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Difference between AC and DC Motor

An electric motor is a machine that converts electric energy into mechanical energy. An electrical signal (voltage) is applied to the input terminals of the motor, and the output of the motor output generates a specific torque according to the motor characteristics.

AC motors and DC motors perform the same function which is to convert electric power into mechanical power, however, they are powered, manufactured and controlled in a different manner. The most primary difference between two of them is the source of power. A.C. motors are operated with alternating current whereas DC motors are operated with direct current, like DC supplies, batteries, or AC-DC converters.

Here, the practical differences between AC and DC Motor are discussed on the basis of important factors like starting mechanism, efficiency, brushes & commutation, response to sudden load change, supply sources, stator & rotor action, speed & torque, armature location, maintenance and uses.The following table explains the key differences between AC and DC Motor.

Difference between AC Motor and DC Motor

CharacteristicsAC Motor (Induction)DC Motor
SupplyWorks on AC supplyWorks on DC supply (mainly from batteries, and cells)
StartingNeeds starting mechanism for single phase NOT for three phaseSelf-starting
Supply systemCan be single and three phaseSingle phase ONLY
Stator & Rotor ActionStator remains stand-still and supplies rotating field while rotor rotatesArmature rotates and field winding remains stand still
Brushes & CommutatorDo not have brushes and commutator; very rigged and have long lifeHave brushes and commutator which limit the speed and reduce the life expectancy
Speed controlBy changing frequencyBy changing armature winding’s current
Speed & TorqueUse for high speed and variable torqueUse for variable speed and high torque
Load changeSlow response to change in loadQuick response to load change
EfficiencyLess efficient because of induction current loss and motor slipMore efficient since no motor slip and induction current loss
Armature LocationArmature remains Stationary and magnetic field rotatesMagnetic field remains stand still while armature rotates
UseAC motors are primarily used in large industries and factoriesThey are used in most of the domestic applications.
Maintenance Requires less maintenance because of absence of brushes and commutation processDemands extensive maintenance frequently

Conclusion

DC motors are typically utilized in applications where the speed has to be controlled by some external source. AC motors operate better in such applications where power efficiency is required for a longer time. AC motors are a single phase as well as three phase whereas DC motors are single phase always.

In certain applications, DC motors are substituted with an AC motor which is connected with an electronic speed controller. This connected assembly is called variable frequency drives (VFD).  DC motors comprise of moving parts that are costly to replace, and repair is generally more expensive than utilizing an AC motor with an electronic controller.

About Ahmad Faizan

Mr. Ahmed Faizan Sheikh, M.Sc. (USA), Research Fellow (USA), a member of IEEE & CIGRE, is a Fulbright Alumnus and earned his Master’s Degree in Electrical and Power Engineering from Kansas State University, USA.

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