Motion System Options for HVAC Gas Valve Actuators
In HVAC systems that provide heat for enclosed spaces, gas and liquid flow must be controlled and metered accurately using sensing and modulating elements such as flow control valves or isolation valves. Some control valves are opened and closed manually at the valve stem, while electronic valves incorporate an actuator to move the stem with control from a motor and drive system. Designers have three motor types to choose from, depending on the application requirements. Here’s a summary of the motion system options:
Brushed DC motor for rotary motion, or linear motion with an external lead screw. This simple DC motor does not require complex electronics to drive the motor. However, if the application calls for linear motion, the DC motor will require an additional lead screw and gearing system to convert the rotational motion to linear motion. It will also need a feedback mechanism like an optical sensor or encoder for accurate linear position control.
DC stepper motor for rotary motion, or linear motion with an external lead screw. A stepper motor rotates in several small, equal and discrete increments called steps, and driving requires a dedicated control unit. The motor can operate in open loop with good accuracy when properly sized, thanks to the discrete steps that provide built-in positioning control. Its design also makes possible the use of detent torque to provide holding position, and the motor offers excellent response for start, stop and reversing operations. If linear motion is required, an additional lead screw and gearing gearing system are necessary to convert rotational motion to linear motion. Typically, stepper motors can provide output speed up to 1,000 rpm and torque up to 170 millinewton-meters. Actual torque and speed also depend on whether the driving mode is full step, half step or micro-step.
Stepper linear actuator with an integrated lead screw. This compact device combines a can stack stepper motor with a threaded rotor and an integrated lead screw to provide direct linear motion. The stepper linear actuator lead screw moves up and down in discrete step increments when electrical pulses are applied. Because a stepper linear actuator can be controlled accurately in an open loop system, no expensive feedback device or braking system is needed for positioning. Typically, linear actuators can deliver output speed up to 80 millimeters per second, and they provide up to 100 Newtons of force. Actual force and speed also depend on lead screw efficiency and driving mode — full step, half step or micro-step.
Key Considerations for Motor Selection
When selecting a motor, keep these considerations in mind:
|•||Maximum and running torque and force required|
|•||Speed requirement: Typically, force and torque is required at low speeds for valve applications.|
|•||Overall price of the system|
|•||Control system: Typically, closed loop control is not required.|
|•||Type of motion needed for actuation: rotary or linear motion|
|•||Expected lifetime of the system before replacement|
|•||The required linear or rotary resolution|
|•||Maximum current and voltage available|
Based on the motion options available and HVAC requirements, a stepper motor is well-suited for a rotary application, and a linear step motor is desirable for a linear application.