INTRODUCTION

INTRODUCTION TO FRICTION LESS BRAKE

Magnetic brakes working on the principle of force interaction between a pair of magnets are nowadays commonly used in the industry as a cheap alternative to the classical brake systems. They are often used as dampers because of their simple design. An eddy current brake, like a conventional friction brake, is a device used to slow or stop a moving object by dissipating its kinetic energy as heat. In an eddy current brake the drag force is an electromagnetic force between a magnet and a nearby conductive object in relative motion due to eddy currents included in the conductor through electromagnetic induction. A conductive surface moving past a stationary magnet will have circular electric currents called eddy currents included in it by magnetic field, due to the Faraday’s law of induction. By Lenz’s law, the circulating currents will create their own magnetic field which opposes the field of magnet. Thus the moving conductor will experience a drag force from the magnet that opposes its motion, proportional to its velocity. The electrical energy of eddy currents is dissipated as heat due to the electrical resistance of the conductor. In an electromagnetic brake the magnetic field may be created by a permanent magnet, or an electromagnet so the braking force can be turned on & off or varied by varying the electric current in the electromagnet’s windings. Another advantage is that since the brake does not work by friction, there are no brake shoe surfaces to wear out, necessitating replacement, as with friction brakes. Eddy current are used to slow the high speed trains and roller coasters, to stop powered tools quickly when the power is tuned off, and in electric meters used by electric utilities.

CONSTRUCTION 

There are three parts to an electromagnetic brake: field, armature, and hub (which is the input on a brake). Usually the magnetic field is bolted to the machine frame (or uses a torque arm that can handle the torque of the brake). So when the armature is attracted to the field the stopping torque is transferred into the field housing and into the machine frame decelerating the load. This can happen very fast (1 -3sec). Disengagement is very simple. Once the field starts to degrade flux falls rapidly and the armature separates. A spring(s) hold the armature away from its corresponding contact surface at a predetermined air gap. 

Fig: Electromagnetic brake

WORKING PRINCIPLE

 The working principle of the electromagnetic brake is based on Right hand thumb rule. If a piece of copper wire was wound, around the nail and then connected to a battery, it would create an electro magnet. The magnetic field that is generated in the wire, from the current, is known as the “right hand thumb rule”. The strength of the magnetic field can be changed by changing both wire size and the amount of wire (turns). The fields of EM brakes can be made to operate at almost any DC voltage and the torque produced by the brake will be the same as long as the correct operating voltage and current is used with the correct brake. A constant current power supply is ideal for accurate and maximum torque from a brake. If a non regulated power supply is used the magnetic flux will degrade as the resistance of the coil goes up. Basically, the hotter the coil gets the lower the torque will be produced by about an average of 8% for every 20°C. If the temperature is fairly constant, and there is a question of enough service factor in the de sign for minor temperature fluctuation, by slightly over sizing the brake can compensate for degradation. This will allow the use of a rectified power supply, which is far less expensive than a constant current supply.

Fig; Right Hand Thumb Rule

Based on V = I × R, as resistance increases available current falls. An increase in resistance, often results from rising temperature as the coil heats up, according to: 

Rf = Ri × [1 + αCu × (Tf – Ti)] 

Where Rf = final resistance,

 Ri = initial resistance,

 αCu = copper wire’s temperature coefficient of resistance, 0.0039 °C-1,

 Tf = final temperature, and 

Ti = initial temperature.

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 mentioned before. Typically, electromagnetic brakes have been mounted in the transmission line of vehicles. The propeller shaft is divided and fitted with a sliding universal joint and is connected to the coupling flange on the brake. The brake is fitted into the chassis of the vehicle by means of anti-vibration mounting.