Brushed DC Motor Basics

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2006 Microchip Technology Incorporated. All Rights Reserved. WebSeminar Title Slide 1

Brushed DC Motor Basics

Hello, My name is John Mouton. I am an Applications Engineer at MicrochipTechnology in the Security, Microcontroller and Technology Division. Thank you fordownloading Brushed DC Motor Basics. This is Part 1 in a 4 part series of web-seminars related to Controlling a Brushed DC Motor using a Microcontroller. Thisweb-seminar will focus on the basics of Brushed DC motor construction and somecommon characteristics. So lets begin!


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Agenda

Topics discussed in this Web-Seminar:- Basics of Brushed DC Motors- Four Types of Brushed DC Motors

Principles of Operation

During this web-seminar I will discuss the main components of a brushed DC motor and how these components work together to function as a motor. We will alsoexplore four types of brushed DC motors and compare common characteristicsbetween them.


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Motor Basics

So lets start off with the brushed DC motor basics.


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Motor Components

1. Stator 2. Rotor / Armature3. Brushes4. Commutator

A brushed DC motor is made up of 4 basic components; the stator, the rotor ( orarmature ), brushes, and commutator.


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Stator

Generates a stationary magnetic field thatsurrounds the rotor, this field is generatedby either:

N S

permanent magnetselectromagnetic windings

First let us look at the stator. As the name implies, the stator generates a stationarymagnetic field that surrounds the rotor and this magnetic field is generated by eitherpermanent magnets or electromagnetic windings.


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Rotor

- Made up of one or more windings- Commutation

N S

Windings

Next is the rotor, also known as the armature. The rotor is made up of one or morewindings.

When these windings are energized they produce a magnetic field. The magneticpoles of this rotor field will be attracted to the opposite poles generated by thestator, causing the rotor to turn. As the motor turns, the windings are constantlybeing energized in a different sequence so that the magnetic poles generated by therotor do not overrun the poles generated in the stator. This switching of the field inthe rotor windings is called Commutation.


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Brushes and Commutator

Commutator

Brushes

-+

Brushed DC motors do not require a controller to switch current in the motorwindings. Instead, it uses a mechanical commutation of the windings. A coppersleeve ( commutator ), resides on the axle of the rotor. As the motor turns, carbonbrushes slide over the commutator, coming in contact with different segments of thecommutator. The segments are attached to different rotor windings, therefore, adynamic magnetic field is generated inside the motor when a voltage is appliedacross the brushes of the motor. The brushes and commutator are the parts of abrushed DC motor that are most prone to wear.


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-+

So, as the rotor rotates inside the stator, the brushes rub the different segments ofthe commutator supplying a charge to that segment and its corresponding winding.


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-+

As the bushes pass over the commutator gaps the supplied electrical charge willswitch commutator segments.


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-+

Thus, switching the electrical polarity of the rotor windings. This will create anattraction of the different polarities and keep the rotor rotating within the stator field.


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-+


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Brushed DC Motor Overview.Stator Magnets

Windings

Brushes

Terminals

Commutator

Rotor

+

Finally, here is a complete overview of a brushed DC motor with all componentslabeled.


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Types of Brush DC Motors

Now let us take a look at the different types of brushed DC motors.


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Four Types of BDCMs

1. Permanent Magnet2. Shunt - wound3. Series - wound4. Compound - wound

Shunt + Series

There are four types of brushed DC motors.

The first type, is the Permanent Magnet Brush DC Motor. Second, the shunt-woundbrushed DC motor. Third is the series-wound DC motor and fourth is the

compound-wound brushed DC motor which is a combination of both the shunt andseries wound brushed DC motors.


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Permanent Magnet(PMDC)

- Most common- Used in fractional horsepower applications

Examples: Toys, RC app., Slot cars, Appliances, etc- More cost effective than wound stators- Torque, limited b/c of stator field

Good low end ( low speed ) torque Limited high end ( high speed ) torque

- Responds very quickly to voltage changes due toconstant

stator field ( good speed control )- Drawback, Magnets lose their magnetic properties over

time

DCVoltageSupply Rotor

Brushes

N S

PermanentMagnetic Poles

Permanent magnet brushed DC motors are the most common. These motors usepermanent magnets to produce the stator field. They are generally used inapplications needing fractional horsepower such as; Toys, Radio Control hobbyapplications, electric slot cars, appliances, etc. It is more cost effective to usepermanent magnets than wound stators because they are cheaper to manufacture.The torque from a permanent magnet brushed DC motor is limited by its stator field,which gives it good low end ( low speed ) torque and a limited high end ( high speed) torque. The permanent magnet brushed DC motor responds very quickly tochanges in voltage. This is due to its constant stator field, thus giving it good speedcontrol capabilities. But, they do have a drawback. Permanent magnet brushed DCmotors will loose their magnetic properties over time, thus degrading its stator fieldand causing the motors performance to decline.


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Shunt-Wound(SHWDC)

DCVoltageSupply

ShuntField

Rotor Brushes

As V Supply , ITot , speed , stator/rotor field , thusTorque

As motor load , IRotor , IShunt , stator field , rotor field ,speed , thus Torque

IRotor ITot

IRotor IShunt

HeavyLoad

ITot

= IShunt

+ IRotor

IShunt

Shunt-wound brushed DC motors have the field coil in parallel ( shunt) with the rotor. The current in the field coil and in the rotor are independent of oneanother, thus, the total current of the motor is equal to the sum of the shunt current (or stator current ) and the rotor current. So, during normal operation, as thesupplied voltage is increased the total current of the motor will increase causing thestator and rotor fields to increase. As total current increases motor speed willincrease, thus motor torque will decrease.

However, once you put a load on the motor the rotor current willincrease causing the rotor field to increase. If the rotor current increases then theshunt current will decrease causing the stator field to decrease. This will cause themotor speed to decrease, thus the motor torque will increase.


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Shunt-Wound(SHWDC)

Decreasing torque at high speed At low speed, higher more consistent torque Excellent speed control Typically used in Industrial and automotive applications No loss of magnetism over time as with PMDC motors More expensive than PMDC motors Motor Runaway, if the shunt current goes to zero

Shunt-wound brushed DC motors have the performance characteristics ofdecreasing torque at high speeds and a high but more consistent torque at lowspeeds. The current in the field coil and in the rotor are independent of one another,thus, the total current of the motor is equal to the sum of the shunt current ( or statorcurrent ) and the rotor current. As a result, these motors have excellent speedcontrol characteristics. Shunt-wound brushed DC motors are typically used inapplications that require 5 or more HP such as industrial and automotiveapplications. As compared to permanent magnet brushed DC motors, shunt woundbrushed DC motors have no loss of magnetism and are more robust. Somedrawbacks are that shunt wound brushed DC motors are more expensive thanpermanent magnet brushed DC motors and have the potential of motor runaway ifthe shunt current decreases to zero. This is a very dangerous condition that canlead the motor to literally brake apart.


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Series-Wound(SWDC)

DCVoltageSupply

SeriesField

Rotor Brushes

As V Supply , IMotor , Stator/Rotor field , speed , thus torque

As motor load , IMotor , Stator/Rotor field , speed , thustorque

IMotor

HeavyLoad

IMotor = ISeries + IRotor

Series-wound brushed DC motors have the field coil in series with the rotor, thus,their field currents become identical. So, the motor current is equal to the sum ofthe stator ( series ) current and the rotor current. Thus, during normal operationwithout a load, as the supply voltage is increased the motor current increases. Thiscauses the stator and rotor fields to increase and the motor speed increases with adecrease in torque. But, as a load is placed on the motor, motor current willincrease also causing the stator and rotor fields to increase. Now the motor speedwill decrease and torque will increase to overcome the load.


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Great slow speed torque ( low end )- But, as load is removed, speed increases sharply

Ideally suited for heavy loads, examples: cranes andwinches

Poor high speed torque ( high end ) More expensive than PMDC Poor speed control due to the series stator field Motor Runaway, if the series field is shorted

Series-Wound(SWDC)

Series wound brushed DC motors have great slow speed ( low end ) torque, but ifthe load is removed, speed will sharply increase. This makes these motors ideallysuited for high-torque applications, such as cranes and winches because thecurrent in both the stator and rotor increases under load. Some drawbacks are thatthey have poor high speed ( high end ) torque characteristics ,thus they must havea load connected to prevent damage under high speed conditions. They are moreexpensive than permanent magnet brushed DC motors, they do not have precisespeed control characteristics like permanent magnet brushed DC and shunt woundbrushed DC motors, and they can potentially go into motor runaway if the seriesstator field is shorted. Again, this is a very dangerous condition that can lead themotor to literally brake apart.


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Compound-Wound(CWDC)

As motor load , IShunt , shunt field , I2 , Series field/Rotor

field , speed , thus torque

ShuntField

Rotor

SeriesField I2 = ISeries + IRotor I2

IShunt = I1

ITot = I1 + I2ITot

HeavyLoad

DCVoltageSupply

Brushes

A compound wound brushed DC motor is a combination of both the shunt woundand series wound brushed DC motors. As you can see, the total current in themotor is the sum of the currents through the shunt field and the series field coils.The current through the series field coil is the same as the current through the rotorfield. So, as a load is placed on the motor, the current through the rotor and seriesfields will increase. Thus, the current through the shunt field will decrease, thespeed of the motor will decrease, and the motor torque will increase.


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Compound-Wound(CWDC)

Has the performance properties of both SHWDC andSWDC motors

High torque at low speed w/ heavy loads, SWDC motors Great speed control, SHWDC motors Great for Industrial and automotive applications,

example: generators Motor Runaway, less likely b/c shunt current must go to

zeroand series field must short

More expensive than PMDC, SHWDC, and SWDC

The compound wound brushed DC motor has the performance characteristics ofboth the shunt wound and the series wound brushed DC motors. It has the hightorque at low speeds with heavy loads as seen with series wound brushed DCmotors and has great speed control as with the shunt wound brushed DC motors.Compound wound brushed DC motors are great for industrial and automotiveapplications such as generators. With compound wound brushed DC motors theproblem of motor runaway is less likely because the shunt current must go to zeroand the series field must be shorted, thus the chances of both of these conditionsoccurring at the same time or very small. However, a compound wound brushedDC motor is more expensive than a permanent magnet, shunt wound, or a serieswound brushed DC motors.


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Performance Comparison

Speed

T o r q u e

Permanent Magnet

Shunt Wound

Compound Wound

Series Wound

S = T

As we compare the performance characteristics of all four types of brushed DCmotors we see that they all show the same basic characteristic, as speed increasestorque decreases. For a permanent magnet brushed DC motor this is a very linearprocess. With the shunt wound brushed DC motor you can see that there is a highbut more consistent torque at low speeds which gives it excellent speed controlcharacteristics. The series wound brushed DC motor graph shows its great lowspeed torque and a very drastic decrease in torque with higher speed, thus if theload was removed, speed would increase very sharply. Finally, with a compoundwound brushed DC motor we see a great combination of both the shunt and serieswound brushed DC motors. The graph shows good low speed torque but with amore linear or gradual decrease of torque as speed increases, thus showing that itis well suited for lifting heavy loads with great speed control.


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Brushed DC Motor TypeComparison

GreatPoor ExcellentExcellentSpeed Control

Low chanceHighchance

High chanceNo chanceSafety( Motor Runaway )

Best at low speedGreat atlow speed

Good,consistent at

low speed.

Good at low speedLess at high speed

Torque vs. Speed

NoneNoneNoneWorstLoss of Magnetism

HighModerateModerateLowCOST

CompoundSeriesShuntPermanentMagnetCharacteristic

Here is a table summarizing the various aspects of each motor type that may makeit easier for you to decide which motor suits your application needs.


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Comparisonwith Other Motor Types

Direct (PWM)

Treadmill exercisersAutomotive starters

Toys and someappliances

High maintenance(brushes)Limited lifespan

Low initial cost

Simple speed control

Brushed DC electric

motor

Multiphase DCHard drivesCD/DVD playersElectric vehicles

High initial costRequires a controller

Long lifespanlow maintenanceHigh efficiency

Brushless DC electricmotor

Multiphase DCPositioning in

printers and floppydrives

Slow speed Requires a controller

Precision positioningHigh holding torque

Stepper DC

Uni/Poly-phase AC

FansAppliancesClocksAudio turntablestape drives

Rotation slips fromfrequency

Low starting torqueMore expensive

Long lifeHigh power high starting torqueRotation in-sync withfreq.long-life (alternator)

AC Induction

Typical DriveTypical ApplicationDisadvantagesAdvantagesType

Now, see how brushed DC motors compare to other types of motors, such asbrushless DC motors, stepper DC motors, and AC induction motors.


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Summary

Basics of Brush DC motors.

4 Types and how they operate. Permanent Magnet Shunt wound Series wound Compound wound

In summary, we covered the basic components of a brushed DC motor, the fourtypes of brushed DC motors ( Permanent magnet, shunt wound, series wound, andcompound wound ) and how they operate.


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Additional Reading

AN905 Brushed DC Motor Fundamentals

For an additional source of information, refer to applications note # AN905 BrushedDC Motor Fundamentals. available at Microchip.com.


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Thank You

Again my name is John Mouton and I thank you for downloading this web seminar.

Documents

Brushed DC Motor Basics