Types of Electrical Machines

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An electrical machine is a device which either converts electrical energy into mechanical energy or mechanical energy into electrical energy. Depending on the type of supply used, connection of winding, or working principle, electrical machines can be classified into several different types such as AC machines, DC machines, induction motor, synchronous machine, and many others.

In this chapter, we will learn about these different types of electrical machines along with their features and applications.

Types of Electrical Machines

The following figure shows the types of electrical machines in different categories −

Lets discuss about each type of electrical machine in detail.

DC Machine

DC machine, also known as a direct current machine, is a type of electrical machine which takes direct current supply as input and converts it into rotational mechanical energy as output. Depending on the function it performs, DC machines are of two types namely,

• DC Motor − A DC motor is nothing but a DC machine which converts input direct current electricity into rotational mechanical energy. DC motors are further classified into three main types which are - DC series motor, DC shunt motor, and DC compound motor.
• DC Generator − A DC generator is a DC machine which converts input mechanical energy into direct current electricity as output. Similar to DC motor, DC generators are also classified into three key types namely, DC series generator, DC shunt generator, and DC compound generator.

Let's get an overview of different types of DC motors and generators.

DC Series Motor

DC series motor is a type of self-excited DC motor in which the field winding is connected in series with the armature winding. Hence, in a DC series motor, the armature winding and field winding carry the same electric current. The connection diagram of a DC series motor is shown in the following figure.

The following are some key features of the DC series motor −

• DC series motor has a very high starting torque.
• The speed of a DC series motor decreases with the increasing load. Hence, it has a poor speed regulation.
• Due to series connection of field and armature windings, a DC series motor draws a high electric current.
• DC series motors have simple design and require less maintenance.
• DC series motors are relatively economical.

DC series motors are mainly used in applications that require high starting torque, such as −

• Electric cars or scooters
• Cranes and hoists
• Electric locomotives
• Grinders, etc.

DC Shunt Motor

DC shunt motor is another type of self-excited DC motor in which the field winding is connected in parallel with the armature winding. Thus, in a DC shunt motor, the armature winding and field winding carry different electric currents depending upon their winding resistances. The connection diagram of a DC shunt motor is shown in the following figure.

The key features of DC shunt motors are highlighted below −

• DC shunt motors operate at a constant speed.
• DC shunt motors provide a moderate starting torque.
• DC shunt motors have a relatively simple speed control mechanism.
• DC shunt motors are self-regulating.

Due to constant speed and moderate starting torque, the following are some common applications of dc shunt motors −

• Lathes and milling machines
• Wood working machines
• Washing machines
• Metal cutting machines
• Small printing presses
• Elevators, etc.

DC Compound Motor

A DC compound motor is a special type of DC motor which has both series and shunt field windings. Hence, this type of dc motor combines the features of both series and shunt motors and provides a proper combination of high starting torque and speed regulation.

DC compound motors have two different types of connections namely,

• Long shunt − In this connection, the shunt winding is connected in parallel with the series combination of armature and series field windings.
• Short shunt − In this connection, the series winding is connected in series with the parallel combination of the armature and shunt field windings.

The circuit diagrams of the long shunt dc compound motor and short shunt dc compound motor are shown in the following figure.

The following are some key features of DC compound motors −

• DC compound motors have a high starting torque.
• DC compound motor provides efficient speed regulation.
• DC compound motor combines the characteristics of series and shunt motors.

DC compound motors are widely used in a verity of applications including the following −

• Industrial machinery
• Power and machine tools
• Shears
• Presses
• Reciprocating machines, etc.

This is all about different types of dc motors, let's now learn about different types of dc generators.

DC Series Generator

A dc series generator is a type of dc generator in which the field winding is connected in series with the armature winding, as shown in the following circuit diagram.

In a dc series generator, the same current flows through the armature winding, field winding, and the load.

Some main features of the dc series generator are listed below −

• The terminal voltage increases with the increase in load current.
• Beyond a certain limit of load current, the terminal voltage drops with the increasing load current.
• DC series generator has a rising voltage characteristic.

DC series generators are mainly used in the following applications −

• Used in DC locomotives to supply field excitation current for regenerative breaking.
• Used as a booster in distribution feeders to compensate voltage drop.
• Also used in series arc lightening, etc.

DC Shunt Generator

A DC shunt generator is a type of dc generator in which the field winding is connected in parallel with the armature winding. Thus, in this generator, the armature current supplies both the field current and the load current.

The circuit diagram of a dc shunt generator is depicted in the following figure.

The key features of the dc shunt generator are given below −

• DC shunt generator provides a constant voltage output across a wide range of load currents.
• DC shunt generator has a good voltage regulation.
• DC shunt generator requires a low current for field excitation during startup.
• The terminal voltage of a dc shunt generator slightly decreases due to internal drops and armature reaction.

DC shunt generators are widely employed in applications that require a constant voltage supply, such as −

• Battery chargers
• General lighting schemes
• Powering up electronic devices and systems, etc.

DC Compound Generator

A DC compound generator is a type of dc generator which has two sets of the field winding namely, series field winding and shunt field winding. Depending on the connection arrangement, there are two types of dc compound generators namely, long-shunt dc compound generator and short-shunt dc compound generator. Both these connection arrangements are shown in the following figure.

DC compound generators are mainly used in the following key applications −;

• DC compound generators are used for supplying power for lighting and heavy power services.
• DC compound generators are also used for driving electric motors.
• DC compound generators are also used in hotels, offices, and other building types for power supply.
• DC compound generators having differential compounding effect are used in arc welding.

AC Machines

AC machines are those electrical machines that convert either alternating current electricity into mechanical energy or mechanical energy into alternating current electricity. These electrical machines have their armature winding always located on the stator and the field winding located on the rotor.

Depending on the function, AC machines can be classified into two types namely AC Motors and AC Generators. Let's discuss about different types of AC machines in detail.

AC Motor

An AC motor is an electrical machine that converts alternating current electricity into rotational mechanical energy. AC motors are broadly classified into two main types namely, induction motors and synchronous motors. Which are explain below in more detail.

AC Generator

An AC generator is an electrical machine which converts rotational mechanical energy into alternating current electricity. Since it generates alternating current electricity, it is also known as alternator. AC generator is also referred to as synchronous generator.

Transformer

An electrical transformer is an electrical machine which has no moving components and used for changing or transforming the levels of voltage or current without changing the frequency and power at the input and output. Electrical transformer is a special machine, as it does not perform electromechanical energy conversion.

Induction Motor

An induction motor is a type of AC electric motor which converts alternating current electricity into mechanical energy by using the principle of electromagnetic induction. This type of AC motor runs at a speed slower than the synchronous speed, hence it is also known as asynchronous motor.

Depending on the power supply used, induction motors are of two types namely, single-phase induction motor and three-phase induction motor. Single-phase motors are used to drive small loads like fans, coolers, air-conditioners, washing machines, etc. while three-phase induction motors are used for operating heavy load like industrial machinery.

The following are some key characteristics of induction motors −

• Induction motors always operate at a speed slightly less than the synchronous speed.
• Induction motors are self-starting.
• Induction motors are highly reliable and simpler in construction.

Synchronous Motor

A synchronous motor is a type of AC electric motor which operates at a constant speed, called synchronous speed. The speed of rotation of the motor shaft is synchronized with the frequency of the electric supply. A typical synchronous motor has two magnetic fields, one is stator magnetic field and another is rotor magnetic field. Hence, a synchronous motor is a doubly-excited machine.

Small synchronous motors are mainly used in timing applications like synchronous clock or timers used in various appliances. While large sized synchronous motors are used in industrial applications like power factor improvement, driving rolling or ball mills, etc.

Some of the key features of synchronous motors are highlighted below −

• Synchronous motors operate at a constant speed.
• Synchronous motors cannot be self-starting.
• Synchronous motors require an additional excitation for rotor circuit.
• Synchronous motors provide facilities to control the power factor of supply.

AC Generator

AC Generator, also known as Alternator or Synchronous Generator, is a type of electric generator that produces alternating current electricity. Hence, we can define it as an electromechanical device that converts mechanical energy into alternating current electricity.

AC generators can be broadly classified into two types namely, single-phase AC generator and three-phase AC generator. A single-phase AC generator is one that produces single-phase electricity and mostly used in domestic and commercial applications, while a three-phase AC generator produces three-phase electricity and mainly used in industrial and large-scale applications.

Single-Phase Transformer

A single-phase transformer is an electrical transformer which has only one pair of primary and secondary windings. It is used in distribution systems and small-scale applications for voltage and current transformation. It has three main components namely, a primary winding, a secondary winding, and a laminated magnetic core as shown in the figure below.

Three-Phase Transformer

A three-phase transformer is a large-sized electrical transformer which consists of three pairs of primary and secondary windings, one for each phase. These three-phase windings are connected in either star or delta connection. Three-phase transformers are mainly used in power transmission and distribution systems for regulating the supply voltage and current.

Autotransformer

An autotransformer is a special type of electrical transformer which has only one winding, acting as both primary and secondary.

In this transformer, the input voltage is applied across two terminals of the primary winding and the output voltage is taken across two terminals of the secondary winding, where terminal is common between the primary and secondary.

Autotransformer transfers electricity from one circuit to another through conduction and induction, because it has only one coil. It is widely used in motor starters, transmission lines and distribution feeders to compensate voltage drops, in testing benches, etc.

Conclusion

In this chapter, we presented an overview of various types of electrical machines along with their circuit diagrams and key features. Here, we touched upon the fundamental concepts of both AC and DC electrical machines, including DC motors, DC generators, AC motors, AC generators, and transformers.