Ironless versus ironcore motor solutions

Oct. 20, 2005
This article presents an engineering-based comparison of the two predominant linear motor types, with the objective of providing the knowledge needed to properly specify the correct type of motor for a given application. Real world examples of both motor types (ironcore and ironless) are presented.

By Herve Stampfli, HST-Etel

LINEAR SERVO motors have now established themselves as the drive technology of choice for high performance motion control applications. From automation to semiconductor and electronics industries, the constantly increasing requirements for throughput improvement, flexibility and dynamics in the machines has led the manufacturers to opt for the “direct drive” approach. In the marketplace today, there are two predominant types of brushless linear motors that are widely utilized: the ironless, or “U-channel” motor, and the iron core or “single sided magnet” type. While they are both linear servo-motors, they have vastly different performance characteristics. That makes the different types suitable for certain types of applications and unsuitable for others.

While the general knowledge of engineers regarding the advantages of linear motors is advancing rapidly, the differences between the main motor types are not well understood. Further to the point, there are many misconceptions as to the advantages and drawbacks of each type. Thus, the wrong type is sometimes misapplied to an application, resulting in inferior performance.

Specific topics this White Paper will address include:

  • Thermal considerations
  • Stiffness, dynamic and static
  • Efficiency
  • Real definition of important, but often misused parameters like cogging, force ripple, commutation ripple, and other terminology, and their effect on motor performance versus the type of motor
  • Force density
  • Accuracy, velocity, stability, settling time
  • Use of Hall effect sensors for initialization and current commutation; effect on performances
  • Influence of the servo controller
  • Magnetic attraction