Selecting motor measurement, analysis instruments
Making precise electrical and mechanical power measurements on motor and variable frequency drive (VFD) systems, especially to calculate energy efficiency, can be done in three easy steps.
In the first part of this three-part series, we examined basic electric motor power measurements and analysis. In this second part, we will examine a three-step process for making precision electrical and mechanical power measurements on a variety of motors and variable frequency drive (VFD) systems. We will also show how these measurements are used to calculate the energy efficiency for motor and drive systems. All this has to happen in a context where waveforms are complex and distorted, so instrument selection for various applications is especially critical.
Different associations have developed testing standards that define the accuracy of instrumentation required to conform to their standard: IEEE 112 2004, NVLAP 160, and CSA C390. All three include standards for the measurement of input power, voltage and current, torque sensors, motor speed, and more (see Table 1). Current transformers (CTs) and potential transformers (PTs) are some of the primary instrumentation devices used to make these measurements.
The corresponding standards are very similar with a few exceptions. The allowable instrumentation errors for IEEE 112 2004 and NVLAP 150 standards are identical. However, CSA C390 2006 has some differences in terms of temperatures and readings.
For example, the input power requirement for CSA C390 2006 is ±0.5% of the reading and must include the CT and PT errors, whereas those for IEEE 112 2004 and NVLAP 150 both require only ±0.5% of full scale (FS).
Current sensors are usually required for testing because high current cannot be brought directly into the measuring equipment. There is a variety of sensors available to match specific applications. Clamp-on sensors can be used with power analyzers. Scope probes can also be used, but they must be used with caution to ensure the instrument is not exposed to high currents.
For CTs, the feed wire can be connected through the window (CTs are typically donut shaped or oblong, with the hole or inner portion referred to as the window), or low current connections can be made to the terminals on the top of the device. Shunts are typically used for dc applications but not ac or distorted frequencies, although they can be used for synchronous motors up to a few hundred Hz. Specialized CTs are available that work well for high frequencies, more commonly found in lighting applications than in motors and drives.
A new type of CT system that delivers high accuracy from dc to the kHz range is now available. It features an active-type transformer that uses a power supply conditioning unit. This type of CT system provides highly accurate measurements-approximately 0.02% to 0.05% of the reading-especially for VFDs, which can have frequencies that range from 0 Hz to the operating speed of the connected motor.
Voltage transformers simply transform a voltage from one level to another. In measurement applications, step-down transformers are sometimes required to reduce the voltage delivered to the measuring instrument. However, many instruments can accommodate relatively high voltages and don’t require a step-down transformer.
An instrument transformer is usually combination of a CT and a voltage transformer. Instrument transformers can reduce the number of required transducers in certain measurement applications.
Selection considerations, cautions
When deciding which device to use, the first question is the frequency range of the parameters to be measured. Typically, dc-to-line frequency sine waves can use dc shunts, which offer high accuracy and simple installation. For ac and dc applications, the Hall effect or active type instrument transformer can be used. Hall effect technology has a lower accuracy rate, while the active type provides more precision. Various instrument transformers can operate at high frequencies of 30 Hz or more, but they can’t be used for dc.
The next consideration is the level of accuracy required. For an instrument transformer, it’s typically specified as the "turns ratio" accuracy. Phase shift is another important factor because many transformers are designed for current measurement only and aren’t compensated for phase shift.
Phase shift is basically an effect of power factor for power measurement, and it will thus have an influence on the power calculation. For example, a CT with a 2-deg maximum phase shift as part of its specification would introduce an error of Cos 2 deg, or 0.06%. The user must decide if that percentage of error is acceptable for the application.
A CT is a current source. According to Ohm’s Law, voltage (E) equals the current (I) through the conductor multiplied by the resistance (R) of the conductor in units of ohms. Opening the secondary of a CT effectively drives the resistance to infinity. This means the internal current will saturate the coil, the voltage will approach infinity as well, and the unit will be damaged or will destroy itself. Even worse, a CT with an accidentally opened secondary could seriously injure workers.
|Never open the secondary circuit of a CT. Users could be seriously injured, and the CT can be damaged or destroyed.|
To determine instrument compatibility, the output level of the CT must be identified. Clamp-on and other types of CTs typically have an output specified in millivolts/amp, milliamps/amp, or amps. A typical Instrument CT output can be specified from 0 to 5 A.
The impedance and load on the CT must be considered. These two factors are affected by how much wire is used to connect the CT to the instrument. This wiring is the resistance, or the burden, on the instrument, and thus could affect its measurements.
Scope probes can present their own set of problems if not used correctly. Many scope probes are designed to work with the input impedance of an oscilloscope. Therefore, the input impedance ranges can be different on a power analyzer, and must be known and taken into account.
Another item to consider when determining instrument compatibility is the physical requirement of the device. Size must be factored along with the type of CT, such as a clamp-on or donut type, each of which will work better in a particular situation.
Example: 3-phase motor system
We will now examine a typical 3-phase, 3-wire motor power measurement using the two-wattmeter method. Blondel’s Theorem states that the number of measurement elements required is one less than the number of current-carrying conductors. This makes it possible to measure the power in a 3-phase, 3-wire system using two transducers when there is no neutral. However, when there is a neutral, three transducers are used as there are now four conductors.
Primarily, 3-phase power is used in commercial and industrial environments-especially to power motors and drives-because it’s more economical to operate large equipment with 3-phase power. To calculate 3-phase wattage, the voltage of each phase is multiplied by the current of each phase, which is then multiplied by the power factor. This value is multiplied by the square root of 3 (the square root of 3 is equal to 1.732).
A power analyzer is used to measure the 3-phase power consumed by a loaded motor. Figure 1 shows a typical measurement connection with the display showing all three voltages, all three currents, total power, and power factor.
Figure 2 shows a 3-phase, 3-wire power measurement accomplished using the two-wattmeter method. All three currents and voltages as well as the total VA and VAR are listed. This configuration can display the individual phase power readings but they should not be used directly, because for this measurement method, only the total power is an accurate reading.
Basically, when using the two-wattmeter method on a 3-wire, 3-phase system, individual phase power can’t be measured directly, nor can any of the phase parameters be measured-including the phase power factors. However, the total of the phase parameters can be measured.
For a 3-phase, 3-wire delta-connected motor, it’s possible to measure line-to-line voltages and individual phase currents. Because there is no neutral, it’s not possible to measure phase voltages. This situation results in some readings that must be explained.
Looking at waveform displays in Figure 3, one sees the line-to-line voltages Vab, Vbc, and Vac. The line-to-line voltages seen by the instrument are 60 deg apart in a balanced system. The currents are phase currents, which are seen by the instruments as 120 deg apart.
Another way to represent this system graphically is with a phasor vector diagram (see Figure 4). The triangle in the upper portion of diagram shows the line-to-line voltage measurements in black, the phase voltage values in red (but these are theoretical because there is no neutral), and the phase currents in blue. The lower portion of the diagram shows the phase differences between the voltages and currents. Again, note that the line-to-line voltages are 60 deg apart while the phase currents are 120 deg apart. Also, if the top diagram represents a pure resistive load, then the blue currents would be in sync with the red voltages. However, with an inductive load, such as a motor, the blue current vectors are out of phase with the voltages.
In addition, for this measurement method, on the bottom diagram, the current vectors will always experience an additional 30 deg shift from the voltages. The bottom line is that a properly configured power analyzer will account for all of these conditions.
What if the phase power and phase power factor must be accurately measured on a 3-phase, 3-wire system, and not just approximated? Figure 5 shows a technique that enables measurement of the phase parameters on a 3-phase, 3-wire motor by creating what is called a "floating neutral."
There are limits to this technique, however. It will work well on the input to an induction motor, a synchronous motor, or a similar motor without a VFD. Caution must be taken when using this technique on a VFD system because the high-frequency distorted waveforms and harmonics can cause inconsistent measurements.
Moreover, the floating neutral technique works only for equipment with sine wave type waveforms. With a pulse-width modulation (PWM) drive, a 500 Hz line filter (low-pass filter) can be enabled, which will then allow the readings to be displayed for the fundamental frequency, but not the harmonics.
Taking 3-wire and 4-wire power measurements
It’s important to recognize that power will read the same regardless if it’s measured in a 3-phase, 3-wire or a 3-phase, 4-wire method. However, using a 3-phase, 4-wire connection, the voltage values being measured are phase voltages from line-to-neutral.
Figure 6 is a screenshot from a power analyzer that shows how similar the power and power factor readings are for a PWM drive operating a motor, comparing a 3-phase, 3-wire 500 Hz filtered input against a 3-phase, 4-wire floating neutral input.
An alternative solution uses a delta measurement function that is found in advanced power analyzers. The delta measurement function uses instantaneous line-to-line voltage and phase current measurements to derive a true line-to-neutral voltage-even if the phases are unbalanced (see Figure 7). A vector amplitude calculation inside the processor makes this possible. This function also provides the phase power measurements on the 3-wire circuit. The delta measurement solution also provides the neutral current.
The final part of our three-part series will cover the very important topic of electrical power measurement on 3-phase motors.
Bill Gatheridge is the product manager for power measuring instruments at Yokogawa Test & Measurement, and has more than 20 years of experience with the company in the area of precision electrical power measurements. He is a member and vice chairman of the ASME PTC19.6 committee on electrical power measurements for utility power plant performance testing.