Electromagnetic brake
On trams and trains, an electromagnetic brake is a track brake where the braking element is pressed by magnetic force to the rail, i.e. the braking is by friction, not the magnetic effect directly.
This is different from an Eddy current brake where there is no mechanical contact between the braking element on the moving vehicle and the rail.
This is also different from other forms of a track brake, where the braking element is pressed on the rail by mechanical means.
Introduction
Imagine the coil shell as a horseshoe magnet (figure A-1) having a north and south pole. If a piece of carbon steel bridges both poles, a magnetic circuit is created. In a clutch, (A-5) when power is applied, a magnetic field is created in the coil. This field (flux) overcomes the air gap between the clutch rotor and the armature. This magnetic attraction pulls the armature in contact with the rotor face. A brake is simpler because the armature is only being pulled against the brake field (A-3). The frictional contact, which is being controlled by the strength of the magnetic field, is what causes the rotational motion to start or stop. Almost all of the torque comes from the magnetic attraction and coefficient of friction between the steel of the armature and the steel of the rotor or brake field. For many industrial brakes, friction material is used between the poles. The material is mainly used to help decrease the wear rate. But different types of material can also be used to change the coefficient of friction (torque) for special applications. For example, if the brake was required to have an extended time to speed or slip time, a low coefficient material can be used. Conversely, if the brake was required to have a slightly higher torque (mostly for low RPM applications), a high coefficient friction material could be used.
The construction of the coil shell for the brake is very similar. The coil shell is made with carbon steel which has a good combination of strength and magnetic properties. Copper (sometimes aluminum) magnet wire is used to create the coil which is held in the shell either by a bobbin or by an epoxy/adhesive. For most industrial brakes, friction material is then placed over the coil and is set between the inner and outer pole. The friction material is flush with the surface of the brake to gain metal-to-metal contact between the coil shell and the armature.
Basic Operation
In a brake, there are only three parts: field, armature, and hub (which is the input on a brake) (B-2). Usually the magnetic field is bolted to the machine frame (or uses a torque arm that can handle the torque of the brake). 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. Like the clutch, this can happen very fast. If required, and within a small range, the brake time to speed and stop can be controlled by voltage/current.
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.
Voltage/Current - And the 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 clutch. If you had a 90 volt clutch, a 48 volt clutch and a 24 volt clutch all being powered with their respective voltages and current, all would produce the same amount of torque. However, if you took a 90 volt clutch and applied 48 volts to it you would get about half of the correct torque output out of that clutch. This is because voltage/current is almost linear to torque.
A constant current power supply is ideal if you want 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 your torque will be by about an average of 8% for every 20°C. If the temperature is fairly constant, but you’re not sure if you have enough service factor in your design for minor temperature fluctuation, you can compensate for flux degradation by slightly over-sizing the brake. This will allow you to use 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.
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 pull in an armature. Within this first scenario there are two factors affecting this. The first one is the amount of ampere turns in a coil which will determine the strength of the magnetic field. The second one is air gap, which is the space between the armature and the face of the brake. Magnetic lines of flux diminish quickly in 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 against either the rotor or the brake face. 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 rotor will wear creating a larger air gap which will change the engagement time of the brake. In high cycle applications where registration is important even the difference of 10 to 15 milliseconds can make a difference in registration of the driven material. 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.
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 accelerate or decelerate. This is referred to as “time to speed” or “time to stop”. In reality, this is what the end customer is most concerned with. Once it is known how much inertia is present for the clutch to start or stop then the torque can be calculated and the appropriate size of clutch can be chosen.
Most CAD systems can automatically calculate component inertia, but the key to sizing a clutch is calculating how much inertial is reflected back to the brake. To do this, engineers use the formula: T = (WK2 × ΔN) / (308 × t) Where T = required torque in lb-ft, 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.
Burnishing - What is it and why is it Important?
Burnishing is the wearing in or mating of opposing surfaces. When armatures, rotors and brake faces are produced the faces are machined as flat as possible. Some manufacturers also lap the faces to get them smoother. But if you were to look at them under a microscope you would see that the machining process leaves peaks and valleys on the surface of the steel. When a new “out of the box” clutch is initially engaged most peaks on both mating surfaces touch which means that the potential contact area can be significantly reduced. In some cases you can have out of box clutches that only have 50% of their torque rating before burnishing.
Burnishing is the process of cycling the clutch to wear down those initial peaks to increase the contact surface between the mating faces.
Even though burnishing is required to get full torque out of the brake, it may not be required in all applications. Simply put, if the application torque is lower then the initial out of box torque of the brake, burnishing would not be required; however, if the torque required is higher, then burnishing needs to be done. In general this tends to be required more on higher torque brakes than on smaller low torque brakes.
The process involves cycling the brake a number of times at a lower inertia, lower speed or a combination of both. Burnishing can require from 20 to over 100 cycles depending upon the size of a clutch and the amount of initial torque required. For bearing mounted clutches where the rotor and armature is connected and held in place via a bearing, burnishing does not have to take place on the machine. It can be done individually on a bench or as a group at a burnishing station. Two piece brakes that have separate armatures should try to have the burnishing done on the machine verses a bench. The reason for this is if burnishing on a two piece brake is done on a bench and there is a shift in the mounting tolerance when that brake is mounted to the machine the alignment could be shifted so the burnishing lines on the armature, rotor or brake face may be off slightly preventing that brake from achieving full torque. Again, the difference is only slight so this would only be required in a very torque sensitive application.
Torque - It may not be what you think
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In the previous paragraph we discussed how burnishing can affect initial torque of a brake but there are also factors that affect the torque performance of a clutch in an application. The main one is voltage/current. In the voltage/current section we showed why a constant current supply is important to get full torque out of the brake.
When considering torque, do you need a dynamic or static torque in your application? For example, if you’re running a machine at relatively low rpm (5 – 50 depending upon size) then you’re really not concerned with dynamic torque since the static torque rating of the brake will come closest to where you are running. However, if you are running a machine at 3,000rpm and you think that you are going to apply the brake at its catalog torque at that rpm you are mistaken. Almost all manufacturers put the static rated torque for their brakes in their catalog. If you are trying to determine a specific response rate you need to know what the dynamic torque rating is for that particular brake at the speed you are running. 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 clutch or brake. (T-1)
Over Excitation - What is it and when it should be used?
Over-excitation involves the momentary application of a higher-than-nominal voltage to achieve a faster response time. Three times the voltage typically gives around 1/3 faster response. At fifteen times the normal coil voltage response time drops by two thirds. For example, if you had a clutch coil that was rated for six volts you would need to apply 90 volts to achieve the three-fold improvement.
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, you want the coil to generate as much of a magnetic field as quickly as possible to attract the armature and start the process of acceleration or 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 provided the higher voltage provided does not cause the coil wire to overheat.
Over excitation can also be used in electromagnetic spring applied holding brakes. In this type of application the increased magnetic field hleps to overcome a large spring force; but once the brake is engaged, coil voltage can be reduced to hold back the force of the springs. This allows design engineers to reduce the size of the brake, saving bosh cost and weight.
Wear
It is very rare that a coil would just stop working in an electromagnetic brake. Typically if a coil fails it is usually due to heat which has caused the insulation of the coil wire to break down. That heat can be caused by high ambient temperature, high cycle rates, slipping or applying too high of a voltage.
The main wear in electromagnetic brakes occurs on the faces of the mating surfaces. Every time a brake is engaged during rotation a certain amount of energy is transferred as heat. The transfer, which occurs during rotation, wears both the armature and the opposing contact surface. Based upon the size of the brake, the speed and the inertia, wear rates will differ. With a fixed armature design a brake will eventually simply cease to engage. This is because the air gap will eventually become too large for the magnetic field to overcome. Zero gap or auto wear armatures can wear to the point of less than one half of its original thickness, which will eventually cause missed engagements.
Designers can estimate life from the energy transferred each time the brake or clutch engages. Ee = [m × v2 × τd] / [182 × (τd + τl)] Where Ee = energy per engagement, m = inertia, v = speed, τd = dynamic torque, and τl = load torque. Knowing the energy per engagement lets the designer calculate the number of engagement cycles the clutch or brake will last: L = V / (Ee × w) Where L = unit life in number of cycles, V = total engagement area, and w = wear rate.
Backlash
Some applications require very tight precision between all components. In these applications even a degree of movement between the input and the output when a brake is engaged can be a problem. This is true in many robotic applications. Sometimes the design engineers will order clutches with zero backlash but then key them to the shafts so although the clutch or brake will have zero backlash but minimal movement remains between the hub or rotor in the shaft.
Most applications, however, do not need true zero backlash and can use a spline type connection. Some of these connections between the armature and the hub are standard splines others are hex or square hub designs. The spline will have the best initial backlash tolerance. Typically around two degrees but the spline and the other connection types can wear over time and the tolerances will increase.
Environment / Contamination
As brakes wear they create wear particles. In some applications such as clean rooms or food handling this dust could be a contamination problem so in these applications the brake should be enclosed to prevent the particles from contaminating other surfaces around it. But a more likely scenario is that the brake has a better chance of getting contaminated from its environment. Obviously oil or grease should be kept away from the contact surface because they would significantly reduce the coefficient of friction which could drastically decrease the torque potentially causing failure. Oil midst or lubricated particles can also cause surface contamination. Sometimes paper dust or other contamination can fall in between the contact surfaces. This can also result in a lost of torque. If a known source of contamination is going to be present many brake manufactures offer contamination shields that prevent material from falling in between the contact surfaces.
In brakes that have not been used in a while rust can develop on the surfaces. But in general this is normally not a major concern since the rust is worn off within a few cycles and there is no lasting impact on the torque.
See also
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