What is Braking Resistor : Working & Its Applications

The controlling of speed in elevators, wind turbines, cranes, lifts, and electric locomotives is very necessary. So a braking resistor is an integral part of these applications because these are the most frequently used high power resistors in electric motor drives to control their speed, within industries like transport, maritime, and construction.

Electric trains are accepted mostly over diesel trains because they are recyclable and also regenerate energy. Generally, these trains produce a huge amount of energy when stopping or slowing down and this energy can be regained or dissipated. So a series of braking resistors are integrated for brakes within an engine system of an electric train. So, Kinetic energy can be changed into electrical at the traction motors of the engine system within dynamic train braking. The energy which is generated can be dissipated in the car body of the train in an onboard sequence of braking resistors. The essential factors for the growth of braking resistors are durability, reliability, and safety. So this article gives brief information on breaking resistor – working & its applications.

What is Braking Resistor?

Braking Resistor Definition: The resistor which is used to slow down or stop the speed of a mechanical system by generating a braking torque is known as a braking resistor. These resistors are designed with some specifications like resistance & average braking power. Braking resistors including smaller ohmic values will help to control the speed of a motor & dissolve more heat.

These resistors provide higher reliability with less service. So, these resistors are mostly chosen over friction brakes to control motors. A braking resistor is required where when there is frequent tripping on the drive due to overvoltage, reduced equipment life or high maintenance costs, or motor and drive damage and/or overheating. In general, we know that resistors consume heat and are used to slow down or stop a mechanical system.

The purpose of a braking resistor is to dissipate the energy which a motor generates during deceleration. As the braking resistor dissipates the energy, it helps prevent damage to the drive which could happen due to overvoltage.

Working Principle of Braking Resistor

The working principle of the braking resistor is, that a braking resistor helps to stop or slow down a motor by dissipating surplus voltage produced by the decelerating electric motor. This dissipated energy can be handled by the resistor to ensure that the increased voltage is kept to secure levels to avoid damage to the drive. So that equipment life can be extended, allow faster braking & avoids the overheating risk.

 The principle behind the braking resistor is very simple. When the motor is in working condition, it draws a high amount of current from the supply. When one wants to stop the load connected to the motor, then all he has to do is open or disconnect the supply to that motor. This small break in power supply will lead to an accumulation of energy by the load. How does this happen? It’s because a motor when suddenly stopped acts as an inductor and not as a resistor.

When an inductive load such as a motor is suddenly disconnected from its power source, it takes time for it to discharge its stored energy back into the power source. During this time, if there is no provision for discharging this energy, then it can lead to severe damage of the equipment connected to that load and even cause physical harm to any person nearby. To avoid such situations, we use braking resistors in series with loads such as motors which need immediate stopping when they are running at high speeds. That’s how a breaking resistor helps in braking a motor.

The braking resistor helps in discharging the stored energy and thus prevents damage.

Difference between Brake Chopper and Brake Resistor

The difference between the brake chopper and brake resistor includes the following.

Braking Resistor Circuit

There are many ways to avoid the failure of a braking resistor from an overload situation like brake-chopper transistor monitoring board, brake resistors using a thermal switch & intrinsically-secure brake resistors. From these three methods, we are explaining one of them like Brake Resistors using a Thermal Switch.

Brake resistors are used with a thermal switch for detecting a potential overload condition. Once a brake resistor is overloaded, then it tries to dissipate more heat than it was designed to do. So in this situation, the thermal switch can be released once the resistor’s temperature turns very hot. So interrupting one of the high voltage supplies to either the resistor or the VFD, the following two diagrams are implemented

In the above circuit, the thermal switch opens up in extreme heat. The control voltage utilized by the main contractor can be disrupted. So this opens the main contactor & detaches the power supply from the VFD.

The thermal switch will be opened from extreme heat. The control voltage used to shut the contactor toward the braking resistor can be interrupted. So this opens the contactor & detaches power from the resistor while the VFD remains powered.

Advantages

The advantages of the braking resistor include the following.

• Braking resistors in a motor control system prevent hardware damage.
• It avoids faults within a VFD.
• Braking resistors within VFD can enhance the capacity of braking torque.
• These resistors generate faster &more controlled braking.
• These resistors can cause large speed variations between motor & drive to feedback extra energy.
• These resistors are most frequently used in the industrial sector.
• These resistors are mostly preferred by experts because they are extensively more fitting for a variety of braking mechanisms.

Disadvantages

The disadvantage of a braking resistor is, that if this resistor is not cooled properly then this will degrade the component’s base material & leads to an early failure.

Applications

The applications of braking resistors include the following.

• Braking resistors are used where the speed of a motor exceeds the speed that is set through the variable frequency drive (VFD) otherwise when rapid deceleration is necessary.
• These resistors are used in a motor control system to avoid hardware damage & faults within a VFD.
• These resistors are applicable in train systems & motor speeds.
• These resistors are required in particular operations because the motor controlled through the variable frequency drive acts as a generator & power supply will be back to the VFD instead of to the motor.
• This resistor will help to stop or slow down a motor by dissipating surplus voltage produced by the decelerating motor.
• Braking Resistors are used in various motor-based applications which are controlled through the driver.
• The function of a brake resistor is to assist in performing motor braking otherwise repairing by absorbing restored braking energy & dissolving it like heat through rheostatic resistors.
• Braking resistors within VFD (Variable Frequency Drive) will enhance the capacity of braking torque, generating more controlled & faster braking.
• This resistor will dissipate regenerated power to maintain the bus voltage from exceeding the speed limit of the drive.

what is the difference between a breaking resistor and a bypass resistor?

A breaking resistor is used in conjunction with a contractor to prevent the current surge that occurs when a coil is de-energized. The resistor will limit the back emf and reduce the current surge. This is probably not required for most applications, but it does help extend contactor life. A bypass resistor is usually placed across a drive to provide reduced voltage to the motor during start-up. This helps prevent overloading of the drive and motor during start-up.

What is a dynamic braking resistor?

A dynamic braking resistor is a resistor that is connected to the motor in parallel to quickly convert its kinetic energy into heat through the resistor. The braking resistor of the inverter motor has two functions:

1) The braking current generated by the motor during deceleration can be converted into heat energy through the braking resistor to ensure that the inverter can be safely and reliably operated.

2. When the power failure occurs, the braking resistor can quickly dissipate the residual kinetic energy of the motor, thereby avoiding damage to the machine.

Thus, a Braking resistor is used in motor brake controlling within VFDs (variable frequency drives) mechanism for changing the energy from regenerated to thermal throughout the electric motor’s braking process. The main functions of the braking resistor are to protect VFDs and power supply lines. Here is a question for you, what is VFD?