainly classified into three types.

  • AC Motors
  • DC Motors
  • Special Motors

AC Motor



The AC electric motor is a device that transforms AC (Alternating Current) electrical energy into mechanical energy. These types of electric motors can be powered by either single-phase or three-phase alternating current. At the core of the AC motor's operation is the creation of a rotating magnetic field (RMF) by the stator winding when an alternating current passes through it. Subsequently, the rotor, possessing its own magnetic field, aligns with the RMF, leading to the initiation of rotation.

The AC motors are further classified into two types

  • Synchronous Motor
  • Asynchronous or Induction Motor


Synchronous Motor



As indicated by its name, a synchronous AC motor operates at a consistent speed known as synchronous speed, which is solely determined by the supply current frequency. These types of electric motors experience speed changes only when there is a fluctuation in the supply frequency and maintain a steady pace regardless of the load variations. Primarily employed in applications requiring a fixed speed and accurate control, synchronous motors share a stator design similar to that of asynchronous motors. They produce a rotating magnetic field when energized with alternating current. Unlike asynchronous motors, the rotor of a synchronous motor varies in design, utilizing a distinct DC excitation to create its magnetic field.


Excited Synchronous Motor

A synchronous motor necessitates DC excitation for operation. This requirement entails providing the rotor with a distinct DC power source to create its magnetic flux. This magnetic flux then interacts with the stator's rotating flux to induce rotation. To facilitate this process, a wire wound rotor with commutator and brush assembly is employed to deliver current to the rotor's windings effectively.


Single Phase Synchronous Motor

A synchronous motor operates on a single-phase AC supply, utilizing two phases where the second is obtained from the first for generating a rotating magnetic field. This design enables the motor to initiate movement in either direction, incorporating an additional starting mechanism to establish a specific direction. The motor's speed is determined solely by the supply frequency, making it ideal for applications in recording instruments and electric wall clocks.

Three Phase Synchronous Motor

The synchronous motor operates efficiently with a three-phase power supply. A notable advantage of utilizing three-phase alternating current is its ability to create a rotating magnetic field in the stator, with the specific phase configuration determining the rotation direction. Unlike other motors, this motor does not necessitate a unique starting mechanism to establish its rotation direction. Nevertheless, the rotor does require an additional DC power 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 motors do not require DC excitation, meaning their rotor does not need a separate DC power supply to generate magnetic flux. These motors utilize squirrel cage rotors similar to the ones found in induction motors.


Reluctance Motor


The single-phase synchronous motor operates by producing torque through magnetic reluctance. It features main windings and auxiliary windings, with the latter crucial for motor startup. This motor possesses a squirrel cage rotor, devoid of windings like those found in induction motors, and is constructed from ferromagnetic material.

The motor starts like an actual single-phase induction motor using the auxiliary winding. Once the motor reaches near synchronous speed, the auxiliary winding is disconnected, and the rotor is locked in synchronism. This occurs due to the rotor's ferromagnetic nature, which strives to keep itself in the position of least reluctance within the rotating magnetic field.


Hysteresis Motor


This type of synchronous motor operates based on the hysteresis loss or residual magnetism found in the rotor. These electric motors can run on both single-phase and three-phase AC supply. In a single-phase hysteresis motor, there is an additional auxiliary winding alongside the main winding, similar to a reluctance motor. The rotor, typically cylindrical in shape, is crafted from a ferromagnetic material known for its high magnetic retentivity or hysteresis loss, such as hardened steel. Additionally, the rotor is upheld by a non-magnetic shaft, providing stability and efficient performance in various electrical applications.

The motor initiates as an induction motor, with the stator's rotating magnetic field inducing eddy currents in the rotor. These eddy currents, along with the hysteresis torque caused by the rotor's high hysteresis loss material, collectively produce torque. It is this torque, stemming from the eddy currents, that allows the motor to exhibit characteristics akin to that of an induction motor.

Once the motor reaches near synchronous speed, the stator's rotating magnetic field effectively pulls the rotor into synchronism. The ferromagnetic nature of the rotor results in opposite magnetic poles due to the interaction with the stator's rotating magnetic field, causing the rotor to behave like a permanent magnet. This alignment leads to no relative motion between the stator and rotor, preventing induction from occurring. As a result, there are no eddy currents or eddy current torque generated. The torque produced by the motor at synchronous speed is primarily due to hysteresis, earning it the designation of a Hysteresis motor.

The biggest benefit of a hysteresis motor is that it's brushless and silent since it doesn't have winding in the rotor, so it runs quietly without making any noise.

Disadvantages

  • It generates very low torque
  • If the load torque increases a certain limit, its speed drops thus it no longer operate as synchronous motor
  • It has less efficiency
  • It is only available in small sizes.

In various applications, precision speed control is essential. For instance, it is crucial in record players to maintain a consistent speed for both recording and playback functions. Similarly, electric clocks rely on constant speed to ensure accurate timekeeping.


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 as it does not require separate rotor excitation. Asynchronous motors are briefly classified into two types: Induction Motor and Commutator Motor.


Induction Motor


The induction motor, widely utilized in various industries, operates as an AC asynchronous motor based on electromagnetic induction between the stator and the rotor. The rotor experiences torque from the induced current generated by the revolving magnetic flux, showcasing the efficiency and reliability of this motor type.

It is mainly divided into two types based on the construction of its rotor.


Squirrel Cage Induction Motor


The rotor of an induction motor bears a striking resemblance to a squirrel cage. Constructed from copper bars interconnected at both ends through a conductive ring, it forms a continuous closed-loop circuit. Interestingly, the rotor operates without any direct electrical linkage.

The stator's varying magnetic field induces a current in the rotor's bars. This induced current creates its own magnetic field in the rotor, interacting with the stator's rotating magnetic field to counteract it by rotating in the same direction. This design is not only simple and cost-effective but also highly reliable. With no electrical connections, commutator, or brush assembly, this system boasts low maintenance requirements.


Slip Ring or Wound Rotor Induction Motor


A slip ring or wound rotor induction motor is another type of induction motor wherein the rotor is composed of windings connected through slip rings. These slip rings facilitate the connection of the windings to external resistors, enabling the control of rotor current and thereby regulating speed and torque characteristics.

Despite sharing the operational principle with a squirrel cage induction motor, the key distinction lies in the ability to manipulate the induced current in the rotor using external resistors. By leveraging external resistance, it becomes feasible to enhance the rotor resistance during motor startup, mitigating high inrush currents.

Additionally, this method augments starting torque, making it adept at lifting high inertia loads. However, slip rings come with drawbacks such as the requirement of regular maintenance due to mechanical wear and tear from constant sliding with brushes.

Moreover, the intricate construction and higher costs associated with slip rings render them more expensive compared to squirrel cage motors.


Capacitor Start Induction Run Motor

It is a type of electric motor known as a single-phase induction motor, distinguished by the use of a capacitor in series with its auxiliary winding to produce additional torque upon startup. The purpose of the capacitor is solely for initiating the motor, and it is disengaged once the motor approaches a speed close to synchronicity, facilitated by a centrifugal switch.


The electric motor features two stator windings known as main windings and auxiliary winding. The auxiliary winding is linked in series with a capacitor through a centrifugal switch. As the motor initiates, electricity courses through both windings, creating substantial starting torque. As the motor hits 70-80% of its full speed, the centrifugal switch cuts off the power supply to the auxiliary windings, enabling the motor to continue running on the main winding alone.


Capacitor Start and Capacitor Run Motor

It is also a single-phase induction motor that utilizes two capacitors during its operation, enhancing its efficiency and performance. The start capacitor is specifically designed to provide extra high starting torque, aiding in the motor's initial startup. On the other hand, the run capacitor plays a crucial role in maintaining smooth operation by being continuously engaged during normal usage. To manage the start capacitor, a centrifugal switch is employed to connect and disconnect it appropriately, ensuring seamless functionality.


When the motor starts, both capacitors are connected to provide high starting torque to the rotor. As the rotor's speed increases, the switch disconnects the starting capacitor. This type of motor utilizes both the main and auxiliary windings throughout its operation, resulting in smoother performance compared to motors that run solely on main windings, such as capacitor run motors.


Commutator Motor


Electric motors are essential components that convert electrical energy into mechanical energy. One specific type is the AC motor, which relies on a commutator and brush assembly to power its rotor. These motors are distinguished by their wound-type rotors, making them reliable and efficient for various applications.

AC Series Motor

As we are aware, electric motors are equipped with two primary types of windings: the stator windings, also referred to as the field windings, and the rotor windings, which are alternatively known as armature windings.



When these both windings are connected in series, it is known as a series wound motor. This type of motor is also referred to as a universal motor because of its ability to operate on both AC and DC power sources.

The field windings and rotor windings carry an equal amount of current in the electrical system. As the brushes transfer current to the armature winding via the commutator, they effectively create a short circuit in the armature windings, resembling a shorted transformer. Moreover, the brushes produce arcs that diminish as the speed rises.





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