January 1997 Volume 3 Issue 2
Adjustable Speed Motor Drive Units
Various energy organizations and government agencies have studied how energy is used in North American manufacturing, and have come to some interesting conclusions.
1. About 70% of the electrical energy purchased by industry is used to drive motors.
2. Almost half of that motor-drive energy could be saved if enough of the right steps were taken.
3. Of the total potential for savings, the three major opportunities for conservation are:
· Energy-efficient motors — 17%
· Drivetrain improvements and better maintenance — 25 %
· Introduction of Adjustable Speed Drives (ASDs) — 40%
Why is it that ASDs have more than twice the potential to save energy as do energy-efficient motors? The answer has its roots in the way air and liquid flows have traditionally been controlled.
FLUID FLOW CONTROL
The ordinary induction motor, when connected directly to 60 Hz power, is a fixed-speed device. For many purposes, fixed-speed motors are fine. But using fixed-speed motors to produce variable air or liquid flows often results in considerable energy waste.
If a constant-speed motor is used to drive a blower or fan, the standard way of reducing air flow is to partially block that flow with a damper — while the motor continues to run at full speed. Similarly, if a constant speed motor drives a centrifugal pump, the standard way of reducing liquid flow is to restrict the flow with a valve in the line — again, while the motor continues to run at full speed. As curve 1 of the graph shows, this approach causes the motor drive energy to remain high even when the flow has dropped to quite low levels. In contrast, note the remarkable reduction in required energy (curve 2) when the flow rate is reduced by slowing down the motor rather than by restricting the flow.
Typical Pump/Fan Drive Energy
At Reduced Fluid Flows
1. Restrictive (Valve/Damper) Flow Control
2. Motor Speed (ASD) Flow Control
% of Maximum Fluid Flow
Actual values depend, of course, on the specifics of the installation. But curve 1 approximates what happens when a throttling valve is used to reduce liquid flow or an output damper is used to reduce air flow. The much more desirable option — operating along curve 2 — is made possible by Adjustable
Speed Drive technology.
WHAT IS AN ASD?
An Adjustable Speed Drive (inverter variety) is a special type of power conversion apparatus. It takes in standard 60 Hz utility power and puts out motor drive power at frequencies adjustable from near zero Hz to a typical maximum of 180 Hz. Fed by this power, the motor runs in a range from near zero to three times normal speed.
Whereas 60 Hz utility power has a smooth sinusoidal waveform, the power coming out of an ASD does not. ASD output power involves pulse waveforms of various kinds, with each type of ASD producing a characteristic waveform:
1. VSI — Voltage Source Inverters (or square-wave inverters) put out a square-wave voltage waveform, and are used to drive motors up to several hundred horsepower.
2. PWM — Pulse Width Modulation inverters produce a complex pattern of short-duration pulses. PWM units cover the same horsepower range as VSI units, but have performance advantages in some situations.
3. CSI — Current Source Inverters deliver a constant-current square wave. They are used primarily to drive motors larger than 200 horsepower.
ASD RETROFIT POSSIBILITIES
Prime candidates for retrofit with an ASD are those variable-flow situations which meet all three of the following criteria:
1. A multi-horsepower constant-speed electric motor drives a centrifugal pump, compressor, blower, or fan for thousands of hours per year, but
2. the actual flow used for a substantial part of that time is much less than maximum flow, and
3. the means by which the flow is currently being reduced is some form of throttling-down — as with a valve (liquid) or a damper (air).
If you have one or more situations of this kind in your plant you might want to make some rough energy-savings calculations. The way this is done (in principle) is to determine
· how many operating hours per year are spent at various flow rates, and
· what the motor input power (kW) would be for each of those flow rates with the present “throttle-down” system and with an ASD system.
· Then, for the two systems, multiply time by power at each flow rate to get the energy (kWh) consumed at that flow rate.
· Finally, add up the energy “chunks” pertaining to each system, and compare the yearly total for the throttle-down system with the yearly total for the ASD system.
Several ASD manufacturers (and others) have software available to help with this sort of analysis. Phone Ron Estabrooks or Mike Proud at 368-5010 (toll free) for more information.
A FEW CAUTIONS
An Adjustable Speed Drive is a major investment, and it is important to give your situation a thorough engineering analysis before committing dollars. This analysis should address:
· Cost-effectiveness of the proposed retrofit, and verification that the ASD approach is the best way of reducing the energy expenditure.
· Mechanical and electrical issues related to the process in question, including redesign to eliminate dampers or valves, and modification of the control system to accommodate motor speed control by the ASD.
· Compatibility of motor and ASD.
· Power supply issues. ASDs sometimes create power quality problems and are sometimes susceptible to them. Will a separate power feed be needed?
For more information, or help in evaluating a situation at your plant, call Mike Proud or Ron Estabrooks at 368-5010 (toll free).