Thursday, January 13, 2011

Electric motor

By Wikipedia

An electric motor converts electrical energy into mechanical energy. Most electric motors operate through interacting magnetic fields and current-carrying conductors to generate force, although a few use electrostatic forces. The reverse process, producing electrical energy from mechanical energy, is done by generators such as an alternator or a dynamo. Many types of electric motors can be run as generators, and vice versa. For example a starter/generator for a gas turbine, or traction motors used on vehicles, often perform both tasks.


Electric motors are found in applications as diverse as industrial fans, blowers and pumps, machine tools, household appliances, power tools, and disk drives. They may be powered by direct current (e.g., a battery powered portable device or motor vehicle), or by alternating current from a central electrical distribution grid. The smallest motors may be found in electric wristwatches. Medium-size motors of highly standardized dimensions and characteristics provide convenient mechanical power for industrial uses. The very largest electric motors are used for propulsion of large ships, and for such purposes as pipeline compressors, with ratings in the millions of watts. Electric motors may be classified by the source of electric power, by their internal construction, by their application, or by the type of motion they give.


Categorization of electric motors

The classic division of electric motors has been that of Alternating Current (AC) types vs Direct Current (DC) types. This is more a de facto convention, rather than a rigid distinction. For example, many classic DC motors run on AC power, these motors being referred to as universal motors.

Rated output power is also used to categorize motors, those of less than 746 Watts, for example, are often referred to as fractional horsepower motors (FHP) in reference to the old imperial measurement.

The ongoing trend toward electronic control further muddles the distinction, as modern drivers have moved the commutator out of the motor shell. For this new breed of motor, driver circuits are relied upon to generate sinusoidal AC drive currents, or some approximation thereof. The two best examples are: the brushless DC motor and the stepping motor, both being poly-phase AC motors requiring external electronic control, although historically, stepping motors (such as for maritime and naval gyrocompass repeaters) were driven from DC switched by contacts.

Considering all rotating (or linear) electric motors require synchronism between a moving magnetic field and a moving current sheet for average torque production, there is a clearer distinction between an asynchronous motor and synchronous types. An asynchronous motor requires slip between the moving magnetic field and a winding set to induce current in the winding set by mutual inductance; the most ubiquitous example being the common AC induction motor which must slip to generate torque. In the synchronous types, induction (or slip) is not a requisite for magnetic field or current production (e.g. permanent magnet motors, synchronous brush-less wound-rotor doubly-fed electric machine).


Comparison of motor types

Type Advantages Disadvantages Typical Application Typical Drive
AC polyphase induction squirrel-cage Low cost, long life,
high efficiency,
large ratings available (to 1 MW or more),
large number of standardized types
Starting inrush current can be high,
speed control requires variable frequency source
Pumps, fans, blowers, conveyors, compressors Poly-phase AC, variable frequency AC
Shaded-pole motor Low cost
Long life
Rotation slips from frequency
Low starting torque
Small ratings
low efficiency
Fans, appliances, record players Single phase AC
AC Induction
(split-phase capacitor)
High power
high starting torque
Rotation slips from frequency
Starting switch required
Appliances
Stationary Power Tools
Single phase AC
Universal motor High starting torque, compact, high speed Maintenance (brushes)
lifespan
Only small ratings economic
Drill, blender, vacuum cleaner, insulation blowers Single phase AC or DC
AC Synchronous Rotation in-sync with freq - hence no slip
More expensive Industrial motors
Clocks
Audio turntables
tape drives
Poly-phase AC
Stepper DC Precision positioning
High holding torque
High initial cost
Requires a controller
Positioning in printers and floppy drives DC
Brushless DC Long lifespan
low maintenance
High efficiency
High initial cost
Requires a controller
Hard drives
CD/DVD players
electric vehicles
DC
Brushed DC Simple speed control Maintenance (brushes)
Medium lifespan
Costly commutator and brushes
Steel mills
Paper making machines
Treadmill exercisers
automotive accessories
Direct DC or PWM
Pancake DC Compact design
Simple speed control
Medium cost
Medium lifespan
Office Equip
Fans/Pumps
Direct DC or PWM

Power

The power output of a rotary electric motor is:

P = \frac {rpm \times T} {5252}

Where P is in horsepower, rpm is the shaft speed in revolutions per minute and T is the torque in foot pounds.

And for a linear motor:

P = F \times v

Where P is the power in watts, and F is in Newtons and v is the speed in metres per second.

Efficiency

To calculate a motor's efficiency, the mechanical output power is divided by the electrical input power:

\eta = \frac{P_m}{P_e},

where η is energy conversion efficiency, Pe is electrical input power, and Pm is mechanical output power.

In simplest case Pe = VI, and Pm = Tω, where V is input voltage, I is input current, T is output torque, and ω is output angular velocity. It is possible to derive analytically the point of maximum efficiency. It is typically at less than 1/2 the stall torque.

[edit] Goodness factor

Professor Eric Laithwaite proposed a metric to determine the 'goodness' of an electric motor:

G = \frac {\omega} {resistance \times reluctance} = \frac {\omega \mu \sigma A_m A_e} {l_m l_e}

Where:

G is the goodness factor (factors above 1 are likely to be efficient)
Am,Ae are the cross sections of the magnetic and electric circuit
lm,le are the lengths of the magnetic and electric circuits
μ is the permeability of the core
ω is the angular frequency the motor is driven at

From this he showed that the most efficient motors are likely to be relatively large. However, the equation only directly relates to non permanent magnet motors.


Uses

Electric motors are used in many, if not most, modern machines. Obvious uses would be in rotating machines such as fans, turbines, drills, the wheels on electric cars, locomotives and conveyor belts. Also, in many vibrating or oscillating machines, an electric motor spins an irregular figure with more area on one side of the axle than the other, causing it to appear to be moving up and down.

Electric motors are also popular in robotics. They are used to turn the wheels of vehicular robots, and servo motors are used to turn arms and legs in humanoid robots. In flying robots, along with helicopters, a motor causes a propeller or wide, flat blades to spin and create lift force, allowing vertical motion.

Electric motors are replacing hydraulic cylinders in airplanes and military equipment.[24][25]

In industrial and manufacturing businesses, electric motors are used to turn saws and blades in cutting and slicing processes, and to spin gears and mixers (the latter very common in food manufacturing). Linear motors are often used to push products into containers horizontally.

Many kitchen appliances also use electric motors. Food processors and grinders spin blades to chop and break up foods. Blenders use electric motors to mix liquids, and microwave ovens use motors to turn the tray food sits on. Toaster ovens also use electric motors to turn a conveyor to move food over heating elements.

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