One definition of “calibrate” is “to determine, rectify, or mark the graduations of something”. An ammeter is an instrument for measuring electric current. Therefore the simple definition of a calibrated ammeter is a current measuring device marked with units of measure, presumably amperes.
In the blower industry, however, the term has developed a specific meaning. A calibrated ammeter is an instrument that measures a blower motor’s current draw and converts the measurement to a display of blower airflow rate. [See Figure 1]
Figure 1: Example of a Calibrated Ammeter.
This simple device has been in use for decades. In many installations the calibrated ammeter with an integral low current switch is the primary surge prevention device. Surprisingly, despite its long history, the calibrated ammeter’s operating principles and limitations are not well understood by end users.
Basis of Operation
The basic ammeter has been used for measuring current since the nineteenth century. Used in conjunction with the current transformer [See Figure 2] it can measure the current draw for motors from fractional to thousands of horsepower. This provides a simple and robust measurement system.
Figure 2: Drawing from a 1928 Patent for a Current Transformer.
Converting amperage to airflow rate is based on the fundamental operating principles of constant speed centrifugal (dynamic) blowers: the pressure ratio and head have a direct, one to one correlation with volumetric airflow rate.
Head, expressed as ft∙lbf/lbm, is a measure of the work done on the gas.
Ws = isentropic head, ft∙lbf/lbm
κ = ratio of specific heats, dimensionless
R ̅ = universal gas constant, ft∙lbf/lbmol∙°R
MW = molecular weight, lbm/lbmol
Ti = inlet temperature, °R
pd, pi = discharge and inlet pressure, psia
Head and flow rate in turn have a direct relationship with power.
P = blower shaft power, bhp
ρ = air density, lbm/ft³
ηs = bare blower isentropic efficiency, decimal
These thermodynamic calculations can be omitted in developing data points for the calibrated ammeter. The starting point for calibration data is the blower performance curve, which shows discharge pressure and power vs. flow rate at specific inlet and barometric conditions. [See Figure 3] The power corresponding to a given ICFM or SCFM can be read directly from the curve.
Figure 3: Example Blower Performance Curve.
For an electric motor at constant voltage, power has a direct relationship with current. The nominal motor efficiency and power factor can be obtained from the motor nameplate or motor data sheet. That permits calculating the current draw from the shaft power.
I = motor current, A
P = blower shaft power from performance curve, bhp
V = system voltage, V
ph = number of phases (typically 3), dimensionless
ηm = motor efficiency, decimal
PF = motor power factor, decimal
The current has a direct correlation to a specific airflow rate at specific inlet conditions.
The calculation can be repeated for any number of points needed. The relationship is essentially linear and intermediate points may also be obtained by interpolation.
Performance curves are often provided using ICFM for the x-axis. The process demand is generally given as SCFM, defining the oxygen content of a cubic foot of air at 14.7 psia, 68 °F, and 36% relative humidity. Most calibrated ammeters show the correlation between SCFM and current.
Ignoring relative humidity, the conversion from inlet volumetric flowrate to SCFM is straight forward:
Note that these relationships ignore the effect of voltage fluctuations and variations in motor properties as well as changes in relative humidity. This obviously results in some inaccuracies, and if precise measurement is needed alternate technology should be investigated. However, the calibrated ammeter has proven to be adequate for aeration process control and blower surge protection in most applications.
Automated Control Applications
The calibrated ammeter is a useful monitoring device for manually throttled blowers. Mounted next to the blower it can be used to modulate the SCFM delivered to the process and more importantly prevent inadvertent operation in surge or overload regions. Most new applications, though, involve a programmable logic controller (PLC) modulating an electrically operated inlet valve to maintain an operator entered flow or pressure setpoint.
Using a PLC to measure motor current can be accomplished simply and easily. Many starters provide an analog signal for motor current as a standard or optional feature. Current transmitters also provide an analog signal. Monitoring one phase provides adequate accuracy, making the instrumentation inexpensive.
The correlation for current and SCFM obtained above can be converted to a linear equation in slope-intercept form, y = mx + b, for PLC programming. One way to accomplish this is to plot several data points in an Excel chart and add a trendline. [See Figure 4]
Figure 4: Linear Correlation of Blower Current to SCFM. Click to enlarge.
It is possible to manually determine the linear equation from only two points. The points can come from the performance curve or values taken from an existing calibrated ammeter.
q = air flow, SCFM
I = motor current, A
m = slope, SCFM/A
b = y intercept, SCFM
It is important to note that the equation derived is most accurate for the specific ambient conditions used to establish the original performance curve or to determine the calibrated meter data under consideration.
Compensating for Temperature
Temperature shifts the correlation between current and SCFM because of the impact on inlet density. Lower inlet temperature increases the density, increasing the head and power draw at a given flowrate. The new correlation can be calculated from the temperature ratio and the linear equation for SCFM vs. current.
Although it may be counter-intuitive, throttling the blower inlet doesn’t affect the correlation between SCFM and motor current. [See Figure 4] The inlet throttling valve creates a pressure drop, which decreases air density. Both the volumetric flow rate and the correlating head and pressure ratio remain the same. Some of the pressure difference across the blower is taken across the inlet valve instead of at the discharge. Because the inlet density is reduced the mass flow rate is reduced. The net result is that the blower shaft power and the motor current are reduced, maintaining the correlation between SCFM and current.
Calibrated ammeters aren’t suitable for positive displacement (PD) blowers or for centrifugal blowers with variable speed or guide vane controls. The correlations behind the calibrated ammeter only apply to constant speed centrifugal blowers.
The head of a PD blower is not a function of flow rate. The discharge pressure and pressure ratio will rise until they are high enough to overcome the resistance to flow. The power demand and motor current are functions of the pressure ratio, and therefore do not correlate with flowrate alone.
The centrifugal blower’s relationship between head and flowrate can be disrupted by the control method. If variable speed is used to modulate blower capacity the performance is governed by the affinity laws. The flowrate is proportional to speed, and the head is proportional to speed squared. Further complication results if a variable frequency drive (VFD) is used to change speed. The VFD output voltage is typically proportional to frequency, which affects the motor current.
Guide vanes also modify the relationship between flow and head. Inlet guide vanes (IGV) and variable diffuser vanes (VDF) change the conversion of kinetic energy produced by the impeller to potential energy. The resulting change in pressure ratio creates a change in head.
It would be possible to correlate speed or guide vane position to current and develop a correlation with flow. Doing so would be complex, however, and the inherent simplicity of the calibrated ammeter would be lost. It is generally preferable to measure airflow by other means for these applications.
The primary sensing element of the system is often a current transformer (C/T). C/Ts are sized based on a 5 A output on the secondary terminals, matching the 5 A input range of most ammeters. For example a 300:5 C/T will output 5 A when motor current is 300 A. Every C/T has a maximum burden rating identifying the maximum secondary volt-amperes (VA). This can be converted to equivalent resistance:
Rmax = maximum allowable total current loop resistance, Ω
Bmax = maximum allowable C/T burden rating, V∙A
I = secondary max current, A, usually = 5 A
Low ratio C/Ts generally have a low maximum burden, and the total loop resistance of the wire from the C/T to the ammeter and back can be exceed the maximum rated VA. This can be overcome by using a higher ratio C/T and multiple turns of the motor wiring through the C/T window, which will change the effective C/T ratio. [See Figure 5]
Figure 5: C/T with Multiple Primary Turns.
The calibrated ammeter is a well-established and robust device for monitoring the air flow rate of throttled centrifugal blowers. The correlation between motor current draw and airflow rate can be readily adapted to PLC or microprocessor based automated control. The system’s accuracy is sufficient for all but the most stringent process control applications.
About the Author
Tom Jenkins has over forty years’ experience in blowers and blower applications. As an inventor and entrepreneur, he has pioneered many innovations in aeration and blower control. He is an Adjunct Professor at the University of Wisconsin, Madison. Tom is the current Chair of the ASME PTC 13 Committee. For more information, visit www.jentechinc.com
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