F.A.Q. - Table of Contents
1. How accurate are Magtorx’s current regulated Digital Power Supplies? Magtorx’s Digital Power Supplies (DPS's) are designed for high performance and high accuracy. Magtorx Power Supplies have a current regulation accuracy lower than 1% of full scale range (some models show an accuracy between 1/4 and 1/2% - see Model EC-120 spec.) Back to Top2. Can Magtorx’s Digital Power Supplies (DPS) be used as a controller - for example, as a tension controller? Yes. Magtorx DPS's provide power and control for torque apparatus. The Programmable DPS allows the user to program and execute the program for white range of applications. It is possible to program (modify) the input signal-output current according to the process requirements. (See Table definition). Also, the Non-Programmable DPS can be used for simple control of tension and torque for many applications. Magtorx’s DPS's are a source of regulated current, specifically for electromagnetic particles, hysteresis, and friction brakes and clutches. The DPS is a device that supplies power for tension applications with open or closed loop control. For DPS utilization potentials also see applications, Current Scaling, Performance Matching and Software. Back to Top3. How can I control Magtorx’s Digital Power Supplies? All Magtorx’s power supplies can be controlled with the RS-232 port, with external control voltage 0 – 5 VDC or with the potentiometer. The 0-5 Vdc is scalable - see current scaling. The Programmable DPS, in addition, can be controlled by the program stored in the DPS memory or from the PC. The Programmable DPS can also control other devices - see Master-Slave applications. Back to TopMagtorx current regulated DPS is a digitally controlled device that has a programmable feature called current scaling. Scaling is an electronic adjustment of the power supply control signal to the desired current level. For example: if the maximum brake current demand is 600mA, the maximum Control Signal of 5Vdc can be scaled to 600mA. In other words, if the DPS maximum current output is 1800mA, the Control Signal level for 600mA without scaling would be 1.667V. After scaling, the maximum Control Signal of 5 Vdc is available for 600 mA. The reason for this feature is to ensure that the highest possible resolution of the Control Signal can be assigned to the full scale of controlled current.
Back to Top5. Why does Magtorx use toroidal transformers for its Power Supplies? The toroidal transformer offers excellent efficiency for a given size and weight. The winding configuration of toroidal transformers results in very low leakage inductance and excellent regulation. Since toroidal winding produces tight coupling, virtually all the flux is utilized - and not left to radiate and interfere with circuitry. This enables tight regulation. In addition, lower copper loss results in less power being wasted as heat. Toroidal transformers are very quiet in operation with low to zero mechanical hum caused by magnetostriction. This makes the toroid transformer a perfect choice for sensitive electronic systems such as the one used by Magtorx’s DPS's. Back to TopSince brake or clutch torque is proportional to current, a constant current is required to provide steady torque. If voltage were to remain constant, the current would drop as the coil resistance increases, therefore, the brake or clutch torque will drift. As an electromagnetic brake or clutch heats up during normal operation, the resistance of the copper coil can increase up to 25%. The increase in the resistance (load) must be compensated for by an increase in the input voltage, thereby maintaining a constant current flow. For the best torque regulation, a DC current regulated Power Supply is recommended. Magtorx current regulated Digital Power Supply, used for controlling magnetic particle or hysteresis brakes and clutches, will provide excellent current regulation for steady and consistent torque output. The DPS has the capability to be programmed for performance matching that is desirable for many applications. Without current regulation, changes in coil resistance due to changes of coil temperature contributes to torque drift. Smooth and regulated current generated by Magtorx’s DPS will eliminate torque drift. Back to Top7. What is the difference between current regulation and voltage regulation of power supplies? The operation of a current regulator is similar to that of a voltage regulator. The basic difference is that one regulates current and the other regulates voltage. Voltage regulation for brakes and clutches has limitations because of a change in coil resistance due to heat. Therefore, voltage regulation is not practical for use with electromechanical brakes and clutches. Back to Top8. What is a “Hysteresis Curve” of electromagnetic brake or clutch? The graph shown below represents a typical (BH) Torque vs. Current curve for an electromagnetic brake or clutch. The torque for a given amount of current is different when the current is increasing than when it is decreasing. This is due to the hysteresis phenomenon of magnetic material.
Back to Top9. What is "Performance Matching” of brakes or clutches ? Performance Matching is the electronic adjustment of the DPS device control signal to the corresponding current output. After matching, the brake or clutch, will perform alike at any point along their Torque-Current curve - with high accuracy. Contrary to electronic matching, the mechanical matching of hysteresis brakes can be performed only at one point of the Torque-Current curve - although with a rough accuracy of 4-10%. Due to the variability of magnetic materials, a coil’s resistance, components, and manufacturing tolerances, the electromagnetic particle and hysteresis brakes and clutches can’t meet the needs of critical tension control processes without a closed loop control. They simply don’t perform the same. In many processes that require multiple pay-offs, the individual control of each pay-off must utilize sensors for feedback, which isn’t economical. Therefore, matching of brake/clutch performances is an ideal solution. In some applications it is very desirable to “match” tension sources. Such applications include: tension control of multiple strings, fibers, cords, woven threads in the manufacturing of fabric, strands, braded wires, tires and other processes requiring multiple pay-offs (unwinding stations). Using “matched” brakes or clutches, it isn't necessary to individually control each pay-off which complicates the system with multiple sensors, which in turn means additional expenses and maintenance. The control of matched tension sources (brakes or clutches) may be simply achieved by using a closed loop control only on one tension device. The rest of them will follow the behavior of one that has a closed loop control. The maximum deviation from brake to brake or clutch to clutch at any point along their torque-current curve will depend on the accuracy of collected data.
The above left graph represents Torque-Current curves before matching and the one on the right represents curves that are matched. As you can see, all three matched curves are identical at any point along their Torque-Current curve. Back to Top10. Can Magtorx’s current regulated Digital Power Supplies be used for DC motors? Yes. Magtorx’s DPS's can be use for some small DC motors wherever there is a need for torque adjustment. The controlled current range has to stay within limit of the power supply. Back to TopHysteresis brakes and clutches rely purely on magnetic action working through an air gap to develop torque. They have an extremely wide torque range. Hysteresis brakes and clutches are generally similar in construction to such magnetic particle devices, except that the magnetic particles are eliminated and magnetic forces—rather than frictional forces—provide the clutching medium. Since torque is produced without physical contact of parts, hysteresis devices are not subject to wear. This feature makes them distinctly superior to mechanical-friction brakes and clutches in life expectancy, servicing requirements and consistency of performance. Since their working members have no physical contact they do not depend on mechanical friction. Therefore, hysteresis units are absolutely and constantly smooth at any slip ratio. Torque is reasonably independent of slip speed and is also directly proportional to coil current, making response time extremely quick. Hysteresis brakes and clutches are also the most repeatable braking and clutching devices known. They will repeat their performance precisely, an indefinite number of times, whenever operating factors are repeated. This makes it ideal for many precision tension control and testing applications. These devices have a number of advantages over magnetic particle brakes and clutches, in particular eliminating the problem of confining the magnetic particles inside the gap. These advantages include long life, environmental stability, precise repeatability and consistency of performance and extremely low power consumption. They can tolerate extreme temperatures and have high heat-dissipation capability. They also have the widest speed range of all electronically torque-control devices. Hysteresis units will outlast any other type of electromechanical unit. The transmitted torque remains constant and smooth as the hysteresis element is forced to rotate within the air gap and will respond to increases and decreases in coil current with corresponding increases and decreases in torque. Unfortunately, hysteresis brakes and clutches also suffer from a problem that is not experienced by magnetic particle devices. Under certain conditions, hysteresis brakes and clutches experience a salient-pole phenomenon called "cogging", an undesirable, pulsating output torque which prevents smooth and efficient operation of these systems - see Cogging Phenomenon. Back to Top12. What is the “cogging” phenomenon in Hysteresis Brakes or Clutches? Under certain operating conditions, it is possible to set up a salient pole condition on the brake rotor that can result in a “cogging” phenomenon referred to as torque ripple. The cogging condition, is often incorrectly diagnosed as a defective brake. Cogging torque in a hysteresis brake or clutch is the result of having established salient poles on the rotor’s magnetic material. The salient pole effect which causes the “cogging” phenomenon is a result of having removed the current from the device coil while its rotor was in a stationary position. The residual impression of alternating north-south polarities around the circumference of the magnetic element such as the rotor, cup, or disk will manifest themselves as numerous evenly spaced bumps, which can be felt throughout the rotation of the brake or clutch shaft. Cogging torque can be as high as about 20% to 30% of the full rated torque. The bumps will be most prominently noted when no power is applied to the brake or clutch coil, and will diminish or disappear altogether, as increasing levels of current are applied while the brake or clutch is in operation - see also de-cogging. Back to Top13. What is “de-cogging” and how can it be performed? The existence of the "cogging” phenomenon limits the utilization and desirability of hysteresis brakes and clutches. The cogged brake or clutch endures a pulsating, cogging torque that can cause problems for a great number of applications. The “de-cogging” of hysteresis brakes and clutches is the process of removing the residual impression of alternating north-south polarities around the circumference of the magnetic element such as the cup, rotor or disk. Performing de-cogging of a brake or clutch by removing or reducing the coil current during shaft rotation is impractical. Removing the cogging by rotating the rotor (even one revolution) using a permanent magnet is impractical, as well. Therefore, the most practical de-cogging solution of hysteresis devices is applying a proper amount of reverse polarity of a magnetic field.
By applying a reverse current to the coil at a very short (controlled) period of time, the magnetic element (rotor, cap or disk) endures reverse magnetization. As a result, the residual impression of alternating north-south polarities around the circumference of the magnetic element will be canceled. Back to TopElectromagnetic particle brakes and clutches operate on the same principles as other electromechanical brakes and clutches, but they are unique in their design because of the wide operating torque range available. In general, these devices are constructed with a gap between two armatures or poles and a shaft-mounted rotary member extending into the gap in spaced relation to the two armatures. The gap between the opposite sides of the rotary member and the adjacent armatures is filled with magnetic particles which function as the clutching medium. Magnetic particles are located in the powder cavity. Without applying any current, they sit in the cavity, however, when current is applied to the coil, the magnetic flux that is created, tries to bind the particles together. As the current is increased, the binding of the particles becomes stronger. Under application of a magnetic field between the armatures, the normally fluid particles become a kind of dense mass, thereby applying frictional forces to the rotary member and hence to the shaft. As the particles start to bind together, a resistant force is created on the rotor. When current is removed from the coil, the rotor is relatively free to turn. Like an electromechanical brake, torque to current is almost linear. Torque can be controlled accurately and can operate in a constant slip mode. Torque is reasonably independent of speed and is proportional to the current applied to the field, allowing stable torque throughout the units operating rpm range. All this makes electromagnetic particle brakes and clutches suitable for tension control applications, such as wire winding, foil and film tension control, and tape tension control. The advantage of using magnetic particle brakes and clutches is that they can create quiet high and reasonably stable torque. One disadvantage is that they also have some type of minimum drag associated with them and a problem of confining (after some period of time) the magnetic particles inside the gap. Another disadvantage of magnetic particle brakes and clutches is that at low speed operation a problem of keeping magnetic particles (powder) homogeneously spread into the working zone causes uneven and unpredictable torque. Low rotation speed (>15 RPM) can result in fluctuation of torque. Therefore, the use of particle brakes or a clutches at very low RPM may not be feasible. Although the particle brakes and clutches can operate well only at horizontal shaft mounting configuration, other mounting arrangements should be consulted with manufacturer of particular device. Back to Top15. What is an electromagnetic friction brake or clutch? Electromagnetic friction brakes and clutches are the most common type of electromechanical devices. The applications for friction brakes and clutches includes packaging machinery, printing machinery, food processing machinery and factory automation. Electromechanical brakes and clutches operate via an electric actuation by transmitting torque mechanically, by friction. When a device is required to actuate, current is applied, the coil is energized creating a magnetic field that turns the coil into an electromagnet that develops magnetic flux. This flux is then transferred through the small air gap between the field armature and the rotor. The rotor portion becomes magnetized and sets up a magnetic loop that attracts the armature. The armature is pulled against the rotor and a frictional force is applied at contact. Since the brake or clutch coil is mounted solidly (most of the time), only the hub and shaft slips or comes to a stop as desired. In some types of brakes and clutches the air gap automatically adjusts as components wear, allowing for a constant air gap. In most designs, springs hold the armature away from the rotor surface when power is released, creating a small air gap. All this enables maintaining a smooth and repeatable operation. The friction armatures engage rather smoothly, but sometimes it will create a chattering noise due to the materials friction phenomenon. An appropriate selection of friction material will help in sustaining quieter operation. Magtorx DPS's used as a power sources for friction brakes and clutches applications allows for capability of Soft Start, Soft Stop and Current Scaling see the spec of Magtorx Model XX-ON-120. Back to Top
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