Difference between Electrical Drives: AC and DC Drives
This post explains the difference between AC and DC Drives. It is highly recommended to read the Electrical Drives: Introduction, Advantages, Classification Post before proceeding further.
A Drive is a combination of various systems combined together for the purpose of motion control. There are three types of drives namely
 Group Drive
 Individual Drive
 Multimotor Drive
Drives employing electric motors are called as Electrical Drives. Based on power supply the electrical drives are classified as
 AC Electrical Drive
 DC Electrical Drive
- Both ac and dc motors are used for electric drives. However, ac system is preferred because the generation of AC power is more natural and efficient than the generation of DC power.
- Also the ac power can be efficiently transmitted at low line losses, the voltage at the consumer premises can be maintained within prescribed limits and there is a possibility of economically stepping up and stepping down of voltages through the use of transformers.
- In spite of all the above the advantages of ac system, the use of dc is essential for some applications.
- The basic criterion in selecting an electric motor for an electric drive system is that it should meet the power level and performance required by the load during steady state and dynamic operations.
- For example, in the applications for which a high starting torque is needed, a DC series motor might be a better choice than an Induction motor (AC motor).
- In constant speed applications, synchronous motor might be a better choice than Induction motor or DC series motor.
If the primary consideration for selecting an electric motor is cost for a particular application, then Induction motor is the right choice.
Because in general, dc motors and brushless motors are most expensive motors whereas squirrel cage induction motors are the cheapest one.
- Environmental factors also will determine the type of motors to be used for the applications.
- For example, in food processing, chemical industries, and aviation the environment must be clean and free from arcs, DC motors cannot be used unless they are encapsulated.
- This is because of the electric discharge that is generated between the motor's brushes and its commutator segments.
- In these cases, the squirrel cage induction motor or other brushless machines are probably the better options.
|DC Drives||AC Drives|
|The Power circuit and control circuits are simple & inexpensive||The power circuit and control circuits are complex|
|It requires frequent maintenance||Less maintenance required|
|The commutation makes the motor bulky, costly and heavy.||These problems are not there. Motors are inexpensive, particularly the squirrel cage motor.|
|Speed and design rating are limited due to commutation.||Speed and design rating have no upper limits.|
|It is used in certain locations only.||It can be used in all locations.|
|Fast response and wide speed range are smoothly achieved by conventional method and solid state control.||In solid state control the speed range is wide and in conventional method it is stepped and limited.|
|The line conditions are very poor. ie, Poor power factor, harmonic distortion of the current.||For regenerative drives the line power factor is poor. For non-regenerative drives the line power factor is better.|
|Power/Weight ratio is small.||Power/Weight ratio is large|