FIVE SOLUTIONS TO BRAKING CONTROL
As we try to improve the performance and productivity in industrial process where stopping or slowing down an overhauling load is important, the capability to manage the load is essential. A few common applications include dynamometers, test stands, and punch presses.
There are many solutions to incorporate braking control into an application. Depending on the desired level of performance and control, the technology being used in the drive, the physical demand and characteristics of the application, this article will help you determine the appropriate solution for the application.
What is braking control?
Braking is essentially the removal of stored motion (kinetic energy) from a mechanical system. When the motor and attached system are brought up to speed, electricity is added and converted in to the motion of the system. To stop or slow down the system, the kinetic energy (that has previously been stored) in the mechanical system must be removed. There are four reasons and examples of applications that may need the addition of braking capability:
- To slow or stop the application – e.g., Conveyors
- To reverse the direction of the application – e.g., Fans
- To hold the application in a fixed position – e.g., Hoist and Cranes
- To provide a load or hold back an overhauling load – e.g., Dynamometers
What happens to the stored energy?
It is important to understand how the energy is removed and what happens to it.
When braking is applied to an application, the kinetic energy is reduced or removed. In a lot of applications, the braking methods that are applied converts the energy into heat. These methods can be wasteful. Other solutions include the use of internal or external methods, and at the point of the motor with a mechanical brake.
With a mechanical brake, the energy is removed directly from the system by the brake shoes or and brake disc that converts the motion in to heat. Alternatively, if a drive is being used, the energy can be removed in the form of electrical energy. Here are some key factors to consider in determining the best solution.
- How much braking is needed? Is full braking torque needed or a slow down?
- How controlled does the stop or slowdown need to be? Tightly controlled stops, or some pre-set level?
- How much shock can the mechanical system take?
- Is there a need to brake continuously, or intermittently?
- How quickly does the brake need to respond?
- How much am I willing to pay to add braking capability to my system? What is the price/performance tradeoff?
Five types of braking control
In order to enhance the performance or if it is necessary in a application to have a form of braking control, then one of five basic types of electrical braking can be implemented:
DC injection braking, quick stop, dynamic brakes/choppers, flux braking, or line regeneration.
One - DC Injection Braking
In DC injection braking is the most basic form of braking, the AC drive regulates a pre-set DC current in the motor windings, making a fixed magnetic field in the motor. The heat is created as the energy in the system is converted in the motor. The DC current also can be maintained after the motor stops to hold it in position. DC injection is a standard feature on most drives today. This form of braking is normally used under light loads and that prolonged usage may cause damage to the motor.
Two - Quick Stop Braking
Quick stop braking provides a performance improvement over DC injection braking, and used with AC Volts/Hertz drives. The quick stop method reduces the drive output frequency and regulates the output current until the motor stops. Since no additional hardware is required, quick stop is easy to implement. You must remember that a "quick stop" converts the system’s energy into heat at the motor and may damage the motor. This type should be used intermittently.
While these braking methods convert the system motion into heat at the motor, other forms of technology extract the energy through the motor leads and bring it into the drive.
Three - External dynamic brakes and choppers
External dynamic brakes and choppers are utilized to remove this energy from the capacitor bank of the drive. The dynamic brake or chopper consists of a power-switching device and a resistor bank. The IGBT or SCR device removes the energy by regulating the current in the resistor bank, where the energy is converted into heat. The dynamic brakes or chopper units can be sized or paralleled to get as much braking as necessary, with the dynamic response of the system limited only by drive performance. One drawback of dynamic braking is that it is inefficient for continuous or high duty cycle operation because it produces excessive heat. Also a large resistor bank is needed for high-duty cycle operation which can become costly.
Four - Flux Braking
Flux braking is a method that can be implemented in drives with field-oriented control. The flux braking method dramatically increases the flux-producing component of the motor current, resulting in increased motor losses. As with several of the above methods, flux braking converts the system energy into heat at the motor, and should be used intermittently to avoid motor damage.
Five - Line Regeneration Braking
Line regeneration is another method of removing energy from the mechanical system. Line regeneration systems feed the energy from the system back onto the AC supply control, and is the most effective method for continuous braking applications such test stands. Depending on the design and application of line regeneration systems, additional benefits can be realized such as power factor correction and line current harmonic reduction. The biggest drawback of line regeneration is that it is the most expensive hardware solution available.
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