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
Characteristics AC Drives DC Drives
Supply Run by AC supply Run by DC supply
Maintenance Requires less maintenance. Requires comparatively more and frequent maintenance.
Use/ Application Are generally used for AC motors. Are used for DC motors normally.
Locational use Are used in almost all the locations. are used in very few locations
Breaking Mechanism Breaking and accelerating when supply frequency (F s ) changes Breaking occurs when resistance is applied at rotor.
Power & Control Circuitry Power 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 Rating Power to Weight ratio is very large. Power to Weight ratio is considerably small.
Components Have converter and inverter. Have converter and chopping circuit.
Commutation There is no commutation which makes AC Drives less bulky and inexpensive. Because of commutation, they are more heavy and costly.
Rectifier There is no need of rectifier circuit Rectifying circuit is necessary
Speed Control Sped control is achieved by changing the frequency Speed control is achieved by armature and field control
Corrosive and Wet Environment Can be used in such environment since their operation is spark free Cannot be used because sparks can be produced in brushes easily.
Cost In AC Drives, motors are less expensive especially squirrel cage motors. In DC Drives, motors are significantly expensive.
Speed limit There is no upper limit for speed. Speed is limited because of commutation process.
Harmonics Converters produce harmonics in supply as well as in load Converters do not produce harmonics
Motor speed Can reach up to 10000 RPM Can reach up to 2500 RPM
Speed regulation ~1% speed regulation is attainable Not possible to achieve 1 % speed regulation
Dynamic response Offer high dynamic response Offer low dynamic response
Starting torque Do not produce high starting torque Produces high starting torque
Battery use Cannot be run directly by batteries without using extra circuitry Can be run directly through batteries without adding an extra components
Noise Operation is noisy which is highly unfavorable in certain applications They are not noisy as compared to AC Drives
Power consumption Consume less power Consume more power
Circuit complexity More complex as it involves inverter (DC → AC) and converter (AC → DC) Less complex because of single power conversion process from AC to DC
Brushes life Very high (~10000h) Low (~3000h)
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.