The Difference Between AC and DC Motors

When choosing a motor for your application, you will come across two different types: AC and DC motors. They have different characteristics of performance, efficiency, reliability, and cost, and choosing between them calls for a clear understanding of the requirements and priorities of your application.

What Do AC and DC Mean?

Fundamentally, the difference between AC and DC motors is that AC motors run on alternating current (AC) and DC motors run on direct current (DC). Alternating current periodically reverses direction (alternating between positive and negative). This is how electricity is distributed through the power grid to homes and businesses.

DC On the other hand, current flows in only one direction, and this is the type of power that’s available from batteries. It’s possible to convert between AC and DC power with varying efficiency depending on the method used.

But what does this really mean for the choice of motor for your application? Well, the type of power that’s used by a motor affects the way it’s designed, which in turn has a variety of impacts on how well it will suit your requirements.

Motor Design

The basic components of an electric motor are the stator and the rotor, which interact through an electromagnetic field to create rotation and torque. The stator is stationary (fixed to the body of the motor), while the rotor is the part of the motor that rotates (and includes the shaft).

In an AC motor, AC current is supplied to the stator. Although the stator does not move, this current produces a magnetic field that revolves according to the frequency of the AC current (how fast it changes between positive and negative). Through an electromagnetic process known as ‘induction,’ this induces a magnetic field in the rotor, and the interaction of these magnetic fields produces rotation and torque.

In a typical brushed DC motor, DC current is instead supplied to the spinning armature through a device known as a commutator, which uses a carbon brush to conduct electricity. The stator’s magnetic field is stationary and may be supplied by permanent magnets or electromagnets. As the rotor spins, the commutator periodically switches the polarity of the electromagnetic field. The armature is attracted and repelled to different elements of the stator, causing it to revolve continuously.

Brushless DC motors are also available, where the permanent magnets are moved to the rotor. The stator instead features electromagnets that switch polarity to create the rotor's continuous rotation. This means that the commutator and brushes, which contribute to a large part of the reliability problems associated with brushed DC motors, are unnecessary.

Performance

One of the main differences between AC and DC motors is speed control. An AC motor runs at the frequency of the AC supply and resists changes to the speed even when the load changes. To change the speed of the motor, it’s necessary to use a Variable Frequency Drive (VFD) which converts the AC supply to DC and back again at a different frequency. However, besides adding to the motor's cost, VFDs have inherent inefficiencies that may create problems such as shaft and bearing currents that can shorten the motor's lifespan if not managed properly. Additionally, AC motors tend to lose torque at higher speeds.

This means that AC motors are ideal for situations where the motor speed is slow to medium and remains constant while the load on the motor varies. This is why AC motors are ubiquitous in heavy-duty, continuous-speed, industrial applications.

DC motors, on the other hand, can easily be speed-controlled by modifying the supply voltage. They provide a consistent amount of torque over their entire speed range, but they are sensitive to changes in the load. This makes them ideal for situations where fine speed control is necessary, and the load does not vary significantly, such as in many domestic appliances or robotics.

Efficiency

AC motors are generally less efficient than DC motors for two main reasons. The first is that they use electromagnets in the stator, which consumes electricity, whereas DC motors often use permanent magnets instead. Secondly, AC motors experience a phenomenon known as slip, which is the difference between the rotor's speed and the speed of the revolving magnetic field in the stator. Although slip is essential for producing torque, it’s directly related to ‘copper losses,’ which are heat-related power losses incurred when electrical currents pass through the motor's field windings.

DC motors also experience some heat losses in their field windings. Brushed versions experience heat loss through resistance in the contact between the commutator and brushes and mechanical losses from this setup.

Cost

DC motors generally cost significantly more than AC motors due to their higher manufacturing costs. Also, because AC induction motors have such widespread use, economies of scale contribute to their relatively lower price. This means that AC motors are used for most large-scale industrial applications, whereas DC motors are found in smaller applications that require fine speed control, such as robotics.

Brushed DC motors have significantly higher servicing and maintenance costs due to the wear and tear from the commutator and brushes. Brushless DC motors do not have this problem; however, they’re more expensive than typical DC motors because of their more complex design.

Reliability and Maintenance

Brushed DC motors usually require much more maintenance and have a shorter lifespan than AC motors. This is because the brushes that press against the spinning commutator experience wear and tear, requiring regular maintenance and replacement.

Brushless DC motors and AC induction motors, on the other hand, do not have parts that rub against each other, which means that they’re quieter and require less maintenance.

Summary

This article has given you an idea of what the key differences are between AC and DC motors and which of them is likely to be more suitable for your application. As always, the motor that does the best job depends on the specific characteristics of your application and which outcomes you are prioritizing.