Electric motors are important components of various motion-control applications across different industries, including food and beverage, manufacturing, medicine, and robotics. Depending on the machine’s function, as well as its accuracy, speed, and torque requirements, engineers usually choose between two types of motors: AC or DC motors.
AC and DC Motors: The Main Differences
Their names highlight their most apparent difference: AC motors operate by applying alternating current to the motor, while DC motors are powered by direct current. AC and DC motors also have different structures. The former has two basic parts: the stator and the rotor. The stator has coils which receive the alternating current to produce a magnetic field. The rotor then spins inside the stator’s coils to produce the required torque. On the other hand, DC motors have six basic parts: the axle, the rotor (also called the armature), the stator, the commutator, the field magnets, and the brushes. Most DC motor designs have at least two high-strength magnets inside the stator to produce a magnetic field.
Types of AC and DC Motors
Design-wise, AC motors have two different types. The first is the induction or asynchronous motor, which uses a magnetic field created by an induced current to spin the rotor. The second type of an AC motor is called a synchronous motor; the speed in which its rotors rotate is directly proportional to the supply frequency.
DC electric motors usually have more designs, including brush and brushless, shunt, series, and compound wound types. Their basic method of operation is similar: a conductor, placed within the magnetic field, delivers the current to rotate the motors. The difference lies in how and where the electromagnetic fields are generated, in the rotor or the stator.
Factors to Consider in Choosing AC and DC Motors
Maintenance
Some engineers prefer AC motors since they require less maintenance compared to brush models of DC motors. However, the brushes used in DC motors are steadily improving in quality over the years, and engineers continue to develop new designs to reduce the wear and tear of brushes. Alternatively, brushless DC motor designs may also be used to eliminate the need for brush maintenance.
Installation
DC motors also have a considerably larger installation base because they have been in use for more than 150 years. This means that engineers and technicians are more familiar with their operations and maintenance compared to AC motors. Replacing DC motors parts is also quicker, easier, and often less expensive than converting to an AC motor and drive.
Speed and Power
If your application needs higher and consistent power performance for longer periods of time, it’s best to use AC motors. For applications that need more speed control and/or speed variance, DC motors are the way to go. You’ll often see DC motors on machines that require constant speed no matter the amount of load, like rock crushers or power tools.
Torque
The simplest definition of torque is the amount of rotational force. In this regard, DC motors have an advantage over AC motors in that it can produce constant torque even as the speed varies; DC motors can also deliver full torque even at low speeds. This makes DC motors ideal for applications that need constant torque loads such as elevators and conveyor belts. However, if you need variable torque, such as in centrifugal fans, pumps, or compressor loads, AC motors, particularly induction motors, are the best choice.
Efficiency
Polyphase AC motors are more efficient since they have lower current requirements to accomplish the same task that a DC motor may require more voltage for. You will also find that most factory-grade machines are driven by AC motors since they have a faster initial speed even as they have a lower power requirement upon start-up. On the other hand, DC motors pack more power in a smaller body. This higher power density becomes a significant benefit in applications that have space constraints or require smaller parts due to their inherent design.
The bottom line is that both AC and DC motors have their advantages and disadvantages, and based on the usage circumstances, one may, in fact, be better than the other. It all depends on the application’s requirements; once you figure them out, the choice between AC and DC will be much clearer and easier to make.
