Motor types

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Servo motor is a electromechanical mechanical actuator with feedback allowing precision closed loop motion control and monitoring.

Construction

Servo motor consists two main parts:

  • Motor part - typically electromagnetic device that produces torque or force when driven with current
  • Feedback device - typically electronic device that outputs measurement information such as shaft angle or velocity

Shapes

Servo motors and feedback devices come mainly in two shapes:

  • Rotary
  • Linear

Shape of motor doesn't change their electromechanical princible so both types of motors can be driven with same drives.

Motor technologies

List of electromagnetic motor part types.

AC

Typical AC (alternating current) servo motor is a 3 phase permanent magnet syncronous machine. This type of motors are driven by 3 wires each driving one phase coil. In ideal case 3 phase AC servo is driven by sinusoidal current waveforms that are synchronized to the permanent magnet rotor.

BLDC

BLDC (brushless DC) is very similar to AC motor with only expection that it is desgined to be driven by trapezoidal current waveforms instead of sinusoidal. BLDC motors can always be driven with same drives regardless of current waveform matching.

Brush DC

Brush DC (direct current) motors typically have permanent magnet stator and rotor with mechanical commutator with brushes.

Comparison

Comparison of motor technologies

Stepping motor DC servo BLDC servo AC servo
Closed loop No Yes Yes Yes
Torque control No, inpractical Yes Yes Yes
Velocity control Yes Yes Yes Yes
Position control Yes Yes Yes Yes
Available speed range Low, Medium Medium High Highest
Rated torque vs size High Low Medium Medium
Dynamic performance Low, Medium Medium High Highest
Motion smoothness Very low to high Medium Medium High
Endurance High Medium High High

Feedback device types

Quadrature encoder

Resolver/synchro

Tachometer

Hall sensors


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