The right motor choice for HVAC control valve actuation

Central to the success of Heating, Ventilation and Air-Conditioning (HVAC) systems are the miniature motors necessary to actuate the control valves. In addition to generating the required torque at low speed to regulate gas flow, the motors must ensure long and reliable life, combined with efficiency and cost effectiveness. Mahesh Dundage and Sajal Kumar discuss the motor selection process for HVAC valve actuation.

An Heating, Ventilation and Air-Conditioning (HVAC) heating system generally includes a gas-driven boiler to heat the media required for circulation. Accurate control of the gas flow required for temperature modulation is typically achieved through a flow control valve. The upward and downward movement of the valve stem, or, on alternative designs, the rotating position of the stem-and-ball, regulates this flow. Crucial to the whole process is the actuation of the valve, which is managed electronically with a motor and drive system.

To select the right motor, the HVAC system designer must determine the torque requirements, alongside the voltage and current range available. While HVAC systems typically operate at low speeds, the speed requirement must also be considered. The application may be based on rotary or linear actuation and while high accuracy position control isn’t always needed, the level of motion precision should be considered. The operating temperature of the application environment impacts the required durability of the motor and its lifespan, and for HVAC OEMs, cost-effectiveness is also a key consideration. To achieve these attributes, a brush DC or stepper motor are the preferred options for HVAC gas valve actuation.

Brush DC motors

Brush DC motors provide simplicity in achieving rotational motion, though linear motion can also be achieved by adding a lead screw and gearing system. When control accuracy is required, a brush DC motor can be combined with a feedback device such as an optical sensor or encoder. Some designers may also add a braking system to improve positioning accuracy due to the high inertia of the motor’s rotor.

Brush DC motors provide a wide range of torque, up to 160mNm, and also a wide speed range, from 1,000 RPM to 10,000 RPM. As a result of their low losses, Portescap’s brush DC motors also provide very high energy efficiency as well as advantages including low friction and good thermal dissipation. Portescap also provides a range of matched gearheads and encoders for simple integration.

Stepper motors

Stepper motors are so-called because they rotate through equal, discrete increments known as ‘steps’. Driving a stepper motor requires a dedicated control unit although the motor’s inherent characteristics mean that while it operates under open-loop positioning, accuracy is achieved. The stepper motor’s discrete movement provides built-in positioning control, though closed-loop positioning can be added with an encoder.

Withholding torque up to 170mNm, the stepper motor can also generate detent torque, making it possible to hold a position at zero speed. Combined with its step positioning, the motor also has response characteristics for applications involving start, stop and reverse. Achieving output speeds up to 1,000rpm, a stepper motor provides rotary motion, however linear motion can be created with the addition of an external lead screw.

Portescap’s can stack stepper motors provide the accuracy required for HVAC applications. Error is non-cumulative as long as step integrity remains and the motors also offer long life, cost-effective solution.

Stepper linear actuators

A stepper linear actuator provides linear motion and comprises a can stack stepper motor with an integrated lead screw in a compact package. Under the control of a driver, when electrical pulses are applied the lead screw moves up and down in discrete step increments. As with a standard stepper motor, an important advantage of the linear variety is the ability for accurate control within an open-loop system, without the cost or footprint required for an external feedback device or braking system.

With speed up to 80mm/s, up to 100N force can be applied. Portescap’s range of stepper linear actuators feature a compact design with better efficiency than a stepper motor with an external lead screw, as well as providing the same long-life technology as included in its range of stepper motors.

Optimised selection for HVAC gas valve control

For most HVAC application requirements, the stepper motor and linear stepper actuator can meet the torque/force to speed requirements for rotary and linear applications. Combined with a compact design for linear actuation, built-in, open-loop position control and long lifetime, these designs create a cost-effective outcome. It’s crucial that the motor is correctly sized and specified, and Portescap’s engineers can work with OEM designers to integrate a stepper motor solution to optimise an HVAC system.

Mahesh Dundage is Lead Design Engineer, and Sajal Kumar, Product Line Manage with Portescap.