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Difference between AC Drives and DC Drives

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The AC drives operate with AC signal or voltage, for instance, 1-phase or 3-phase AC voltages whereas DC drives operate with DC signal or voltage, for instance, DC supplies, and Batteries.

Generally, a DC drive changes an Alternating Current (AC) into Direct Current (DC) using a converter (Rectifier) to operate a DC motor while the AC drive changes the input AC current to DC using converter (Rectifier) and then changes this DC to AC, using an inverter, to operate the motor.

The key differences between AC and DC Drive are discussed in this article on the basis of real-world factors such as supply source, maintenance, uses, breaking mechanism, power & control circuitry, size & weight, commutation process, speed control & limit, harmonic distortion, speed regulation, starting torque, response to dynamics, noise, power consumption, brushes life, and application.The following table explains the key Differences between AC Drives and DC Drives.

Difference between AC Drives and DC Drives

CharacteristicsAC DrivesDC Drives
SupplyRun by AC supplyRun by DC supply
MaintenanceRequires less maintenance.Requires comparatively more and frequent maintenance.
Use/ ApplicationAre generally used for AC motors.Are used for DC motors normally.
Locational useAre used in almost all the locations.are used in very few locations
Breaking MechanismBreaking and accelerating when supply frequency (F s ) changesBreaking occurs when resistance is applied at rotor.
Power & Control CircuitryPower and control circuits are difficult and quite complex in design.Power and control circuits are simple to design and less expensive as compared to their counter-parts.
Size in terms of Power RatingPower to Weight ratio is very large.Power to Weight ratio is considerably small.
ComponentsHave converter and inverter.Have converter and chopping circuit.
CommutationThere is no commutation which makes AC Drives less bulky and inexpensive.Because of commutation, they are more heavy and costly.
RectifierThere is no need of rectifier circuitRectifying circuit is necessary
Speed ControlSped control is achieved by changing the frequencySpeed control is achieved by armature and field control
Corrosive and Wet EnvironmentCan be used in such environment since their operation is spark freeCannot be used because sparks can be produced in brushes easily.
CostIn AC Drives, motors are less expensive especially squirrel cage motors.In DC Drives, motors are significantly expensive.
Speed limitThere is no upper limit for speed.Speed is limited because of commutation process.
HarmonicsConverters produce harmonics in supply as well as in loadConverters do not produce harmonics
Motor speedCan reach up to 10000 RPMCan reach up to 2500 RPM
Speed regulation~1% speed regulation is attainableNot possible to achieve 1 % speed regulation
Dynamic responseOffer high dynamic responseOffer low dynamic response
Starting torqueDo not produce high starting torqueProduces high starting torque
Battery useCannot be run directly by batteries without using extra circuitryCan be run directly through batteries without adding an extra components
NoiseOperation is noisy which is highly unfavorable in certain applicationsThey are not noisy as compared to AC Drives
Power consumptionConsume less powerConsume more power
Circuit complexityMore complex as it involves inverter (DC → AC) and converter (AC → DC)Less complex because of single power conversion process from AC to DC
Brushes lifeVery high (~10000h)Low (~3000h)

Conclusion

Although the DC drives are quite renowned for offering high starting torque, bearing simplified circuitry and are effective with constant speed applications, they are believed to have a lot of problems because of commutation process and brushes in the DC Motors which require periodic and extensive maintenance. On the other side, AC drives are quite energy efficient and can manage abrupt speed changes very well. They frequently have numerous programmable parameters for protections.

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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.