Techno Linear Motion Catalog26Technical InformationPermanent Magnets(Rotor)Brushless MotorArmature(Stator)CoilsCommutatorBrushBrushBrushed DC MotorHeatHeathigh to prevent motor shaft oscillation at resonance points. Finally, stepper motors may perform poorly inhigh-speed applications. The maximum steps/sec rate of the motor and drive system should be considered,carefully.Servo motorsare more robust than stepper motors, but pose a more difficult control problem. They areprimarily used in applications where speed, power, noise level as well as velocity and positional accuracyare important. Servo motors are not functional without sensor feedback; they are designed and intendedto be applied in combination with resolvers, tachometers, or encoders (closed loop). There are severaltypes of servo motors, and three of the more common types are described as follows.The DC brush type are most commonly found in low-end to mid-range CNC machinery. The brush refersto brushes that pass electric current to the rotor of the rotating core of the motor. The construction consistsof a magnet stator outside and a coil rotor inside. A brush DC motor has more than one coil. Each coil isangularly displaced from one another so when the torque from one coil has dropped off, current isautomatically switched to another coil which is properly located to produce maximum torque. The switchingis accomplished mechanically by the brushes and a commutator as shown below.There are distinct advantages to using DC brush servo motors. They are very inexpensive to apply. Themotor commutates itself with the brushes and it appears as a simple, two-terminal device that is easilycontrolled. Among the disadvantages is the fact that they are thermally inefficient, because the heat mustdissipate through the external magnets. This condition reduces the torque to volume ratio, and the motorperformance may suffer inefficiencies. Also, the brushed motor will require maintenance, as the brusheswill wear and need replacement. Brushed servo motors are usually operated under 5000 rpm.The DC brushless type offers a higher level of performance. They are often referred to as inside out DCmotors because of their design. The windings of a brushless motor are located in the outer portion of themotor (stator), and the rotor is constructed from permanent magnets as shown below. DC brushless motorsare typically applied to high-end CNC machinery, but the future may see midrange machinery use brushlesstechnology due to the narrowing cost gap.