DC Motors versus AC Motors: What You Need to Know

October 14, 2020 Cadence PCB Solutions

Key Takeaways

  • Learn about DC and AC motor functionality.

  • Learn about the different types of DC and AC motors. 

  • Gain a greater understanding of the different methodologies employed by DC and AC motors.

 AC servo motor, DC brushless motor, and a stepping motor.

Various AC and DC electric motors.

In 1834, Thomas Davenport developed the first electric motor powerful enough to perform an actual task. However, there were earlier iterations developed by Michael Faraday and Joseph Henry, but their devices were not powerful enough to perform a task.

Fast-forward to the present day, and you will see electric motor technology in use in a nearly endless list of applications. Whether it is an electric ride-on car (DC motor) or in a Tesla Roadster utilizing an AC induction motor with a copper rotor, electric motor technology is everywhere.

The Electric Motor

As I am sure you are aware, we can power an electric motor with either DC (direct current) or AC (alternating current). History confirms that DC motors came before AC motors, and they came into existence only 30 years after the invention of the battery. However, like virtually every device in the field of electronics, each type possesses specific advantages and disadvantages.

Although both types of motors work differently, they have one functional characteristic in common, both utilize the power of an electromagnetic (EM) field. To fully comprehend the functionality of motors, we must first examine the fundamental principals of EM fields and the components that these motors are made of.

An AC electric motor utilizes both a secondary and primary winding (magnet) to achieve functionality. The primary is affixed directly to a generator or the AC grid power, where it receives its energy. The secondary winding obtains its energy from the primary without the need for direct contact. This complex and vital phenomenon is called induction.

Induction and Electric Motor Functionality

Electromagnetic induction is the creation of current in a conductor as it travels within a magnetic field. A magnet generates lines of magnetic force and iron filings will align themselves along these invisible lines of flux.

According to Faraday's Law of Induction, if you move a wire back and forth within a magnetic field, you will cut across these lines of flux. The magnetic field applies a force to the electrons within the metal. As you might know, copper possesses 27 electrons, and the last two in orbit move easily to adjacent atoms. This active movement of electrons is called electrical flow.

When you move a full coil of wire within a magnetic field, you generate a robust flow of electrons. In summary, the strength, or power, of your motor is dependent on these following parameters:

l = The conductor length within the magnetic field

v = The speed of the rotor or velocity of the conductor

B = The strength of the EM field

e = efficiency

Therefore, the efficiency of the electric motor or generator is as follows:

e = B * l * v

Electric Motor Functionality

Although there are two overall categories for electric motors, and various sub-types, they all share similar components. For example, all motors use a stator comprised of wound, insulated wires or a permanent magnet. Motors also use an armature (rotor) that typically sits in the middle and is subject to the stator's magnetic field. The poles of the armature rotate and are repelled and attracted by the various poles of the stator.

Wire length in an electromagnet within the stator, along with voltage, determines the torque or strength of the motor. Also, the longer the wire, i.e., the more coils in the stator, the greater the magnetic field. This translates into increased power to rotate the rotor.

Another vital component in a motor is its windings. The coil or winding, which is typically copper wires wound around a core, either generates or receives EM energy. Furthermore, the wire in use in windings must be insulated, and the most common material in use for windings is copper. Although you will encounter aluminum in use, it requires an increased thickness to safely accommodate the same electrical load as its copper counterpart. As I am sure you can imagine, this translates into reduced motor size when utilizing copper windings versus aluminum windings.

DC Motors versus AC Motors

In general, a DC motor's advantages include:

  • Higher starting torque

  • Faster start and stop

  • Faster reversing

  • Speeds vary with changes in the input voltage

  • Ease of control

  • Cost-effectiveness in required control methodology

Overall, we divide DC motors into two sub-categories.

1.   Brushed, which is divided further into types, such as:

  • Permanent magnet

  • Shunt-wound 

  • Series-wound 

  • Compound-wound

  • Separately excited 

2.   Brushless, which is also divided into sub-types, such as:

  • Single-phase

  • Two-phase

  • Three-phase

The Brushed DC Motor

A brushed DC motor consists of four components:

Stator: Generates a stationary magnetic field which encompasses the rotor, and it generates this field by either utilizing permanent magnets or electromagnetic windings.

Rotor or Armature: Consists of one or more windings. As stated earlier, whenever the windings energize, they create a magnetic field. The magnetic poles of the rotor field are attracted to the opposite poles produced by the stator, which promotes rotor rotation.

Now, as the motor turns, the windings receive constant energization at different sequences to ensure that the magnetic poles produced by the rotor are not overrunning the poles created in the stator. Also, the switching of the field within the rotor's windings is what we refer to as “commutation.”

Brushes: Note, a brushed DC motor does not need a controller to switch the current in the motor windings. As an alternative, it utilizes the mechanical commutation of its windings. A commutator or copper sleeve resides on the rotor's axle. When the motor rotates, the carbon brushes slide over the commutator and make contact with the various sections of the commutator.

These sections attach to various rotor windings, producing a magnetic field (dynamic) in the motor when we apply a voltage across the motor's brushes. In terms of DC motor issues, the commutator and brushes are the two most common causes of motor failure due to wear.

Commutator: When the rotor turns inside the stator, the brushes rub the various sections of the commutator, thus supplying a charge to that section and the corresponding winding. Also, when the brushes pass over the commutator gaps, the electrical charge it provides switches commutator sections or segments.

The result of this action switches the electrical polarity of the rotor windings. It also creates an attraction of the different polarities and keeps the rotor turning within the stator field. So long as there is a supply voltage available, the process will continue.

In terms of disadvantages of the brushed DC motor, its brushes produce both sparks and friction. The result of these side-effects typically translates into overheating issues in the device, thus limiting maximum rotation speed. Another disadvantage of the Brushed DC motor also derives from its sparks production. These sparks produce radio frequency interference (RFI).

The Brushless DC Motor

As I am sure you are aware, the brushless DC motor is available in three configurations, with the three-phase configuration being the most common.

The primary difference in design between the two motor types (brushed and brushless) is the replacement of the mechanical commutator with an electric switch circuit. Note, a brushless DC motor is, as the name implies, without brushes. It is characteristically a synchronous motor because the stator's magnetic field and the rotor rotate at the same frequency.

A Brushless DC motor consists of the following components:

Stator: The physical characteristics of the stator in a brushless DC motor mirror that of an induction motor. It consists of stacked laminations (steel) with cut slots (axially) for its winding. However, the winding of a brushless DC motor differs slightly from that of a typical induction motor.

Rotor: The rotor component of a brushless DC motor consists of a permanent rare-earth alloy magnet. Also, depending on the application, the number of poles will vary between two and eight and utilize an alternating placement for the north and south poles. Another configuration for the rotor in a brushless DC motor is magnetic-embedded, where they embed the rectangular permanent magnet(s) inside the core of the rotor. In the other configuration for the rotor, they insert the magnets inside the iron-core of the rotor.

Position sensor: Brushless DC motors controls its commutations electronically since it does not possess brushes. To rotate its rotor, it energizes the stator's windings sequentially. This requires knowledge of the rotor position to precisely energize specific stator winding sets. This is the function of the position sensor; it detects the position of the rotor and transforms it into an electrical signal.

AC Motor

The advantages of the AC motor include minimal maintenance and less demand for power on starting. In general AC motors are efficient, durable, and quiet, thus making them highly desirable for some applications. 

The two types of AC motors are:

  1. Synchronous: As the name implies, this motor rotates at the frequency of the supplied current and is composed of a rotor and stator. 

  2. Induction: These are among the most common motor types in use and are both simple and rugged. These motors are composed of a rotor and a wound stator. Electromagnetic induction in the stator winding is what creates the force to turn the rotor. 

Overall, both DC and AC motors serve the same function, which is the conversion of electrical energy into mechanical energy. The most fundamental difference, of course, is their power source. Also, specific types of DC motors require more maintenance, have limited speed, and shorter lifecycles. However, AC induction motors are incredibly rugged with longer lifecycles. Finally, another essential difference is speed control. In comparison, DC motors utilize rotor winding's current, whereas AC motors control speed by varying the frequency of the applied current.

A DC motor graphic with DC excitation current.

An internal view of a DC motor.

Regardless of what motor type you choose to be the right one for your electronic design, having the right set of design and analysis software is the only way to guarantee a successful implementation. OrCAD PCB designer has a full suite of design and analysis features to ensure your PCB is done right the first time. 

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts

 

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