A motor suddenly loses its load and responds with a burst of acceleration. Because very few applications desire such a reaction, engineers must find a way to control or prevent it. While this is only one example of speed control, other applications usually require a similar form of control. A way to provide it is to add braking to the motion system.
Braking is essentially the removal of kinetic energy from a mechanical system. As a motor and its load accelerate, electrical energy is added and stored as kinetic energy. Braking removes the kinetic energy by slowing the motor or bringing it to rest. Braking also removes energy when it reverses motor direction, stops and holds the motor in a fixed position, or holds back an overhauling load.
Braking action can be achieved mechanically through a conventional brake that operates through friction, turning nearly all of the kinetic energy into heat and moving it into the frictional elements.
Electrical braking is another method. By using a motor drive, this method dissipates the energy as heat or as electrical energy that is eliminated or returned to the ac utility. For this method, engineers have several electrical solutions to choose from.
System considerations
Which solution is chosen, though, depends on how it will affect system performance. For example, engineers need to determine whether the system needs just enough braking torque to gradually bring down motor speed or whether it should be maximum braking torque.
Second, the amount of stopping torque may need to be tightly controlled, applied gradually, or activated at some pre-set level to prevent damaging shock loads. How much shock the mechanical system can handle will influence this consideration.
Then, engineers should decide whether the application needs continuous or intermittent braking. If the load is overhauling, or if the drive is a load to another piece of equipment, continuous braking is necessary. Stopping a spindle for a tool change is an example of an operation needing intermittent braking.
Another factor to consider is how quickly the brake must respond. For example, is a delay of up to 100 msec acceptable or does the application need faster response? The last consideration is to determine how much it will cost to add braking and whether this meets price/performance criteria.
A look at the options
There are four basic types of electrical braking: dc injection braking, dynamic brakes or choppers, flux braking, and line regeneration.
Dc injection braking. Additional devices, such as power switches or resistor banks are not needed for this method. Only the drive is used. To slow or brake a motor, it sends a preset dc current into one set of motor windings. This alters the motor's magnetic field, which results in the motor slowing or stopping. Engineers must pre-program the magnitude of the current and how long it will be applied, which means they need to know the worst-case stopping conditions beforehand. Once the motor stops, though, the dc current can hold it in position.
One drawback is that it may be difficult to control the amount of braking as the dc current is pre-set. Also, because the energy dissipates in the form of heat in the motor, high duty cycle applications may reduce motor life.
Dynamic brakes and choppers. While dc injection and flux braking methods turn the system energy into heat in the motor, the drive also can extract the energy through the motor leads and bring it in, where external devices such as dynamic brakes and choppers remove the energy.
A dynamic brake or chopper consists of a power switching device and a resistor bank. The power switching device removes the energy from the drive by regulating the current in the resistor bank, where the energy turns into heat. Engineers can size or group any number of these units in parallel, up to the current limits of the drive and motor, to get as much braking as necessary. Only the capabilities of the drive limit dynamic response of the system.
Dynamic braking can produce excessive heat in the resistor grid, so it can be inefficient for continuous or high-duty-cycle operation. To operate in these applications, the system needs a large, expensive, resistor bank. If the resistor is mounted indoors, an air conditioning system often ends up removing the excess heat as it controls the temperature of the room. However, this arrangement is not very efficient.
Flux braking. Drives with fieldoriented, or flux vector, control can implement flux braking. The drive independently controls the flux-producing and torque-producing currents in the motor. It increases the flux-producing component when it's time to brake, resulting in increased motor losses in the form of heat. As with dc injection braking, this method converts the system energy into heat in the motor, so it should be used intermittently to avoid reducing motor life.
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Line regeneration. This type of braking essentially removes energy from the mechanical system and moves it into the capacitor bank. The bank then feeds the energy, now in the form of voltage, back into the ac supply system.
Depending on the design and application, line regeneration systems can enhance power factor and reduce line current harmonics. This method is currently the most efficient for continuous braking applications, such as dynamometers and test stands.
The biggest drawback is that it is also the most expensive hardware solution available. The electrical savings, though, may justify this expense in high duty cycle or continuous braking applications.
Stretching performance
The saran wrap that protects your food is plastic blown-film. So is the gigantic tarp that protects larger objects, such as lawn furniture, boats, and food crops. To attain the flawless and uniform thickness of each sheet or roll of film requires precise web tensioning. At Brampton Engineering, Brampton, Ontario, however, the drives controlling the nip and roller equipment would often trip, resulting in film with uneven thickness and other imperfections.
Too often, the drives sensed an overvoltage condition when haul-off equipment and rollers moved film from the four-story tall extruder to the nips and winder equipment. From their analyses, engineers discovered that the overvoltage was the result of an overhauling load. Apparently, a section of film on a winder would pull on another section creating the excessive load, which eventually tripped the drive.
In 1994, engineers installed adjustable-speed ac drives to gain better speed control, but the drives were often ineffective in the lower speed ranges. So the engineers looked into braking. They decided against line regeneration drives because of the cost and chose dynamic braking instead. It made the drives so responsive, that the engineers could reduce the acceleration and deceleration times from a few seconds to as little as 0.5 sec, reducing the potential for overhauling loads.
Doug Weber is a product manager at Allen- Bradley Standard Drives Business, Rockwell Automation, Mequon, Wis.