Ohm’s Law

+

-

RE

I

E = I * RI = E / RR = E / I

E = VoltageI = CurrentR = Resistance

Bar Magnet

N S

Lines of Flux

Flux Density = # of Flux Lines / per unit area

Magnetic Poles

S NS N

Unlike Poles Attract

Like Poles Repel

N SS N

Current Carrying Conductor

Lines of Flux

Conductor

Current Flow

Electromagnetic Coil

Lines of Flux

Flux DensityDependant on:

• Current• # of Coils• Core Material

Electro-magnetic Coils

Direction of Current determines Magnetic Polarity

DC Motor Rotation Considerations

• Speed

• Torque

Speed

Rpm - revolutions per minute

Rotation of the shaft

Torque Torque is the product ofForce x Lever Arm Length (Radius)

Clockwise and Counter-Clockwise efforts are distinguished by differences in sign, + or -

DC Motor

• 1) Armature

• - coils of wire on the shaft

• 2) Field (Shunt Field)

• - coils of wire built into stationary frame

A DC motor consists of two electromagnetic fields

Force Effect of Magnetic Fields

Cancellation

Reinforcement

Mechanical Effects of Magnetic Fields

Rotation is a function of two fields pushing or pulling each other.

Principle of a DC Motor• A DC motor has two independent

electromagnetic fields– Controlled independent of each other

• Either field can influence the performance of the motor– Speed (rpm)– Torque (ft lb)

Motor Armature, Commutator and Field Wiring Arrangement

F2Brush

F1

Brush

Main FieldMain Field

Commutator Bars

A1

A2

ArmatureCoils

Motor General Equation

EETT = K = KMMN + IN + IAARRAA

ET = Armature (Terminal) VoltageKM = Motor Constant = Motor Field Flux DensityN = Motor SpeedIA = Armature CurrentRA = Armature Resistance

DC Motor Speed

N = EN = ETT

KKMM

Motor Speed Varies by:

ET = Armature (Terminal) VoltageKM = Motor Constant = Motor Field Flux DensityN = Motor Speed

DC Motor Torque

T = KT = KTTIIAA

DC Motor Torque varies by:

T = Motor TorqueKT = Motor Constant

(# poles, armature conductors) = Motor Field Flux DensityIA = Armature Current

DC Motor Horsepower

DC Motor Horsepower Can be Determined By:

HP = T x N 5252

HP = Motor HorsepowerT = Motor TorqueN = Speed

Speed Power CurveArmature Voltage Contro l

Constant F ie ld Current

F ie ld Current Contro l

Constant ArmatureVoltage

Constant Power

Speed (% of Base Speed)

Po

we

r (%

of

Ra

ted

)100

100

Speed Power CurveArmature Voltage Contro l

Constant F ie ld Current

F ie ld Current Contro l

Constant ArmatureVoltage

Constant Power

Speed (% of Base Speed)

Po

we

r (%

of

Ra

ted

)100

100

Types of DC Motors• Shunt Wound

– Straight Shunt

• Compound Wound– Stabilized Shunt

• Permanent Magnet

• Series Wound

Most DC Motors are:

Note: Straight Shunt must be used with reversing/regen

DC Motor Review• Speed is primarily determined by

Armature Voltage

• Torque is determined by

• Armature Current

DC Motor Review• Speed is primarily determined by

Armature Voltage

• Torque is determined by

• Armature Current

One possibility…

• Connect motor directly to the I/O pins

Two directions:

• PD2: 1; PD3: 0

• PD2: 0; PD3: 1

DC Motor Control

What is wrong with this implementation?

• Our I/O pins can source/sink at most 20 mA of current

• This is not very much when it comes to motors…

How do we fix this?

Simple H -B ridge

+ 5 V

What happens with these inputs? 1 0

Simple H -B ridge

Pulse Width Modulation (PWM)

Time On Time On Time On

Total Cycle Time Total Cycle Time Total Cycle Time

Time OnTotal Cycle Time

Duty Cycle =

When we wish to control the speed of a motor we adjust its voltage. This being the age of digital electronics we have found a very fast and efficient way to vary a motor’s voltage. Using powerful transistors (MOSFETS), we switch the voltage supplied to the motor off and then back on very fast (sometimes millions of times a second). The amount of time the voltage is switched on compared to the amount of time it is switched off is also controlled. This is referred to as Pulse Width Modulation (PWM). The most important factor of the PWM signal is the duty cycle.

L293 H-bridge chip