AC Servomotors Selection GuideAC Servomotors Selection GuideAC Servomotors Selection Guide

Image Credit: KEB America, Inc. | Torque Systems | Exlar 

 

AC servomotors are AC motors that incorporate encoders are used with controllers for providing feedback and closed-loop control. These motors can be positioned to high accuracy, meaning that they can be controlled exactly as required for the application. Often servomotors have better bearings or higher tolerance designs, and some smaller designs also use higher voltages in order to achieve greater torque. AC servomotors are commonly used in robotics, automation, CNC machinery, and other applications requiring versatility and a high level of precision.

 

Servomotor vs. Stepper Motor

Stepper motors and servomotors both belong to a class of motors designed for precise postion control. Stepper motors are brushless DC motors that move a rotor in small angular steps or fractions of steps, while providing fine motion control like servomotors. The advantage of servomotors over stepper motors is the incorporation of constant positional feedback, which results in higher torque at higher speeds. Servomotors also draw less current than stepper motors, resulting in higher efficiency and lower heat production. However, stepper motor systems are simpler and typically less expensive than servomotor systems, especially for smaller-sized applications. Because of this, stepper motors are often preferred for cost-conscious, low to medium speed control applications; servomotors are the best option for high-speed and high-volume requirements.

 

Types of AC Servomotors

The main difference between an AC servomotor and all other AC motors is the incorporation of an encoder and controller. The motor itself can be any type of AC motor.

 

Construction

There are two distinct types of AC motors based on construction: synchronous and induction.

  • Induction motors are constructed of shortened wire loops on a rotating armature. Voltage is 'induced' in the rotor through electromagnetic induction. They are rugged, versatile, and can provide considerable power as well as variable speed control.
  • Synchronous motors are constructed of a wound rotor where coils of wire are placed in the rotor slots. These motors are designed to operate at a specific speed, in step with the rotating magnetic field.

For a more complete overview of these different types of AC motors, visit the AC Motors Selection Guide on GlobalSpec.

 

Gearing

AC motors can also incorporate gearing to increase torque output, reduce speed, and simplify motor design. There are a number of different gear assemblies that can be used:

  • Spur
  • Planetary
  • Worm
  • Harmonic

Specific advantages that gearmotors offer to a servo application include:

  • Operation of the motor over its optimum speed range
  • Minimizing motor size by multiplying torque
  • Minimizing reflected inertia for maximum acceleration
  • Providing maximum torsional stiffness

A more complete overview of gearhead selection can be found in the Gearmotors Selection Guide on GlobalSpec.

 

Specifications

The most important part of the process of selecting a servomotor is determining the appropriate specifications.

 

Power Requirements

The only characteristic difference between AC servomotors and DC servomotors is the source of power. Unlike DC servomotors which run on batteries, AC motors run on power typically supplied by a wall outlet. AC servomotor power requirements are thus more complex, because performance depends on both the frequency and voltage of the power supplied rather than just the voltage.

 

In addition to the power supply available, a servomotor needs to be compatible with the amplifier and controller in the associated system. This involves current ratings, voltage ratings, and switching frequency.

 

Performance Specifications

AC servomotors share many performance specifications that are applicable to all types of AC motors. To properly size a motor, these specifications must be matched according to the load requirements of the application.

  • Shaft speed (RPM) defines the speed at which the shaft rotates, expressed in rotations per minute (RPM). Typically, the speed provided by the manufacturer is the no-load speed of the output shaft, or the speed at which the motor's output torque is zero.
  • Torque is the rotational force generated by the motor shaft. The torque required for the motor is determined by the speed-torque characteristics of the various loads experienced in the target application.
    • Starting torque - The torque required when starting the motor, which is typically higher than the continuous torque.
    • Continuous torque - The output torque capability of the motor under constant running conditions.
  • Horsepower - The mechanical power output of the motor at rated speed and voltage, expressed in horsepower (hp). Horsepower is the product of shaft speed and torque, and is used as a gauge of the work output of the motor.
  • Efficiency - Motor efficiency indicates the percentage of input electrical energy that is converted into output mechanical energy. Comparing two motors with the same horsepower, the one with higher efficiency will consume less power.

Selection Tip: Servomotors should have enough speed and torque capability for the application, with a 20-30% margin between the load requirements and motor ratings to ensure reliability. Exceeding these margins by too much reduces cost effectiveness.

  

Inertia Ratio

Inertia defines how difficult it is to change the rotating velocity of an object (in the case of a motor, the motor shaft). Inertia increases with increasing mass. Servomotors are thus designed lightweight in order to be able to stop, start, and reverse direction quickly. The inertia ratio, another specification important to sizing servomotor systems, is defined as the proportion of load inertia (the inertia of the system load reflected in the motor shaft) to motor inertia (the inertia of the motor shaft itself).

 

There are a lot of variables which affect inertia matching (balancing the inertia ratio), and there is no rule of thumb for an ideal. All other factors being equal, a lower inertia ratio correlates to better performance. However, an excessively low inertia ratio can indicate an oversized, expensive, and energy intensive motor with little performance increase. If inertia ratio is the limiting sizing factor, it's important to fully understand the application's performance requirements before ruling out a higher ratio.

 

Servo System Compatibility

An industrial servomotor is almost always part of a larger servo system. It is important to ensure the motor is compatible with the associated system before making a selection. This includes compatibility with the controller, amplifier, and programmable logic controller (PLC) components of the system. In operation, the PLC sends position commands specified by the user to a servo controller. These commands are interpreted by the position controller and sent to an amplifier which processes them for use and response by the motor. As the motor carries out these commands, the encoder sends position signals back to the controller, which processes them for the next set of commands.

 

AC Servomotors Selection Guide

Image Credit: TryPLC.com

 

Feedback Devices

Servomotors can incorporate either encoders or resolvers as feedback devices to sense angular shaft speed, direction, and position.

 

Encoders

Encoders are low weight devices, meaning they contribute significantly less revolving inertia. This makes them more suitable for high acceleration/deceleration applications. They are also immune to electrical noise. Two types of encoders include incremental encoders and absolute encoders.

  • Incremental encoders are the most prevalent devices used on servomotors today, offering a standard interface that can be used on any servo drive. They are typically the lowest cost and most versatile, and are used when retaining absolute position is not necessary. They may be designed as optical encoders which use a light as the sensing/measuring mechanism, or magnetic encoders which use a magnetized rotor and magnetoresistive sensor for feedback generation. Magnetic encoders are more environmentally durable than optical encoders.
  • Absolute encoders retain position information even when powered off. They also have no need for homing sensors or "homing" the axis when powered on. They are also less prone to the effects of EMI. Absolute encoders may be designed as single-turn or multi-turn. Single-turn encoders are only capable of retaining position within one revolution, while multi-turn encoders can keep track of the number of revolutions.

Resolvers

Resolvers are rotating transformers. They are much more rugged than encoders and are made of tough materials that can withstand harsh environments and high temperature over time. Resolvers are also less sensitive than encoders to shock and vibration.

 

 

Resolver

Encoder

Angle measurement

Absolute

Absolute or incremental

Absolute resolution

16 bits

13 bits

Incremental resolution

N/A

10,000 lines/revolution

Accuracy (arc minutes)

4 to 40

.25 to 6

Electronic interface

R/D converter

Direct

Noise immunity

Sensitive

Best

Output signal

Analog

Analog or digital

Construction materials

Robust

Fragile

Weight

Heavy

Lighter

Inertia

High

Low

Longevity

Very High

High

Shock/vibration

Rugged

Limited

Temperature

+150°C

+100°C

Contamination

Immune

Vulnerable

Implementation

Complex

Simple

Interchangeability

Limited

Vast

Retrofit/upgrade

Fixed

Open

Cost

Higher

Less

Table Credit: Donald E. Barnett - optoresolver.com

 

Other feedback devices include tachometers which sense only rotational speed (rpm). Modern tachometers are actually rotary incremental encoders which indicate the position, speed, and direction of rotation.

 

Features

Servomotors often require modifications to meet the requirements of different applications, typically in the form of different options and features.

  • Environmental sealing provides protection from dust, water, and other forms of environmental contamination.
  • Fail-safe brakes, mechanical or electronic, are often built in to servomotors to prevent over-rotation.
  • Fan cooling can be incorporated to provide temperature control and cooling for larger servomotors.
  • Shaft, shape, and mounting modifications are made in order to properly fit/attach the motor to the system.

Environment and Housing Factors

In certain applications it may be important to consider the environmental criteria and ratings of the motor.

  • Operating temperature(s) specifies the rated temperature or range of temperatures that the motor is designed to operate at. 
  • Shock rating is the maximum shock the motor can withstand, commonly expressed in g's as a multiple of the earth's standard acceleration due to gravity.
  • Vibration rating is the maximum vibration the motor can withstand and still meet operating specifications. Vibration is expressed in the same units as shock, based on acceleration.
  • Enclosure ratings are used to rate the adequacy of motor enclosures to protect from various environmental conditions. Enclosures may be dust-proof, drip-proof, explosion-proof, or water-proof according to different standards.

Some motors may also be rated for use in special or extreme environments such as clean rooms, vacuum chambers, and cryogenic temperatures.

 

References

 

Feedback Options - Parker Hannifin Corporation

 

Selection of Servo Motors and Drives (pdf) - Drivetech, Inc.

 

Servo Motor Sizing Concepts - Design News

 

Stepper or Servo (pdf) - Techno Inc.

 

Stepper vs. Servo Motors - CNC Router Source

Read user Insights about AC Servomotors

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  • AC Motors

    AC motors include single, multiphase, universal, induction, synchronous, and gear motors. They also include servomotors.  

  • DC Motors

    DC motors are most commonly used in variable speed and torque applications. They include brushless and gear motors, as well as servomotors.

  • DC Servomotors

    DC servomotors are generally small and powerful for their size, and easy to control. Common types of DC servomotors include brushless or gear motors.

  • Gearmotors

    Gearmotors consist of an AC or DC motor with an integral gearbox or gear head typically used to adjust the motor's output speed and torque.

  • Stepper Motors (rotary)

    Stepper motors use a magnetic field to move a rotor in small angular steps or fractions of steps. They provide precise positioning and ease of use, especially in low acceleration or static load applications.