What is Braking Resistor? – When the drive often trips owing to overvoltage, shortened equipment life or high maintenance costs, damaged motor and drive, and/or overheating, a brake resistor solution is required. Generally speaking, we are aware that resistors generate heat and are used to halt or slow down mechanical systems. A brake resistor’s job is to dissipate the energy that a motor produces when it slows down. The brake resistor helps shield the drive from harm that may result from overvoltage by dissipating the energy.
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What Is a Braking Resistor?
A mechanical system can be made to move more slowly by taking use of resistors’ capacity to dissipate heat. This technique is known as dynamic braking, and the resistor in question is known as a dynamic braking resistor (or simply a brake resistor). Kinetic energy is converted back into electrical energy to slow down an electric motor. With the use of a power resistor, this energy is lost. Rheostatic and regenerative dynamic braking are also possible. The energy lost during rheostatic braking is converted to heat in a resistor. The electric power is recycled back into the system during regenerative braking. The cost of the last choice is often greater. In addition to being employed in huge structures like trains and trams, brake resistors are also used in (small) motion systems. The speedier deceleration and reduced wear and tear over friction braking systems are significant advantages.
How Does A Braking Resistor Work?
The motor is stopped or slowed down by a brake resistor if the decelerating motor produces too much electricity. When a motor’s electricity is turned off, the load is reduced and the motor starts to revolve more quickly. In this manner, the motor begins to function as a generator, producing electrical energy that subsequently returns to the power circuit.
If the voltage increases too much, the motor may trip due to overvoltage or the drive may be harmed. By dispersing the energy produced by the sluggish motor, a brake resistor helps in this situation. By making sure the voltage rise stays within the limit, a brake resistor guards against damage to the drive. By allowing for quicker braking, reducing the wear on brake components, and removing the possibility of overheating, it also contributes to equipment longevity.
Brake Resistors For Variable Frequency Drives
We are aware that when disconnected from the power source, the majority of DC MOTORs begin to function as generators. Their persistent magnets are the cause of it. A power resistor can be connected as a load in order to disperse the generated energy.
The stator of these motors contains spinning magnetic fields that produce an induced magnetic field, which is crucial to comprehend. Because of this, brake resistors are very helpful in situations when a quick deceleration is required or if the motor speed exceeds the speed limit imposed by VDF. Even at higher torques, brake resistors can still offer regulated braking. When the rotating speed is greater than the synchronous speed, the device begins to function as a generator. Additionally, the extra energy will be sent into the VFD at that point.
In other words, if the speed differential between the motor and the drive is greater, more energy will be fed back. Therefore, experts advise selecting a braking resistor of the proper size. A resistor will quickly stop a motor if its ohmic value is lower (which will eventually generate more heat). The brake resistor must be larger in order to solve this problem. To make sure it stays within the predetermined temperature limits when braking, the resistor’s power rating is calculated by the designer.
Brake Resistors in Trains
A significant amount of energy must be lost in order to stop a train. Simply put, conventional disc brakes need too much wear and upkeep. Therefore, dynamic braking is frequently utilized as an extra mechanism. The locomotive’s traction motors convert the train’s kinetic energy into electric current during dynamic train braking. Frequently, this device is added to a friction braking system. Large onboard banks of resistors dissipate the produced current under the locomotive car body. The resistors are protected by large air blowers that cool them. Thermal monitoring is a further defense mechanism. The train will switch to friction braking if the resistors get too warm. Dynamic brakes are currently a common feature on diesel trains, but in the past they were only utilized in mountainous areas where extended descents may cause the standard brakes to overheat. One may hear the humming sound of the dynamic brakes being used when a train with them is going by.
When Choosing A Brake Resistor, What Should Be Taken Into Account?
The resistance value and the resistor’s ability to dissipate power are the two key considerations once it has been determined that a brake resistor is necessary for the application.
The Lowest Resistance Value
A “chopper circuit” or brake transistor will be included in VFDs that employ a brake resistor. The brake transistor shunts current from the DC bus over the brake resistor when the voltage on the DC bus rises too much. The maximum current value and duty cycle will normally be provided by the VFD manufacturer. This braking transistor circuit has current limits.
Since V=IR, a reduced resistance will result in a greater current if the voltage remains constant. It is therefore easy to determine the minimal resistance that would maintain the current value below the braking transistor’s maximum current rating if the maximum voltage is known to be the KEB over voltage level of 840VDC. The minimum resistance value is necessary to make sure the resistor functions correctly with the VFD even though it has no impact on how well it operates or can dissipate power.
Capacity for Dissipating Power
The capacity of the brake resistor to dissipate power is the second factor to be taken into account. The power that KEB brake resistors may safely dissipate when utilized continuously (PD) as well as three values for intermittent duty are mentioned. The total number of seconds that the resistor is utilized over the course of two minutes is shown by each of the numbers in P6, P25, and P40. For instance, the KEB 10BR100-1683 resistor can complete two cycles of three seconds each over the course of two minutes, or it can safely dissipate up to 2200W for one stretch of six seconds.
It must be taken into account how much energy will be created back toward the drive that will need to be dissipated now that it is understood what resistance values will safely function with the VFD and the power dissipation capacities of the individual resistors. This will guarantee that the brake resistor of your choice has the capacity to safely dissipate the energy produced by the motor. Calculation is the first method of doing this. If the mass moment of inertia of the motor and load, the motor torque, the speed change, and the duration of the deceleration are all known, the power produced by the motor can be calculated. The brake resistor handbook has further details on how to do these calculations. However, it might be challenging to determine and/or compute the mass moments of inertia in real-world applications, particularly for the load. Because of this, it is frequently required to utilize a testing procedure to establish the correct power size of the brake resistor.
As a general rule, more power has to be dissipated the heavier the load and the quicker the deceleration. However, to obtain a more precise picture of the brake resistor consumption, it is possible to record the drive’s DC bus voltage during operation by using the scope feature in the Combivis 6 software. With the scope, it is possible to check if a resistor with more power dissipation is needed or whether the current resistor is large enough. In the latter case, it could be feasible to modify the procedure to enhance performance, for example, by accelerating the deceleration.
FAQs about Barking Resistor
1. What does an inverter brake resistor do?
DBRs (dynamic braking resistors) for DC drive systems and inverters A generator can also be used as a driving motor. All of the energy is used to heat the resistor; some of it is lost instantly, and the rest is lost throughout the course of the cooling process.
2. What’s the mechanism of a dynamic braking resistor?
Using a train vehicle’s electric traction motors as generators to slow down the locomotive is known as dynamic braking. If the produced electrical power is lost as heat in the brake grid resistors, it is referred to as rheostatic, and if it is restored to the supply line, it is referred to as regenerative.
3. What purpose does DBR serve?
In addition to producing braking torque, dynamic braking resistors (DBRs) also serve to absorb the substantial energy released while stopping electric motors. Elevators, cranes, and trains all have variable-speed drive systems that employ them.
4. How does a braking resistor work?
A dynamic braking resistor’s job is to slow or abruptly halt a motor by dissipating extra electricity and maintaining safe limits. The surplus voltage is released as heat by our rheostatic resistors.
5. What is a regenerative resistor?
Regenerative resistors are frequently used in servo systems to capture the kinetic energy of braking or decelerating servo axes. A servo drive with a motor can function as an energy source or a generator of energy.
6. How do railway brakes operate?
For freight trains, a fully charged brake pipe is normally 70-90 psi (4.8-6.2 bar; 480-620 kPa), and for passenger trains, 110 psi (7.6 bar; 760 kPa). When the engineer switches the brake handle to the “service” position, the brakes are activated, resulting in a drop in pressure in the train pipe.
7. What benefits does the regenerative braking system offer?
A regenerative brake is an energy recovery device that slows down a moving item or vehicle by transforming its kinetic energy into a form that may either be utilized right away or stored for later use.
8. What does a hybrid car’s regenerative braking system do?
They are aware that the term “regenerative” in the context of regenerative braking refers to the process of collecting the kinetic energy (vehicle momentum) and converting it into electricity that recharges (regenerates) the onboard battery when the vehicle slows down or stops.
9. Which automobiles employ regenerative braking?
The name of this system is regenerative braking. These brakes are now most common in completely electric automobiles like the Tesla Roadster and hybrid cars like the Toyota Prius. Maintaining a fully charged battery is crucial in cars like this.
10. What exactly is KERS?
The kinetic energy of a moving vehicle can be recovered via an automobile device called a kinetic energy recovery system (commonly abbreviated KERS or kers). The recuperated energy is kept in reserve (such as a flywheel or high-voltage batteries) for use during acceleration in the future.
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