DC Drive System Control Methods

The basics of DC drives have already been covered. At this point, it would be helpful to review several DC applications and summarize the characteristics of each system.

--- Transmit and receive fiber-optic connections ( Made by ABB Inc.). DC drive system in a printing press Color 1 Inkwell Color 2 Inkwell Color 3 Inkwell Color 4 Inkwell To Additional Process Accumulator Unwind Stand

Printing Press

A simplified diagram of a printing press, using a DC motor and drive system.

DC drives have been traditionally used in applications that require high starting torque. Printing presses have long been a prime candidate for DC drives and motors.

A very simplified drawing of a four-color offset press. In an actual press, more rollers would be seen, as well as a variety of sensors, limit switches, and transducers. In some cases, the press may be connected to a common line shaft (i.e., meaning only one motor, connected to a long shaft; each ink station would have a drive shaft connected to the common shaft by a gear box).

Each of the color stations impress a specific color onto the paper web. The key to the success of the press is the tension control and coordination between all of the stations. Each inkwell station and roller set is operated by an individual DC drive and motor. In many cases, the drive is located inside the machine, in a clean and dry cubicle with a constant stream of filtered air.

The lower pinch roll is operated by the drive, which is similar to how a surface-driven winder would be controlled. The upper roll carries the ink plate with the specific color. The accumulator is available to take up any slack in the web, before it moves to the next process (i.e., coating, folding, cutting, bundling, etc.).

This is a prime example of where high-speed fiber-optic communications is essential. Each drive needs to operate at a slightly faster speed than the previous drive. If that ratio of speed is done by each drive, the proper tension will be maintained on the web. If too much tension occurs, the web breaks and the machine must be stopped and re-threaded, which may take up to 1/2 hour to accomplish. If the web has too little tension, the web starts to bunch with the result being uneven ink transfer and wrinkled paper sent to the cutting process. In addition, the register could be off.

(The synchronization of all colors means colors printing exactly where they are suppose to print. Off register printing leaves the "shadow" effect, with a blurry image.) If the printing system is "tuned" properly, all the operator has to do is operate the system speed, to increase or decrease production. The entire coordination is in the automated electronics, not in manual manipulation of torque, speed, tension, and ink coloration.

As time goes on, some of the DC printing systems are being retrofitted with flux vector AC drive systems. As the technology of torque control with AC drives improves, this trend will continue for many years to come.

--- Ski lift system (chair lift)

--- Hand/auto control: 3-Phase; DC Dr DC Motor Process

Equip Voltage Frequency Variable Speed Proc Out Process Processing and logic Internal to Drive Hand Before Auto Before Hand Auto Start Fwd/Rew Start Fwd/Rew

Operator Coal Unload Movement Coal Unloader Coal Barge Movement Feed Conveyer

Ski Lifts

Another traditional DC drive system is that seen in ski lift units. These types of applications are also found at state fairs, theme parks, and any other location where above ground "people movers" are found. A simplified version of a ski lift system.

Additional components are found in ski lift systems. Sensors, current limit devices, safety limit switches, and monitors are just a few of the additional items found in modern lift systems. Typically the drive is located inside a clean, dry, heated room, which may or may not be in the loading building.

Quite often, the drive system is asked to start up at full torque, with rated capacity of people on board the chairs. In some cases, the entire system is manually controlled by an operator located in the loading building or a control station. The operator's job is to observe the system and change speed if necessary. In other cases, the system is operated automatically from the control station (auto), or by remote, from another location-possibly at the drop-off point (hand). This is only one instance where hand/ auto control is possible. There are a variety of other applications where hand/auto functions are required for convenience of the operators: indicates a diagram of hand/auto control.

Another DC system application is found in material handling. A barge unloader application with coal being the material. Coal is delivered by ship and brought to the utility by barge, which can maneuver into tighter locations compared with a freighter. This application would be found at coal-fired power utility plants or at any factory where material is delivered by ship or barge.

As seen, the barge, connected by cable to a DC motor, is pulled into position by a DC drive. Another cable, connected to the other end of the barge, would also be operated by a DC motor and drive (not shown). The function of that drive is to stabilize the barge and to control back tension. As the barge moves in the direction indicated, the other DC drive slowly moves the motor in the opposite direction, maintaining tension at all times. As the barge is slowly moving, the coal unloader rotates like a Ferris wheel, in the direction indicated. Using swivel-type scoops, coal is deposited onto the feed conveyor to be trans ported to the coal yard for storage or directly to the utility boiler system.

The operator controls the process, manually moving the coal unloader up and down and the gantry back and forth, to achieve maximum removal.

In this application, DC drives and motors work well for tension control of the coal barge. In this case, 50- to 100-HP DC motors can operate the application satisfactorily. The gantry motor and drive can also be DC or even AC and as low as 50 HP. Since precise torque or tension control is not required in the gantry system, a standard AC PWM drive will perform the functions required. Limit switches and joystick control are standard manual operator devices. (Note: Joysticks have a center off position. When pushed away from the operator, the drive speed is forward. When the joy stick is pulled toward the operator, the drive speed is reversed.) This automated control comes where the two DC drives and motors have to slightly "fight" each other to provide adequate tension control. There fore the DC drives have to coordinate in speed and torque reference to maintain the tension required. Too little torque could mean an unstable barge and coal not removed by the semi-automated process. Extra time and effort would be required to manually unload any remaining coal.

There are many more DC applications that use torque, tension, and precise speed control throughout the process. The above are only a few, but they do highlight the torque and tension capability of DC systems, as well as low-speed control.

--- Common DC bus configuration: Common DC BUS Supply section Diode or Thyristor Drive sections Braking sections Control Transformer 230/115 VAC Supply

Unit Inverter Braking chopper Resistor PLC Drives Tools; Monitoring .