DC Servomotors

DC servomotors have an output shaft that can be positioned by sending a coded signal to the motor. As the input to the motor changes, the angular position of the output shaft changes as well. Servomotors are generally small and powerful for their size, and easy to control. Common types of DC servomotors include brushless or gearmotor types.

Important performance specifications to consider when searching for DC servomotors include shaft speed, terminal voltage, continuous current, continuous torque, and continuous output power.  The shaft speed is the No-load rotational speed of output shaft at rated terminal voltage.  The terminal voltage is the design DC motor voltage.  Continuous current is the maximum rated current that can be supplied to the motor windings without overheating.  The continuous torque is the output torque capability of the motor under constant running conditions.  Continuous output power is the mechanical power provided by the motor output.

Motor construction choices for DC servomotors include permanent magnet, shunt wound, series wound, compound wound, disc armature, and coreless or slotless.  A permanent magnet motor is a motor with permanent magnets embedded into the rotor assembly.  The rotor aligns itself with the rotating magnetic field of the stator windings.  PM motors exhibit constant speed with varying load (zero slip) and provide relatively high torque, good efficiency, and lower current draw than comparable synchronous motors.  Shunt wound motors exhibit minimum speed variation through load range and can be configured for constant horsepower over an adjustable speed range.  Frequent applications include machine tools, fans, and blowers.  Series wound motors exhibit high starting torques for permanently attached loads.  Frequently used in heavy industrial applications.  Compound wound motors are designed with both a series and shunt field winding.  They are often used where the primary load requirement is heavy starting torque, and adjustable speed is not required.   They can exhibit speed variation from no-load to full-load.  Applications include elevators, hoists, and industrial shop equipment.  Disc armatures are flat, pancake-shaped rotors that are driven by an axially, rather than radially, aligned magnetic field.  The thin construction of these armatures can result in low inertia with resulting high acceleration.  Coreless and slotless motors incorporate a cylindrical winding that is physically outside of a set of permanent magnets.  The winding is not held by a slotted iron cage but is laminated together.  In a slotless motor, the magnets attached to the rotor rotate, while in a coreless motor, the windings rotate around the permanent magnet stator.  Commutation choices include brush or brushless.  Brush motors have the armature windings on the rotor.  The magnetic fields are commutated via direct contact of brushes with the rotor commutator.  Brushless 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.

The motor configuration in DC servomotors includes motor only or gearmotor.  Gearmotors include units with single integral gearheads, or replaceable / interchangeable gearhead options.  Gearing choices, if applicable to the DC servomotor include spur, planetary, harmonic, worm, and bevel.  The gearbox ratio of input to output speed is also important to consider.  Gearbox efficiency is the percentage of power or torque that is transferred through the gearbox.  Shaft options for DC servomotors include in-line, offset or parallel, right angle, single-ended, double-ended or hollow.  Feedback for the motor can be integral encoder, integral resolver, or integral tachometer.

Other important parameters to consider when specifying DC servomotors include housing and enclosure features such as the design units, motor shape, diameter or width, housing length, NEMA frame size, enclosure options and special or extreme environments the motor might need to operate in.  Common features include integral driver electronics, integral brake, integral clutch, and integral brake or clutch combination.  Important environmental parameters to consider include operating temperature, shock rating, and vibration rating.

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Part Numbers for DC Servomotors