Fundamentals of AC vs DC motorsSeth Price | March 12, 2021
Electric motors are commonplace in all automation and motive power applications. The primary purpose of all electric motors is to turn electrical energy into mechanical energy. Often, the motors turn to provide a rotating motion, where torque and power output can be matched mechanically with pulleys and gears. Other motors are gear fed to a track to translate the rotational motion into linear motion.
There are a wide variety of electric motors, and the proper motor choice makes the difference between a long-lasting, efficient motor and a troublesome, inefficient machine. One of the important motor characterizations is whether the motor operates on alternating current (AC) or direct current (DC). Both types of motors have their advantages and disadvantages, as well as some special considerations for their use in an industrial setting.
AC motors are able to turn by converting an alternating electrical current into a rotating magnetic field. The rotating magnetic field is then used to turn the rotor, which is attached to the output shaft. The output shaft is then attached to a flywheel, blower motor cage and pump to perform useful work.
There are several key parts to AC motors. AC enters the stator, which consists of coils of wire. As the current changes direction in the stator, it generates the magnetic field. The stator does not move, but the AC flowing through it creates the changing magnetic field. The magnetic field pushes against the rotor, causing its rotation and turning the output shaft.
The image below shows an AC motor with a section cutaway. Inside, the stator and its windings are visible, as are the magnets on the rotor.
DC motors also have a stator, and as its name implies, it does not move. However, in a DC motor, the stator is a set of two permanent magnets (of opposite polarity) instead of the coil of wire used by an AC motor. In between the two permanent magnets is an armature (rotor) that consists of wire loops wound around a piece of metal. At the end of the armature is a communicator, which is simply a set of thin, flat plates for current to flow through as the motor spins. Finally, the DC power supply is connected to a set of carbon brushes, which transfer current to the communicator.
The figure below shows the armature of a small, DC motor. The wrapped wires form the electromagnet that repel and are attracted to the permanent magnets of the stator. The communicator is the series of flat copper plates. The carbon brushes connect and disconnect from these, polarizing different loops and causing the electromagnet to switch directions.
When the DC power supply is turned on, current flows through the carbon brush, into the communicator, then into the wire loop. This generates a magnetic field that repels away from one permanent magnet, and attracts to the other. As the motor turns, contact is broken between the carbon brush and the communicator, then reconnected to the opposite side. This reverses the polarity of the armature, causing it to continue spinning.
Comparison of AC and DC motors
AC motors are great for applications where the speed of the motor might need to be variable. AC motor speeds can be controlled using a variable frequency drive (VFD), where the change in frequency will change the speed of the motor. The VFD can be controlled with precision using advanced control algorithms, such as proportional, integral and derivative (PID) control. For example, imagine an assembly line that must repeatedly start and stop. Now, imagine that assembly line carrying upright glass bottles. Starting and stopping must be performed gently to not tip over the bottles.
In general, AC motors are physically lighter for their output. This is because the DC motor of equivalent output requires a large permanent magnet for its stator, which is heavier than the coils of wire used in the stator for the AC motor.
DC motors can start up quickly, and deliver high startup torque. This is particularly useful for large, stationary equipment. However, DC motors are also used on devices that are powered by batteries, such as electric cars, and portable electronics. Most small, battery-powered consumer goods that have moving parts contain DC electric motors.
The temptation is to use AC motors for vehicles, such as electric cars and trains. Their lightweight and precise speed control seem like a good fit in the transportation industry, where reducing weight means longer travel times, and speed control makes smoother starts and stops. However, the conversion from the DC power source (alternator/battery) to AC is not very efficient, meaning some of the gains found by reducing weight are lost anyhow. Also, as power electronics improve, dynamic braking and dynamic resistance to DC motors have reduced the jerkiness of early DC motors in the transportation industry. For now, the standard is to use DC motors for transportation.
Many children have played with toy cars equipped with battery-powered, small DC motors. These toys do not require high torque to turn the wheels. However, when a DC motor is scaled up, they turn much bigger devices, and require high startup torque. To accommodate this high torque, the DC motor will draw more current.
There is a natural limit to this, where the motor can only safely draw a certain current. To reduce these high startup currents, a starter is added in series with the armature winding. The starter adds a series resistance, reducing the startup current to a safe level.
This additional series resistance should not remain in place during steady-state operation. Thankfully, physics takes care of this problem. As the armature spins inside a magnetic field, a voltage is produced that is in the opposite direction, called the “back electromotive force,” or “back EMF.” As it turns out, the back EMF varies directly with armature speed, meaning it is zero when the motor is stationary, and increases as the motor speeds up. The back EMF works to reduce the effective resistance of the starter, meaning the faster the motor spins, the lower the resistance of the starter.
In terms of maintenance, the biggest problems are failed windings and failed shaft bearings. These are often due to overheating from lack of lubrication in the bearings, or excess current draw on the wiring. Bearings can be lubricated as part of regular motor maintenance, and, on more expensive motors, accelerometers can measure vibration. When there is a new vibration signature or excess vibration, the technicians can start troubleshooting the problem. Wire overheating can be detected early by measuring and archiving current data to the motor. DC motors may also require maintenance to the carbon brushes and springs that push the carbon brushes against the communicator.
AC motors can often be wired in several ways, depending on whether they are used in a single phase or three-phase configuration. DC motors often run clockwise with one wiring and counterclockwise with the other polarity, but individual motors may vary.
Choosing the proper electric motor for a given application requires careful thought and some understanding of the physics around electromagnetic fields. Motor size, torque and speed requirements must be researched and compared to the specific application. It also requires an evaluation of the site, power source and control system necessary to run the motor.
Once a motor has been chosen and installed, it is up to the engineers and technicians to develop a regular maintenance schedule and log motor health data so that problems can be detected early.