The fundamentals of pumped water systems.
month, we began a discussion on variable frequency drives with a look at how
three phase AC induction motors work and how they are controlled by a VFD. We
said the speed at which a three-phase motor runs is determined by the number of
magnetic poles in the motor and the frequency of the power supplied to the
motor. The number of poles is fixed by the motor design, but the frequency, and
thus the speed, can be adjustable by a VFD. We also took a brief look at the
economics of VFDs in pumping applications, and at some residential uses,
present and future. This month’s article will explore the basic concepts in more
Meeting Variable Loads
Pumping system designers, engineers and contractors are constantly facing the
challenge of meeting a varying load with a fixed capacity pump. Pumps are sized
to meet the maximum foreseeable load, but that maximum load may only occur a
small percentage of the time. The application might be in a household with
loads varying from drip irrigation in the garden to light household usage to
lawn watering to a back-up fire system. Or, it could be a golf course with
zones having widely varying flow requirements. You would hope though that the
golf course irrigation system designer had put some effort into balancing the
The Control Valve Option
After this series on VFDs, we will take a close look at the control valve
solution to dealing with varying flow demand. Briefly, a control valve can be
used to automatically throttle back excess flow and maintain a constant
pressure on the demand side of the valve. As the control valve reduces system
flow, the pressure on the pump side of the valve goes up following the pump
curve and the operating point moves to the left. Figure 1 shows a generic pump
curve and two system curves with the corresponding operating points, “A” for
the control valve in the open position, and “B” when the valve is in the
throttle position. The system curves depict the static head plus the friction
losses in the piping system, including those of the control valve.
we move toward the left side of the curve, the horsepower required to run the
pump drops. This may be counter intuitive to some, but the fact is that most of
a pump’s energy goes into producing flow, not pressure. Figure 2 is the pump
curve for a 3 horsepower pump and includes a shaft power curve. Notice that as
the flow is reduced the horsepower required to run the pump drops dramatically.
At 90 GPM, it takes 3 horsepower to run this pump and at 20 GPM, only about
half that amount. This is the order of magnitude of energy savings that can be
realized using a throttling valve.
VFDs, on the other hand, move the operating point straight down the system
curve to Operating Point C on Figure 1. How is horsepower affected? To answer
this question, we turn to the Affinity Laws.
The Affinity Laws
The Affinity Laws describe what happens when the speed of a centrifugal pump
changes. (The same laws apply to changing impeller sizes in a particular pump.
Just substitute impeller diameter for motor speed.)
Affinity Law No. 1: Flow (F) is directly proportional to motor speed (S). That
is, S1/S2=F1/F2. As an AC induction motor slows, the flow rate drop-off is
proportional to the speed. If the RPM is cut in half, the flow rate drops in
Affinity Law No. 2: Pressure (P) is proportional to the square of the speed.
That is, (S1/S2)2=P1/P2. As the RPM is reduced, the pressure drops off as the
square of the speed. If the RPM is cut in half, the pressure output of the pump
drops to one forth of its full speed capability.
Affinity Law No. 3: Horsepower is proportional to the cube of the speed. That
is, (S1/S2)3=HP1/HP2. As the RPM is reduced, the horsepower required to operate
the pump drops as the cube of the speed. Cutting the RPM in half reduces the
horsepower to one-eighth of that required to run the pump at full speed.
For example, let’s say we have a 100 horsepower pump, pumping 1000 GPM at 100
PSI at 3450 RPM. If we cut the speed in half to 1725 RPM, the flow rate will
drop to 500 GPM, the pressure to 25 PSI and the horsepower to 12.5.
Much is made of the energy savings potential of VFD systems. It is easy to
generalize about cost savings when comparing a VFD system to conventional
constant-speed pumping systems and to systems utilizing a throttling valve.
Just look at the 100 horsepower example above, where cutting the flow in half
cut the horsepower by 87.5 percent. Even after you factor out the drive’s 3
precent operating efficiency loss, you still have a substantial energy savings.
Compare that to the earlier example in Figure 2, where using a throttling
valve, a reduction of 75 percent in the flow yielded a 50 percent drop in
horsepower. So, why isn’t everyone using a VFD? Here are a few reasons.
1. Not enough pressure: The second Affinity Law states that the pressure drops
as the square of the speed. If you need most of the pressure your pump produces
most of the time, you can’t use a VFD. If, on the other hand, your load varies
from a low pressure, low flow drip system to a high pressure, high flow fire
system, perhaps you should consider a VFD.
2. Turn down ratio: This is the ratio of the maximum versus minimum flow rates
your system needs to provide. 4:1 is a typical ratio for VFDs. For submersible
motors, Franklin recommends 2:1 and motor cooling may be a factor at lower flow
rates. If you need to throttle your system more than these ratios, go with a
3. Environmental conditions: VFDs use computers and computers don’t like high
ambient temperatures. You may need to add an AC cooling system, so factor in
some added cost. Also figure in some filtration if you are in a dusty
4. Pump cable length and harmonics: Most VFDs have a limit on the length of
power cable between the drive and the motor. This may be a factor for
submersible applications. Also, make sure you discuss the existence of any
harmonics and other electrical noise associated with your drive and the effect
on nearby electronic equipment.
Other VFD functions provided include control of the motor acceleration and
deceleration, torque control and motor protection. In pumping applications, the
motor acceleration is normally controlled, often in several steps. With a
submersible pump, it is important to accelerate to at least half speed within
one second for motor bearing lubrication, then accelerate to the desired speed
in another 10 seconds or so. Deceleration, on the other hand, is left to nature
in a pumping application, unless water hammer is a factor. As to motor
protection, modern VFDs include phase loss and balance protection, high voltage
protection and current protection. It is usually not necessary to provide
additional motor protection when employing a VFD, but many system designers do
it just the same, believing that you can never have too much protection.
We have covered most of the basic concepts having to do with variable frequency
drives in these last two articles. Next month we will conclude our series on
VFDs with a look at some of the applications. Till then … ND
If you would like to have a copy of all of my articles for reference or
training purposes, they have been compiled into a book, titled “The Pump Book,”
which is available for $20. Send an email firstname.lastname@example.org
the book, and I’ll mail you a copy.