Description

Linear motors generate force only in the direction of travel. A linear motor applies thrust directly to a load, and does not require any intermediate mechanism to convert rotary motion into linear motion. Linear motors are capable of extremely high speeds, quick acceleration, and accurate positioning.

Types

Choices for linear motors include:

• Moving coil - In a moving coil design the coil moves and the magnet is fixed, such as an audio speaker.
• Moving magnet - In a moving magnet design the magnet moves and the coil is fixed.
• AC synchronous design - AC synchronous motors are a class of motors that operate at constant speed up to full load. The rotor speed is equal to the speed of the rotating magnetic field of the stator; there is no slip. Reluctance and permanent magnet are the two major types of synchronous motors.  Synchronous linear motors are often used where the exact speed of a motor must be maintained.
• AC induction or traction design - AC induction or traction design motors are a class of motors that derives its name from the fact that current is induced into the rotor windings without any physical connection with the stator windings (which are directly connected to an AC power supply); adaptable to many different environments and capable of providing considerable power as well as variable speed control.  Typically there is "slip," or loss of exact speed tracking with induction motors. Typically rolled flat version of rotary AC induction motors.
• Linear stepping design - Stepper motors use a magnetic field to move a rotor in small angular steps or fractions of steps. Stepper motors provide precise positioning and ease of use, especially in low acceleration or static load applications.
• DC brushed design - Brush motors have the armature windings on the rotor.  The magnetic fields are commutated via direct contact of brushes with the rotor commutator.
• DC brushless design - Brushless linear motors have their armature windings on the stator and the field on the rotor.  They rely on internal noncontact sensing devices to activate external commutating electronics.

Specifications

Performance Specifications

Important performance specifications to consider include:

• Rated continuous thrust force - The maximum rated current that can be supplied to the motor windings without overheating.
• Peak force - The maximum force of the linear motor.
Nominal stator length - The length of the fixed magnet or coil.
Slider or carriage travel - The range of travel of the moving coil or magnet.
Slide or carriage width and length - The dimensions of the moving coil or magnet.
Maximum speed
Maximum acceleration

Electrical Properties

Important electrical properties to consider when specifying linear motors include:

• Rated current per phase - The maximum rated current per phase or winding for a stepper motor.
• Number of leads - Specifies unipolar = 6 leads, bi-polar = 4.
• Continuous current
• Motor force constant

Mechanical Properties

Mechanical properties to consider for linear motors include:

• Linear stepper resolution - Units are typically in 'distance per step' or 'steps per unit distance'.
• Design units
• Maximum coil temperature

Features

Common features include:

• Forced air-cooling
• Water-cooling
• Balanced design
• Integral position feedback
• Modular stator

Environmental Parameters

Important environmental parameters to consider for linear motors include:

• Operating temperature
• Maximum shock
• Maximum vibration

References

Image Credit:

Aerotech, Inc.