The key objective of this project is to introduce and prepare a working model of eddy current brakes. The problems faced in conventional frictional brakes; i.e. fading, overheating, very short life span etc. precedes the motivation of the work, presented in the report, followed by the statement of problem and objectives.
A brake is a device which inhibits motion. Most commonly brakes use friction to convert kinetic energy into heat, though other methods of energy conversion may be employed. For example regenerative braking converts much of the energy to electrical energy, which may be stored for later use. Other methods convert kinetic energy into potential energy in such stored forms as pressurized air or pressurized oil. Still other braking methods even transform kinetic energy into different forms, for example by transferring the energy to a rotating flywheel.
Brakes are generally applied to rotating axles or wheels, but may also take other forms such as the surface of a moving fluid (flaps deployed into water or air). Some vehicles use a combination of braking mechanisms, such as drag racing cars with both wheel brakes and a parachute, or airplanes with both wheel brakes and drag flaps raised into the air during landing.
Since kinetic energy increases quadratically with velocity (K = mv2 / 2), an object traveling at 10 kilometers per second has 100 times as much energy as one traveling at 1 kilometer per second, and consequently the theoretical braking distance, when braking at the traction limit, is 100 times as long. In practice, fast vehicles usually have significant air drag, and energy lost to air drag rises quickly with speed.
Friction brakes on automobiles store braking heat in the drum brake or disc brake while braking then conduct it to the air gradually. When traveling downhill some vehicles can use their engines to brake.
When the brake pedal is pushed the caliper containing piston pushes the pad towards the brake disc which slows the wheel down. On the brake drum it is similar as the cylinder pushes the brake shoes towards the drum which also slows the wheel down.
General Principle of Brake System
The principle of braking in road vehicles involves the conversion of kinetic energy into thermal energy (heat). When stepping on the brakes, the driver commands a stopping force several times as powerful as the force that spots the car in motion and dissipates the associated kinetic energy as heat. Brakes must be able to arrest the speed of a vehicle in short periods of time regardless how fast the speed is. As a result, the brakes are required to have the ability to generating high torque and absorbing energy at extremely high rates for short periods of time. Brakes may be applied for a prolonged periods of time in some applications such as a heavy vehicle descending a long gradient at high speed. Brakes have to have the mechanism to keep the heat absorption capability for prolonged periods of time.
Conventional Friction Brake
The conventional friction brake system is composed of the following basic components:
The “master cylinder” which is located under the hood is directly connected to the brake pedal, and converts the drivers’ foot pressure into hydraulic pressure. Steel “brake hoses” connect the master cylinder to the “slave cylinders” located at each wheel. Brake fluid, specially designed to work in extreme temperature conditions, fills the system. “Shoes” or “pads” are pushed by the slave cylinders to contact the “drums” or “rotors,” thus causing drag, which slows the car. Two major kinds of friction brakes are disc brakes and drum brakes.
Disc brakes use a clamping action to produce friction between the “rotors” and the “pads” mounted in the “caliper” attached to the suspension members. Disc brakes work using the same basic principle as the brakes on a bicycle: as the caliper pinches the wheel with pads on both sides, it slows the vehicle.
Drum brakes consist of a heavy flat-topped cylinder, which is sandwiched between the wheel rim and the wheel hub. The inside surface of the drum is acted upon by the linings of the brake shoes.
When the brakes are applied, the brake shoes are forced into contact with the inside surface of the brake drum to slow the rotation of the wheels.
Air brakes use standard hydraulic brake system components such as braking lines, wheel cylinders and a slave cylinder similar to a master cylinder to transmit the air-pressure-produced braking energy to the wheel brakes. Air brakes are used frequently when greater braking capacity is required.
Brake Fading Effect
The conventional friction brake can absorb and convert enormous energy values (25h.p. Without self-destruction for an 5-axle truck, Reverdin 1974), but only if the temperature rise of the friction contact materials is controlled. This high energy conversion therefore demands an appropriate rate of heat dissipation if a reasonable temperature and performance stability are to be maintained. Unfortunately, design, construction, and location features all severely limit the heat dissipation function of the friction brake to short and intermittent periods of application. This could lead to a ‘brake fade’ problem (reduction of the coefficient of friction, less friction force generated) due to the high temperature caused by heavy brake demands. The main reasons why conventional friction brakes fail to dissipate heat rapidly are as follows:
- Poor ventilation due to encapsulation in the road wheels,
- Diameter restriction due to tire dimensions,
- Problems of drum distortion at widely varying temperatures.
It is common for friction-brake drums to exceed 500 °C surface temperatures when subject to heavy braking demands, and at temperatures of this order, a reduction in the coefficient of friction (‘brake fade’) suddenly occurs (Grimm, 1985). The potential hazard of tire deterioration and bursts is perhaps also serious due to the close proximity of overheated brake drums to the inner diameter of the tire.
Applications of Eddy Currents
Working of induction furnace is based on the heating effects of Eddy Currents.
Voltage is induced when a magnet moves towards or away from a coil, inducing a current in the coil. Faster the magnet’s motion, the greater the induced current.
FIG 3.3 INDUCTION OF EDDY CURRENT
The induced voltage in a coil is proportional to the product of the number of loops and rate at which the magnetic field changes within the loops.
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