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Another type of control valve uses electric motors to open or close it. The electric motor is geared down so that its shaft moves slowly and it's limited to one revolution. The time for one revolution will be 10-20 seconds. That is, the valve will require 10-20 seconds to move from open to closed or from the closed position to the open position. Ill. 1 shows a examples of this type of motor-driven and other electrically-driven valves.
A typical application for these types of motors is to set the position for the dampers in heating or air-conditioning systems. The motor is proportional, which means that it can position the dampers in any position between full open and full closed. The motor can be operated on any AC or DC voltage but 24 volts ac is a common control voltage for air-conditioning and heating systems. The basic principle of operation for this type of motor is that its shaft will turn as long as power is applied to the motor, and the shaft will remain in position anytime power is disconnected. Limit switches are also used to detect the maximum travel in either direction. When the motor shaft rotates one complete revolution, it will move against the limit switch. This will open and stop the motor's travel in the clockwise direction and set the limit switch contacts so that the motor will be energized in the counterclockwise direction when power is applied again. When the motor shaft moves full travel in the counterclockwise direction, it will hit that limit switch. This will de-energize the motor in the counterclockwise direction and set it so it will start in the clockwise direction when power is applied again.
Potentiometers or other position sensors can be used to provide feedback to tell the exact position of the damper. In many cases, the feedback mechanism can be a temperature sensor. If the dampers are used to control the temperature in an air-conditioning system, the damper motor would open the dampers farther if the temperature is not cold enough. If the temperature is getting too cold, the dampers could be closed off more to limit the amount of cold air entering the conditioned space.
Above: Ill. 1: Varoius electric actuators for valves.
Dampers are used in many air supply systems for industrial processes. Any type of sensor can be used to provide feedback to the system, such as an air flow sensor. A pressure sensor could also be used to determine the back pressure in the system. This would indicate how far closed the dampers are and how much back pressure or pressure differential is built up across the dampers.
In some applications, limit switches are used as end switches. The limit switches have both NC and NO contacts that are positioned so that the NC set will be opened when the damper travels to its maximum. The NO set will close at this same point to ready the motor for reverse travel when the controller provides the reverse voltage.
This type of motor is also used to move the actuator or valve stem in the valves shown in this figure and this figure. The valves shown in these figures use air to move the valve actuator. The electric motor can be used to move the valve actuator the same way that the pneumatic diaphragm does. The main difference is that the motor-driven valves are slow to respond, which means that the process they are controlling will have a lot of dead time. Note that dead time will cause the amount of integral action and derivative action in the controller to change. When one is tuning a control system that has dead time, one will need to increase the integral value (time) to make the system control correctly and respond smoothly.
In some applications such as filling a very large tank with liquid, the amount of dead time will not be a problem and the longer actuation time will be acceptable. The electric motor-driven valve is often used when a source of control air is not readily available.
|Calculating mA or psi for I/P Transmitters||Using a Proportional Amplifier with a Motor-Driven Valve|