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
and

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



HTML Symbol Entities

HTML supports a variety of entity symbols which can be entered using either numbers or an entity name.  The number or name is preceded by the ‘&’ sign...

Lead Acid Batteries

Lead acid batteries are cost effect and reliable, making them suitable for many applications.This note examines topics of interest associated with the...

Dielectric loss in cables

Dielectrics (insulating materials for example) when subjected to a varying electric field, will have some energy loss.   The varying electric field causes...

Fault Calculations - Introduction

Fault calculations are one of the most common types of calculation carried out during the design and analysis of electrical systems. These calculations...

Laplace Transform

Laplace transforms and their inverse are a mathematical technique which allows us to solve differential equations, by primarily using algebraic methods...

8 Motor parts and common faults

Straight forward list of some common motor faults.  If I have missed any other common faults, please take a bit of time to add them in as a comment below...

Thomas Edison

American inventor Thomas Alva Edison was born in Milan, Ohio on February 11, 1847. He was the youngest of seven children and received little formal schooling...

GE's Shingijutsu Factory

GE's latest thinking on product manufacturing is he Shingijutsu philosophy or Lean production system. They have started applying this at the Louisville...

Photovoltaic (PV) - Utility Power Grid Interface

Photovoltaic (PV) systems are typically more efficient when connected in parallel with a main power gird. During periods when the PV system generates energy...

IEC Reference Designations

The IEC publishes a series of documents and rules governing the preparation of documents, drawings and the referencing of equipment.   Depending on country...

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