ASHRAE Journal - October 2012 - 82

emerging technologies
ficiencies benefit from being coupled with a variable speed drive. Energy standards are beginning to recognize this fact. For example, though Phases 2 and 3 of the European Union regulation EC No. 640/2009 mandate use of IE3 (Premium efficiency) class motors starting in 2015 and 2017, they include the following “or” option: “or meet IE2 (High efficiency) efficiency and be equipped with a variable speed drive.”2 Permanent Magnet (PM) Motor Brushed dc permanent magnet (PM) motors have magnets affixed to the stationary stator. Commutating magnetic fields are then created by reversing the electric field to the windings through slip rings and brushes. This creates the rotating magnetic fields that interact with the opposing poles of the magnets, thus causing rotation. Most stationary HVAC&R applications require longer lifetimes than typical brush wear lives, so brushed dc motors are seldom used for these applications. Brushless dc (BLDC) PM motors have dedicated controllers which create the rotating field for commutation using solid state switches in bridge configurations. Hall-effect position sensors for monitoring of the voltage in the unenergized field winding are used to determine when to commutate. This makes them more complicated and expensive. The tradeoff is higher reliability and life of the motor itself, due to the noncontact position sensors and the lack of brushes. Additionally, higher efficiencies and better overall control are achieved.3 BLDC motors can accurately control connected loads due to the linear relationships between applied voltage, motor speed, current and torque. With their controller replacing a variable speed drive (VSD), they can control the motor to deliver known torque output. For variable air volume applications this allows for the ability to set fan flow rates, minimizing air distribution system balancing.3 As noted previously, for small motors, the combined system efficiency of the brushless dc motor is higher than the efficiency of an induction motor with comparable speed and power output.
82 ASHRAE Journal
Housin g Stator

Rotor

Figure 1: A basic motor schematic. Variable Speed Drives VSDs also known as variable frequency drives, or VFDs) for ac induction motors dominate the marketplace. They can operate in several modes, including scalar and vector. In scalar control, the voltage and frequency are only varied in magnitude proportionally together. This method is based on steady-state characteristics of the motor with no inner current feedback and the assumption that the stator voltages and currents are sinusoidal. The drawbacks are poor reaction time for load changes and poor efficiency during these operation points. For so-called variable torque applications, such as fans, blowers and pumps, scalar control is generally adequate. In vector control, the individual phases can additionally be controlled to allow more precise control of torque with higher efficiency. It employs sophisticated algorithms that provide the following advantages, which are highly relevant to refrigerant compressor drives:4 • full motor torque capability at low speed; • better dynamic behavior; • higher efficiency for each operation point in a wide speed range; • decoupled control of torque and flux; • short-term overload capability; and • four-quadrant operation. For brushed permanent magnet (PM) dc motors, the drive provides dc by means of a variable dc power supply. The quality of the drive determines the amount of ripple, which affects how well the motor performs.
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Some drives are unfiltered SCR (Thyristor) type, while others employ pulse width modulation (PWM) for better performance. As mentioned previously, BLDC PM motors have dedicated controllers instead of a separate drive. Therefore, when discussing ac IM vs. dc PM motors and drives, we focus on brushless dc, with controllers as drives.

Energy Savings Potential
Electric motors are significant energy consumers. In 2006, global motor energy consumption was approximately 4310 TWh for residential motor use, and 3690 TWh for commercial and public sector motor use. Thirty-eight percent of commercial motor use could be attributed to HVAC applications, while 20% to 25% of residential motor use could be attributed to refrigerators, freezers, and HVAC equipment.5 Through using more efficient and variably controlled motors significant energy savings can be achieved. For example, electronically commutated motors (ECMs) used in commercial refrigeration consume 15% less power than permanent split capacitor motors (PSC) used for the same application. One company’s EC motors consume less power and generate less heat when run below 100% motor speed, and their electronically controlled design allows them to be run at preprogrammed variable speeds.7 Motor choice has a significant impact on system efficiency. On a larger scale, the motor electricity consumption in the EU by 2015 is estiOctober 2012

ASHRAE Journal - October 2012

Table of Contents for the Digital Edition of ASHRAE Journal - October 2012