Page 2 of 2

The simplest type of speed control relies on a potentiometer located on the blower. Speed can be adjusted without the need for an external controller. This control scheme is common for fixed-speed applications.

One alternative is a two-wire control scheme that includes a simple controller “on board” the motor and wires that serve as DC power-supply connections. The blower's speed is directly proportional to the supply voltage, and a separate speed command signal becomes unnecessary.

More advanced control schemes incorporate a more sophisticated “on-board controller.” The controller provides a variety of speed commands for a blower, whether via a DC signal, a pressure sensor in the air circuit (to adjust the blower-speed command according to the pressure), pulse-width modulation with tachometer feedback (to confirm blower speed), or mechanically with a potentiometer.

When Size Matters

Commercial boiler systems have been trending toward smaller design envelopes; BLDC premix blowers have been traveling along the same path. (Compact combustion systems especially make the grade when retrofitting older buildings that may provide access only through typically sized door openings.)

A blower's size generally will be specified relative to the size of the combustion system. The limiting factor usually is not the size of the blower, but the size of the venturi/gas valve required to keep the gas mixture on target. Advanced BLDC blowers utilize a venturi/gas valve to mix the air and gas to the desired 10-1 turndown ratio and mate the outlet to a cylindrical burner used to heat the hot water.

BLDC premix blowers ultimately offer practical airflow delivery in a compact package. For burner applications, 500,000 Btu requires 100 cfm of airflow. By using a BLDC premix blower, the airflow needs for a 400,000-Btu system can be satisfied with a 6-lb assembly, just a quarter of the typical weight of traditional blowers. For example, one company has been able to reduce the width of its 2-million-Btu boiler by switching to a blower half the size. Additionally, a manufacturer has been able to integrate the same size blower into its mid-size boiler line, simplifying production and service and reducing the parts count, which also reduces costs.

Some advanced blower types benefit from an outside rotor design that allows for higher speeds than conventional inside rotor designs. Faster rotational speeds, along with fan-design optimization, can deliver higher performance (pressure and flow) while taking up less real estate. This is a plus in combustion applications in which high system impedance occurs and more performance is needed to make full use of the heat exchanger. Additionally, higher pressure overcomes the challenges presented by longer flue lengths often found in larger commercial buildings.

Conclusion

Looking ahead, numerous technological improvements are on the horizon. For example, one advanced blower design uses a digital-signal-processing motor chip that can enable advanced programmable control of a blower. The blower's performance, inputs, and outputs can be tailored to meet specific system needs. Option cards also broaden the potential for blower customization.

As BLDC motor technology is applied to more combustion and industrial motor applications, the overall cost of these blowers will decline. Additional volume has helped BLDC blowers stay competitive with more traditional alternatives, including AC/VFD blowers.

The push toward greener products coincides with the introduction of BLDC motors to the commercial boiler market. The quick response time of BLDC blowers when used in a premix combustion system minimizes the flue-gas-emission problem that occurs with traditional slower-reacting blowers. Using a gas valve/venturi/boiler controller with a BLDC blower provides an economical and environmentally friendly solution for current boiler and water-heater manufacturers.

Every application, of course, will carry its own set of requirements and demands. Various factors will influence blower specification, including appropriate pressure and flow rate, available design envelope (governing blower size), desired service life, input voltages, and control scheme, among others. Choices will broaden with the capability to customize.

By consulting with a manufacturer early in the specification stage or selection process, users can arrive at the best-equipped BLDC-premix-blower solution for a commercial boiler application.

Did you find this article useful? Send comments and suggestions to Associate Editor Megan White at megan.white@penton.com.

Global combustion market manager for AMETEK Technical & Industrial Products, Jodie McLay has 10 years of experience selling brushless direct-current motors and blowers and 24 years of experience in technical sales. He earned his bachelor's degree in commerce and engineering from Drexel University in Philadelphia. He can be contacted at jodie.mclay@ametek.com.