Chemical Engineering February 2013 - 25

SOLVING THE POWER EQUATION IN PUMP MOTORS
rocess control via traditional adjustable-speed drives
(ASD) with centrifugal devices has an inherent problem
of attempting to use a linear equation to solve a nonlinear
problem. Traditional proportional integral derivative
(PID) methods control frequency directly. Controlling frequency
directly produces flows and pressures along a nonlinear
curve and an individual machine's impeller design.
Toshiba created VLP (Virtual Linear Pump) Drive Software
to offset this issue. According to a white paper published
by Toshiba, the pump industry improved the mechanical
aspects of pump design through increasingly sophisticated
computer CAD/CAM modeling and improved machining,
and the use of ASDs achieved better energy efficiency as the
kilowatt usage drops off along a voltage/frequency ratio.
However, the aspect that was missing, which the Toshiba
software addresses, was the ability to solve indirectly for
power (P) in the Pump Affinity Laws for each machine, individually,
whether the machine operates alone, in series or in
parallel.
P
The VLP algorithm starts with a fixed VLP number, according to
the white paper. If the output of the electrical current is less than
the VLP number, the frequency is increased. Otherwise, if the output
current is greater than the VLP number, the frequency is decreased.
The frequency (in Hz) turns into rpm to meet the current draw (amp
draw). This is true on centrifugal devices. Amp draw is linear to both
pressure and flow, and non-linear to speed (N). This is in contrast
to the VLP algorithm, which controls power directly to meet pressure
or flow.
The VLP number is a percentage of available amps of power
of a specific ASD. When a motor/pump is first setup in the VLP
wizard, the user is instructed to enter the motor's full-load amps
into the drive. The ASD temporarily sets the VLP Max variable
to the entered motor amps divided by the ASD full output amps.
The operator can then adjust the MAX VLP number to achieve the
maximum desired set point for flow, pressure and so on. At this
point, the ASD has sorted the limits of where the pump can operate.
On multiple-pump systems, this is repeated so each machine
has the same operational set points. The VLP numbers, which all
effect the corresponding amp draw and frequencies of operation
at any given point, will be different because of mechanical differhighly
reliable performance in the 125to
450-hp range, while exhausting 90%
of the drive-generated heat outside the
enclosure via the backchannel for high
energy efficiency.
Also, to create more efficiency in
pump motors, Toshiba International
Corp. (Houston; www.toshiba.com) is
offering its P9 low-voltage adjustablespeed
drive. The drive incorporates
Toshiba's proprietary Virtual Linear
Pump (VLP) Technology, allowing the
drive to linearly control pressure, temperature
or flow (see the excerpt from
a Toshiba white paper in the sidebar
above, " Solving the Motor Power
Equation in Pumps " for more on how
this works).
" Every pump has a curve, a non-linear
curve, " says Kevin Nguyen, application
engineer with Toshiba. " And, it's
very hard to predict that curve using
the linear equations of traditional PID
(proportional integral derivative) programs
and get the best efficiency from
The new VLP
algorithm
provides enhanced
control,
expanded
configuration
options and
improved energy
efficiency
over the conventional
PID
method
Toshiba
ences in piping, impeller wear or trim, and motor efficiency. Next,
a Virtual Linear Pump curve is created in the software, where a
percentage of each pump will run at the required power in order
to precisely match the other pumps' set points.
If controlled from a common external analog signal, A VLP MIN
of 0% will cause the pumps to run at the MIN VLP set point. The
same is true at 100% where all pumps will be running at their
individual MAX VLP. What is important is that for each change in
set point, the resulting flow or pressure will be linear, whereas the
frequencies and amp draws will be non-linear.
This is important on a single machine when the frequency is
changed, as in standard ASDs the output is non-linear and energy
is wasted searching to find the appropriate speed needed to satisfy
the control loop. Because VLP produces a virtual linear performance
curve, the first calculation from an external PID controller
will most likely be the most accurate calculation. The VLP software
changes the power of the motor to produce the desired pressure or
flow. No time or energy is wasted accelerating and decelerating
past the needed frequency.
This new VLP algorithm for the Toshiba P9 drive provides enhanced
control, expanded configuration options and improved
energy efficiency over the conventional PID method, according to
the company's white paper.
❏
Baldor Motor
FIGURE 5. Investment
costs for a motor are a
small fraction of the total
life cycle costs
that pump. PID looks at the
rate of error and compensates
for it, which leads to
overshooting, speeding up
and slowing down until it
eventually levels out to the
correct pressure, flow or rate
of speed. We set out to find
a better method that would lead to
more efficiency and came up with VLP,
which helps increase the efficiency of
the pump motor, " he says.
Obviously these solutions come with
a higher price tag than lower-efficiency,
standard-induction motors, motor systems
and drives, but like most things
in life, you get what you pay for. For
example,
John Malinowski, senior
product manager for AC motors with
Baldor Electric Co., suggests looking
at the bigger picture during a motor
upgrade. " If you look at the pie chart of
a motor's 20-yr life cost (Figure 5), you
can see that the initial purchase price
is only about 2%, " he says. " Ninetyseven
percent of the lifetime cost of a
motor is the electricity used to operate
it. Yes, you will pay more upfront for
a premium motor, but you'll pay significantly
more later for the electricity
used to run a less efficient motor. " ■
Joy LePree
CHEMICAL ENGINEERING WWW.CHE.COM FEBRUARY 2013 25
http://www.toshiba.com http://WWW.CHE.COM
Chemical Engineering February 2013

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