PID error (set point minus input) is ramped slowly into a PID regulator, which arrives at set point without overshoot, undershoot, or cycling.

The weighted sum of proportional, integral, and derivative functions is used to correct process variation. With those three functions tuned, system performance can be controlled for responsiveness and oscillation.

VARIABLE-SPEED DRIVES

PID calculations adjust motor speed as necessary to maintain a set point. As system dynamics, such as valve and damper operation, ambient temperature, and flow rate, change, the motor speed required to maintain a set point differs. PID performance is based on ability to maintain a set point, not on motor speed.

A variable-speed drive may be capable of serving as a PID controller. A set-point value can be provided by a wired signal to the drive's input/output terminals or sent by serial communications.

FIGURE 4: PID-control concept.

The use of PID control with a variable-speed drive is illustrated in Figure 4. A variable-speed drive uses a reference set point to compare feedback. Scaling of PID parameters is required to suit the application or the range of the sensor providing the feedback signal (or both). Examples of PID parameters include 0- to 20-psi pressure, 0- to 500-gpm flow, and -100°F to 300°F temperature.

For variable-speed drives, speed reference-level parameters must be selected as a percentage of the analog voltage input range used as the feedback input signal. The output-frequency level is entered in hertz and set as a function of the speed reference, while the internal PID set point is entered as a percentage of the PID feedback range. To simplify the setup of PID control for a variable-speed drive, values as close to the maximum limit as possible and that remain within powers of 10 with respect to actual values should be used.

UTILIZING PID CONTROL FOR HVAC APPLICATIONS

Proportional control provides the output capacity of HVAC equipment matching the load requirements of a building. Output capacity is controlled using a percent of full output or, in the case of motor speed, 0 to 60 Hz, similar to a two-position switch by which mechanical equipment is either fully on (100 percent) or fully off (0 percent).

HVAC applications with variable-speed drives benefitting from PID regulation include:

• AHUs

  A pressure sensor placed inside of an air duct can provide feedback to a drive PID regulator, comparing desired pressure with actual pressure and optimizing by adjusting fan-motor speed.

• Chilled-water pumps

  Chilled-water-pump speed is controlled based on chilled-water-line-to-AHU-cooling-coil differential pressure. A pressure sensor can be placed in a chilled-water line so that a drive PID regulator can compare actual pressure with desired pressure and adjust chilled-water-pump motor speed to minimize variance.

• Cooling-tower fans

  By adjusting cooling-tower-fan-motor speed, the speed of a cooling-tower fan and the temperature of water discharged from the cooling tower can be controlled to maintain a desired temperature set point.

BENEFITS OF PID CONTROL

Incorporating PID control with variable-speed drives results in HVAC systems that are more efficient and save money. Digital controls, throttling valves, and dampers that can be opened vary hot- or chilled-water-pump-motor speed or AHU-fan speed, allowing temperature to be controlled. Savings can be realized in three major areas:

• Maintenance
  

  Motor speed as a function of control point for heating.

  Reducing dependence on throttling valves and dampers for the mechanical adjustment of airflow output saves on the maintenance of temperature-control equipment.

• Energy

  Open-loop control equipment cycles on and off constantly, causing a peak in-rush current during startup. Closed-loop control saves energy by providing continuous operation of AC motors at lower speeds.

• Money

  Reducing pump- and fan-motor speed reduces electricity costs.

For past HPAC Engineering feature articles, visit www.hpac.com.

Harvey Eure is a product manager focusing on enclosed variable-frequency drives for Schneider Electric. He can be reached at harvey.eure@us.schneider-electric.com.