Eddy Current Brakes-Seminar Paper

eddy current brake


This is an electric braking system which works on the principle that eddy current produced in it opposes the driving torque. This opposing torque is used to brake the automobiles. Mainly this system is purely based on Faraday’s laws of electromagnetic induction and Lenz’s law. 

                For operating this a control switch is provided on the steering column in a position for easy manual. The skidding and complexity of mechanical braking system can be minimized by this system. Also the wear and tear of the vehicles can be reduced.
               Since researches are going on to eliminate some of the disadvantages of this system, we can accept it to be the norm one in a few years of time.


Many of the ordinary brakes, which are being used now days stop the vehicle by means of mechanical blocking. This causes skidding and wear and tear of the vehicle. And if the speed of the vehicle is very high, the brake cannot provide that much high braking force and it will cause problems. These drawbacks of ordinary brakes can be overcome by a simple and effective mechanism of braking system ‘The eddy current brake’. It is an abrasion-free method for braking of vehicles including trains. It makes use of the opposing tendency of eddy current

           Eddy current is the swirling current produced in a conductor, which is subjected to a change in magnetic field. Because of the tendency of eddy currents to oppose, eddy currents cause energy to be lost. More accurately, eddy currents transform more useful forms of energy such as kinetic energy into heat, which is much less useful. In many applications, the loss of useful energy is not particularly desirable. But there are some practical applications. Such an application is the eddy current brake. 

Eddy current brake works according to Faraday’s law of electromagnetic induction. According to this law, whenever a conductor cuts magnetic lines of forces, an emf is induced in the conductor, the magnitude of which is proportional to the strength of magnetic field and the speed of the conductor. If the conductor is a disc, there will be circulatory currents i.e. eddy currents in the disc. According to Lenz’s law, the direction of the current is in such a way as to oppose the cause, i.e. movement of the disc.
             Essentially the eddy current brake consists of two parts, a stationary magnetic field system and a solid rotating part, which include a metal disc. During braking, the metal disc is exposed to a magnetic field from an electromagnet, generating eddy currents in the disc. The magnetic interaction between the applied field and the eddy currents slow down the rotating disc. Thus the wheels of the vehicle also slow down since the wheels are directly coupled to the disc of the eddy current brake, thus producing smooth stopping motion.
 CONSTRUCTION                                                                                                                                                                          Essentially an eddy current brake consists of two members, a stationary magnetic field system and a solid rotary member, generally of mild steel, which is sometimes referred to as the secondary because the eddy currents are induced in it.  Two members are separated by a short air gap, they’re being no contact between the two for the purpose of torque transmission. Consequently there is no wear as in friction brake.

                        Stator consists of pole core, pole shoe, and field winding. The field winding is wounded on the pole core.  Pole core and pole shoes are made of east steel laminations and fixed to the state of frames by means of screw or bolts.  Copper and aluminium is used for winding material the arrangement is shown in fig. 1. This system consists of two parts.
1.         Stator
2.         Rotor

              It is supported frame members of the vehicle chassis. It has introduced  magnetic poles energized by windings. Current is supplied to the winding from the battery.
     It is a rotating disc, which is fitted on the line of crankshaft with small air gap to stator. When disc rotates a flux change occur in the section of the disc passing the poles of stator. Due to the  flux change there is a circulatory  or eddy current in the disc around the magnetic lines of force. The effect of this eddy current induces ‘N’ and ‘S’ poles at the surface  of the disc. Then there will be a ‘drag’ or braking  effect in between eddy current  induced  poles and magnetic  poles in the stator. By changing current from the battery we can change the braking force. In this breaking  system kinetic  energy of the vehicle is converted  to heat and this heat is dissipated  through the rotating  disc.
   Total resistance of field winding
R = Lr/A
where,                         L          =          total length of field winding in meter.
                        r          =          Resistivity of the wire in ohm meter
                        A         =          the area of cross section of field winding in m2
Total no: of terms = total length /mean length of one term
            The rotor is a rotating disc on shaft, which is placed very near to the stator with small air gap (1 mm to 2 mm). Rotating disc may be one or both side of stator.
            The two units have common ring member, poles cores on which winding are provided being fixed to ring number.  If a malleable casting is employed, then the pole core could be cast integrally with the right.  After fitting the windings on the cores, poles shoes are fitted to provide pole faces of appropriate shape and area.  The rotor disc should  be provided with properly designed fins for faster heat removal.  The magnetic circuits of the two units are substantially the same, non-undue and thrust would be imposed on the motor bearings.  Slight axial displacement of rotor could however, cause quite appreciable discrepancy, the air gap of two units. The effect would be to increase the magnetic pull in one air gap and diminish it on other which could give to rise to excessive and thrust on rotor bearing to overcome the inherent defect, the air gaps of both units could be put in series by making the central number non magnetic and providing a continues pole core for each pair axially opposite poles.  This modification could possibly reduce the length of the combined pole course or permits a larger winding length.
The maximum diameter of the eddy current brake is decided by
1.         The spacing of vehicle chassis frame.
2.         Vehicle floor clearance

WORKING                                                                                                        When the vehicle is moving, the rotor disc of eddy current brake which is coupled to the wheels of the vehicle rotates, in close proximity to stationary magnetic poles. When we want to brake the vehicle, a control switch is put on which is placed on the steering column in a position for easy operation.

        When the control switch is operated, current flows from a battery to the field winding, thus energizing the magnet. Then the rotating disc will cut the magnetic field. When the disc cuts the magnetic field, flux changes occur in the disc which is proportional to the strength of the magnetic field. The current will flow back to the zero field areas of the metal plate and thus create a closed current loop like a whirl or eddy. A flow of current always means there is a magnetic field as well. Due to Lenz’s law, the magnetic field produced by the eddy currents works against the movement direction. Thus instead of mechanical friction, a magnetic friction is created. In consequence, the disc will experience a “drag” or the braking effect, and thus the disc stops rotation. The wheels of the vehicle, which is directly coupled to the disc, also stop rotation. Faster the wheels are spinning, stronger the effect, meaning that as the vehicle slows, the braking force is reduced producing a smooth stopping action.
            The control switch can be set at different positions for controlling the excitation current to several set values in order to regulate the magnetic flux and consequently the magnitude of braking force. i.e. if the speed of the vehicle is lpw, a low braking force is required to stop the vehicle. So the control switch is set at the lowest position so that a low current will be supplied to the field winding. Then the magnetic field produced will be of low strength, so that a required low braking force is produced.
             When the control switch is operated during the standby position of the vehicle, the magnet will be energized and magnetic field is created. But since the wheels are not moving, magnetic lines of force are not cut by it, and the brake will not work. However, a warning lamp is provided on the instrument   panel to indicate whether the brake is energized. This provides a safe guard for the driver against leaving the unit energized.
        When control switch is put in any one of the operating positions, the corresponding conductor in the contractor box is energized and current flows from the battery to the field winding to the contractor box.  This current magnetizes the poles in stator, which placed very near to the rotor.  When rotor rotates it will cut magnetic lines and eddy current will set up in the rotor.  The magnetic field of this eddy current produces a breaking force or torque in the opposite direction of rotation disc.  This kinetic energy of rotor is converted as heat   energy and dissipated from rotating disc to surrounding atmosphere.  Current in the field can change by changing the position of the controls switch. Thus we can change the strength of the braking force. 

            In the case of trains, the part in which the eddy current is induced is rail. The brake shoe is enclosed in a coil, forming an electromagnet. When the magnet is energized, eddy currents are induced in the rail by means of electromagnetic induction, thereby producing braking action.

            There are two types of eddy current brakes according to the method of excitation.
1.Electrically excited eddy current brake
2.Permanent magnet eddy current brake

   Recently, permanent magnet eddy current brakes have been developed for subways, trams and local trains. These brakes need a mechanical actuator to turn the magnets jn an on and off position. The main advantage of this type of brake is safety. i.e. it does not need electrical power supply to energize the magnet.

 Braking Power Calculations
            Sophisticated calculation methods for the determination of braking forces of eddy current brakes are important for the design of the brakes.
            For  a simple eddy current brake employing a thin non-magnetic disc as copper the drag or braking force on the disc

H = Magnetic field strength in Webers.
A = Pole force area in cm2.
V = Velocity in cm/sec of mean radius of disc under the poles.
T = disc thickness in cm.
r = specific resistance of disc material at its operating temperature in micro ohms/cm3.
Torque = F * R Nm
Where R = mean pole radius in meter.
Power P = 2 p NT/60 watts.

An eddy current brake is an energy converter functions is to convert  the kinetic energy of a vehicle into heat and dissipate it such a rate to maintain the temperature of unit with in reasonable limits under maximum and prolonged braking conditions. The energy absorbed by the brake is transformed into heat by the currents induced in the motor, and this is heat manly dissipated in surrounding air through the medium of suitable designed fins the rotating member.
              In mountains area, continues braking force is needed for a long time  (say about half an hour), at this condition, eddy current braking is more suitable to function without over heating. The use of these retarders is by no means limited to mountain on country. They can be advantageously employed on public service vehicles on city routes without frequent  stops. But  in this braking system there with out be any braking force in the vehicle is rest. So the eddy current brake is used as an auxiliary heavy-duty retarder. By using auxiliary retarder very smooth retardation is assumed and likely hood of skidding on slippery roads  surfaces is minimized. Smooth braking action cuts down tier were and since brake is used as an auxiliary heavy-duty retarder. By using auxiliary retarder very smooth retardation is assumed and likely hood of skidding on slippery roads  surfaces is minimized. Smooth braking action cuts down tier were and since the conventional brakes are relieved of heavy duty being required only to bring vehicles to rest.

            A typical mounting of an eddy current brake consists of  two discs in which pole salient type, supported between the frame numbers of a vehicle chassis.  Rotor is coupled  to road wheels being often mounted on a shaft that is interposed between the gearbox and propeller shaft and stator is mounted on the frame of the vehicle.
              The driver who can select one or four  excitation settings according to the breaking effect required mounts a control switch on a steering column in a position for easy operation.  In the operative positions of this switch 1, 2, 3, 4 contractors are energized to supply current to the excitation windings of the retarder. Warning lamp is also provided on the instrument panel to indicate when the retarder is energized.  This provides a safe guard for the driver against leaving the unit energized when the vehicle is stationary.

  Less maintenance
  Wide range of braking force available within the temperature limit
  Prolonged braking is possible
  Long life
  Less strain to the operation
  Smooth retardation, which cuts down, the tire wears
            Eddy current brakes offer smooth retardation of vehicles without skidding. It is totally free of wear and tear. So it has long life compared to ordinary brakes. These need less maintenance.
            In mountain areas continuous braking is needed for a long time. At this condition, eddy current braking is more suitable to function without overheating.
            By changing the excitation current to the field winding, i.e. by adjusting the position of the control switch, we can vary the braking force to required range. The operation of eddy current brake is very simple. i.e. the control switch is a soft switch which can be operated without any strain.
            Eddy current brakes works even under the toughest  environmental conditions. For example, in larger water slides and water coasters where the humidity   would immediately result in reduction of friction and thus impair the effectiveness of ordinary brakes. It works even in highly corrosive environments and heavily contaminated areas.

DISADVANTAGES                                                                                                                                No breaking force at rest                                                                                                                                                                        Need of electric power
The main disadvantage of the edyy current brake is that it needs electric power to work. Researches are going on to overcome this disadvantage by making the brake regenerative i.e. by converting the kinetic energy of the vehicle into electric energy and storing it back into the battery.

  For additional safety on long decants in mountain area
  For high speed passenger and goods vehicle
  Eddy current brakes are best substitutes for ordinary brakes, which are being used nowadays in road vehicles even in trains, because of their jerk-free operation. In mountain areas where continuous braking force is needed, for a long time, the eddy current braking is very much useful for working without overheating. Eddy current brakes are very much useful for high-speed passengers and good vehicles. It can also be used to slow down the trolleys of faster roller coasters.


 Eddy current brakes are the best choice when demands for reliability and safety are the highest. They work even in the toughest environmental conditions. Even the strike of lightning will not result in the loss of the braking force.

            Eddy current braking system is not popular now a days. But we hope that the eddy current braking system which is simpler and more effective will take the place of the ordinary braking system and we can do expect it to be the norm one in few years of time.

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