Electric motors are a part of many industrial and residential systems. They convert electric energy into mechanical energy by producing rotational force. The two main types of electric motors are AC motors (Alternating Current) and DC motors (Direct Current). Both have varied power sources, starting torque, speed control, and applications.
AC motors generate higher torque than DC motors and offer better efficiencies at high-rated speeds. However, this does not mean that DC motors are inefficient. There are many advantages of DC motor for a variety of applications. Let’s explore them below.
What Is AC Motor and How Does It Work?
An AC motor changes the input power into mechanical energy in the form of rotating movement. It has a stationary part, the stator, and a rotating part, the rotor. The AC motor can operate on a single-phase alternating current or three-phase current.
AC motors give out a magnetic flux and induced current within the motor through the actions of the stator and the rotor. In an AC motor, a ring of electromagnets is arranged on its exterior. They make up the stator. They produce a rotating magnetic field. A strong metal axle, a loop of wire, a coil, and a squirrel cage rotor consisting of metal bars and interconnections rest inside the stator.
The alternating current is supplied to the coils that make up the stator. It energizes the stator and creates a magnetic field that rotates around the motor’s outside.
What Is DC Motor and How Does It Work?
A DC motor converts direct current electrical energy into a rotating motion. Its rotor, which contains coil windings, is present in the appliance. When a DC current powers the motor, a magnetic field forms within the stator. It attracts and repels magnets on the rotor, which then starts rotating. This magnetic field repels the original field from the permanent magnet and causes the rotor to spin.
When the rotor contacts the magnetic field, it stops spinning. However, in a DC motor, a mechanical device called a commutator reverses the current through the stator. So, the magnetic field reverses and the rotor keeps spinning.
A brushless DC motor is a type of DC motor with no commutator. The rotor is a permanent magnet and the coils are fixed in place on the stator. The rotating movement happens by controlling the magnetic fields generated by the rotor. The magnetic field generated by the stationary magnets stays stationary. If you want to change the speed of rotation speed, you need to change the current supplied for the coils.
With the brushed motor, rotation is achieved by controlling the magnetic fields generated by the coils on the rotor while the magnetic field generated by the stationary magnets remains fixed.
What Is the Difference Between AC and DC Motor?
The major difference between AC and DC motors is the type of electricity they use. While AC motor uses an alternating or consistent flow of current, DC motor uses direct current. It remains consistent and follows a single direction. Due to this reason, a DC motor has a higher starting torque (rotational or twisting force) than an AC motor.
The direction of current in a DC motor is regulated by the commutator. This device periodically reverses the flow of the current between the rotor and the external circuit, ensuring that a torque-producing magnetic field stays in a single direction relative to the rotor. This facilitates a smooth and consistent torque.
In contrast to the DC motor, an AC motor consists of a fixed stator and a moving rotor. The windings in the stator create a continuous electromagnetic field of rotation when supplied with alternating voltage. It causes the rotor to rotate.
The starting torque of an AC motor is lower than DC motor because its rotor is not directly linked to the stator’s magnetic field. It is indirectly linked to it through electromagnetic induction. As a result, it reduces the level of torque that the AC motor can produce at start-up.
The difference in the nature of current and torque production makes AC and DC motors suitable for different applications. When high starting torque and speed control are required, the advantages of a DC motor are more. It is thus used for applications like:
- Vehicles
- Cranes
- Elevators
- Winches
- Crushers
- CNC
- Conveyors
Contrarily, the low starting torque of the AC motor makes it suitable for the following applications:
- Pumps
- Compressors
- Fans
- Washing machines
- Refrigerators
- Air Conditioners
- Vacuum cleaners
Differences Between AC and DC Motors
| Point of difference | AC Motor | DC Motor |
|---|---|---|
| Type of current | Alternating current | Direct current |
| Types | Asynchronous induction motor and synchronous motor | Brushed DC motor and brushless DC motor (BLDC) |
| Type of power supply | Single-phase or three-phase | Single-phase |
| Commutation requirement | No | Yes |
| Self starting | Single-phase motor is not self-starting but three-phase motor is self-starting | Yes |
| Starting torque | Lower | Higher |
| Speed control | Done through VFD | Done by adjusting the voltage |
| Efficiency | 75% to 90% | 85% to 90% with BLDC |
| Torque changes | Yes | No |
| Type of applications | High power | low power |
Which Is Better? AC Motor Vs. DC Motor
To effectively compare AC motor vs DC motor, we will use essential parameters like design, speed control, starting mechanism, toque characteristics, and maintenance requirements.
Motor Design and Construction
AC motors have a simpler design than DC motors. They have a single moving part, the rotor, to which the current is supplied. Besides this, they have a terminal box, rotor sheet package, and ball bearing.
This design contrasts with that of a DC motor, which has a commutator and carbon brushes that reverse the flow of current. They tend to wear out and need replacement from time to time.
However, brushless DC motors have a simple design because they lack brushes and commutators, which are the major wear components. The design is similar to AC induction motors.
Speed Control
DC shunt motors, in which the field winding is connected parallel to the armature winding, can regulate their speed over and under the rated values by modifying the supplied current. This is not the case with AC motors.
The AC motor operates at the frequency of the current supplied and is resistant to changes in speed. To vary the input’s frequency, Variable Frequency Drives (VFDs) are used.
This means that precise speed control with consistent torque is only possible with a DC motor. If you want to adjust their rates of output for various shifts or seasons, a DC motor will be ideal.
Starting Mechanism
A DC motor is self-starting because it does not need any external devices to start and operate, and it has a commutator and brushes. Contrarily, the rotating rotor inside a magnet on a single-phase AC motor does not move independently. It needs a rotating magnetic field to rotate. The rotor needs a push to get it moving.
Efficiency
A motor’s efficiency is the effectiveness with which electrical force gets converted to mechanical energy. More efficient motors require less energy to produce a rotating motion.
A brushless DC motor, which lacks rotor windings, has a stationary magnet rotor. It dramatically minimizes the slip between the rotor and stator. By slip, we mean the difference between the rotor’s speed and the speed of its magnetic field. Consequently, the electromechanical conversion rates of such a DC motor are between 85% and 90%.
This is the same principle on which the AC motor’s efficiency is considered. Since it does not have a commutator, the energy loss associated with it is removed.
Another factor related to efficiency is how fast the motor can adjust its speed for energy savings and better efficiency. A brushless DC motor often surpasses an AC motor in this case because its speed is controllable through a controller that uses Pulse Width Modulation (PWM).
Maintenance Requirements
A traditional DC motor needs more maintenance than an AC motor because of physical brushes. However, brushless DC motors are as low-maintenance as AC motors. Both types of motors have mechanically fewer components. So, they only need periodic lubrication to work optimally.
Key Advantages of DC Motor: Where Do They Excel?
Now that you know the working principle of the DC motor and how its features compare to the AC motor, let’s dive into the seven key advantages of DC motors.
1. Better Efficiency and Performance
The DC motor design has greatly benefitted from material science and electronics advances. New optimization techniques, like improved magnetic circuit design, thermal management, and reduced frictional losses, have made them highly efficient power transmission tools.
The integration of Smart IC (Integrated Circuit) technology controls and drives the running DC motors more efficiently and intelligently than AC motors. The tech integration does several things, like adjusting motor speed control, current detection, mechanical fault protection, and communication with external devices.
2. High Torque-to-Weight Ratio
Today’s DC motors are equipped with an axial flux design. It facilitates a high torque-to-weight ratio. Brushless DC motors have fewer components. It leads to less energy loss due to friction, thereby generating high torque.
The motor produces significant rotating force relative to its size. So, you can use compactly made DC motors and get the benefits of increased power density and efficient use of space. This advantage of a DC motor is particularly great for the aerospace and automotive industries.
3. Faster Response Times
A DC shunt motor responds quickly to changes in voltage supplies. It enables quick acceleration and deceleration. The motors can also reverse direction if needed. These advantages of DC motors make them most appropriate for applications in electric vehicles and industrial machinery.
4. Harmonic-free Operation
Harmonics are the layers of additional electrical waveforms that come on top of the main power signal in an electrical system. The main power signal is the fundamental frequency or the standard voltage pattern on which your motor operates. Harmonics hamper power quality, leading to torque pulsations in motors. They cause losses in the stator windings, rotor circuits, and iron cores, which reduces the motor’s energy conversion capability. In DC motors, there are no issues related to harmonics because they don’t have a switching nature, unlike AC motors.
5. Advanced Cooling Systems
Modern DC motors have advanced cooling systems personalized to the motor’s heat dissipation requirements. Features like liquid-cooled stators or rotor cooling channels, hybrid cooling with pipes, and improved ventilation designs prevent overheating and increase the motor’s longevity. They also result in optimal performance levels.
6. Easy Installation
DC motors do not need as much electronic and rectification on the power system circuit as an AC motor. So, you can fit it quickly and start using it by supplying electricity. To use an AC drive, the incoming alternating current should be rectified to create a direct current, which is then changed to alternating current for the AC motor.
But with a DC motor, you just need to rectify the alternating current before passing it to the DC motor. These motors can also be supplied with power from varied sources, including batteries.
7. Quieter Operation
Brushless DC motors operate quietly and smoothly because there is no contact between the brushes and the commutator. They also produce no noise or vibration when they work. Thus, they are appropriate for applications requiring low noise levels, like patient hoists.
Concluding Words
High starting torque, efficient speed control, energy efficiency, and hassle-free installation are some key advantages of DC motor. This is why they are used widely in power tools, compressors, cranes, elevators, and vacuum pumps. With steady advancements in DC motor technology, the scope of applications is growing. If you need a highly precise DC motor for your operation, feel free to contact us. As an authorized distributor of electric motors by major brands, we stock many types of motors and provide complete assistance in choosing the right one.
