Designing and Fabrication of Electro Magnetic Brake

CHAPTER 1 INTRODUCTION

1.1 Friction Braking of Vehicles and Its Disadvantages

Road, rail, and air vehicles all rely mainly or solely on mechanical friction brakes. These brakes are composed of two functional parts: a rotor connected to the wheels and a stator fixed to the chassis of the vehicle. The rotor is either a drum or a disc generally made of cast iron for road and rail vehicles, and carbon fiber for aircraft. The stator comprises shoes (drum brakes) or pads (disc brakes) made of a soft friction material and an actuator, generally a hydraulic piston.

Although the principle is the same for drum and disc brakes, the terminology used from there on refers to disc brakes. The contact between the soft material of the pads and the surface of the rotor is characterized by a high friction coefficient. When braking is commanded by the driver, the actuator presses the pads against the rotor, thus inducing a friction force tangential to the surface of the rotor, which opposes the motion of the vehicle (Fig. 1). The braking force is proportional to the normal force developed by the actuator pressing the pads against the rotor and the coefficient of friction: F (braking) = f. N (actuator)

Fig. 1.1: Forces involved in friction braking

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Friction braking is dissipative: the vehicle‟s kinetic energy is dissipated as heat at the contact surface between the pads and the disc. Kinetic energy dissipation results in a very significant increase of the disc and pads temperatures. Although most of the heat is ultimately dissipated through forced convection by the disc‟s cooling fins, during a braking phase the temperature of the disc and pads may rise to several hundreds of Celsius degrees. The friction coefficient between the pads and the disc, and therefore the maximum braking force obtainable depends on temperature, increasing slightly from room temperature to a maximum, and decreasing rapidly beyond a certain point (Fig. 2). The decrease in braking force at high temperature is a phenomenon called “fading”. It is usually encountered when a vehicle is driven downhill because of the supplementary force due to the grade and the weight of the vehicle. In some dramatic cases, the brakes can lose all braking capability and the vehicle is totally brakeless. Another effect of temperature is disc warping, which occurs as a result of very high disc temperature during heavy braking. This phenomenon is rarely encountered in road or rail vehicles, but has prompted the replacement of cast iron discs by carbon fiber discs on aircraft.

Fig. 1.2: Temperature dependence of friction coefficient The combined effects of friction, heat, exposure to water and dirt result in the abrasion of the pads. Consequently, the pads must be changed regularly, whereas the rotors 2

sometimes need to be resurfaced. The cost of new pads and maintenance personnel costs are significant for heavy-duty vehicles (trucks and buses). Furthermore, the dust from the pads may be harmful to the environment and the health of populations. Heavy and/or fast vehicles require very large braking forces to bring them to a complete stop. Such forces are usually beyond the capabilities of a human operator, which has prompted the installation of power assistance on nearly all road, rail and air vehicles. The assistance mechanism is usually pneumatic for gasoline vehicles and trains or hydraulic for diesel vehicles and aircraft. The assistance mechanism requires many parts, which are often redundant for safety reasons. There is therefore a significant increase in complexity due to this additional hardware. Furthermore, the pumps required on diesel vehicles and aircraft take their toll on the fuel economy of the vehicle. It is worth noting that hybrid vehicles would also require an assistance pump to guarantee assistance even when the engine is shut down.

There is also a propagation time associated with the assistance mechanism, which delays the application of braking from the time the driver pushes the pedals. This delay is significant in buses, trucks, and trains where the brake fluid lines are long. Delays in brake application result in significant increases in braking distances and increased control complexity. Brake controls are required to balance the braking forces between the rear and front axles depending on vehicle load, speed and road conditions, but also to prevent wheel-lock. Anti-lock controls, know as ABS are an important safety feature of modern road vehicles. In addition, the latest generation of automobiles incorporates dynamic stability control systems that correct the driver‟s mistakes to a certain point and maintain the vehicle on a safe trajectory even under harsh road conditions. The interface between the electronic controls and the hydraulic circuit is ensured by electrically 5 actuated valves that operate in a switching mode, either open or shut. There are additional nonlinearities in the brake system due to delays in fluid ducts, nonlinear contact between pads and discs, etc…, which render brake control delicate.

In conclusion, although friction brakes are compact and effective, they suffer from several disadvantages, some a mere annoyance and some a real burden on users and owners, whether private or commercial. While these problems are being dealt with currently, it is at a cost, which it would be beneficial to reduce. 3

1.2 Project plan

The purpose of the present report is to design an electromagnetic brake for a given braking torque and speed, validate it experimentally, provide a conceptual design, and study its integration in automobiles. The project plan is decomposed as follows:

- Theoretical analysis: An analytical model is derived for the electromagnetic brake and its fundamental physics are investigated. The model provides a preliminary sizing of the brake and critical information about the sensitivity to design parameters.

- Experimental validation: A test bed has been built based on specifications from numerical data calculated and data has been gathered and compared to the numerical analysis data. The objective is to validate the accuracy of numerical analysis and to explain potential divergences. This validation is necessary to establish the ability of numerical analysis to model electromagnetic brakes.

- Expansion of the concept: Several additional innovations are analyzed and incorporated in the integrated brake in order to make the concept more complete and more relevant to real world applications.

- Integration study: The use of the integrated brake in conventional and hybrid automobiles is investigated. Specifications for the respective sizing of the friction, regenerative and eddy-current brake are derived through an optimization study and the gains achieved are analyzed.

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CHAPTER 2 LITERATURE REVIEW

General principle of electromagnetic brakes

2.1. Introduction

Electromagnetic brakes have been used as supplementary retardation equipment in addition to the regular friction brakes on heavy vehicles. The working principle and characteristics of electromagnetic brakes are given below.

2.2. 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 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.

2.3. 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

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“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).

Figure 2.1: Disc Brake (Limpert 1992)

Drum brakes consist of a heavy flat-topped cylinder, which is sandwiched between the wheel rim and the wheel hub (see Figure 2.2). 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 (Limpert 1992). 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.

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Figure 2.2: Drum Brakes (Limpert 1992) 2.4. “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: 7

- poor ventilation due to encapsulation in the road wheels, - diameter restriction due to tire dimensions, - width restrictions imposed by the vehicle spring designer; - 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.

2.5. Retarders Retarders are means of overcoming the above problems by augmenting a vehicle‟s foundation brakes with a device capable of opposing vehicle motion at relatively low levels of power dissipation for long periods. There are several retarder technologies currently available. Two major kinds are the hydrokinetic brake and the exhaust brake. Hydrokinetic brake uses fluid as the working medium to oppose rotary motion and absorb energy (Packer 1974). Hydrodynamic brakes are often built into hydrodynamic transmissions (Foster, 1974). Exhaust brakes use a valve which is fitted into the exhaust pipe between the exhaust manifold and silencer. When this valve is closed air is compressed against it through the open exhaust valve by the piston rising on the exhaust stroke. In that way the engine becomes a low pressure single stage compressor driven by the vehicle‟s momentum, resulting in a retarding effect being transmitted through the transmission to the driving road wheels. The power-producing engine is converted into a power absorbing air compressor (Smith, 1974). This approach could put a lot of stress on the cylinder and exhaust system. So it may require extra engineering efforts to implement this system. As a brake applied to the engine, exhaust brakes can only absorb as much power as the engine can deliver. But the power absorbed in braking is usually greater than the power absorbed in driving. Compared with these retarders, electromagnetic brakes have greater power capability, simplicity of installation and controllability.

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2.6. General Principle and Advantage of Electromagnetic Brakes (retarders) 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.

The brake is fitted into the chassis of the vehicle by means of anti-vibration mounting. The practical location of the retarder within the vehicle prevents the direct impingement of air on the retarder caused by the motion of the vehicle. Any air flow movement within the chassis of the vehicle is found to have a relatively insignificant effect on the air flow around tire areas and hence on the temperature of both front and rear discs. So the application of the retarder does not affect the temperature of the regular brakes. In that way, the retarders help to extend the life span of the regular brakes and keep the regular brakes cool for emergency situation.

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. 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). 9

A typical retarder consists of stator and rotor. The stator holds 16 induction coils, energized separately in groups of four. The coils are made up of varnished aluminum wire mounded in epoxy resin. The stator assembly is supported resiliently through antivibration mountings on the chassis frame of the vehicle. The rotor is made up of two discs, which provide the braking force when subject to the electromagnetic influence when the coils are excited. Careful design of the fins, which are integral to the disc, permit independent cooling of the arrangement.

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 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. The braking effect of the engine even with a fitted exhaust brake is approximately 150 h.p. The brakes, therefore, would have to absorb 300 h.p, meaning that each brake in the 5 axles must absorb 30 h.p, which is beyond the limit of 25 h.p. that a friction brake can normally absorb without self-destruction.

The electromagnetic brake is well suited to such conditions since it will independently absorb more than 300 h.p (Reverdin 1974). It therefore can exceed the requirements of 10

continuous uninterrupted braking, leaving the friction brakes cool and ready for emergency braking in total safety. The installation of an electromagnetic brake is not very difficult if there is enough space between the gearbox and the rear axle. It does not need a subsidiary cooling system. It does not rely on the efficiency of engine components for its use, as do exhaust and hydrokinetic brakes. The electromagnetic brake also has better controllability. The exhaust brake is an on/off device and hydrokinetic brakes have very complex control system. The electromagnetic brake control system is an electric switching system which gives it superior controllability. From the foregoing, it is apparent that the electromagnetic brake is an attractive complement to the safe braking of heavy vehicles.

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 11

brake as supplementary 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 (Reverdin1974). 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. The braking effect of the engine even with a fitted exhaust brake is approximately 150 h.p. The brakes, therefore, would have to absorb 300 h.p, meaning that each brake in the 5 axles must absorb 30 h.p, which is beyond the limit of 25 h.p. that a friction brake can normally absorb without self-destruction. The electromagnetic brake is well suited to such conditions since it will independently absorb more than 300 h.p (Reverdin 1974). It therefore can exceed the requirements of continuous uninterrupted braking, leaving the friction brakes cool and ready for emergency braking in total safety.

The installation of an electromagnetic brake is not very difficult if there is enough space between the gearbox and the rear axle. It does not need a subsidiary cooling system. It does not rely on the efficiency of engine components for its use, as do exhaust and hydrokinetic brakes. The electromagnetic brake also has better controllability. The exhaust brake is an on/off device and hydrokinetic brakes have very complex control system. The electromagnetic brake control system is an electric switching system which gives it superior controllability. From the foregoing, it is apparent that the electromagnetic brake is an attractive complement to the safe braking of heavy vehicles.

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Thermal Dynamics

Thermal stability of the electromagnetic brakes is achieved by means of the convection and radiation of the heat energy at high temperature. The major part of the heat energy is imparted to the ventilation air which is circulating vigorously through the fan of the heated disc. The value of the energy dissipated by the fan can be calculated by the following expression:

Q M Cp = D q (2.1) Where: M = Mass of air circulated; Cp = Calorific value of air; Dq = Difference in temperature between the air entering and the air leaving the fan;

The electromagnetic brakes has excellent heat dissipation efficiency owing to the high temperature of the surface of the disc which is being cooled and also because the flow of air through the centrifugal fan is very rapid. Therefore, the curie temperature of the disc material could never been reached (Reverdin 1974). The practical location of the electromagnetic brakes prevents the direct impingement of air on the brakes caused by the motion of the vehicle. Any air flow movement within the chassis of the vehicle is found to have a relatively insignificant effect on the air flow and hence temperature of both front and rear discs. Due to its special mounting location and heat dissipation mechanism, electromagnetic brakes have better thermal dynamic performance than regular friction brakes.

2.7 Review of existing electromagnetic brake

Electromagnetic brakes operate electrically, but transmit torque mechanically. This is why they are used to be referred to as electro-mechanical brakes. Over the years, EM brakes became known as electromagnetic, referring to their actuation method. There are three parts in 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 13

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 (0.1-3sec). `Disengagement is very simple. Once the field starts to degrade flux falls rapidly and the armature separates. A spring holds the armature away from its corresponding contact surface at a predetermined air gap.

Fig 2.3: Electromagnetic brake

Concept: Voltage/Current - And the Magnetic Field

Fig 2.4: Right hand rule

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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”. (V-1) The strength of the magnetic field can be changed by changing both wire size and the amount of wire (turns). EM clutches are similar; they use a copper wire coil (sometimes aluminum) to create a magnetic field. 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. If a 90 volt brake had 48 volts applied to it, this would get about half of the correct torque output of that brake. This is because voltage/current is almost linear to torque in DC electromagnetic brakes. 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 design 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. 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. An electromagnetic brake is a new revolutionary concept. They work on the principle of electromagnetism. These are totally frictionless.

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Types of Electromagnetic Brakes 1. Electromagnetic Power off Brake:

Fig 2.5: Electromagnetic power off brake spring set Introduction - Power off brakes stop or hold a load when electrical power is either accidentally lost or intentionally disconnected. In the past, some companies have referred to these as "fail safe" brakes. These brakes are typically used on or near an electric motor. Typical applications include robotics, holding brakes for Z axis ball screws and servo motor brakes. Brakes are available in multiple voltages and can have either standard backlash or zero backlash hubs. Multiple disks can also be used to increase brake torque, without increasing brake diameter. There are 2 main types of holding brakes. The first is spring applied brakes. The second is permanent magnet brakes. How It Works: Spring Type - When no electricity is applied to the brake, a spring pushes against a pressure plate, squeezing the friction disk between the inner pressure plate and the outer cover plate. This frictional clamping force is transferred to the hub, which is mounted to a shaft. Permanent Magnet Type – A permanent magnet holding brake looks very similar to a standard power applied electromagnetic brake. Instead of squeezing a friction disk, via springs, it uses permanent magnets to attract a single face armature. When the brake is engaged, the permanent magnets create magnetic lines of flux, which can turn attract the armature to the brake housing. To disengage the brake, power is applied to the coil which sets up an alternate magnetic field that cancels out the magnetic flux of the permanent magnets. 16

Both power off brakes are considered to be engaged when no power is applied to them. They are typically required to hold or to stop alone in the event of a loss of power or when power is not available in a machine circuit. Permanent magnet brakes have a very high torque for their size, but also require a constant current control to offset the permanent magnetic field. Spring applied brakes do not require a constant current control, they can use a simple rectifier, but are larger in diameter or would need stacked friction disks to increase the torque.

2 . Electromagnetic Particle Brake:

Fig 2.6: Electromagnetic Particle Brake Introduction : Magnetic particle brakes are unique in their design from other electromechanical brakes because of the wide operating torque range available. Like an electromechanical brake, torque to voltage is almost linear; however, in a magnetic particle brake, torque can be controlled very accurately (within the operating RPM range of the unit). This makes these units ideally suited for tension control applications, such as wire winding, foil, film, and tape tension control. Because of their fast response, they can also be used in high cycle applications, such as magnetic card readers, sorting machines and labeling equipment.

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How It Works: Magnetic particles (very similar to iron filings) are located in the powder cavity. When electricity is applied to the coil, the resulting magnetic flux tries to bind the particles together, almost like a magnetic particle slush. As the electric current is increased, the binding of the particles becomes stronger. The brake rotor passes through these bound particles. The output of the housing is rigidly attached to some portion of the machine. As the particles start to bind together, a resistant force is created on the rotor, slowing, and eventually stopping the output shaft. When electricity is removed from the brake, the input is free to turn with the shaft. Since magnetic particle powder is in the cavity, all magnetic particle units have some type of minimum drag associated with them. 3. Electromagnetic Hysteresis Power Brake:

Fig 2.7: Electromagnetic Hysteresis Power Brake Introduction: Electrical hysteresis units have an extremely wide torque range. Since these units can be controlled remotely, they are ideal for test stand applications where varying torque is required. Since drag torque is minimal, these units offer the widest available torque range of any of the hysteresis products. Most applications involving powered hysteresis units are in test stand requirements.

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How It Works: When electricity is applied to the field, it creates an internal magnetic flux. That flux is then transferred into a hysteresis disk passing through the field. The hysteresis disk is attached to the brake shaft. A magnetic drag on the hysteresis disk allows for a constant drag, or eventual stoppage of the output shaft. When electricity is removed from the brake, the hysteresis disk is free to turn, and no relative force is transmitted between either member. Therefore, the only torque seen between the input and the output is bearing drag. 4 Multiple Disk Brakes: Introduction: Multiple disk brakes are used to deliver extremely high torque within a small space. These brakes can be used either wet or dry, which makes them ideal to run in multi speed gear box applications, machine tool applications, or in off road equipment.

Fig 2.8: Electromagnetic Multiple Disk Brake

How It Works: Electro-mechanical disk brakes operate via electrical actuation, but transmit torque mechanically. When electricity is applied to the coil of an electromagnet, the magnetic 19

flux attracts the armature to the face of the brake. As it does so, it squeezes the inner and outer friction disks together. The hub is normally mounted on the shaft that is rotating. The brake housing is mounted solidly to the machine frame. As the disks are squeezed, torque is transmitted from the hub into the machine frame, stopping and holding the shaft. When electricity is removed from the brake, the armature is free to turn with the shaft. Springs keep the friction disk and armature away from each other. There is no contact between breaking surfaces and minimal drag. Particle brakes are unique in their design from other electro-mechanical brakes because of the wide operating torque range available. Like an electro-mechanical brake, torque to voltage is almost linear; however, in a magnetic particle brake, torque can be controlled very accurately (within the operating RPM range of the unit). This makes these units ideally suited for tension control applications, such as wire winding, foil, film, and tape tension control. Because of their fast response, they can also be used in high cycle applications, such as magnetic card readers, sorting machines and labeling equipment. How It Works: Magnetic particles (very similar to iron filings) are located in the powder cavity. When electricity is applied to the coil, the resulting magnetic flux tries to bind the particles together, almost like a magnetic particle slush. As the electric current is increased, the binding of the particles becomes stronger. The brake rotor passes through these bound particles. The output of the housing is rigidly attached to some portion of the machine. As the particles start to bind together, a resistant force is created on the rotor, slowing, and eventually stopping the output shaft. When electricity is removed from the brake, the input is free to turn with the shaft. Since magnetic particle powder is in the cavity, all magnetic particle units have some type of minimum drag associated with them. 2.8 Engagement Time There are actually two engagement times to consider in an electromagnetic brake. The first one is the time it takes for a coil to develop a magnetic field, strong enough to

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pull in an armature. Within this, there are two factors to consider. The first one is the amount of ampere turns in a coil, which will determine the strength of a magnetic field. The second one is air gap, which is the space between the armature and the coil shell. Magnetic lines of flux diminish quickly in the air. The further away the attractive piece is from the coil, the longer it will take for that piece to actually develop enough magnetic force to be attracted and pull in to overcome the air gap. For very high cycle applications, floating armatures can be used that rest lightly against the coil shell. In this case, the air gap is zero; but, more importantly the response time is very consistent since there is no air gap to overcome. Air gap is an important consideration especially with a fixed armature design because as the unit wears over many cycles of engagement the armature and the coil shell will create a larger air gap which will change the engagement time of the brakes. In high cycle applications, where registration is important, even the difference of 10 to 15 milliseconds can make a difference, in registration of a machine. Even in a normal cycle application, this is important because a new machine that has accurate timing can eventually see a “drift” in its accuracy as the machine gets older. The second factor in figuring out response time of a brake is actually much more important than the magnet wire or the air gap. It involves calculating the amount of inertia that the brake needs to decelerate. This is referred to as “time to stop”. In reality, this is what the end-user is most concerned with. Once it is known how much inertia is present for the brake to stop then the torque can be calculated and the appropriate size of brake can be chosen. Most CAD systems can automatically calculate component inertia, but the key to sizing a brake is calculating how much inertia is reflected back to the brake. To do this, engineers use the formula: T = (WK2 × ΔN) / (308 × t) Where T = required torque in lbft, WK2 = total inertia in lb-ft2, ΔN = change in the rotational speed in rpm, and t = time during which the acceleration or deceleration must take place. Inertia Calculator There are also online sites that can help confirm how much torque is required to decelerate a given amount of inertia over a specific time. Remember to make sure that the torque chosen, for the brake, should be after the brake has been burnished.

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2.9 Torque When considering torque, the question of using dynamic or static torque for the application is key? For example, if running a machine at relatively low rpm (5 – 50 depending upon size) there is minimal concern with dynamic torque since the static torque rating of the brake will come closest to where it is running. However, when running a machine at 3,000rpm and applying the brake at its catalog torque, at that rpm, is misleading. Almost all manufacturers put the static rated torque for their brakes in their catalog. So, when trying to determine a specific response rate for a particular brake, the dynamic torque rating is needed. In many cases this can be significantly lower. It can be less than half of the static torque rating. Most manufacturers publish torque curves showing the relationship between dynamic and static torque for a given series of brake. 2.10 Over Excitation Over-excitation is used to achieve a faster response time. It is when a coil momentarily receives a higher voltage than its nominal rating. To be effective, the overexcitation voltage must be significantly, but not to the point of diminishing returns, higher than the normal coil voltage. Three times the voltage typically gives around 1/3 faster response. Fifteen times the normal coil voltage will produce a 3 times faster response time. With over-excitation, the in-rush voltage is momentary. Although it would depend upon the size of the coil, the actual time is usually only a few milliseconds. The theory is, for the coil to generate as much of a magnetic field as quickly as possible to attract the armature and start the process of deceleration. Once the over-excitation is no longer required, the power supply to the brake would return to its normal operating voltage. This process can be repeated a number of times as long as the high voltage does not stay in the coil long enough to cause the coil wire to overheat.

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2.11 Summary

With all the advantages of electromagnetic brakes over friction brakes, they have been widely used on heavy vehicles where the „brake fading‟ problem is serious. The same concept is being developed for application on lighter vehicles.

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CHAPTER 3 EXPERIMENTAL SET UP AND METHODOLOGY USED

3.1 Objective and experimental procedure

The objective of the experiment is to establish the ability of the theoretical analysis to model the performance and behavior of electromagnetic brake excited by electromagnet and using a disc made of ferromagnetic material. Therefore, the theoretically modeled brake and the experimentally tested brake must be as similar as possible. Because of some constraints imposed by the assumptions taken, there will be some error arose.

Most soft magnetic materials applications require a combination of a high relative permeability with an electrical conductivity as low as possible. Such is the case for transformers, inductor cores, and electric motors. Therefore, magnetic properties are listed only for useful materials, which have a high relative permeability. However, theoretical analysis indicates that electromagnetic brakes require low permeability soft magnetic materials with a high electrical conductivity. There are indications that cast iron and some steels do possess these properties. So mild steel, which has the magnetization saturation of 23T is chosen as the disc material.

3.2 Methodology - electromagnetic brake system •

How does a magnetic brake system work? Magnetic resistance works by passing a spinning metallic disk through a

magnetic field. The magnetic field provides resistance to the spinning disk thus slowing it‟s rotation. The amount of resistance can be increased or decreased by varying the strength of the magnetic field. Field strength is controlled by changing either the power of the magnet or the distance between the magnet and the spinning disk. •

Resistance Formula: Resistance is determined by three factors:

Disk RPM, MAGNET POWER and the DISTANCE between the magnet and disk.

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The three factors are expressed as a ratio of one to one to one squared:

RPM : MAGNET POWER : DISTANCE = RESISTANCE The distance value is the most important part of the formula because it‟s value is squared. This means that very small changes in DISTANCE make very large changes in the resistance level.

Because distance is such an important part of the resistance formula small variations can make large differences in the amount of resistance. This can make it difficult to adjust the resistance level by useful amount.

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Many systems place the magnets at the edge of the flywheel. As the distance between the flywheel and magnets increases the resistance level quickly decreases. This makes fine adjustment of the resistance level difficult to achieve resulting in an uneven resistance curve. Users can feel this as resistance that varies little at lower settings but then spikes suddenly at high resistance settings.

The M‐Force difference By placing magnets on the sides of the Eddy Current disk the patented M‐Force magnetic brake system maintains a consistent distance between the disk and magnets at all times. Maintaining a consistent distance allow for fine adjustment of the resistance 26

level simply by adjusting the magnetic field power. This results in a smooth resistance curve. Users can feel resistance that increases evenly from low to high settings. 3.3 – Test bed design The test bed general architecture is shown on Fig. 3.1. It includes a motor drive, a torque meter, the electromagnetic brake, a handheld ammeter and two regulators. The operation of the electromagnet brake will only be investigated at steady state. Therefore, a three phase AC motor with the stabilizer is provided to provide torque to the test bed. The characteristics of the AC motor are summarized in TABLE 3.1 below: TABLE 3.1 AC motor characteristics PARAMETER

VALUE

Nominal armature voltage

220 V

Nominal armature current Armature resistance

6A 285 Ω

Maximum speed Nominal shaft power

2800 rpm 18 w

Maximum torque

0.0614 N.m

Torque constant

0.0.0102 N.m/A

The eddy-current brake is designed to fit the characteristics (maximum torque and speed) of the ac motor drive. TABLE 3.2 gives the geometric characteristics of the test eddycurrent brake. The geometry is the same as that shown on Fig. 3.1 . TABLE 3.2 Test electromagnetic brake characteristics PARAMETER electromagnet residual flux Number of turns of coil Resistivity of disc material Number of electromagnet used Arc between north and south pole Airgap width Disc thickness Disc inner radius Disc outer radius

SYMBOL B n ρ p

VALUE 10.4 T 1900 1068Ω×− m. 4

τ

80 degree

g e Rinner R outer

2.5 mm 3.5 mm 5.5 cm 9 cm

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Fig 3.1: Experimental set up 3.4 –Disc Material Evaluation

The braking disc for an electromagnetic brake must be made of a highly conductive material in order to efficiently host Eddy currents with the least amount of resistance possible and also should have good magnetic properties. For the application of our project we will also need an extremely strong material that can withstand the stresses produced by the high rate of rotation, described in the constraints. We feel the main challenge in finding a disc material is finding a conductor that can withstand those stresses due to the high rate of rotation. We investigated the strengths, electrical resistivity, along with statement of other mechanical properties required in each metal for design and analysis.

A major challenge in determining a disc material is the balance between strength magnetic and conductivity properties. The three metals that will be discussed for tradeoff study have very low electrical resistivity to optimize eddy current flow. The different magnetic and other properties are analyzed below. 28

TABLE 3.3 Disc Material Comparison

Material

Ult. Tensile strength

Yield Strength

Poisson’s Ratio

Shear Strength

Shear Modulus

Electrical Resistivity

1018 Mild steel

63.8 kpsi

53.7 kpsi

0.303

42.0 kpsi

14.2 kpsi

1.18 × 107 Ωm

6061- T6 Aluminum

45.0 kpsi

40.0 kpsi

0.330

30.0 kpsi

3.77 kpsi

2.65 to 2.82 × 10-8 Ω·m

A36 Mild steel

59-79.8 kpsi

36.3 kpsi

0.300

40.0 kpsi

7.25 kpsi

7.2 × 10-7 Ωm

From the above we choose mild steel by comparing strength, magnetic and other properties.

Disc Orientation

The disc will have only one orientation. It will mount perpendicularly to the drive shaft and the electromagnets while mounted in the middle of the paired coils. One major concern comes in the form of how we will fasten the disk to the drive shaft. We will most likely pressure fit a collar around the drive shaft thereby attaching the disc by way of a pressure fit collar. If this is not feasible, we will conduct further review and brainstorming for a new way to fasten the disc to the drive shaft.

3.4 Electromagnet Design

The construction is having four and the pairs will be oriented with the polarities aligning North to South and the disc spinning between the pairs.

a change of

polarity in electromagnets applied to electromagnetic brakes will produce a higher force than only one direction of polarity. The electromagnet is mounted at the peripheral side of the disc as shown in fig 3.1: however upon testing it may conclude that having the magnets closer together may create the maximum torque. Further testing will either prove or disprove this concept. 29

The electromagnets generate the magnetic field needed. We will use standard coated aluminum wire coiled around a ferrous metal core. Coating the aluminum wire will prevent corrosion and increase the life of the electromagnets and maintain the efficiency of the overall braking system. The number of turns of copper around our ferrous material will determine the strength of the induced magnetic field. With the theoretical calculation: it determines how many turns are needed per coil as well as the amperage needed to provide the needed magnetic field, which determines the force generated.

After researching electromagnet design we determined a ferrous material, such as mild steel or iron, ideal for a metal core for electromagnets. When constructing the electromagnet components, the aluminum wire is coated and exposed core with a protective epoxy coating as to not leave the electromagnets exposed to the environment. The electromagnet is mounted in pairs in series connection one after another. By pairing the electromagnets and aligning north-south polarity it will direct and concentrate the magnetic field to ensure a perpendicular magnetic field with maximum possible magnitude. The magnetic pairs will produce a similar magnetic field to the illustration in figure 3.4.

Figure 3.4: Magnetic field lines of Coil Pairs

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Electromagnet Core Material

The electromagnet core material must have a high magnetic permeability which in most cases calls for a low carbon content. In determining the ideal core material we took into consideration two types of metals: Cast Iron and Iron Silicon magnetic soft steel.

Iron Silicon magnetic soft steel The soft steel in consideration is Kinetics MIM 2.5% Si-Fe Soft Magnetic Steel with material properties given in table3.4.

This material is commonly used in

Solenoid construction along other electromagnet applications such as armatures, relays, and other applications where low core loss and high electrical resistivity in AC and DC applications are required. We will also give property values for alternative soft steels which will take the place of Kinetics MIM 2.5% Si-Fe Soft Magnetic Steel if not available. The alternative soft steels are: •

AISI type 430 F(Se)

•

AISI 434

•

AISI 435

•

AISI 436

Table 3.4: Electromagnet Core Material Properties Material AISI 430 F(Se)

Composition Carbon: