2004
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17
f e a t u r e p u m p m o n i t o r i n g
T
he use of power sensing
techniques in pump perform-
ance monitoring is non-
intrusive as the sensor only needs to be
wired into the pump supply and there
are no probes in the fluid to get
contaminated or holes to cut in pipes.
This ensures that sensor installation
can be achieved with minimum
downtime and very low cost.
We concentrate here on the front end
of the process, the sensor, as it
matters not what sophisticated
instrumentation or computers are used
to process the sensors signals; unless
the sensor can respond quickly and
accurately to changes in whatever
process is being monitored then it will
not be possible to make sensible
process control decisions.
Power transducers
& load controls
Sensing motor power is an extremely
clever way to get feedback about
machine, pump or process perform-
ance. Monitoring the load on the
motor can give you valuable
information since this motor reflects
the changes that are taking place as
they happen.
On a mixer or agitator, for example, as
the viscosity increases, it will take
more power to stir the mixture. The
beginning and end of processes can be
detected precisely with ease. In the
case of pumps, sensing power allows
the detection of overloads, caused by
blockages or mechanical problems,
and under-load situations such as loss
of prime or cavitation. In addition,
changes in pump fluid flow conditions
are reflected in levels of motor power
allowing rapid warning of possible
problems.
A power sensor like the Digital Pump
Load Control senses the load and also
has display, delay timers and built-in
relays to sound alarms, stop the pump,
change feed rates, stop the machine,
etc.
A Hall effect power sensor such as
Load Controls UPC (Universal
Power Cell) can measure these load
changes very precisely and send
a signal to meters, computers,
programmable controllers, recorders
or data collection systems.
The Hall effect is observed when an
electric current flows through a
conductor in a magnetic field. The
magnetic field exerts a transverse force
on the moving charge carriers, which
tends to push them to one side of the
conductor, positive charge carriers
moving one way and negative charge
carriers the other. This build-up of
opposing charges produces a
measurable voltage between the two
sides of the conductor: the Hall effect.
Electricity, motors
and power
Nearly all industrial motors are three-
phase induction motors. The three-
phase power creates a rotating field in
the stator, which induces the rotor to
rotate. To measure three-phase power:
P = EI(cos
)(1.73)
where P = power in watts; I = current
in each phase (amps); E = voltage
phase to phase (V); cos
= power
Monitoring pumps
In this article,
Cliff Wyatt
discusses methods of performance monitoring and
protection of industrial pumps, utilizing power sensing techniques that
accurately measure electrical power drawn by pump motors. The focus is on
products and techniques developed by specialist manufacturer Load Controls Inc
of Sturbridge, MA, USA.
Power factor
0.9
0.1
No load
Full load
Amps
Power
No load
Full load
No load
Full load
No sensitivity
for low loads
Power is linear
Equal sensitivity at
both low and high loads
(a)
(b)
(c)
Figure 1. Change in (a) power factor, (b) current and (c) power as load is increased. www.worldpumps.com
WORLD PUMPS
December 2004
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f e a t u r e p u m p m o n i t o r i n g
factor (ranges from 01) and 1.73 is
the multiplication factor for three
phases =
3 (NB: 1 HP = 746 W).
What is the power
factor?
In an induction motor, the current
always lags the voltage. The power
factor is the cosine of this angular lag.
For a lightly loaded motor, the power
factor can be as low as 0.1. You can
think of this low power factor as
electrical inefficiency. Current is
flowing to the motor but it is not doing
useful work (power). As the load
increases, the power factor improves
and is typically 0.9 for a fully loaded
motor.
Why monitor power
instead of amps?
As you start to load a motor, the power
factor improves rapidly while the
current doesnt change significantly
until the motor reaches maybe 50% of
capacity (Figure 1). Power is linear
with load and change in load is a
change in power (HP or kW).
Measuring power gives you the signal
that you need for monitoring and
control of pump, machine or process.
When the load is low, power is low.
When the load is high, power is high.
At light loads, power is perhaps ten
times more sensitive than amps.
We often get calls saying, I put a
clamp-on ammeter on the motor and
Im getting 50% of full load amps. Your
device is only reading 10%. Whats
going on?
If you look at the curve in Figure 1b,
you can see that for a lightly loaded
motor the current is high. Why? The
power factor is low! As you start to
load the motor, the power factor
increases, but the current doesnt
change much. This is the advantage of
sensing power, which is directly
proportional to load, rather than
amps.
Two ways to
measure power
Single element
wattmeter
In many of the Load Controls
units, the single element wattmeter
technique for sensing power is used
(Figure 2).
Voltage
A 120 V voltage signal is taken from
two phases with a transformer. The
assumption is: the 120 V signal changes
in the same ratio as the primary
voltage. This 120 V signal typically
comes from a control transformer that
is also used for other instrument supply
purposes.
Current
The current signal is taken from
the remaining phase with a current
sensing toroid that is either built
into the control or located extern-ally.
For large motors a current transformer
is used together with the toroid.
Power factor
The control calculates the power factor
by sensing the zero crossing of the AC
voltage (AC voltage changes from + to
and back) and the zero crossing of the
AC current. This lag is the power
factor.
Assumption
The technique assumes that the load is
balanced. Since a load control is
normally used on a single motor, the
imbalance between phases is small. If
more than one motor is being sensed
from a single location, use the second
method of measuring power, the Power
Cell, which measures current and
voltage in all three phases. The single
element wattmeter technique also
doesnt work well on variable frequency
drives (VFDs). If you are using a VFD
drive, you need to use a Power Cell.
Power Cell
Traditional techniques do not work for
measuring the power from a
VFD.
Current transformers (and clamp-
on ammeters) do not work at very
low or high frequencies.
Voltage transformers do not work at
very low or high frequencies.
The wave shape as it leaves the
drive is too distorted to use zero-
crossing techniques.
Many lower-cost instruments are
designed for sine wave calculations
of values of current and voltage.
COM 5 A 15 A
L1 L2
Chassis
GND 120 V AC
L3
L2
L1
Voltage from
2 phases
Motor
Current from
remaining phase
Voltage
transformer
with 120 V
secondary
Figure 2. Single element wattmeter technique. Power sensing is done by monitoring the voltage between two of the
phases and current in the remaining phase. WORLD PUMPS
December 2004
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f e a t u r e p u m p m o n i t o r i n g
Traditional schemes for measuring
the power on the input to the drive
are also not reliable.
The drive doesnt take its power in
sine waves. It takes power only
during part of the cycle as it is
charging capacitors.
The drive often takes power at a
high power factor regardless of the
motor load. This wont give a true
value for lightly loaded motors.
The solution is to use the Power Cell,
since the Hall effect sensors are not
affected by the odd wave shapes and
frequency. Also, no current trans-
formers (CTs) and voltage trans-
formers (VTs) are used.
The bottom line is that this may be
the only sensor that works properly on
the output of a drive.
Unique Sensor
The Power Cell (Figure 3) is a unique
power sensor. It uses three balanced
Hall effect devices. Hall effect
semiconductors have two char-
acteristics: they sense a magnetic
field and they can multiply two
signals. When a current-carrying
conductor passes through a magnetic
flux concentrator and the Hall
effect sensor is placed in a gap in
the concentrator, the signal is
proportional to the current. The Hall
effect sensor is also excited with a
signal that comes from the voltage
sample for that phase. The Hall
device multiplies these two signals.
The resulting output is then
proportional to power (volts x amps).
This is an instantaneous vector
multiplication, which also calculates
the lag or lead of the current (power
factor). The signals for each of the
three-phases are summed and the
analogue output signal is
proportional to the three-phase
power (HP or kW).
Using Hall effect devices instead of
the traditional CTs and VTs greatly
simplifies installation. Accuracy is
also improved by eliminating the
phase shift errors from CTs and VTs,
which can be large at low power
factors. The Hall devices will also
work on the output of VFDs. The
analogue output can be connected to
meters, computers, programmable
controllers, chart recorders and data
loggers. It can also be used together
with a V series load control if trip
points and relay outputs are required.
Pump performance
monitoring
Although these power monit