Difference Between DC Motors and AC Motors

Edited by Diffzy | Updated on: May 28, 2023

       

Difference Between DC Motors and AC Motors

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Introduction

AC Motors and DC motors are the electric current sources of the Electric motors. First of all, let's understand what Electric Motors are.

Electric Motors are electrical engines that transform electrical energy into mechanical energy. Mainly, electric motors function when the motor's magnetic field and electric current act together in a wire twirling to generate force in the form of torque used on the motor's shaft. Electric motors generate linear or rotatory force planned to drive some external mechanism. 

Applications of Electric motors include machine tools, household appliances, industrial fans, vehicles, disk drives, pumps and blowers, and power tools. Electric motors can be utilised in reverse, as generators to recover energy that might otherwise result in vanishing as heat and friction. The working of the electric motor relies on the communication of magnetic and electric fields. The frequently used parts used in electric motors are- Central Motor shaft, Windings, Bearings (for reducing friction and wear), Armature (located on the rotor or the stator), Brushes (only in DC motors), Frame, Terminals, and End Shields.

Electric motors may get categorised by considering the following aspects - power source type, construction, application and type of motion output. They can get motorized by AC or DC, be brushed or brushless, axial or radial flux, have two-phase, three-phase, single-phase, and can also be air-cooled or liquid-cooled. 

There are two major kinds of Electric Motors: 

  1. AC motors or Alternating Current Motors
  2. DC motors or Direct Current Motors. 

These are employed in the majority of electric motor applications and are adjusted to serve the industry and requirements of the product. Let's have a look at the difference between DC Motors and AC Motors.

DC Motors vs AC Motors

AC and DC both are electric motors but are very different from each other. DC motors use direct current whereas AC motors utilise alternating current. In DC motors current is transformed which is not the case with AC motors. On one hand, DC motors require high maintenance because of the presence of commutators and brushes. On the other hand, due to the absence of commutators and brushes, AC motors are low to maintain. In AC motors the armature stays still and the magnetic field rotates. In DC motors, the armature revolves but the magnetic field remains still. The efficiency of DC motors is high because there is no loss of induction current and magnetic field. AC motors have less efficiency because of the induction current loss and motor slip.

We shall dicuss about the differences between DC motors and AC motors further in detail.

Difference Between DC Motors and AC Motors in Tabular Form

Parameters of ComparisonDC MotorsAC Motors
DefinitionDC Motors are machines which convert electrical energy into mechanical energy using direct current.AC motors are the ones that work with an alternating current to generate mechanical energy using magnetism.
TypesThe types of DC motors include- Series, Geared, and Shunt. It can have other types depending on the basis of classification.AC motors are majorly of two types- Synchronous and Induction or Asynchronous.
CurrentDC motors use direct current.AC motors work on alternating current.
InitiationDC motors are self-starting in nature.A single-phase AC motor is not self-initiating and a three-phase AC motor can be self-starting.
ArmatureThe armature revolves but the magnetic field remains still in DC motors.In AC motors the armature stays still and the magnetic field rotates.
Load switchDC motors respond fast to load changes.AC motors are less responsive to load switches.
UsesDC motors are usually used in small appliances.AC motors are utilised in large appliances.
Commutator & BrushesCommutator and Brushes are present.Commutator and Brushes are absent in AC motors.
MaintenanceDC motors require high maintenance because of the presence of commutator and brushes.Due to the absence of commutator and brushes, they are low to maintain.

What is DC Motors?

DC Motors are machines which convert electrical energy into mechanical energy using direct current. In a DC motor, a current-bearing conductor develops a magnetic field. When this conductor gets positioned in an external magnetic field, it will undergo a force proportional to the current in the conductor and to the power of the external magnetic field. DC motors can vary in size and strength from small engines in toys and devices to giant mechanisms that power vehicles, pull elevators, and move steel rolling mills.

History of DC Motors

British scientist William Sturgeon invented the first commutator DC electric motor that could turn machines in 1832. After him, American inventor Thomas Davenport and Emily Davenport built a commutator-type DC electric motor and got it patented in 1837. The motors raced at up to 600 revolutions in 60 seconds, and powered machine tools and a printing press. The motors were a commercial failure because of the high cost of main battery power and bankrupted Davenport. No other practical commercial market appeared for these motors as there was no electricity distribution system during that period.

After many failed attempts by many scientists, Prussian/Russian Moritz von Jacobi invented the first actual rotating electric motor in May 1834. His motor set a world record as it expanded exceptional mechanical power output. The second motor that he developed was powerful; it could drive a boat with 14 people across a wide river. 

A turning point arrived in 1864; Antonio Pacinotti first described the ring armature. Zénobe Gramme rediscovered Pacinotti's plan and adopted some explanations by Werner Siemens in 1871. It was deemed the first commercially victorious DC Motor. Frank Julian Sprague conceived the first practical Direct Current motor in 1886. It was a non-sparking device that harbored relatively consistent speed under varying loads. Sprague's motor invention led to an outbreak of interest and usage in electric motors for the industry. The evolution of electric motors of acceptable efficiency was delayed for several decades by failure to perceive the extreme significance of an air gap between the rotor and stator.

Types of DC Motors

DC motors are categorized into the following types:

  1. Series DC Motors- A Series wound DC motor is a motor where field winding is joined internally in sequence to the armature winding. In Series DC motors, the field windings are united in series with the armature. The field strength is different in varying armature currents. The series motor provides high starting force but must never run without a load. It is capable of moving enormous shaft loads when first loaded with energy.
  2. Geared DC Motors- Geared DC Motors reduce the pace of the motor but with an increase in torque. Geared motors consist of a DC brush motor and a gearbox affixed to the shaft. It is applicable in many places due to its fee of designing, reduces complexity, and is used in constructing applications such as industrial equipment, actuators, medical tools, and robotics.
  3. Shunt DC Motors- In Shunt motors, the field windings are linked in parallel with the armature. It may have a lower current because of the current being parallel. It is helpful in applications that require continuous torque.

What is AC Motors?

AC or Alternating Current Motors are the ones that work with an alternating current to generate mechanical energy using magnetism. The AC motor structure comprises - Coils to develop a rotary magnetic field within an affixed Rotor toward an output shaft to generate a second magnetic field.

These motors have been employed for several years by inventors and engineers to use in various applications. They are very helpful in generating torque, are simple to handle, and are less costly.

History of AC Motors

During the 1880s, many inventors tried to invent AC motors because AC motors' benefits in long-distance high-voltage transmission were neutralized by the incapability to operate motors on AC. In 1885, Galileo Ferraris developed the first AC commutator-less induction motor. In 1886, he improved his first design by creating more advanced steps.

Nikola Tesla independently invented his induction motor in 1887 and patented it in 1888. Tesla presented  A New System of Alternate Current Motors and Transformers describing the three patented two-phase four-stator-pole motor types including- one with a four-pole rotor developing into a non-self-starting reluctance motor, the second with a wound rotor introducing a self-starting induction motor, the third a true synchronous motor with unconnectedly excited DC supply to the rotor winding.

George Westinghouse achieved his first discovery of a practical induction motor in 1892. In 1893, he designed a line of polyphase 60-hertz induction motors. In 1889, Mikhail Dolivo-Dobrovolsky invented the three-phase induction motor of both types- cage rotor and wound rotor. In 1891, The General Electric Company began generating three-phase induction motors. 

The improvements at that time because of the inventions and innovations were so good that a 100-horsepower induction motor presently has the exact mounting dimensions as a 7.5-horsepower motor in 1897.

Types of AC Motors

The types of AC (Alternating Current) motors are:

  1. Synchronous Motors- The function of Synchronous AC Motors is to ensure that the shaft rotates at the same frequency as the speed of the current supply; with the rotation period being equivalent to the integral number of AC cycles. When the power supply is provided to Synchronous motors, then only a rotating field can be generated. It is not a self-initiating motor.
  2. Induction Motors- They are also known as Asynchronous Motors. This is the standard AC motor used, since it relies on alternating current connected to the stator for its power supply. The armature winding in this motor performs like both the windings like armature & field. Induction Motors are also classified into two kinds-
  • Single-phase induction motors- This AC motor is smaller in size and less costly. The rotor spins according to the sum of two contrarily rotating fields, which is the double-revolving field theory. Therefore, the torque produced is opposite and equal.
  • Three-phase induction motors- They are similar to single-phase motors in terms of their operation. This system has a group of equal voltages at the same frequency that is positioned to have an equivalent phase distinction between the adjoining electromagnetic fields. It is also known as a Polyphase induction motor.

Main Differences Between DC Motors and AC Motors (In Points)

  • DC Motors are mechanical devices which transform electrical energy into mechanical energy using direct current. AC motors work with an alternating current to generate mechanical energy using magnetism.
  • DC motors operate only when there is a one-phase supply of current. On the other hand, AC motors can work on three-phase as well as single-phase supplies.
  • In AC motors the armature stays still while the magnetic field rotates. This process is the exact opposite in DC motors. The armature revolves while the magnetic field remains still in the DC motors. 
  • DC motors are preferred when there is a need for altering the speed or high torque. When there is a requirement for pace or inconsistent torque, AC motors are preferable.
  • DC motors respond fast to load changes. On the contrary, AC motors are less responsive to load switches.
  • AC motors are used in large industrial applications and DC motors are used in small domestic appliances.
  • Commutators and Brushes are present in DC motors therefore, they are high in maintenance. Due to the non-availability of Brushes and commutators, AC motors are low maintenance.
  • The efficiency of DC motors is high because there is no loss of induction current and magnetic field. AC motors have less efficiency because of the induction current loss and motor slip.

Conclusion

Understanding the concept and difference between DC motors and AC motors is essential not only from the technological front but also for its application in practical life. As mentioned earlier in the article, efforts were made by many scientists to develop better and better motors for the convenience of people, understanding this topic is important from the future point of view. Development starts with a developing brain. Electric motors were invented by some great inventors who changed the fate of their countries. Hopefully, this article will help clear the air of confusion between DC and AC motors. The definitions and differences have been vividly illustrated in the article, along with their histories and types.


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"Difference Between DC Motors and AC Motors." Diffzy.com, 2024. Fri. 19 Apr. 2024. <https://www.diffzy.com/article/difference-between-dc-motors-and-ac-motors>.



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