Types of electric motors and their characteristics

Electric motors convert electrical energy into mechanical energy. They are based on the principle of electromagnetic induction. They can be used in a variety of applications, for example, as a drive solution in industrial machines.

Electric motor types

There are different types of electric motors, each of which has different functions and properties. The types of electric motors used today are categorically divided into DC Motors and AC motors. For AC motors, a distinction is made between synchronous and induction motors.

The construction of electric motors is usually as follows:

• Stator: The stator is an electrical or permanent magnet that is permanently, e.g. rigidly, connected to the motor housing.
• Rotor: The rotor is an electromagnet that sits between the poles of the stator on the motor axis. Depending on the design, a distinction is made between rotary motors (rotating motor axis) and linear motors (sliding axis).
• Commutator (pole inverter): The commutator consists of two or more segments of a slip ring, isolated from each other, via which current is supplied to the rotor. Two sliding contacts, which are connected to a power source, press against it from the outside.

• 1 - Rotor
• 2 - Brush
• 3 - Coil
• 4 - Air gap
• 5 - Permanent magnet

Various open-loop and/or closed-loop control methods are used to control electric motors. Common methods include pulse width modulation (PWM), direct voltage control, or field-oriented regulation.

DC motors (DC motors)

DC motors operate with a constant current direction and generate torque. They are useful in applications that require precise speed and torque control, such as CNC machines. The following motors are examples of DC motors:

• DC motor with brushes: The commutator works with contacts that are contacted by spring pressure. These are called brushes and are made of carbon, for example. Thanks to the brushes, the power can be easily regulated by the applied operating voltage.
• DC motor with brushes: Instead of the mechanical commutator, an electric sensor measures the rotor angle of the brushless on DC motors.
• Gear motor: A gearbox is used to lower - or in rare cases also to increase - the original speed of rotation to a defined value.
• Stepping motor: Stepping motors are available as rotary motors and as linear motors. The motors can be adjusted to small positioning steps by means of a high number of pole pairs (tooths). They are usually controlled with rotary or linear position encoders. These motors are suited demanding positioning work, such as 3D printing It should be noted that overloading/undersizing can lead to step errors.

Advantages and disadvantages of DC motors

The advantages of DC motors include:

• High starting torque, making it suitable for heavy-duty applications.
• They can be precisely controlled, which is mainly achieved with speed of rotation. This makes them suited for applications that require accurate positioning and smooth operation.
• Very reliable, simple torque control even possible at low speed of rotation.
• Reversibility: The direction of rotation can be easily changed by inverting the current direction.
• A wide speed range is available.
• Brushless DC motors are durable, low-maintenance and quieter

Disadvantages are:

• The use of brushes results in frequent inspection intervals, usually every three months, which leads to higher maintenance costs.
• Susceptibility to harsh environmental conditions: Relatively open design makes it easier for dust and dirt to penetrate.
• The purchase is often expensive, especially with brushless DC motors since they require a more complex control than a simple brush control.

AC motors

In alternating current motors, the voltage changes regularly due to constant changes in the current direction. The speed of rotation of the motor is determined by the mains frequency and the number of poles. There are single-phase and three-phase AC motors. Single-phase AC motors are mostly used in smaller machines and household appliances, while three-phase AC motors are mainly used in industrial applications due to their higher power rating and efficiency. The following motors are examples of AC motors:

• Induction motor: The speed of rotation of the motor is not exactly synchronized with the frequency of the connected alternating current. This difference allows the rotor to rotate. They often work with three-phase alternating current, which is widely used in industry, which is why the induction motor is one of the most commonly used motors. Induction motors can be divided into single-phase and three-phase.
• Synchronous motor: The rotor speed is synchronous to the rotating magnetic field of the stator. This results in high accuracy and constant speed of rotation, which is important for machine tools, for example. Synchronous Motors can be divided into single-phase and three-phase.

Servo motors:

A servo motor can be a DC motor, asynchronous or synchronous motor. These motors have a servo drive and usually their own control electronics. A servo drive is an electronic control system that positions a motor precisely and in a controlled manner and also permits speed changes to perform precise motions in various applications such as robotics, CNC machines and automation systems.

Advantages and disadvantages of AC motors

The advantages of AC motors include:

• They are more durable and have less maintenance if brushes are not used
• The closed design results in less dirt ingress
• Higher efficiency
• High speeds are possible
• They are generally more cost-effective

Disadvantages are, for example, that they have a higher noise development due to stronger vibrations. Due to the design, the speed of rotation can only be controlled by changing the frequency and/or can be controlled in increments by switching the pole pairs. Additional components may be required for precise control. In addition, AC motors are not suited for mobile applications because batteries supply DC voltage (or is this only possible with additional components, such as inverters).

Depending on the local conditions and the desired application, a wide variety of electric motor designs can be used, which is why MISUMI provides a wide range of motors.

Operation principles of an electric motor using the example of a DC motor

When an electric current flows through the rotor coil in a DC motor, the rotor starts to rotate. Each half turn causes the commutator to reverse the polarity to ensure continuous motor movement. Otherwise, the motor would only rotate until the north pole of the rotor faces the south pole of the stator and comes to a standstill in this position. A motor with a two-pole rotor does not start automatically in every position, which is why rotors with three or more poles are often used in electric motors. The principle is somewhat different for linear motors: Instead of a rotor, this application uses a linearly guided rotor (carriage) equipped with several coils that moves along a straight line. Permanent magnets are installed at regular intervals. By purposefully controlling the coils, the rotors are moved by magnetic fields according to the same operating principle as in the rotary motor. The controlled coils can be positioned on the rotor and also on the linear stator path in a reversed design.

Factors characterizing electric motors

The characteristics of motors are critical to their performance and application in various technical and industrial areas.

Torque and speed

The torque of a motor indicates how much force it can exert on a shaft. The speed of rotation refers to the speed at which the shaft or rotor member of a linear motor moves. Motors are designed for different torques and speeds of rotation/velocities depending on the application. The speed of rotation-torque characteristics are important for motor sizing. They represent the ratio of speed of rotation to torque and may differ significantly in some cases for different (electric) motor types and operating points.

Power density

The power density of electric motors refers to the ratio between the generated power of the motor and its size or mass. Modern electric motors can have a high power density and thus can deliver a lot of power in a compact design. The power density is usually measured in watts per kilogram (W/kg) or watts per cubic centimeter (W/cm3), depending on whether the engine weight or volume is being considered.

Controllability

The controllability of electric motors refers to the ability to control or regulate the speed, torque and other operating parameters of the motor in a certain way. This is particularly useful in applications that require precise control.

• Speed of rotation: By regulating the speed of rotation, the motor can be adapted to various speed requirements.
• Torque: The control of the torque is helpful for variable loads, for example. Electric motors can be controlled to provide the torque needed to move or overcome loads.
• Direction: By changing the current direction or by using special circuits, a motor can be customized for direction-dependent applications. These include, for example, drive motors in conveyor belts or elevators.
• Positioning and precision: Precise positioning and control are decisive for CNC machines, for example. Stepping motors or servo motors are often used here.

In order to integrate your electric motors safely and reliably, we offer a large number of transmission parts in our shop, including toothed belts, gear wheels, belts and more.

Service life and maintenance

The life of an electric motor depends on various factors. The motor type, operating conditions, maintenance and manufacturing quality can have an impact, for example. Here are some of the key factors that affect the service life of an electric motor:

• Motor type: Some motor types are more robust and durable than others. For example, due to their brushless design, brushless DC motors (BLDC) generally have a longer service life than direct current motors with brushes.
• Operating conditions: Factors such as temperature, humidity, shock loads and vibration can significantly influence the service life. Motor protection devices such as overheating protection and overvoltage protection also serve this purpose.
• Duty cycle: The length and frequency of operation affect the service life. Motors operated in continuous operation often have a shorter service life.
• Maintenance In general, electric motors are low-maintenance - where electric motors with brushes/abrasive contacts are the exception. However, regular inspection and maintenance, e.g. of bearings and wear parts, can significantly extend the life cycle of a motor.

Electric motor cooling

Cooling electric motors is critical to ensuring that they operate efficiently and remain within safe operating temperatures. Overheating can significantly shorten the service life of an electric motor and lead to malfunctions. Here are some common methods for cooling electric motors:

• Air cooling
• Cooling fins and heat sinks
• Coolant
• Oil Cooling
• Liquid cooling
• Temperature sensors and controls