Industrial controls literature frequently promises that adjustable speed drives reduce motor-related energy costs by 10 – 60%, recouping your investment within 24 months [i] [ii] [iii] [iv]. So, the more drives you have, the more energy savings you’ll redeem, right?
Despite what industrial controls salespeople may tell you, every motor does not need a drive; and, in some applications, a drive may cause costlier maintenance problems than it ever generates in energy savings.
This post — the first in a three-part series — presents three questions for assessing whether your motor-driven system will benefit from a drive.
What’s the difference between adjustable speed and variable frequency drives?
Adjustable speed drives (ASDs) refers to the class of motor controllers that vary motors’ rotational speed. There are eight basic types of ASDs; today, the most common type of ASD is the variable frequency drive (VFD).
Induction motors rotate at a speed proportionate to the frequency of incoming power [v]. VFDs use sequential inverters to convert incoming, 50- (Europe) or 60-Hertz (Americas) alternating current (AC) power to direct current (DC) and then back to AC at the frequency required to spin the motor at the desired speed.
1. Do you need precise control of this motor and motor-driven system?
Perhaps your application’s duty cycle involves accelerating, decelerating, reversing direction, braking, and/or holding a constant torque at low speeds, like an elevator. Or, maybe your system has several parallel motors that need to supply one system in a coordinated manner.
If either of these situations sound familiar, or if you have another motor-driven application that requires very precise control, you may benefit from adding a drive. First, drives may enable you to replace fickle DC motors with more-common, lower-cost AC induction motors. Additionally, drives allow you to coordinate how multiple motors work together across a system, optimizing how loads are split between component motors and preventing motors from fighting each other.
2. Do you currently use mechanical means to regulate this application?
Drives produce the greatest energy savings where the application currently uses mechanical means — such as gearboxes, throttling or bypass valves, dampers, and hydraulic or magnetic couplings — to regulate load or flow. Wherever you have one of these mechanical devices, you’re paying for an oversized motor to produce too much pressure or flow, and you’re throwing away the excess.
For example, consider a pump and throttling valve, or a fan and damper, set-up. In both cases, the motor and pump or motor and fan raises the necessary volume of liquid and air, respectively, to a much higher pressure than the application requires; then, the throttling valve and damper drop the pressure of the flow to the application’s requirements. With a drive, you could eliminate the valve or damper and run the motor at a reduced power and/or speed to precisely meet your application’s pressure and flow requirements.
3. Does this motor supply a variable load?
Consider, for example, a chilled water system where the system needs to maintain pressure regardless of demand. Or, an HVAC air handler, where ASHRAE standards require a certain minimum number of air changes per hour in addition to the hourly conditioning demands required to maintain the temperature set-point. Drives allow you to tailor supply precisely to demand and redeem energy savings any time a motor would otherwise operate at a partial load — whether, as in the previous question, there’s a mechanical device regulating the application or the load simply varies throughout the day. If your load varies throughout the day like either of these applications, you may benefit from a drive.
Next week’s post will review drive contraindications: situations where drives create more challenges than opportunities. In two weeks, we’ll post ways to optimize your motor-driven systems without a drive. For assistance prioritizing which of your motors would benefit from a drive, and to find out how to reduce your motor-related energy expenses by up to 30%, check out www.motorsatwork.com or email me.
Nicole Dyess is the Director of Client Solutions for Motors@Work, which provides cloud-based energy management solutions for maintaining and operating motors and motor-driven systems at their peak efficiency and lowest cost.
[i] Siemens, “Variable Frequency Drives and Energy Savings” (2010).
[ii] Honeywell, “VFD Energy Savings and Payback Calculator” accessed 2016.
[iii] Rockwell Automation, “Energy savings with variable frequency drives: Invest in energy management with intelligent motor control solutions,” (2007).
[iv] ABB, “Energy efficiency: Using drives to control motors can lead to big savings,” accessed 2016.
[v] Speed (in rpm) equals 120 times the frequency of incoming power (in Hertz) divided by the number of magnetic poles in the motor windings, which is always an even integer because there’s always both a north and a south magnetic pole; in other words:
