Introduction to NSF SPIRIT Workshop 2006 DC ELECTRIC MOTORS

Motors Everywhere! The fan over the stove and in the microwave ovenmicrowave oven The dispose-all under the sink The blender The can opener The washerwasher The electric screwdriverelectric screwdriver The vacuum cleaner and the Dustbuster mini- vacvacuum cleaner The electric toothbrush The hair dryerhair dryer Source:

More Motors... The electric razorelectric razor Power windows (a motor in each window)Power windows Power seats (up to seven motors per seat) Fans for the heater and the radiatorFans Windshield wipers Most toys that move have at least one motor (including Tickle-me-Elmo for its vibrations)toys Electric clocks The garage door opener Aquarium pumps TEKBOT In Short, EVERYTHING THAT MOVES uses some type of motor!

DC Electric Motors What They Are How They Work Their Components Motor Ratings –Torque, Speed, Voltage, Current, Gear Ratio Motor Control

DC Electric Motors Electric Motors convert electrical energy to mechanical motion DC Electric Motors use Direct Current (DC) sources as their source of electrical energy Mechanical motion results when electrical current in a wire produces a magnetic field which in turn, produces a force that moves the shaft of the motor in rotational motion.

How DC Electric Motors Work The motion of a DC motor is caused by the interaction of 2 magnetic fields housed inside the motor. A permanent magnetic field exists in the stationary portion of the motor, called the Stator. The opposing armature magnetic field exists on the rotating portion of the motor, called the Rotor. 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!

Electromagnet How do we change the magnetic poles of the armature magnetic field? –We can control the poles of the armature magnetic field because it is created using electromagnetic windings. –An electromagnet is a magnet created when electricity flows through a coil. It requires an DC power source (such as a Battery) to set up the magnet. –This contrasts to a permanent magnet (on household refrigerators) that exists all the time.

A Simple Electromagnet A Nail with a Coil of Wire Q - How do we reverse the poles of this electromagnet? A – By reversing polarity of the battery!

DC Motor Operation Principles In a motor, the permanent magnetic field of the Stator surrounds the Armature field like shown here:

Brushed DC Motor Components

(Permanent Magnet) Brushed DC Motor Components Stator is a Permanent Field Magnet Armature –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

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.

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. 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) shown.

TekBot Motor Ratings “GM8 - Gear Motor :1 Offset Shaft” 143:1 gear motor spins at 70RPM at 5V, drawing 670mA at stall generating 43 in*oz torque (free running at 57.6mA). Manufactured by Solarbotics

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 or 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 or 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.

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

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 – –Better suited for applications that require a large speed range –Extra electronics 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

For Further Information Basics of Design Engineering - DC Motors uiEdits/Content/bdeee3/bdeee3_5.aspx uiEdits/Content/bdeee3/bdeee3_5.aspx Overview of Motor Types Tutorial R How Stuff Works