Saturday, December 31, 2011

Using Ecm Motors in HVAC System

HVAC (Heating, Ventilating and Air Conditioning) refers to systems that heat or cool a designated environment. HVAC systems are especially important when it comes to designing large office buildings or climate controlled environments, such as some aquatic enclosures at the zoo. To achieve heating or cooling, an HVAC system depends heavily upon the quick movement of air from one location to another. AC motors have been used in past applications to serve as the primary air driving force, but they are not always the most efficient choice because they run continuously at full power. Electronically commutated motors (ECMs) were developed to offer a greater range of operability choices, and to minimize noise.

ECM Basics

ECMs are DC motors that function using a built-in inverter and a magnet rotor, and as a result are able to achieve greater efficiency in air-flow systems than some kinds of AC motors. (Although AC current is used for ECM, the ECM’s internal rectifier converts the current to DC voltage). Permanent split capacitor (PSC) motors, often used in conjunction with electronic SCR motors, are somewhat inefficient when used in air control systems because the fan motor noise requires the motor to run at less than a full load. When turned down, PSC efficiency suffers and falls in the range of 12 to45 percent. ECMs, on the other hand, maintain a high level (65 to 75 percent) of efficiency at a variety of speeds. As a result, ECMs are cost and energy efficient and can reduce operating costs. Additionally, ECMs are not prone to overheating and do not require additional measures to offset the generation of heat, as PSCs often do.

ECMs are also relatively low-maintenance; the use of true ball bearings reduces the need for oiling, and varied start-up speeds reduce stress on mounting hardware. The operating range is significant enough to enable one ECM to replace two induction-style models, which simplifies the replacement, maintenance, and installation processes, and minimizes product choices. However, not all ECM motors run at variable speeds and selection depends heavily upon application specifications. The initial cost of an ECM can be high, but is typically balanced by overall energy savings in the long run.

ECM in HVAC Systems

When considering an ECM for application in an HVAC system, there are several factors to keep in mind. Although ECMs are often selected because many models run at variable speeds, in certain condenser applications it is preferable to select and ECM that runs at a fixed speed—an ECM running at a fixed speed s in a condenser unit still uses less energy than a typical PSC motor running at a fixed speed in a similar unit. As a result of increased energy savings, a condenser operating with an ECM will have a higher SEER (seasonal energy efficiency ratio) rating. In other HVAC units, an ECM can run at variable speeds but depends on a controller that pre-programs speed, including the rate at which the motor ramps up. Whereas typical PSC motors start and almost immediately run at full capacity, an ECM can start slowly and stop slowly, which can help reduce humidity. Additionally, the control can be set to alter the amount of air an ECM motor drives through the system, which enables a greater range of possible air-flow rates.

A typical ECM operating in an HVAC system will go through several stages, as determined beforehand by a manufacturer or a preprogrammed controller. In its first stage, and ECM runs at a lower speed to remove humidity (this is especially important in a cooling system). Next, the ECM reaches its designated peak speed, as specified for the application, maintaining high efficiency despite any shifts in operating speed. When the ECM stops, it can be programmed to stop slowly (called a soft stop).

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