Allen Bradley's "Energy Savings with Adjustable Speed Drives"
The paper written by Allen Bradley "Energy Savings with Adjustable Frequency Drives" is incorrect.
The paper from AB has six pages on how important Total Dynamic Head or (TDH) is
for pumps. The example is for a pump to operate a system that requires 250 feet
of head at 250 gallons per minute. Then a later explanation states that a static
head or (TDH) does not exist in this example. A TDH always exist with pumping
systems. If head is not required, then a pump is not needed, because water will run
down hill by itself.
If 250 feet of head is required then 250 feet of head is always present. A 250
feet deep well will always be 250 feet deep. 250 feet is the same as 108 PSI. If
sprinklers at a golf course need 108 PSI to operate then it does not matter if only
one of the 50 GPM sprinklers is running or all five of the 50 GPM sprinklers are
running. You must always supply 108 PSI or 250 feet of head even as the flow
required varies from 50 GPM to 250 GPM.
The pump in this example can be throttled from 250 GPM to 100 GPM, will still be
43% efficient, therefore the power required will be reduced from 25 HP to 17 HP.
(100 GPM X 290') divided by (3960 X .43) equals (17 HP)
The 250 feet of head must be maintained in this variable speed system example.
Because head is reduced by the square of the speed, this pump can only be slowed
from 3,560 RPM to 3,310 RPM. A 7% reduction in speed (or 3,310 RPM) is as slow
as this pump can spin or the 250 feet of head required is no longer present.
If 7% reduction in speed is as slow as you can go, then according to the Affinity
Law and reducing power requirements by the cube of the speed, this variable speed
pump will only drop in power required from 25 horses to 20 horses. In this case
the pump will use less power when throttled with a valve (17 HP) than it does when
being slowed by a drive (20 HP).
(See also "TDH Limits RPM for VFD") This paper shows the correct way to figure
minimum possible pump speed. You cannot use the Affinity Law to figure
horsepower requirements until you have determined the minimum possible speed
that the pump can run and still produce the required TDH.
Customers become very unhappy when they purchase a drive and expect to see a
25 HP slow down to a 1.6 HP load, and then realize that because TDH is always
required this pump can only be slowed to a minimum of 20 HP. How can you claim to
have a fair comparison if you do not maintain the mandatory TDH throughout the