District Energy - 2010 4th Quarter

Table 1. Controlling Multiple Motors With a Single VFD: Advantages and Disadvantages.

Advantages

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 smaller VFDs.

It offers the potential for lower installation costs.

There is no increased risk of harmonics.

Disadvantages

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.

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Kattner Associates gives you both.

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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!
Click. Click.

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 stredinnick@syska.com.