DC Motor Operation 

By on

Coils of wire on the rotor carry a d.c. current which generates a magnetic field. A stator magnetic field is created using either permanent magnets or electrical field windings.

Any current-carrying conductor placed within an external magnetic field experiences a force. Torque is produced in the motor by this mechanism.

As a d.c current provides a constant magnetic field, this needs to change each half revolution and is called commutation. Commutation is typically accomplished by metal brushes against the contacts. Where a field winding is used to generate the stator magnetic field, different motor characteristics can be obtained depending on how this winding is connected.

Operation Principals 

DC Motor Operation PrincipalsFor the equivalent circuit the parameters are given by:


myElectrical EquationWhere:

myElectrical Equation       - lap winding

myElectrical Equation -       wave winding

given that

  • n = number of conductor
  • p = number of pole pairs
  • ω = motor speed, rad s-1
  • φ = flux per pole, Wb
  • T = torque Nm

myElectrical Equation

The power input to armature is given by:      myElectrical Equation

The armature power loss is given by:           myElectrical Equation

The output power is given by:                         myElectrical Equation

The torque is given by:                                     myElectrical Equation

Typically, IaRa is ±3% of Va and the power dissipated in field winding ±3.5% of the motor rating.

Field Windings

Permanent magnets or field windings of a dc motor provide the necessary magnetic fields for the motor to operate(often call excitation):

  • permanent magnets - provides a continuous and constant magnetic field by the use of permanently magnetized materials (typically on only the smallest motors)
  • separately excited - separate windings and power supply provide the magnetic field for the motor
  • shunt winding - this is connected in parallel with the motor armature to provide the excitation
  • series winding - this is connected in series with the motor armature to provide the excitation
  • compound winding - is a hybrid between shunt and series where to field windings are connected both in parallel and in series with the armature

Characteristics of DC Motors


Speed Control of DC Motors

Separately excited, shunt

  • Constant power drive - Ia,Va is held constant and If varied
  • Constant torque If is held constant and Ia, Va varied

DC Converter output Voltages

Semi converter,       myElectrical Equation         [DC converter equations do not look correct.  Please can someone check.]     

Full converter, myElectrical Equation - with infinite inductive filtering

Steven McFadyen's avatar Steven McFadyen

Steven has over twenty five years experience working on some of the largest construction projects. He has a deep technical understanding of electrical engineering and is keen to share this knowledge. About the author

myElectrical Engineering

comments powered by Disqus

Fault Calculations - Typical Equipment Parameters

A frequent problem in fault calculations is the obtaining of equipment parameters.  While it is always preferable to use the actual parameters of the equipment...

Bows and Arrows

It starts with me reading one of the Horrible History books with my son (Groovy Greeks). Arrows were mentioned which lead to the discussion of the bodkin...


Multimeters are undoubtedly the most common item of electrical test equipment in use.  Often it is the first piece of equipment people will turn to when...

Electromagnetic Compatibility (EMC)

Electromagnetic compatibility (EMC) is the study of coordinating electromagnetic fields give off equipment, with the withstand (compatibility) of other...

Low Voltage Circuit Breakers

Circuit breakers are switching devices whose primary function is to isolate parts of an electrical distribution system in the even of abnormal conditions...

Cable Insulation Properties

Cable insulation is used to provide electrical separation between conductors of  a cable.  During the historical development of cables, numerous types...

9 power supply issues solved by using a UPS

Installation of a UPS can help in reducing problems due to issues with the power supply.  A lot of people relate this to nine key issues.  Depending on...


When current flows within a wire, a magnetic field is created. The potion of this magnetic field perpendicular to the wire is called the magnetic flux...

Autonomous Vehicle Challenge

Two driverless and solar power vans have departed from Italy on their way to China via the silk road. During the 13,000 kM trip the vans will drive themselves...

Questions - Reputation and Privilege

Our question and answer system while letting you do exactly what it says, is much more.  It is a dynamic user driven system, where our users not only ask...

Have some knowledge to share

If you have some expert knowledge or experience, why not consider sharing this with our community.  

By writing an electrical note, you will be educating our users and at the same time promoting your expertise within the engineering community.

To get started and understand our policy, you can read our How to Write an Electrical Note