Motor Categories | Characteristics and Applications of Electromagnetic Motors

Although there are many different types of motors, the term “motor” generally refers to electromagnetic motors. Electromagnetic motors convert electrical energy into a rotating force by utilizing electromagnetic induction.

Electromagnetic motors are categorized into three types according to the power source and control method: DC motors, AC motors, and stepping motors. DC motors are used in toys and home appliances, AC motors are used in home air conditioners and washing machines, and stepping motors are used in applications requiring precise position control.

According to their structures and mechanisms, these motors are further classified as shown below:

Types and applications of DC motors

DC motors run on DC power, and their rotational speed can be controlled by a voltage. Thanks to their simple structure, DC motors are used in a variety of products, such as toys, home appliances, and car power windows.

Depending on whether they have brushes (electrodes), DC motors are divided into the two types:

1. Brushed DC motors

Brushed DC motors, also known simply as DC motors or brushed motors, operate simply by connecting directly to a power source, such as a dry cell battery. They are small in size and have a simple structure that controls the rotational speed through a voltage, and are used in craft models such as plastic models, electric toothbrushes, etc.

Brushed DC motors are driven by contact between the commutators and brushes during rotation. This has disadvantages such as electrical noise, unwanted sounds, sparks, and short lifetimes due to wearing of the brush and commutator.

Categorization by magnetic field generation method

Brushed DC motors can be further classified into two types depending on how the magnetic field is generated.

• ・Wound field type: A magnetic field is generated by windings (electromagnets).
• ・Permanent magnet field type: A magnetic field is generated by permanent magnets.

Wound field designs are used for large brushed DC motors, while permanent magnet field designs are used for small- to medium-sized brushed DC motors.

2. Brushless DC motors

Brushless DC motors literally have no brushes (electrodes). This eliminates the wear problems associated with brushed DC motors, resulting in motors that are long-lasting, highly efficient, energy-efficient, quiet, and low-noise. Brushless motors are used in a wide range of products such as HDDs, automobiles, EVs, air-conditioning equipment, and home appliances.

Brushless DC motors do not rotate simply upon connection to a power source; they require a drive circuit. This makes the structure more complex than that of brush motors.

Brushless motors basically use permanent magnets and are divided into two types depending on the permanent magnet position.

• ・Surface permanent magnet (SPM) type: Permanent magnets are placed on the outer circumference of the rotor.
• ・Interior Permanent Magnet (IPM) type: Permanent magnets are embedded in the rotor.

SPM motors are generally used for small-capacity products. IPM motors are used for large-capacity and high-speed applications such as automobiles and EVs, because there is no risk of magnets fragmenting and scattering even at high rotation speeds.

[Related Articles]

For details on brushed motors, refer to Brushed DC Motors, which has the following contents:

• ・Construction of Brushed Motors
• ・Power Generation Principles
• ・Short Braking
• ・Driving Brushed DC Motors with an H-Bridge: Switching Output States
• ・Driving of Brushed DC Motors Using BTL Amplifier Circuits: Linear Voltage Driving
• ・Single-Switch Circuit Driving and Half-Bridge Circuit Driving

For details on brushless DC motors, refer to 3-Phase Brushless Motors.

• ・Structure of 3-Phase Full-Wave Brushless Motors
• ・Position Detection in 3-Phase Full-Wave Brushless Motors
• ・Driving 3-Phase Full-Wave Brushless Motors
• ・Driving 3-Phase Full-Wave Brushless Motors: Sensorless 120° Commutation Driving

Types and applications of AC motors

AC motors run on AC power. Since homes and factories are powered by AC power and the voltage can be easily changed with a transformer, AC motors are widely used in home appliances and industrial machinery.

AC motors are classified into three types: induction motors, synchronous motors, and commutator motors.

1. Induction motors

Induction motors use a rotating magnetic field. When an alternating current is applied to stator coils, a rotating magnetic field is generated, producing an induced current and electromagnetic force in the rotor (a conductor such as iron) inside the stator, causing the rotor to rotate. Since the rotational speed of the rotor is slightly slower than the synchronous speed at which the rotating magnetic field rotates, induction motors are also called asynchronous motors.

Induction motors have a relatively simple and durable structure and are easy to maintain. Induction motors are classified into single-phase induction motors and 3-phase induction motors, according to the power source used.

1-1. Single-phase induction motors

Single-phase induction motors are driven by a single-phase alternating current. With a simple structure and relatively low cost, these motors are used in home appliances such as washing machines, electric fans, cooling fans, and pumps.

1-2. Three-phase induction motors

Three-phase induction motors are driven by a three-phase alternating current. These motors feature lower energy loss and provide more power than do single-phase induction motors. This makes them ideal for powering industrial machinery. There are squirrel-cage type and wound type motors according to the shape of the rotor, with the squirrel-cage type being the mainstream in recent years.

2. Synchronous motors

Synchronous motors, like induction motors, utilize a rotating magnetic field, but differ from induction motors in that the synchronous speed and rotational speed are equal. Since there is no “slip,” which is the difference between rotational speed and synchronous speed, there is no loss of energy.

Among synchronous motors, “PM motors” that use permanent magnets in the rotor have achieved higher efficiency and greater energy savings than induction motors, thanks to improved technology. PM synchronous motors are suitable for powering industrial machinery such as cranes, metal processing machines, and pumps, as well as home appliances. In addition to PM motors, there are reluctance motors that do not have permanent magnets embedded in the rotor, and hysteresis motors that utilize hysteresis characteristics.

3. Commutator motors (universal motors)

Commutator motors, also called universal motors or AC commutator motors, can operate on both DC and AC power sources. Because of their high speeds and ability to use a household power supply (100 VAC), they are suitable for home appliances that require light weight and high output, such as hair dryers, vacuum cleaners, mixers, and electric drills.

Commutator motors can be used with DC power sources. In reality, however, nearly all commutator motors are made for AC power sources. For this reason, this article categorizes commutator motors as AC motors.

Types and applications of stepping motors

A stepping motor rotates by a predetermined number of grooves on its rotor according to pulse signals from drivers. So, stepping motors are also called pulse motors. While DC motors and AC motors rotate continuously and are used to power products, stepping motors rotate intermittently and are not suitable as a mechanical power source, but are used for position control due to their characteristic of rotating through fixed angles.

An example of a stepping motor application is an analog clock. The second hand of a clock is rotated 6° every second by a stepping motor. Stepping motors are also incorporated into ATMs, vending machines, medical equipment, laser printers, 3D printers, and ticket gates.

Stepping motors can control speed and position with high precision, but they have the disadvantage of requiring a controller and a driver, which in turn increase the overall size of the motor. Also note that each rotation produces noise and vibration.

Stepping motors are classified into three types, variable reluctance (VR) motors, permanent magnet (PM) motors, and hybrid (HB) motors, depending on the rotor.

1. Variable reluctance (VR) motors

A VR (Variable Reluctance) type motor uses a gear-shape iron core as the rotor. This has the advantage of enabling smaller step angles through machining, but there are the drawbacks of a somewhat smaller torque, making it difficult to achieve both compact size and a large torque.

2. Permanent magnet (PM) motors

A PM (Permanent Magnet) type motor can achieve large torques even when small in size through the use of a permanent magnet for the rotor, but there are limits to reduction of the step angle.

3. Hybrid (HB) type motors

A hybrid type stepping motor combines advantages of both VR type and PM type stepping motors. The structure of the gear-shape iron core allows fine adjustment of the rotation angle, and the use of permanent magnets provides high torque. Because of their excellent performance, hybrid-type stepping motors have become popular in recent years.

[Related Articles]

For details on stepping motors, refer to Stepping Motors, with the following contents:

• ・Structure of Stepping Motors
• ・Basic Operating Principles of Stepping Motors
• ・Stepping Motors: Microstep Operation Principles
• ・Basic Characteristics of Stepping Motors
• ・Structure and Operating Principles of Hybrid Type Stepping Motors
• ・Stepping Motor Driving: Bipolar Connections and Unipolar Connections
• ・Driving 2-Phase Bipolar Stepping Motors

Summary of electromagnetic motors

The three types of electromagnetic motors explained above are summarized in the table below.

[Supplement]

Servomotors

Like stepping motors, servomotors are used as control devices. Unlike stepping motors, the motors do not have grooves on the rotor. Instead, they have encoders that act as a sensor to detect the position and speed of the motor. The encoder informs the driver of the detected motor speed and position information. The driver makes adjustments so that there is no difference between the detected motor position and speed and the control command information.

Servomotors can thus automatically correct positional deviations and provide finer position control than stepping motors. However, they are more complex in structure and thus cost more.

Linear motors and direct drive motors

Linear motors and direct drive motors are both types of servomotors, described above. Linear motors perform linear motion, while direct drive motors perform rotational motion.

In addition, there are electrostatic motors and ultrasonic motors with different operation principles.

Selection of motors

When selecting a motor, the following steps should be taken:

1. Determine the drive mechanism

To determine the drive mechanism, consider the dimensions and mass of objects to be moved, and the friction coefficients of sliding sections of moving parts.

The drive mechanism of the motor may use ball screw or a belt, instead of a simple rotor. A ball screw drive is suitable for high-precision positioning and transporting large loads, while a belt drive is suitable for high-speed positioning and light load transportation.

2. Confirm the specifications required for the motor

Check the specifications required for the motor, such as the equipment operating speed and time, positioning distance and time, stopping accuracy, the operating environment, and the supply voltage.

3. Calculate the load

Calculate the load torque and load moment of inertia at the motor output shaft. For the calculation formulas for each drive mechanism, refer to the manufacturer’s website.

4. Select a motor model

Select the most suitable motor according to the requirements determined above.

[Related Articles]

The following website compares small motors and can be used as a guideline for selection: Comparison of the Features, Performance, and Characteristics of Small Motors.