Table 1. Controlling Multiple Motors With a Single VFD: Advantages and Disadvantages.
It is a simple way of controlling
multiple motors operating at exactly
the same speed for a common
application or process.
A single large VFD should be more
efficient when compared to multiple
It offers the potential for lower
There is no increased risk of
This method must be used for the
correct application; it cannot control
each motor or fan individually. All
motors start as one and operate at
the same speed.
Each motor will require its own
individual thermal overload
protection (TP) devices.
Codes may require individual motor
disconnect switches (DSW).
Adding motor TP and external
DSW devices to meet local code
requirements will reduce and may
even eradicate any cost savings,
depending on the quantity and size
of the motors.
If something in the VFD fails and you
switch to bypass mode, all motors will
operate at 100 percent speed.
gives you both.
WE KNOW THE INDUSTRY –
with 25 years of proven, long-term
relationships with clients in the
district energy/combined heat and
WE GET TO KNOW YOU –
your asset management issues,
strategic planning goals, technical
challenges and customer needs.
carefully thought out to determine the
suitability for every operating hour and
load condition. The application just may
not be worth the risk of having the VFD
fail and, as a result, not having sufficient
control or losing the process entirely.
As always, be careful what you
wish for – it may never be the paradise
you envisioned! Having all your motors
operating at the same speed may be
analogous to having all the cars on the
interstate going the same speed in all
lanes when you are in a hurry. If the cars
are spaced side by side, you would never
get the opportunity to pass. Sometimes
having everything moving at the same
speed is not beneficial!
So, while whistling while you work
is totally acceptable, if you start hearing
voices saying “we hear and obey,
master,” it may be time to get some
professional help. As for me, my kids
already think I am deluded because I
keep pointing the many remotes we have
at them and clicking various buttons with
a wicked yet hopeful grin on my face.
Alas, no success yet; but who knows?
One day it may work. Like winning the
lottery, you have to play to win!
Based in Madison, Wis.,
Steve;Tredinnick,;PE, , is
vice president of energy services
for Syska Hennessy Group,
which has more than 16 locations across the U.S. He has
more than 27 years’ experience
related to building heating,
ventilation and air-conditioning systems. The past 16
years of his work have been focused on district energy systems. Tredinnick is a graduate of Pennsylvania
State University with a degree in architectural engineering. A past IDEA board member and a member
of ASHRAE, he is currently immediate past chair and
handbook chair of ASHRAE TC 6. 2 District Energy and
current chair of the Chilled Water Plant Subcommittee
of ASHRAE TC 6. 1 Steam and Hydronic Systems. He
may be reached at email@example.com.
That’s why we can help you effectively
■ reposition your system for even
greater value – from customers,
investors or campus leaders;
■ develop and implement
sales and marketing plans that
■ analyze new business opportunities;
■ evaluate the effectiveness of
■ design service agreements and rate
■ train marketing and sales personnel.
All with a quick learning curve.
Your district energy management and
marketing resource. Contact us today for
insight when and where you need it.
John F. (Jack) Kattner
Kattner Associates LLC
(612) 338-5956 • firstname.lastname@example.org