Resources for the Resourceful – Motor Operating Costs

I’ve mentioned Electrical Apparatus
magazine in a previous
, but I thought I would highlight an article in the
December issue because it has a lot of good information that would
be useful to anyone who is about to purchase a motor or assess a
motor in terms of its operating cost.

The article looks at motor operating costs via a list of
Do’s and Don’ts for arriving at the true cost of
operating a motor, including the reasons why a particular item
matters.  For instance one of the Don’ts is Don’t take it
for granted that higher efficiency means higher power
.  The paragraphs that follow go on to discuss why
some aspects of motor design targeted at improving efficiency will
lower power factor while others will raise it.

The following is a list of the Do’s and Don’ts that are
discussed in the article. I thought I would provide the list
because for one thing, its a great checklist for anyone considering
a motor purchase or assessment.  In addition, it will provide
you with a pretty good window in to the content of the
article.  (Rather than using a bunch of quotation marks, I’ll
simply state that what follows is generally made up of the opening
sentences in the bulleted list in the article.)


Do know your load
Do measure voltage in all phases
Do recognize that distortions in voltage
or wave form can significantly affect motor efficiency
Do use accurate instruments if you are
doing measurement and verification (M&V)
Do verify instrument calibration prior to
performing M&V
Do be aware that a more efficient motor
may use more power rather than less when driving the same

(Incidentally, this is the topic brought up in my post titled
Optimization; Same Efficiency + Different Slip = Different
 and is also the focus of the string of recent
posts starting with Pump
and Motor Interaction

Do earn about how the
utility bills for power factor and how your motor will impact
Do be careful with the assumptions you
make about motor life
Do use an appropriate accounting method
for assessing motor economics (life cycle cost vs. simple


Don’t assume the motor
nameplate efficiency is the exact value to use in operating
cost evaluation
Don’t assume an efficiency for an older
motor just because its of a certain age, type, or make
Don’t assume that the motor always
runs at a constant load or at the horsepower marked on its
Don’t expect motor losses or efficiency
to vary with load in the same way for all motors
Don’t take it for granted that higher
efficiency also means higher power factor
Don’t assume some average utility rate
and don’t expect it to remain unchanged throughout a  motor’s
probably life span
Don’t assume phase unbalance is not a
factor for a given location
Don’t expect to save energy
by applying a reduced-voltage starting method to any motor
Don’t consider energy usage to be
directly proportional to current
Don’t use new or higher efficiency as an
excuse to neglect sound installation and maintenance practices
Don’t forget to assess the capabilities
of repair centers when you send a motor out for

If this list tweaked your interest and you don’t
already have a
subscription to the magazine
, you can read a summary of the
article or order a
copy for $5
.  For those who are wondering if it’s worth
the $5, I would say that the knowledge gained by reading the
article and having it on hand will pay for itself the first time
you need to assess a motor. And by taking a few minutes to see if you qualify for
a free subscription
(I suspect many of the folks that read this
blog would), you can begin to build up a library of useful
information like this for no cost at all.

Senior Engineer – Facility Dynamics

here for an index to previous posts

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3 Responses to Resources for the Resourceful – Motor Operating Costs

  1. Even though ‘know your load’ is mentioned, it is very important
    that the starting torque requirements are specifically addressed.

  2. Even though ‘know your load’ is mentioned, it is very important
    that the starting torque requirements are specifically addressed.

  3. You make a great point Laszlo. Motors with identical horsepower
    ratings can have very different starting torque capabilities. If
    you don’t pay attention to that when you replace an existing motor
    with a new one, you may find that you can’t accelerate the load up
    to speed with the new motor. In HVAC, this can especially be an
    issue with large, slow speed fan wheels like return fans where the
    wheel mover a lot of air at a low static pressure and thus does not
    require a lot of power to run. But the large size of the wheel
    means it’s heavy and thus has a lot of inertia and can take a lot
    of torque to get it started. I am currently working on a string of
    posts about motor performance curves and will make a point of
    bringing your insight up as a part of that discussion. Thanks for
    commenting, David

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