AC and DC Servomotors, Stepper Motors, and More: A Comprehensive Look

Posted on Jan 21, 2025 in Audiovisual Communication and Journalism

Working Principle of AC Servomotor

As an ordinary induction motor, the stator windings are excited by voltages of equal RMS magnitude and 90° phase difference. These results in exciting currents i1 and i2 that are phase displayed by 90° and have equal RMS values. These current give rises to a rotating magnetic field of constant magnitude. The direction of rotation depends on the phase relationship of the two currents (or voltages). The exciting current produces a clockwise rotating magnetic field and phase shift of 180° in i1 will produce an anticlockwise rotating magnetic field.

DC Servo Motor

Construction

A small DC motor consists of a feedback potentiometer, gearbox, motor driver circuit, analogue feedback control, etc.

Principle of DC Servo Motor

The feedback signal corresponding to the present position of load is obtained by using a position sensor. The sensor is normally a potentiometer that produces the voltage corresponding to the absolute angle of the motor shaft. By that, it is sent to the feedback section and controlling is done.

Field Controlled DC Servo Motor

Rotation of the motor can be controlled by an amplified signal if the constant armature current is large enough. Every little change in the field current forces a cosmetic change in torque on the motor shaft. The direction of rotation can be changed by the polarity of the field.

Armature Controlled DC Servo Motor

The DC motor armature is less inductive and the time constant of the armature winding is more than enough. Discourses to change of armature current produce a sudden change in armature voltage. Rotation can be changed by reversing the polarity of the error signal.

Permanent Magnet Armature Controlled DC Servomotor

Usually manufactured in 60-28 volt ratings in fractional horsepower sizes, 150 volt rate and integral up to 2 HP. The field structure is usually Alnico VI alloys. Well compensated by means of commutating the commutation windings to avoid demagnetization of field magnets when the motor is suddenly reversed.

Series or Split Field DC Servo Motor

Some can operate with a fractional kilowatt excited field controlled motor. A motor with respect to field write with some fractional kilowatt. They have a typical torque speed curve which denotes high stall torque and rapidly reduces with high speed.

Stepper Motor

  • High accuracy: Operate under open loop
  • Reliability: Stepper motors are brushless.
  • Load independent: Holding torque: For each and every step, the motor holds its position without brakes.

Types of Stepper Motors

  1. Variable Reluctance Motor (VRM)
  2. Permanent Magnet Stepper Motor (PMSM)
  3. Hybrid Stepper Motor (HSM)

Variable Reluctance Motor – It consists of a soft iron multi-toothed rotor and a wound stator. When the stator windings are energized with DC current, the poles become magnetized. Rotation occurs when the rotor teeth are attracted to the energized stator poles.

Permanent Magnet Stepper Motor – A stepper motor using a permanent magnet in the rotor is called a PMSM. The rotor no longer has teeth as with the VRM. Instead, the rotor is magnetized with alternating north and south poles situated in a straight line parallel to the rotor shaft.

Hybrid Stepper Motor – Operated with the combined principles of the permanent magnet and variable reluctance motors. Standard HSM have 50 rotor teeth and rotate at 1.8 degrees per step. If excitation is removed from phase A and phase B is excited, then pole 2 produces south pole and pole 4 produces north pole resulting in rotor movement of 900 in the clockwise direction. Step length = 90degree/Nr

Advantages

  • Brushless design for reliability and simplicity.
  • High torque per package size.
  • Holding torque at standstill.

Disadvantages

  1. Resonance
  2. Vibration
  3. Torque ripple

Stepper Modes of Excitation

  1. Full step
  2. Half step
  3. Micro step

Full step excitation mode is achieved by energizing both windings while reversing the current alternately. Essentially one digital input from the driver is equivalent to one step. If two phases of the hybrid stepper motor are excited, the torque produced by the motor is increased, but the power supply to the motor is also increased.

Half Step Excitation Mode: Two phases ON and a single phase ON. This increases the angular resolution, but the motor also has less torque (approx 70%) at the half step position (where only a single phase is ON). This may be mitigated by increasing the current in the active winding.

Micro Step Excitation Mode

Micro step is typically used in applications that require accurate positioning and a fine resolution over a wide range of speeds. The major disadvantage of the micro step drive is the cost.

Unipolar Driver

  1. Winding with a center tap, or two separate windings per phase.
  2. Two switches per phase
  3. Utilizes only half the available copper volume of winding
  4. Incurs twice the loss of a bipolar drive at the same output power

Bipolar Driver

  1. One windings per phase
  2. Four switches per phase, in the form of an H-bridge
  3. Motor winding is fully energized
  4. Loss is minimum compared to unipolar drive

AC Series Motor

The single-phase series motor is a commutator-type motor. If the polarity of the line terminals of a DC series motor is reversed, the motor will continue to run in the same direction. Motors that can be used with a single-phase AC source as well as a DC source of supply voltages are called universal motors.

  1. Its efficiency is low due to hysteresis and eddy-current losses.
  2. The power factor is low due to the large reactance of the field and the armature winding.
  3. The sparking at the brushes is excessive

Construction

  • The stator is made up of laminations and has a salient pole structure
  • Two or four pole structure is used to reduce winding reactance
  • Machine wound field coils are provided on the stator poles
  • The rotor is laminated and has slots to accommodate armature windings. The slots are skewed by one pole pitch. (Skewing results in considerable reduction in excitation and saturation)
  • Commutator assembly is mounted on the shaft
  • Armature assembly is mounted on the shaft. Armature coils are connected to commutator segments which are insulated from each other.
  • High resistance brushes and leads are used. Armature and field windings are connected in series through these brushes.
  • Compensator windings is distributed over the stator bore and connected in series with the armature winding

Principle of Working

The basic principle of a single phase AC series motor is that a current carrying conductor placed in a magnetic field experiences a mechanical force. During the positive half cycle of the supply, the upper stator pole becomes the north pole and the lower the south. The armature conductors carry current in the direction shown. Then according to Fleming’s left hand rule, the armature conductors experience a torque in the CCW direction. As the armature conductors are on the rotor, the rotor rotates in CCW direction.

Characteristics of AC Series Motor

  • T ∝ΦI, if phase angle between flux and current is neglected, T∝I2
  • Speed is proportional to counter EMF or proportional to supply voltage minus voltage drops
  • The reactance drops are lowest and counter EMF is highest at light loads

Universal Motor

A universal motor is a special type of motor which is designed to run on either DC or single phase AC supply. These motors are generally series wound (armature and field winding are in series), and hence produce high starting torque.

Types and Construction

  1. Non compensated concentrated field type
  2. Compensated distributed field type

Advantages

  • Able to produce more power in small size
  • High starting torque
  • High-speed operation is possible
  • Speed control is easy

Limitations

  • Regular Maintenance is required
  • High noise at high speeds
  • Brush sparking causes radio and TV interferences
  • Perfect balancing is required to avoid vibrations

Hysteresis Motor

The working of the motor depends on the working of the continuously revolving magnetic flux. For the split phase operation, the stator winding of the motor has two single phase supply.

Construction of Stator of Hysteresis Motor

The motor produces a rotating magnetic field and is almost similar to the stator of the induction motor. Thus, the stator of the motor is connected either to single supply or to the three phase supply. The stator winding of the single-phase hysteresis motor is made of permanent split capacitor type or shaded pole type.

Construction of Rotor of Hysteresis Motor

The rotor of the hysteresis motor consists of the core of aluminum or some other non-magnetic material which carries a layer of special magnetic material. The outer layer has a number of thin rings forming a laminated rotor. The rotor of the motor is a smooth cylinder, and it does not carry any windings.

Advantages
  • No mechanical vibration due to absence of teeth and winding in the rotor
  • Noiseless operation due to no vibration
  • Multi speed operation possible due to gear train
Disadvantages
  • Low efficiency
  • Low power factor
  • Low torque
Applications
  • Good quality record player
  • Instruments which are used for sound generating
  • Sound recording instruments
  • Electrical clock