Difference between AC Drives and DC Drives

In this article, we will have a detailed discussion regarding electric drives, their types, and how AC Drives and DC Drives differ in their features.

What is an Electric Drive?

An electrical or electronic device which is used for controlling the speed of an electric motor is known as an electric drive. The electric drives are constant and variable speed controllers and extensively used in industrial automation and robotics. The basic block diagram of an electric drive is shown in the figure below.

Based on the nature of electric current, the electric drives are of following two types −

• AC Drives (Alternating Current Drives)

• DC Drives (Direct Current Drives)

What is an AC Drive?

The type of electric drive which is used for controlling the speed of AC motors (especially 3-phase induction motors) is known as AC drive. The operation of an AC drive can be described in the following steps −

What is an AC Drive?

The type of electric drive which is used for controlling the speed of AC motors (especially 3-phase induction motors) is known as AC drive. The operation of an AC drive can be described in the following steps −

• Step 1 − AC drive converts input AC supply into DC using rectifier circuit.

• Step 2 − Inverts the DC back to the AC using inverter circuit to control the speed of the AC motor.

AC drive is also known as variable frequency drive (VFD) because it changes the frequency of the AC supply to control the speed of the AC motor.

What is a DC Drive?

A type of electric drive which is employed for controlling the speed of a DC motor is known as DC drive. The DC drive converts the input AC supply to the DC supply output using the rectifier circuit to control the speed of the DC motors. The DC drives are of two types viz.: analog DC drive and digital DC drive.

Differences between AC and DC Drives

The main differences between AC drives and DC drives are shown in the following table.

Point of Comparison	AC Drives	DC Drives
Definition	The electric drive which is used for controlling the speed of an AC motor is known as AC electric drive.	The electric drive which is used for controlling the speed of a DC motor is known as DC electric drive.
Operation	AC drive converts the AC supply into DC using rectifier circuit, then convert it back to AC (frequency is changed) to control the speed of the AC motor.	DC drive uses mercury arc rectifier circuits to convert the input AC into DC at different voltages to control the speed of DC motors.
Converter circuit	AC drives have two converter circuits viz. rectifier for AC to DC conversion and inverter for DC to AC conversion.	DC drive involves only one converter circuit that is rectifier for AC to DC conversion.
Controlling quantity	AC drive controls the AC output from the AC input.	DC drive controls the DC output from the AC input.
Self-starting	AC drives are not self-starting.	DC drives are self-starting.
Drive power supply	AC drives are operated by AC power supply either 1-phase or 3-phase.	DC drives are operated by DC power supply such as batteries, etc.
Circuit complexity	AC drives involve complicated circuit design due to presence of rectifier circuit and inverter circuit.	The circuit of DC drive is relatively less complex due to the single power conversion, i.e. from AC to DC only.
Parameter of speed control	In AC drives, the speed control is performed by changing the supply frequency.	In DC drives, speed control is achieved by armature and field control.
Limitation of speed	In AC drives, maximum speed can be obtained.	In DC drives, speed is limited due to commutator used in the DC motors.
Speed of motor	Using AC drives, motor speed can be obtained up to 10,000 RPM.	With DC drives, motor speed can be obtained up to 2500 RPM.
Speed Regulation	There is approximately 1% speed regulation is achievable in the AC motors driven by AC drives.	The speed regulation of 1% cannot be achieved in DC motors driven by DC drives.
Speed torque curves	Using AC drives, the adjustment of speed-torque curves of motor is complex.	With DC drives, it is easy to adjust the speed-torque curves of the motor.
Control and power circuits	The control and power circuits of an AC drive are complex.	The control and power circuits of DC drives are simple.
Braking	The braking mechanism of AC drives are controlled by changing the supply frequency.	The braking mechanism of DC drives is controlled by applying resistance at rotor side of motor.
Power consumption	AC drive consumes less power.	DC drive consumes comparatively more power than AC drive.
Noise	AC drives have noisy operation.	DC drives have less noisy operation.
Sparking	The operation of AC drives is spark free, therefore, can be used in wet areas.	In DC drives, the spark is produced the brushes, hence they cannot be used in wet areas.
Maintenance	AC drives require less maintenance.	DC drives require frequent and more maintenance.
Weight and Size	The AC drives are heavy and larger in size.	The DC drives are light and smaller in size.
Power rating	AC drives have high power rating.	The power rating of DC drives is comparatively smaller.
Cost	AC drives are more expensive.	DC drives are cheaper than AC drives.
Presence of Harmonics	In AC drives, the inverter circuit produces harmonics on both supply and load sides.	In DC drives, only rectifier converter is used which does not produce harmonics.
Applications	AC drives are used for speed control of AC motors.	DC drives are used for speed control of DC motors.

Conclusion

From the above comparison, we can conclude that AC drives are employed for controlling the speed of AC motor, these drives find extensive application in industries because the three phase induction motors are used widely to perform various industrial processes.

On the other hand, DC drives are efficient, reliable and easy to operate, less expensive and easy to implement. However, the DC drives are used in few areas as compared to AC drives due to the limited use of DC motors in industries.