Variable Speed Drives are a Powerful Efficiency Tool
Among the biggest users of electricity in a building are the motors being used by the building itself — for HVAC and other duties — and by tenants for their businesses.
Both should be tracked by energy and facility managers. Motors — or, more specifically, the ways in which they are controlled — is evolving. The overall goal is to more closely match the speed of the motor at a given moment to the job at hand. This is a far more efficient approach than simply having the motor run at full throttle from the moment it is turned on.
This granular control is added by equipment that attaches to the motor. Austin Energy suggests that such equipment can be used with motors of all sizes. Examples are those used in air handling units, cooling tower fans and variable-flow primary and secondary water pumps.
The first challenge to understanding how this world is changing is the nomenclature. At Consulting-Specifying Engineer, two GRAEF employees — Mechanical and Plumbing Group Leader Jason Gerke and Electrical Consulting Engineer Robert Jeffers — wrote that there are several names for this class of controller, including variable speed drives and variable frequency drives.
The key is the word “variable.” It actually refers to two distinct ways in which motors use energy: What they need when they are first turned on to do their ongoing job. In the past, motors would start at 100 percent power and simply continue that way until turned off. The new generation can manage and moderate both processes according to need:
Motors have always been one of the largest energy users in commercial buildings. That fact, coupled with a higher level of energy and environmental awareness in recent decades, necessitated that the industry revisit the way in which motors are operated and controlled. VFDs have been used in many applications over a number of years. Technological advances over the last number of decades have reduced VFD costs and sizes to now make them the preferred choice for motor control.
At Control Design, Star Manufacturing Controls Engineer Thomas Stevic does a nice job of explaining what a variable speed motor does. Simply, incoming alternating current (AC) is converted to direct current (DC). The speed control element of the VFD control then creates pulses of varying frequency. The variations in frequency dictate the speed with which the motor works.
An important subsequent question is what size VFD to use. The sense is that the constant changes in how hard the motor is working causes more stress than running at a single speed:
How big should the VFD be? The size of the VFD should be chosen based on maximum motor current at peak demand and not chosen based upon motor horsepower. Constant starting, stopping and dynamic loads affects the electronics inside the VFD far more than the effect they have upon the local power bus and a full voltage motor starter. Therefore, peak demand current should be used. Manufacturers may continue to list hp ratings more as an historical rating than as a useful one.
The bottom line is very clear: Motors are the muscles of a facility. Like a muscle, a motor is used for myriad purposes. Finding a way to systematically reduce the amount of energy that is consumed when they accomplish their unending list of tasks will add up, bit by bit, into significant savings. Putting in place electronics that match the energy expenditure to the task is the key to accomplishing that goal.
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