Types of Electric Motor

There are a number of different types of electric motor. Also there are a design factors when considering the different types of electric motor:

  • Application
  • Environment
  • Commutation method
  • Duty cycle
  • No-load speed
  • Weight
  • Stall torque
  • Lifetime
  • Load (operating) point
  • Torque ripple
  • Power source
  • Controllability
  • Envelope (volume)
  • Heat dissipation

AC Induction Motors

These are generally used for constant speed applications where a fixed frequency power source such as 60 Hz or 400 Hz is available. Typical applications are fans and pumps. Motor construction consists of windings on the stationary part of the motor and copper shorting bars on the iron laminations of the armature. The AC voltage applied to the windings induces a current in the armature of the rotor, creating a magnetic field. This field reacts with the field in the stationary part of the motor to create torque.

  • High torque and high rpm applications


  • Motor lasts as long as the bearings hold up as there are no brushes to wear out.
  • Rugged

Induction motor noise has tonal components at the rotation speed due to imbalance. The electromagnetic excitation is also strong at a frequency that is the product of the number of rotor conducting bars and the “slip frequency”. The slip frequency is the difference between the rate of rotation of the stator′s magnetic field and of the rotor.

Electrical Generation – An induction machine either squirrel cage type or wound rotor type is forcibly driven above its synchronous speed at which point it acts as a generator of electrical power into whatever electric system it happens to be connected. The electric system to which the motor is connected supplies the necessary excitation current to the induction machine.

Slip – The difference between the speed of the rotating magnetic field and the rotor in a non-synchronous induction motor. This is expressed as a percentage of synchronous speed. Slip generally increases with an increase in torque.

Brush DC Motors

The Brush DC motors use commutators and carbon brushes to apply current through the windings as the motor rotates. The Brush DC motor utilizes wound elements in the rotor and permanent magnets attached to a stationary stator ring. In a Brush DC motor, electrically separated motor windings are connected to the commutator ring. Current is carried by spring loaded brushes, through the commutator into the windings of the rotor. The current in the windings creates magnetic fields, which react with the stator’s permanent magnetic field. The magnetic repulsion causes the rotor to rotate. This rotation causes the brushes to make and break connections through the commutator with different winding pairs. The moving magnetic field provides the torque necessary to rotate the motor’s armature.


  • Limited life applications
  • Low rpm applications


  • Low cost
  • Simplicity
  • Availability


  • Brush dust
  • Brush to commutator arcing and wear
  • Electromagnetic interference
  • Mechanical noise
  • Short motor life
  • Low efficiency
  • Limited speed
  • Poor thermal characteristics in vacuum

Electrical Time Constant – For DC motors this is the ratio of electrical inductance to armature resistance.

Brushless DC Motors

The BLDC motor uses electronic commutation to control the current through the windings. The BLDC motors use permanent magnets on the rotor. The BLDC motor contains rotor position sensor electronics so that the power input wave form to the windings is in sequence with the proper rotor position. Motor efficiency is enhanced because there is no power loss in the brushes. In the BLDC motor, the stator is wound with electromagnetic coils that are connected in a multiphase configuration, which provides the rotating field, and the armature consists of a soft iron core with permanent magnet poles. Sensing devices define the rotor position. The commutation logic and switching electronics convert the rotor position information to the correct excitation for the stator phases. Sensing devices include hall-effect transducers, absolute encoders, optical encoders, and resolvers. The electronic controller can be separate or packaged with the motor.

  • High rpm applications
  • Light weight applications
  • Low thermal emission applications


  • High speed (up to about 100000rpm)
  • High torque at high speed
  • Approximately double the output torque of Brush DC motor of the same size
  • Improved heat dissipation over of Brush DC motor as the windings are on the stator.
  • Motor lasts as long as the bearings hold up as there are no brushes to wear out.
  • Higher efficiency
  • Vacuum compatible


  • Higher electronic cost
  • Greater motor drive complexity

Stepper Motors

Stepper motors are a special case of Brushless DC Motors. Construction is identical except that they contain no position sensors. Excitation is sequentially applied to the windings, creating the rotating field to produce torque. Uses:

  • Low torque applications
  • Open loop micropositioning
  • Timer switching


  • Simplicity
  • Compatability with digital control schemes


  • High continuous power dissipation
  • High ripple torque

Used for noncritical, low power applications, since positional information is easily lost if acceleration or velocity limits are exceeded.

Within the different types of electric motor there are a number of variations where certain aspects of the performance are optimised. Based on the large number of applications there is a rather large number of different types of electric motor.

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