Simplified guide to energy saving with variable frequency drives

An introduction on how the installation of variable frequency drives saves significant sums of money when applied to fan and pump motors, and how you can convince your manager.  Let’s start with an analogy:


Imagine running a car at full throttle and controlling the speed by using the brakes.  In addition to the noise, vibration and waste of fuel, think how quickly everything would wear out.
It sounds crazy but that’s what we do with electric motors because we didn’t have a choice.   Standard induction motors are cheap and reliable, but they rotate at a fixed speed: if you needed a slower speed you used a gearbox.  However, by installing a variable frequency drive, you can now electronically control a motor’s speed from zero to maximum.


Now the important question: are there areas of your plant where you control the flow of fluids (gases or liquids) with either valves or dampers?  If there are, or you have a motor that could be run 10% slower without affecting performance, read on.


Cube Law

It’s obvious that reducing the speed of a motor reduces the power consumed by the motor.   As motor speed falls, the reduction in energy required to drive the motor is determined by the cube law.  Simply put, reducing the motor speed by 10% reduces the power required by the motor to 90% x 90% x 90% =72.9% of full speed: a saving of 27.1%.  By the same law, if you reduce the motor speed to 60% then 60% x 60% x 60% = 21.6% or a reduction of 78.4%.


Payback time

Now consider a modern 22kW IE2 (91.6% efficient) motor running at full load for 10 hours a day 250 days a year.  This will use 22 x 10 x 250/91.6% = 60,000kW of electricity in a year.  Assume £0.10 per kW that’s a running cost of about £6,000.  From the previous paragraph, by running it at 90% of full speed you save 27.1% of your energy consumption or £1626 a year for as long as the motor runs.  By coincidence, £1626 is about the cost of a 22kW inverter from 999 Inverters, so you know the payback time too.


Borrow one and try

This is why almost everyone including the UK Government and the Carbon Trust is incentivising the use of variable speed drives.  If you don’t believe it ask us and we’ll loan you an inverter to try it for yourself.  If your accountants tell you no capital expenditure, then we’ll lease you one and it will pay for itself from the savings.


If you would like more information, send us a comment or contact us at:


Tel: 0116 2999 222
Fax: 0116 2999 223
By Post: 999 Automation
Ross Walk

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