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What is the Difference Between a DC Motor and an AC Motor?

Difference Between AC and DC Motors

AC and DC motors both convert electrical energy into mechanical power. But do you know, “What is the difference between a DC motor and an AC motor?”

The answer is straightforward. The basic difference that we can easily distinguish from their names, AC motors are powered by alternating current while DC motors are powered by direct current. Also, AC motors’ efficiency and power output are much higher than DC motors. It makes AC motors an ideal choice for industrial and residential use. 

That’s not all. There are several other differences between AC and DC motors that include.

  1. Commutation Process.
  2. Magnetic Field.
  3. Speed Regulation, etc. 

In this article, I will discuss every difference in detail. But first, let’s discuss the types of electric motors.

Types of Electric Motors

 

There are two types of electric motors: AC motors and DC motors. 

What are DC Motors?

Type Of Dc Motors DC motors are a type of electric motor that converts electrical energy into mechanical rotational energy. DC motors are powered by direct current. The DC motors are further classified into:
 
  1. Permanent Magnet DC Motors
  2. Series Wound DC Motors
  3. Shunt Wound DC Motors 
  4. Compound Wound DC Motors
  5. Brushless DC Motors 

What are AC motors?

 

Ac Motor Picture

AC motors are electric motor types that convert electrical power into mechanical power using electromagnetic induction. AC motors are powered by an alternating current. Hence, the name. The AC motors are further classified into:

  1. Induction Motors (asynchronous): They convert electric power into mechanical power using electromagnetic induction.
  2. Synchronous Motors: They convert electrical energy into mechanical energy and operate at a synchronous speed. 

What is the Difference Between AC and DC Motors?

 

The purpose of both electric motors (AC and DC) is to convert electrical energy into mechanical power. But their power, construction, and speed control differ significantly from one another. Let’s discuss how AC and DC motors differ from each other in detail. 

Nature of Input Power

 

Input Power

AC Motors

We know that the primary input power is the alternating current when we speak of AC motors. Thus, AC motors operate on the principle of electromagnetic induction. 

The supply sources of AC motors are from single-phase or polyphase, or three-power supply mains. The polyphase or three-power source features various voltage inputs, operating at different angles (120 or 2π/3 radians).

DC Motors

The primary input current in DC motors is the direct current that does not change its direction. I.e., they are unidirectional. The energy in DC motors is usually obtained from batteries, DC power cells, or AC to DC power converters. 

Commutation Process

 

Motor Commutator

AC Motors

In AC motors, the input supply to the armature is the alternating current. Thus, the commutation process is absent. 

Also, AC motors supply the input to the stator, which is stationary. Hence, carbon brushes and commutators are absent. 

DC Motors

The input supply to the armature in DC motors is the direct current. Thus, the commutation process is necessary. DC motors supply the input to the rotor that rotates. Hence, carbon brushes and commutators are essential for DC motors.

These carbon brushes require regular maintenance and add the additional maintenance cost of DC motors. In addition, they also affect the lifespan of DC motors.

Magnetic Field

 

AC Motors

In AC motors, the alternating current supplies current to the stator coils. In return, it produces a rotating magnetic field that generates an electromotive force (EMF) in the rotor coils. The EMF produces a current in the rotor, and it rotates. 

DC Motors

However, the magnetic field is stationary in a DC motor, i.e., it does not rotate. But the current supply is to the armature, which causes the armature’s rotation.

Speed Regulation

 

Motor Speed Controller

AC Motors

You can control the speed of AC motors by varying the input frequency. 

When you supply the input frequency of the alternating current to the stator, it is directly proportional to the speed of the motor. It means when you increase the frequency, the engine’s speed also increases.

To control the speed of AC motors, we use a variable frequency drive controller (VFDs).

DC Motors

You can control the motor speed by varying armature current and voltage for DC motors. The speed control process in DC motors is easier through PWM. 

PWM or Pulse width modulation is a standard method to control DC motor speed with a microcontroller.

Maintenance

 

Dc Motor Maintenance

AC Motors

If you have an AC motor, chances of maintenance are pretty low. The absence of commutators and brushes in AC motors makes them less costly and more effective than DC motors.

For the maintenance of AC motors:

  1. Lubricate the bearings properly.
  2. Change the bearings occasionally.
  3. Maintain a uniform gap.

And your motor would last you years. 

DC Motors

For DC motors, the maintenance process is costlier and more frequent. The brushes and commutator in the DC motors make them high maintenance and limit the speed of motors. Ultimately making the lifespan of DC motors shorter than AC motors.

Torque Performance

 

AC Motors

As you increase the speed of AC motors, the torque decreases. Hence, torque is inversely proportional to the motor speed in AC motors. 

DC Motors

DC motors are more effective for applications where we need speed control because they provide a constant torque over the speed range. Also, DC motors’ response to load changes is faster than AC.

Input Terminals

 

Rs 555sh Back View Prorich Motor

AC Motors

In AC motors, there are three input terminals, RYB, respectively. 

RYB stands for Red, Yellow, and Blue. Usually, we separate the three phases by 120° angle in a three-phase AC motor. Also, we allot a specific color to each phase (R, Y, or B) for easy identification. 

DC Motors

There are two input terminals in DC motors, i.e., positive and negative terminals.

Startup Mechanism

 

AC Motors

When it comes to AC motors, not all are self-starting. For example, polyphase AC motors are self-starting and demand no external equipment to start. 

But single-phase AC electric motors need additional electronic equipment, such as a capacitor.

DC Motors

All DC motors are self-starting. It means that they do not require any extra equipment to start. 

Field of Application

 

AC Motors

AC motors are suitable for large industrial applications such as drill machines, water pumps, motorized off-road equipment, ovens, and water heaters.

Water Pump

DC Motors

We use DC motors in precise position control and high torque applications, including cranes, kids toys, small motors, elevators, rolling mills, and excavators.

Robot Car

Advantages and Disadvantages of AC Motors

 

Here are some of the advantages and disadvantages of AC motors.

Advantages: 

  1. AC motors are flexible and easy to control.
  2. They offer durability and a greater lifespan.
  3. AC motors demand little to no maintenance.
  4. They have lower startup power demands. 
  5. AC motors control speed and torque at different levels.
  6. Capable for various configurations.

Disadvantages: 

  1. AC motors are unable to operate at low speeds.
  2. They have poor positioning control 
  3. They produce back EMF, which slows the motor down. 

Ac Motor

Advantages and Disadvantages of DC Motors

 

Here are some of the advantages and disadvantages of DC motors.

Advantages: 

  1. DC motors feature a cheap and straightforward drive design.
  2. The installation of DC motors is simple. 
  3. They provide excellent speed control. 
  4. The startup power and torque are higher.

 Disadvantages: 

  1. The maintenance of DC motors is high and costly.
  2. They are vulnerable to dust which decreases their performance. 

Conclusion

 

We use AC and DC motors in a variety of applications. The motors’ main goal is precisely the same, i.e., to convert electrical energy into mechanical power. 

However, it’s crucial to understand the differences between AC and DC motors to ensure you pick the right one.

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