DC Machines

What are DC Machines?

DC machines are electrical devices that convert mechanical energy into electrical energy (DC generators) or electrical energy into mechanical energy (DC motors) using direct current (DC). They operate based on the principles of electromagnetism, where a magnetic field interacts with current-carrying conductors to produce motion or electricity.

Why DC Machines?

DC machines are used in applications where precise speed control, high starting torque, and smooth operation are required. Some key reasons to use DC machines include:
Speed Control – They allow smooth and precise speed regulation using voltage or field control.

High Starting Torque – DC motors provide high torque at low speeds, making them ideal for applications like traction (trains, trams) and elevators.
Reversibility – DC machines can be easily reversed in direction.
Steady and Smooth Operation – They provide smooth and consistent motion, making them suitable for robotics and conveyor systems.
Battery-Powered Applications – DC motors are used in electric vehicles, drones, and appliances powered by batteries.

How Do They Operate?

DC machines work based on Faraday’s Law of Electromagnetic Induction and Lorentz Force Law:

DC Motor Operation

  • When a DC voltage is applied to the armature winding, current flows through the conductors.
  • The current interacts with the magnetic field from the stator, producing a force (Lorentz force).
  • This force creates torque, causing the rotor (armature) to rotate.
  • The direction of rotation is determined using Fleming’s Left-Hand Rule.

DC Generator Operation

  • A prime mover (engine or turbine) rotates the armature within a magnetic field.
  • Due to electromagnetic induction, an EMF (voltage) is generated in the armature windings.
  • The generated EMF is collected through commutators and brushes as DC output.
  • The direction of induced EMF follows Fleming’s Right-Hand Rule.

Motor Action

• When a current-carrying conductor is placed in a magnetic field, the conductor experiences a mechanical force.

• Fleming’s left-hand rule gives the direction of this force.



Single loop rotor coil situated in a two-pole field




Generator Action (Motional EMF)

  • When a conductor moves through a magnetic field, an electromotive force (emf) is induced in it.
  • This is described by Faraday’s Law:

where:

    • e = induced emf (voltage)
    • N = number of turns in the coil
    • Φ = magnetic flux
    • ​ = rate of change of magnetic flux

  • The magnitude of the induced voltage depends on:
    • The number of turns in the coil (series-connected turns contribute to the total voltage).
    • The rate of change of flux through the coil.

A diagram of a magnet showing a finger and a pen

Construction of a DC Machine

 


• Stator

− Consists of yoke and poles (made of cast steel)

− Provides mechanical support

− Field poles are made of thin laminations to reduce magnetic losses

− Pole shoes help maintain uniform flux Density around the air gap

• Field winding

− Poles are made electromagnets by DC

− Shunt, series, compound and separately Excited configurations

− DC servo motors have permanent magnets Instead

• Armature 

− Rotating part of the motor 

− Composed of thin, highly permeable, and electrically insulated circular steel    laminations 

− Features axial slots to accommodate the armature coils, which are made of insulated copper wires 

• Commutators 

− Constructed from wedge-shaped, hard-drawn copper segments 

− One end of the armature coil connects to a copper segment of the commutator 

− Rotates with the armature, maintaining a sliding contact with the brushes 

• Brushes 

− Secured in a fixed position by means of brush holders 

− An adjustable spring inside the brush holder applies constant pressure to ensure proper contact between the commutator and brush 

− Made of carbon or carbon graphite

DC Machine Types

Classified based on the field excitation.


Equivalent Circuit

Considering a separately excited DC machine in Motor mode



Steady State Equations

A diagram of mathematical equations

Power Flow in a DC Motor

In Generator Mode

A diagram of a company's financial loss AI-generated content may be incorrect.

Copper/ Electrical Losses

• Armature copper loss

– contributes about 30 to 40% to full load losses

– variable and depends upon the amount of loading of the machine.

• Field copper loss

– It contributes about 20 to 30% to full load losses.

• Loss due to brush drop

– relatively small and depends on the material

– Generally for metal graphite brushes, the brush drop is taken as 0.5 V.

– And for electro-graphitic and graphite brushes a value of 2.0 V is taken.

Core/Iron Losses

• Losses occur in the iron core is categorized under here.

– Hysteresis loss

– Eddy current loss

• Typically considered as fixed losses.

Mechanical/ Rotational Losses

• Losses due to friction in bearings and commutator.

• Air friction loss of rotating armature also contributes.

• These losses are about 10 to 20% of full load losses.

Stray/ miscellaneous Losses

• These losses are difficult to account.

• They are usually due to inaccuracies in the designing and modeling of the machine.

• Generally, stray losses are assumed to be 1% of the rated power output.

Starting a DC Motor

• Lock-rotor or Blocked-rotor parameters.

• High starting current

• Normal Operation -> Arcing and burning, mechanical damages