Understanding Electric Motor and types

Main Parts of an Electric Motor

Electric motors convert electrical energy into mechanical motion. An electric motor, regardless of its specific type, consists of several key components that work together:

Power Connections

The motor receives electrical power through its power connections. These connections can supply either alternating current (AC) or direct current (DC), depending on the motor design. The power flows into the stator windings, which create a magnetic field essential for motor operation.

Stator and Rotor

The stator is the stationary part of the motor that houses the windings. The rotor, connected to the rotor shaft, is the rotating component. Depending on the motor type, the rotor may contain permanent magnets or electromagnets. The interaction between the magnetic fields of the stator and rotor generates torque, causing the rotor shaft to turn.

Cooling System

Motors often work under demanding conditions, making cooling a critical factor. Effective cooling helps maintain performance and prolongs motor life. Many motors include a fan at the front and air intake vents at the back to facilitate airflow and dissipate heat during operation.

Encoder

Some motors are equipped with an encoder, a device that monitors the position of the rotor shaft in real time. This feedback is vital for precise control in applications requiring accurate positioning or speed regulation.

Brake

While not typically used to slow the motor down, a brake serves as a holding mechanism. It keeps the motor shaft steady when the motor is stopped, preventing unwanted movement especially in vertical applications and ensuring safe operations.

Electric motors are broadly categorized based on their power source—Direct Current (DC) motors and Alternating Current (AC) motors—and further classified by construction and control methods.

DC motors operate on direct current and are common in applications requiring variable speed and torque control. They come in four main types:

• Brushed DC Motors: These traditional motors use carbon brushes and a commutator to deliver current to the rotor windings. They are simple, low-cost, and easy to control, but brushes wear out over time, requiring maintenance. They are often found in household appliances, toys, and hand tools.
• Brushless DC Motors (BLDC): A modern alternative where the rotor contains permanent magnets, and the stator has windings. Electronic controllers replace brushes for commutation, providing higher efficiency, less noise, and reduced maintenance. BLDC motors are compact, capable of high speeds, and used in drones, electric bikes, and compact robots.
• Stepper Motors: Specialized DC motors designed for precise positioning without feedback in many cases. They move in discrete steps, making them ideal for controlled motion applications like 3D printers, compact CNC machines, and robotics. Hybrid stepper motors combine features of reluctance and permanent magnet motors for enhanced performance.
• Linear DC Motors: These convert electrical energy directly into linear motion, useful in applications requiring precise linear positioning.

AC motors run on alternating current and are widely used in industrial and commercial applications.

Asynchronous (Induction) Motors
Asynchronous motors operate with the rotor spinning at a speed slightly different from the stator’s magnetic field—a difference known as slip. This slip induces current in the rotor, generating torque. The design is robust and simple but offers less precise speed control compared to synchronous motors. Types include:

• Classic AC Motors (asynchronous): These are the traditional induction motors where the rotor spins slower than the stator’s magnetic field, creating the slip necessary for torque production. Classic AC motors are self-starting, reliable, and widely used across various industries in applications such as pumps, fans, compressors, and conveyors. Modern versions have been optimized for high efficiency, reaching up to 95%.
• Asynchronous Servo Motors: These are induction motors equipped with encoders for position feedback, enabling dynamic response and precise control. They are commonly used in machine tools and high-power servo applications.
• Asynchronous Linear AC Motors: These motors operate on the same slip principle but produce linear motion instead of rotary. They are robust and suitable for applications requiring moderate precision and high force, such as material handling and conveyor systems. However, their dynamic response and accuracy are generally lower than synchronous linear motors.

Synchronous Motors
Synchronous motors have rotors that spin at the exact same speed as the stator’s magnetic field, allowing for precise speed control. Types include:

• Classic Synchronous Motors: Equipped with electromagnets on the rotor, powered via slip rings, these motors deliver high power and precise speed control. They are commonly used in heavy industry and applications requiring constant speed.
• Synchronous Reluctance Motors: Featuring rotors without windings or magnets, these motors generate torque through magnetic reluctance. They offer high efficiency and robustness but require external electronics for starting.
• Synchronous Servo Motors: Compact motors with permanent magnet rotors, combining high power density, efficiency, and precise speed and torque control. Ideal for robotics and advanced CNC machines.
• Synchronous Linear AC Motors: Similar in principle to linear DC motors but operating with AC, these motors provide high dynamic response and accuracy, making them suitable for precision applications such as electronics manufacturing and printing.

Applications: Driving vs. Controlled Motion

When selecting an electric motor, understanding the application's nature is critical. Applications generally fall into two categories:

1. Drive Applications

In drive applications, the motor's primary role is to provide power to keep systems running at a consistent speed or torque without the need for precise positioning. Examples include:

• Conveyors
• Pumps and fans
• Compressors
• Household appliances

Key considerations here are robustness, efficiency, and cost-effectiveness. Motors like brushed DC, asynchronous induction, and synchronous reluctance are common choices due to their simplicity and durability.

2. Controlled Motion Applications

These require precise positioning, repeatability, and dynamic control of speed and torque. Examples include:

• Robotics and pick-and-place systems
• CNC machines and machine tools
• 3D printers and compact industrial robots
• Printing systems requiring high accuracy

Motors used here typically involve feedback systems (encoders) and advanced controllers. Stepper motors, brushless DC motors, synchronous servo motors, and linear motors are popular for these applications.

Key criteria for selecting the right electric motor

Choosing the optimal motor involves balancing multiple factors:

Mechanical and Performance Factors

• Torque and Power: Required to move the load efficiently.
• Speed Range: Maximum and minimum speeds needed.
• Inertia: Load inertia affects acceleration and deceleration.
• Repeatability and Accuracy: Crucial for positioning tasks.
• Dynamic Response: Ability to respond quickly to control inputs.

Electrical and Environmental Considerations

• Supply Voltage: Availability of AC or DC power, voltage levels (e.g., extra low voltage <72 V DC).
• Control Complexity: Some motors require complex drives and software.
• Maintenance: Brushless motors generally require less maintenance.
• Efficiency: Energy consumption impacts operating costs and environmental footprint.
• Size and Footprint: Space constraints may limit motor options.
• Size and proximity to the controls cabinet.
• Environmental Conditions: Temperature, cooling needs, and protection ratings (e.g., IP65).

Economic Factors

• Initial Cost: Motor and controller price.
• The time taken to wire the machine and cost/size of containment
• Total Cost of Ownership: Includes energy consumption, maintenance, and downtime.
• Integration and Expertise: Availability of support, software tools, and ease of commissioning.

Real-World Example: Sizing the Optimal Motor for a Life Sciences Application

To illustrate how to select the best motor for a specific task, let’s consider a typical life sciences application involving a liquid handling head dispensing into multiple vials.

Application details:

• The system requires a single horizontal axis with a payload of about 1 kg and a stroke of approximately 400 mm. The motion involves precise incremental positioning steps (e.g., 3 mm moves), pauses, and a return to the home position, forming a complete motion profile.

Sizing process using Festo’s electric motion sizing tool:

• This online tool requires no downloads and is always up to date. Users start by selecting the type of movement—in this case, linear motion where rotary motor movement converts to linear via ball screws or toothed belts.
• Next, the user inputs key application data such as stroke length, payload, axis orientation (horizontal), and detailed motion steps including travel times and pauses. The tool can also accommodate advanced parameters like external forces and acceleration profiles.

Results and selection:

• Based on the input, the tool evaluates over 12,000 possible motor and drive combinations in real time. It ranks solutions by criteria such as cost-effectiveness and performance. For this example, a stepper motor solution emerged as optimal, with detailed specifications including motor type, supply voltage, and workload distribution across motor, controller, and mechanics.

Additional benefits:

• The tool provides comprehensive motion profiles with timestamps, speed, acceleration, and jerk values, helping to size power supplies and ensure system reliability. It also generates a complete bill of materials with pricing, availability, and connectivity options.
• Importantly, Festo offers extensive application notes, setup guides, and software plugins to simplify commissioning and integration.

Decentralized Drives

Decentralized or cabinet-free drive solutions integrate the motor and controller into a single unit installed directly in the field. This approach reduces:

• Cabinet size and complexity
• Wiring and installation time with single-cable daisy-chaining
• Cooling requirements and associated costs

Festo’s latest decentralized servo motor solutions support multiple motor types (steppers, brushless DC) and communication protocols (Profinet, EtherCAT, Modbus TCP), providing flexibility and simplified machine architectures.

Summary: Matching Motor Technology to Your Application

The optimal electric motor choice depends on a clear understanding of your application’s requirements. For power-driven applications with less need for precision, robust and cost-effective motors like asynchronous induction or brushed DC motors are suitable. For controlled motion requiring accuracy and dynamic response, brushless DC, stepper, and synchronous servo motors are preferred.

Emerging technologies such as hybrid stepper motors with servo drives and decentralized drive solutions offer new opportunities to enhance performance while reducing costs and complexity.

To make the best choice:

• Assess load characteristics, cycle times, and environmental factors.
• Consider total cost of ownership including energy and maintenance.
• Utilize sizing and selection tools to evaluate options.
• Collaborate with technology-neutral experts for tailored advice