Drive System Control Methods--Review/Quiz

Summary

Expenses incurred in selecting and specifying drive systems tend to be lowest for the type of equipment that is the most familiar to the specifier.

The equipment supplier can help to reduce this expense by providing application information and assistance. Ultimately, it’s the user that must be satisfied with the purchase of the drive system. The most efficient use of system capabilities will be obtained if up-front time is taken to review the application, and match the system with the requirements.

Review

Drive systems operate in a coordinated fashion, with control between the controller (PLC) and the drive unit. A variety of sensors, switches, and transducers are a part of the overall scheme of automation.

Proportional integral derivative control is used when automatic control of some quantity is required. Temperature, pressure, and humidity levels are just a few of the items that can be conveniently controlled by PID.

Tension control is a major part of any coordinated system that processes web material. Dancer control is similar to tension control, in that a separate regulator signal is fed back to the drive for correction to take place.

Proportional gain and integration time play a part in the tuning of a web system.

A variety of remote operator devices are available for interfacing signals to the drive unit. Remote-operator stations are the simplest form of remote control. Standard I/O would include start/stop, speed reference, digital inputs, analog outputs, and relay outputs. Both sinking and sourcing control are used in industry today.

Serial communications is the simplest form of communication link to a drive. Typically, multiple drives are controlled by one system controller, which could be a computer that is set up to talk to the protocol that is installed in the drive. Fiber-optic communications has the highest immunity to noise compared with other forms of drive communications. Optical fibers are connected in a ring structure and can be connected with plastic or glass fiber. Building automation systems or Ethernet systems are able to talk to many drives on the market today.

DC systems have traditionally been associated with printing press, ski lift, and material handling applications. A major benefit of DC is the high starting torque at zero speed. AC systems have their roots in energy-saving applications. Substantial energy savings can be realized using VFDs instead of fixed-speed outlet damper control. Additional applications for AC drives include conveyors, overhead gantry units, overhauling loads, etc.

A multitude of questions should be asked when comparing AC systems with DC systems. Initial procurement, operating, and maintenance costs need to be analyzed over the life of the equipment to be installed.

QUIZ

1. Name three or more devices that are used in closed-loop systems for setpoint or feedback conditions.

2. Why is proper tension control important in a web-fed system?

3. What is a dancer control?

4. What are jumpers or DIP switches used for when connected to analog input signals?

5. What is the difference between sinking and sourcing control logic?

6. What is meant by serial communications? How is it used with drives?

7. What are termination resistors and why are they used?

8. What is the advantage of using fiber optics instead of serial communications?

9. What is the benefit of using DC drives in applications such as printing and ski lifts (chair lifts)?

10. What is the common DC bus configuration and why is it used?

11. How is a VFD used to save energy in an outlet damper application?

12. What are the most significant items to be reviewed when evaluating AC or DC drive systems?

-- Answers --

1. Pressure, humidity, temperature transducers, potentiometers, a variety of electro-mechanical devices that change voltage output given the input signal.

2. Tension control is required or the product quality will suffer. Too little tension on the web will cause bunching and poor quality of the product.

Too much tension could stretch the material, causing a thinner product, or in the worst case, cause the web to break.

3. A dancer is a device that changes resistance per the amount of force being applied. This device is typically used to monitor and regulate the amount of tension placed on web-fed material.

4. Jumpers or DIP switches match the analog input signal with the drive input setting. Typical input signals are 4(0) to 20 mA and 0 to 10 VDC.

5. Sinking logic is where the control voltage is tied directly to circuit common. A command is done when a contact closure is between the terminal and ground. Sourcing logic is where the control voltage is directly applied to the appropriate terminal. All circuit commons and ground are tied together.

6. Serial communications is the transmission in sequence of bits of data to or from a controller device. Communications speeds range from 4800 to 19,200 baud. Drives are wired in parallel (daisy chained) to the control device. The controller could be a computer or any device that talks the same language as the drive. The drive may need a language "interpreter" if the language is not pre-loaded in the drive.

7. Termination resistors reduce the introduction of electrical noise into a communication system. They are placed (terminated) on the first and last drive of a communication network. The in-between drives are left nonterminated. These resistors also "close" the communication circuit.

8. Fiber optics are extremely immune to electrical noise. Since they use light as the transmission medium, little to no low-frequency radiation can affect the quality of the transmission (i.e., 60 Hz).

9. DC motors offer very high torque at low speeds. Printing and ski lifts are demanding applications. The system may need to move a fully rated load from zero speed, up to set point. Standard AC PWM, voltage-regulated drives cause the motor to slip to develop torque. However, the newer flux vector, or direct torque-control drives, have very similar operation, com pared with a DC drive system.

10. Common DC bus arrangements are used when the likelihood exists for an overhauling load. The extra energy generated by the motor is fed back to the common bus and then transferred to another inverter unit connected to the bus. This increases the overall efficiency of the system.

11. The VFD is used to vary the speed per the feedback from the pressure transducer in the duct (if PID control is used). The outlet dampers are fixed in the open position, with the drive varying the speed of the fan per the feedback received from the transducer. Lower fan speed and pressure in the duct is the result. The fan only supplies the airflow required to meet the application needs.

12. Most significant factors are motor, its options and accessories, transformer and other power equipment, PF and harmonic correction, wire, cable, and conduit costs, installation and commissioning expenses, and overall operating expenses.