NSF S.P.I.R.I.T. Workshop 2007 DC ELECTRIC MOTORS.

NSF S.P.I.R.I.T. Workshop2007

DC ELECTRIC MOTORSDC ELECTRIC MOTORS

Motors Everywhere!The fan over the stove and in the

microwave ovenThe dispose-all under the sink The blender The can opener The washer The electric screwdriver The vacuum cleaner and the Dustbuster

mini-vac The electric toothbrush The hair dryer

Source: http://electronics.howstuffworks.com/motor2.htm

More Motors . . .The electric razor Power windows (a motor in each window) Power seats (up to seven motors per seat) Fans for the heater and the radiator Windshield wipersMost toys that move have at least one motor

(including Tickle-me-Elmo for its vibrations) Electric clocks The garage door opener Aquarium pumps Your TEKBOT In Short, EVERYTHING THAT MOVES

uses some type of motor!

Important ConceptsHow Motors WorkThe Components of a DC Electric Motor

What’s inside the motor box?The parts of the motor and what they do!

Motor Control How to change directions

Concepts of Speed, Torque, Gear RatioPractical considerations

Noise, slippage, limits of operation: speed, torque

DC Electric MotorsElectric Motors or Motors convert electrical energy

to mechanical motion

Motors are powered by a source of electricity – either AC or DC.

DC Electric Motors use Direct Current (DC) sources of electricity: Batteries DC Power supply

Principle of How Motors Work: Electrical current flowing in a loop of wire will

produce a magnetic field across the loop. When this loop is surrounded by the field of another

magnet, the loop will turn, producing a force (called torque) that results in mechanical motion.

Motor BasicsMotors are powered by electricity, but rely

on principles of magnetism to produce mechanical motion.

Inside a motor we find: Permanent magnets, Electro-magnets, Or a combination of the two.

MagnetsA magnet is an object that possesses a magnetic

field, characterized by a North and South pole pair.

A permanent magnet (such as this bar magnet) stays magnetized for a long time.

An electromagnet is a magnet that is created when electricity flows through a coil of wire. It requires a power source (such as a battery) to set up a magnetic field.

A Simple ElectromagnetA Nail with a Coil of Wire

Q – How do we set up a magnet?A – The battery feeds current through the

coil of wire. Current in the coil of wire produces a magnetic field (as long as the battery is connected).

A Simple ElectromagnetA Nail with a Coil of Wire

Q - How do we reverse the poles of this electromagnet?

A – By reversing the polarity of the battery!

+ -

S N

The Electromagnet in a Stationary Magnetic Field

If we surround the electromagnet with a stationary magnetic field, the poles of the electromagnet will attempt to line up with the poles of the stationary magnet.

The rotating motion is transmitted to the shaft, providing useful mechanical work. This is how DC motors work!

OPPOSITE POLES ATTRACT!

DC Motor Operation PrinciplesOnce the poles align, the nail (and shaft) stops rotating. How do we make the rotation continue?

By switching the poles of the electromagnet. When they line up again, switch the poles the other way, and so on.

This way, the shaft will rotate in one direction continuously!

Motor TerminologyThus, the motion of a DC motor is caused by the

interaction of two magnetic fields housed inside the motor.

These two magnetic fields can be described by where they are located inside the motor.

The stationary parts of the motor make up the STATOR.The Stator Stays Put!

The rotating parts of the motor constitute the ROTOR.The Rotor Rotates!

The Stator houses the Permanent Field Magnet.The electronically-controlled magnet, called the

Armature, resides on the Rotor.

Motor TerminologyThe magnetic poles of the Armature field will

attempt to line up with the opposite magnetic poles on the Stator. (Opposites ATTRACT).

Once opposite poles align, the movement of the motor would stop.

However, to ensure continuous movement of the motor, the poles of the Armature field are electronically reversed as it reaches this point, so it keeps turning to keep the motor shaft moving along in the same direction!

This electronic switching of the Armature poles is accomplished using Brushes and Commutators.

Brushed DC Motor Components

Brushed DC Motor ComponentDescriptionsThe Stator is a Permanent Field MagnetThe Armature

Is an electromagnet comprised of coils wound around 2 or more poles of the metal rotor core

CommutatorAttached to the rotor and turns with the rotor to

mechanically switch direction of current going to the armature coils

BrushesStationary attached to battery leads. These metal

brushes touch the Commutator terminals as it rotates delivering electric current to the commutator terminals.

Axle or ShaftMoves in rotational motion

Brushed DC Motor Components

How the Commutator WorksAs the rotor turns, the commutator

terminals also turn and continuously reverse polarity of the current it gets from the stationary brushes attached to the battery.

Controlling Motor DirectionTo change the direction of rotation:

Simply switch the polarity of the battery leads going to the motor (that is, switch the + and – battery leads)

+-

-+

CW CCW

Direction of Rotation

Controlling Motor DirectionIn the TekBot, this switching is done using an

“H-bridge” motor control circuit.A signal is sent from your hand-held tether to

the TekBot when you tell each wheel to go forward or reverse.

This signal goes to the H-bridge circuitry on the TekBot which sends the correct polarity to the battery leads wired to the TekBot motors to accomplish the desired rotation.

Inside a Toy Motor(Similar to TekBot Motor)

Toy DC Motor, cont.End Views of Motor

AxleBattery Leads

Axle will turn if connect battery leads to a 9V battery

Reverse battery leads and axle will turn the Opposite direction!

The white nylon cap on the motor can be removed to reveal…

A View of the BrushesInside the Nylon

cap are the Brushes

Brushes can be made of various types of metal.

Their purpose is to transfer power to the commutator as it spins.

Inside the Motor, cont.The Axle is the rotating part

of the motor that holds the armature and commutator.

This armature is comprised of 3 electromagnets. (3-Pole DC Motor)

Each electromagnet is a set of stacked metal plates with thin copper wire wound around each.

The two ends of each coil wire is terminated and wired to a contact on the commutator.

Thus, there are 3 commutator contacts in all.

Inside the Motor, cont.The final piece is the

stator, a permanent field magnet.

It is formed by the motor enclosure and two curved permanent magnets (2 Pole: 1 North, 1 South) shown.

Torque ConceptsThe movement of the motor comes from

the interaction of magnetic fields. A magnetic force that is perpendicular to

the magnetic field and the current in the coils delivers a rotational force - torque - that turns the axle of the motor.

Intuitively, the higher the torque the greater the force of rotational movement.

The higher the motor input current, the greater the torque on the output.

Speed ConceptsSpeed of rotation of the output shaft is

measured in RPM – “Revolutions Per Minute”.The speed of rotation is directly proportional

to the voltage applied to the armature windings.This is a linear relationship up to the motor’s max

speed.These motors produce high speed, low torque

axle rotation, which is improved by a gear reduction to reduce speed and increase torque on the output shaft.

TekBot Motor Ratings“GM8 - Gear Motor 8 - 143:1 Offset Shaft”143:1 gear motor (“gear ratio”)spins at 70RPM at 5V, (maximum speed)drawing 670mA at stall (“stall current”)generating 43 in*oz torque (free running

at 57.6mA).

Manufactured by Solarboticshttp://www.solarbotics.com/

Characteristics of Brushed DC MotorsVery commonly used in everything from toys

to toothbrushes, electric toys to mobile robots.

Easy to control using simple control circuitry

Small, Cheap

Generally not used in industrial applications

DC Motor VarietiesBrush-type DC Motor

Used for RPM under 5,000 Simpliest to control Very common choice for hobby use

Brushless DC Motor (a.k.a AC Synchronous Motor)– Better suited for applications that require a large range &

precise speed Extra electronics for control and position sensors are required

Wound-field DC Motor Common in industrial applications Allows for wide range of precision speed control & torque

control Permanent Magnet DC Motor

The field magnet is a permanent magnet and does not need to be activated by a current

Intermittent vs. Continuous Duty Continuous Duty motors can operate without an off period.

Electric motor power rating hp = (torque X rpm)/5,250

References / For Further InfoBasics of Design Engineering - DC Motors

http://www.electricmotors.machinedesign.com/guiEdits/Content/bdeee3/bdeee3_5.aspx

Overview of Motor Types Tutorialhttp://www.oddparts.com/acsi/motortut.htm#DC_MOTOR

How Stuff Works - Motors http://electronics.howstuffworks.com/motor4.htmMagnets Definedhttp://en.wikipedia.org/wiki/Magnet

Presentation Created by: Alisa N. Gilmore, P.E., University of Nebraska-Lincoln, NSF SPIRIT, July 2007

3 Pole DC Motor

http://www.solarbotics.net/starting/200111_dcmotor/200111_dcmotor2.html