Ducts and Duct Systems:Damper Motors and Actuators

Damper Motors and Actuators

Damper motors or actuators are devices used in heating, ventilating, and air-conditioning systems for the following applications:

• Diverting airflow

• Decreasing airflow

• Controlling ventilation

• Zoning

Motorized dampers are available in a number of different designs, but they can be grouped into two general classes: (1) single- blade dampers and (2) opposed-blade dampers. There are two types of single-blade dampers. One type is opened and closed by the damper motor. It remains closed when not engaged by the damper motor. Another type of single-blade damper uses the damper motor to open it and a spring to close it. An opposed-blade damper has narrow panels covering slots. The panels are opened and closed by the damper motor.

A damper motor or actuator can be used to position a diverting damper when a parallel airflow pattern is required. The damper is used to direct airflow through either the heating or cooling unit. The damper motor is connected to the heat-cool switch on a semi- automatic changeover thermostat or in parallel with the cooling contact of an automatic changeover thermostat.

In modern forced-air heating systems, zoning is commonly accomplished by a thermostat for each zone. When there is a call for heat, the thermostat sends a signal to a control panel. The con- trol panel then opens the appropriate zone damper and turns on the furnace or air handler. Some of the older systems do not have a control panel. Instead, the furnace or air handler is turned on by damper drip switches. A control panel is used in modern heating and air-conditioning systems to control the changeover from heating to cooling or from cooling to heating.

A damper motor or actuator can also be used to operate a bypass damper for the following purposes:

• To bypass a cooling coil for dehumidification

• To bypass the heat exchanger on cooling

• To bypass the heat exchanger on heating

It is sometimes necessary to decrease the airflow during the heating cycle in some systems. This can be accomplished by using a damper motor or actuator to position a resistance damper.

Systems that require the introduction of outdoor air during the cooling season, but not during the heating season, can use damper motors or actuators to control the ventilation.

Damper motors or actuators are also frequently used in zoning a duct system by opening or closing dampers to the various zones.

The small, compact electric motors used to provide proportional control of dampers can also be used to drive valves or step controllers in heating, cooling, and ventilating systems. For this reason, additional information about these motors is included in Chapter 9 of Volume 2 (V“ alves and Valve Installation).

An example of a motor used to drive a damper is found in Figure 7-17. This is a cutaway of a Honeywell modutrol motor,

which functions as the drive unit in a modulating control circuit.

The following are the basic components of this motor:

• Reversible motor

• Balancing relay

• Feedback potentiometer

• Gear train

The balancing relay controls the motor, which turns the motor drive shaft through the gear train. The motor is equipped with switches that limit its rotation to 90° or 160°. The gear train and other moving parts are immersed in oil to eliminate the need for periodic lubrication.

The motor is started, stopped, and reversed by the single-pole, double-throw contacts of the balancing relay. The balancing relay consists of two solenoid coils with parallel axes, into which are inserted the legs on the U-shaped armature. The armature is pivoted at the center so that it can be tilted by the changing magnetic flux of the two coils to energize the relay. A contact arm is fastened to the armature so that it may touch either of the two stationary con- tacts as the armature moves back and forth on its pivot. When the relay is balanced, the contact arm floats between the two contacts, touching neither of them.

A feedback potentiometer consisting of a coil of wire and a slid- ing contact is included in the Honeywell modulating motor. The sliding contact is moved by the motor shaft so that it travels along the coil and establishes contact wherever it touches, according to the position of the motor.

Figure 7-18 shows a typical wiring diagram for a Honeywell modutrol motor. Note that there are two separate circuits in the modulating motor powered from T1 and T2. The motor circuit consists of the reversible motor, the rotational limit switches, and the contacts of the balancing relay. The control circuit includes the feedback potentiometer, the coils of the balancing relay, and the controller potentiometer.

The control circuit offers two paths for current flow—done through each side of the balancing relay. Increasing resistance in the B leg of the motor control circuit by changing the setting of the controller potentiometer will run the motor toward the closed position. Adding resistance to the W leg runs the motor open.

As the motor shaft turns, it moves a wiper over the feedback potentiometer. This makes the resistance in each side of the circuit the same. When the resistances are equal, the current flow through both sides of the balancing relay is equal. The balancing relay contacts open, stopping the motor. The circuit is said to be balanced.

The motor of a damper actuator should be completely sealed and immersed in oil. Such a motor can operate without maintenance or service for the life of the unit. An example of this type of motor is found on the ITT General Controls DHO series damper actuator shown in Figure 7-19. These DHO series motors are avail- able in two-position type (energized and deenergized) or three- position type (deenergized, first stage open, and second stage open). The first-stage intermediate position can be adjusted.

It is important that the DHO series damper motors work within the manufacturer’s specified load limit during all phases of operation in order to ensure delivery of rated forces under usual voltage variation. Maximum workload can be determined from the data given in Table 7-3. Load is the deadweight pull at the particular stroke position. The imposed load can be measured with a small spring scale.

The Honeywell M833A damper actuator shown in Figure 7-20 is used to regulate duct damper condition according to zone thermostat requirements. It attaches directly to a damper shaft 1⁄2 inch in diameter or a 3⁄8-inch shaft with adapter provided. It mounts in any position directly on a duct, or inside a standard wiring junction box where Class 1 wiring is required.

Gas Burners:Venting and Ventilation and Safety Precautions

Basic Electricity:Capacitance

Analog Sensing for Programmable Controllers

Air-Conditioning Equipment:Electric Motors and Troubleshooting Electrical Motors

Steam Heating Systems

Steam and Hydronic Line Controls:Thermostatic Traps

Controls:Basic Control

Hydraulic Turbines:The Pelton turbine

Pipes, Pipe Fittings, and Piping Details:Wrought-Steel Pipe