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



Medium Voltage Switchgear Room Design Guide

Many medium voltage (MV) indoor switchgear rooms  exist worldwide. The complexity of these rooms varies considerably depending on location, function and...

Fault Calculation - Symmetrical Components

For unbalance conditions the calculation of fault currents is more complex. One method of dealing with this is symmetrical components. Using symmetrical...

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...

Inductance

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...

Restricted Earth Fault Protection

The windings of many medium and small sized transformers are protected by restricted earth fault (REF) systems. The illustration shows the principal of...

Maxwell's Equations - Introduction

Maxwell's Equations are a set of fundamental relationships, which govern how electric and magnetic fields interact. The equations explain how these fields...

Lightning Risk Assessment (IEC 62305)

IEC 62305 'Protection against lightning' requires a risk assessment be carried out to determine the characteristics of any lightning protection system...

Windows Live Writer and myElectrical

When making adding a Note to our site we have a great online WYSIWYG editor and things are pretty simple.  However, if you prefer you can write, manage...

What is a rectifier transformer?

I've recently come across this question a couple times browsing the internet. Decided to give a quick answer here. A rectifier transformer is a transformer...

How to measure power supply quality

If your are ever called out to troubleshoot something on your electrical system, one of the first things consider is the supply voltage. You want to ensure...

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