section_path: Advanced types/Rotary/Pancake or axial rotor motor
section_name: Pancake or axial rotor motor
paragraph_text: Pancake or axial rotor motor
  
  The printed armature or pancake motor has windings shaped as a disc running between arrays of high-flux magnets. The magnets are arranged in a circle facing the rotor spaced to form an axial air gap. This design is commonly known as the pancake motor because of its flat profile.
  
  The armature (originally formed on a printed circuit board) is made from punched copper sheets that are laminated together using advanced composites to form a thin, rigid disc. The armature does not have a separate ring commutator. The brushes move directly on the armature surface making the whole design compact.
  
  An alternative design is to use wound copper wire laid flat with a central conventional commutator, in a flower and petal shape. The windings are typically stabilized with electrical epoxy potting systems. These are filled epoxies that have moderate, mixed viscosity and a long gel time. They are highlighted by low shrinkage and low exotherm, and are typically UL 1446 recognized as a potting compound insulated with 180 °C (356 °F), Class H rating.
  
  The unique advantage of ironless DC motors is the absence of cogging (torque variations caused by changing attraction between the iron and the magnets). Parasitic eddy currents cannot form in the rotor as it is totally ironless, although iron rotors are laminated. This can greatly improve efficiency, but variable-speed controllers must use a higher switching rate (>40 kHz) or DC because of decreased electromagnetic induction.
  
  These motors were invented to drive the capstan(s) of magnetic tape drives, where minimal time to reach operating speed and minimal stopping distance were critical. Pancake motors are widely used in high-performance servo-controlled systems, robotic systems, industrial automation and medical devices. Due to the variety of constructions now available, the technology is used in applications from high temperature military to low cost pump and basic servos.
  
  Another approach (Magnax) is to use a single stator sandwiched between two rotors. One such design has produced peak power of 15 kW/kg, sustained power around 7.5 kW/kg. This yokeless axial flux motor offers a shorter flux path, keeping the magnets further from the axis. The design allows zero winding overhang; 100 percent of the windings are active. This is enhanced with the use of rectangular-crosssection copper wire. The motors can be stacked to work in parallel. Instabilities are minimized by ensuring that the two rotor discs put equal and opposing forces onto the stator disc. The rotors are connected directly to one another via a shaft ring, cancelling out the magnetic forces.