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

Motors Everywhere! The fan over the stove and in the microwave oven The 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 wipers Most 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 Concepts How Motors Work The 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 Ratio Practical considerations Noise, slippage, limits of operation: speed, torque

DC Electric Motors Electric 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 Basics Motors 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.

Magnets A 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 Electromagnet A 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 Electromagnet A 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 Principles Once 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 Terminology Thus, 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 Terminology The 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 Component Descriptions The Stator is a Permanent Field Magnet The Armature Is an electromagnet comprised of coils wound around 2 or more poles of the metal rotor core Commutator Attached to the rotor and turns with the rotor to mechanically switch direction of current going to the armature coils Brushes Stationary attached to battery leads. These metal brushes touch the Commutator terminals as it rotates delivering electric current to the commutator terminals. Axle or Shaft Moves in rotational motion

Brushed DC Motor Components

How the Commutator Works As 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 Direction To change the direction of rotation: Simply switch the polarity of the battery leads going to the motor (that is, switch the + and – battery leads) Direction of Rotation CW CCW + - - +

Controlling Motor Direction In 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 Axle Battery 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 Brushes Inside 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 Concepts The 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 Concepts Speed 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 Solarbotics http://www.solarbotics.com/

Characteristics of Brushed DC Motors Very 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 Varieties Brush-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 Info Basics of Design Engineering - DC Motors http://www.electricmotors.machinedesign.com/guiEdits/Content/bdeee3/bdeee3_5.aspx Overview of Motor Types Tutorial http://www.oddparts.com/acsi/motortut.htm#DC_MOTOR How Stuff Works - Motors http://electronics.howstuffworks.com/motor4.htm Magnets Defined http://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