03-12-2012, 01:14 PM
General Principle of Electromagnetic Brakes
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Introduction
Electromagnetic brakes have been used as supplementary retardation
equipment in addition to the regular friction brakes on heavy vehicles. We
outline the general principles of regular brakes and several alternative
retardation techniques in this section. The working principle and
characteristics of electromagnetic brakes are then highlighted.
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
puts the car in motion and dissipates the associated kinetic energy as heat.
Brakes must be able to arrest the speed of a vehicle in a 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 “rotor”
and the “pads” mounted in the “caliper” attached to the suspension members
(see Figure 2.1). 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 (Limpert 1992).
Installation Location
Electromagnetic brakes work in a relatively cool condition and satisfy all
the energy requirements of braking at high speeds, completely without the use
of friction. Due to its specific installation location (transmission line of rigid
vehicles), electromagnetic brakes have better heat dissipation capability to
avoid problems that friction brakes face as we mentioned before. Typically,
electromagnetic brakes have been mounted in the transmission line of
vehicles, as shown in figure 2.2. The propeller shaft is divided and fitted with a
sliding universal joint and is connected to the coupling flange on the brake.
Working Principle
The working principle of the electric retarder is based on the creation of
eddy currents within a metal disc rotating between two electromagnets, which
sets up a force opposing the rotation of the disc (see figure 2.3). If the
electromagnet is not energized, the rotation of the disc is free and accelerates
uniformly under the action of the weight to which its shaft is connected. When
the electromagnet is energized, the rotation of the disc is retarded and the
energy absorbed appears as heating of the disc. If the current exciting the
electromagnet is varied by a rheostat, the braking torque varies in direct
proportion to the value of the current. It was the Frenchman Raoul Sarazin who
made the first vehicle application of eddy current brakes. The development of
this invention began when the French company Telma, associated with Raoul
Sarazin, developed and marketed several generations of electric brakes based
on the functioning principles described above (Reverdin, 1974).
Electric Control System
The electric wiring diagram of the installation is shown in figure 2.4. The
energization of the retarder is operated by a hand control mounted on the
steering column of the vehicle. This control has five positions: the first is ‘off’,
and the four remaining positions increase the braking power in sequence.
This hand-control system can be replaced by an automatic type that can
operate mechanically through the brake pedal. In this case, the contacts are
switched on successively over the slack movement of the brake pedal. The
use of an automatic control must be coupled with a cut-off system operating at
very low vehicle speed in order to prevent energization of the retarder while the
vehicle is stationary with the driver maintaining pressure on the brake pedal.
Both the manual control and the automatic control activate four solenoid
contractors in the relay box, which in turn close the four groups of coil circuits
within the electric brake at either 24 volts or 12 volts, as appropriate (Reverdin
1974 and Omega Technologies).
Characteristic of Electromagnetic Brakes
It was found that electromagnetic brakes can develop a negative power
which represents nearly twice the maximum power output of a typical engine,
and at least three times the braking power of an exhaust brake (Reverdin
1974). These performance of electromagnetic brakes make them much more
competitive candidate for alternative retardation equipments compared with
other retarders. By using the electromagnetic brake as supplementary
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retardation equipment, the friction brakes can be used less frequently, and
therefore practically never reach high temperatures. The brake linings would
last considerably longer before requiring maintenance, and the potentially
“brake fade” problem could be avoided. In research conducted by a truck
manufacturer, it was proved that the electromagnetic brake assumed 80
percent of the duty which would otherwise have been demanded of the regular
service brake (Reverdin 1974). Furthermore, the electromagnetic brake
prevents the dangers that can arise from the prolonged use of brakes beyond
their capability to dissipate heat. This is most likely to occur while a vehicle
descending a long gradient at high speed. In a study with a vehicle with 5 axles
and weighing 40 tons powered by an engine of 310 b.h.p traveling down a
gradient of 6 percent at a steady speed between 35 and 40 m.p.h, it can be
calculated that the braking power necessary to maintain this speed is the order
of 450 h.p.