Fluke has come out with another superb electrical measuring instrument. It is the Fluke 438-II Motor Analyzer. Owners of the previous Fluke 435-II Power Quality Analyzer can download an upgrade that will add Motor Analyzer functions to their existing equipment. Full documentation including specifications and owner’s manuals for both of these members of the oscilloscope family of instruments is available free of charge from the Fluke website.
The 438-II, like its predecessor is a hand-held, battery-powered instrument and consequently you don’t have to worry about exposing it (and yourself, and the device under test) to a dangerous arc fault as with an AC-powered bench-type instrument when connecting to a voltage that is referenced to but floats above utility ground.
The case is an impact, moisture and dust-resistant enclosure, so there is every reason to believe that with careful handling it will last indefinitely.
The Fluke 438-II Motor Analyzer is equipped with voltage and current clamp-on leads as required for three-phase work. It can also analyze single-phase power sources and motors, but in all cases a load is required so that voltage and current can be read simultaneously as needed to measure power quality and analyze the condition of a motor.
The Motor Analyzer measures mechanical power, torque and rpm direct on-line asynchronous (induction) motors and asynchronous motors driven by a variable-frequency drive (VFD). In conjunction with electrical power and power-quality measurements, the Motor Analyzer feature provides abundant information about a motor’s mechanical and electrical operational variables and efficiency. There is no need to use additional mechanical sensors or to shut down the process.
Configurations that are supported include three-phase delta, three-phase open leg, three-phase It and two-element hookups. Specifications are valid for motors at stable operating temperatures. The motor to be analyzed should be run in advance of the test for at least one hour at full load. If the motor is 50 hp or higher, it should be run for two to three hours at full load prior to analysis.
Rated torque is calculated from rated power and rated speed. Update rate of motor measurements is once per second. Default trend duration is one week.
To prepare for a motor measurement, the first step is to consult the motor name plate. To comply with the National Electrical Code, all motors (except small clock motors and the like) are required to have attached metal name plates displaying information applicable to the type of motor.
To begin a motor analysis, open the Motor Setup Screen. On the motor analyzer keyboard press Menu, which opens the Menu Screen. Then press F2 to change the page view. Use the Up and Down keys to move through the menu selections and highlight Motor Analyzer. Push Enter to open the Motor Setup Screen.
Use the Up and Down and Left and Right buttons to enter the motor name plate parameters. The Motor Analyzer supports motor designs according to NEMA and IEC design types. If the design type is unknown, press Other. In this event, you must consider an additional error of five percent for the mechanical motor parameters.
Default values are available for the nameplate settings and ranges. Use F1 (Unit Setup) to open the Setup Screen to configure these values. The selection depends upon the default motor frequency. The function keys guide the user through the screen setups:
Press F1 and select Unit Setup screen to open the Motor Analyzer Function Preference screen. This permits the user to select metric or imperial units for the mechanical motor power, torque and default motor frequency. Press F2 and select Trend Setup screen. This permits the user to adjust the duration of automatic trend recording. The default is seven days. Press F4 and select Defaults for the Motor Setup screen. This permits the user to set the default values defined in the Function Preferences screen included in the Unit Setup. The defaults depend upon the default motor frequency. Press F5 to start measurements. This starts the Motor Analyzer function.
Supported VFD characteristics include:
Drive output frequency – 41 Hz to 69 Hz
Drive type – voltage source inverter only
Drive control method – V/f only, open-loop vector control, closed-loop vector control and drive with and without encoders.
Use the Setup screen to set the Analyzer limits and defaults. F1 sets the Analyzer limits. F2 sets the 50 Hz defaults. F3 sets the 60-Hz defaults. F5 goes back to the previous screen.
If you change the 50-Hz or 60-Hz defaults, go to the Motor Setup screen to activate the new defaults.
The user can adjust the default limit values that display the system performance in bar graphs. These limits adjust where the bar graph displays the transition from orange to red. Use the Up, Down, Right and Left keys to change a default value. Press F4 to restore the default values. The Trend Setup screen modifies the Timed Start screen. In Motor Analyzer mode, there is Immediate Start (not Timed Start) mode only. Duration can be adjusted.
The Motor Analyzer screen shows mechanical and electrical parameters. Mechanical parameters include power, torque and speed as well as efficiency (ratio between electrical power supplied and mechanical power delivered in percentage). A bar graph shows the performance of the motor compared to its rating. The actual value for each parameter displays the system performance. Green = good, yellow = adequate, orange = poor, red = bad. Use the Up and Down keys to navigate between electrical and mechanical parameters screens. The Electrical and Power Factor screen shows the voltage unbalance and harmonics voltage factor in accordance with NEMA M61.
The limit values where the bar graph turns from orange to red are set to NEMA and industry standards. Press F1 to open the Analyzer Limits menu and view these limits.
The next screen shows electrical power and power factor. It displays voltage unbalance and harmonics voltage factor according to NEMA MG1. Use the side arrow keys to navigate between electrical and mechanical parameters screens. The relevant function keys are:
F2 – View Analyzer Limits screen
F2 – Select Meter screen
F3 – Select Derating screen
F4 – View Motor Setup screen
F5 – Stop and Save measurements
The View Analyzer Limit screen has the same information as the Set Analyzer Limits screen, but it is read only. The single active function key is F5 ( OK or Back), which navigates to the Motor Analyzer screen. To change information, go to the Motor Analyzer or Meter screen. Then press F5 to stop the process. The other parameters are existing logger parameters in the 430 Series-II instrument: Active power, Apparent power, Reactive power, Power factor, Cos φ/DPF, Harmonic power, Unbalance power, Voltage, Current, Volt THD, Amp THD, Unbalance.
Relevant function keys are:
F1 –Use Up and Down arrow keys to scroll through the motor screen
F2 –Opens the Analyzer screen
F3 – Opens the Trend screen
F4 – Opens the Motor Setup screen
F5 – Stops and saves measurements
The Trend screen is the same as in the 430 Series-II instrument. It shows the recorded measurement data over time.
When unequal line voltages are applied to a three-phase motor, unbalanced currents flow through the stator windings. A small voltage unbalance will result in a much larger current unbalance. The temperature rise, therefore, in a motor operating at a given load and voltage unbalance is greater than in the same motor operating under the same conditions with balanced voltages. If the voltages are unbalanced, the maximum permitted load must be de-rated. Often a three-phase power source will exhibit a small voltage unbalance and the load will exhibit a different unbalance. Depending upon the wiring terminations, these errors may add or subtract. If they add, this condition is far more harmful, while if they subtract, it is reduced. Therefore, the installation may be improved by re-arranging the terminations. It any one pair is reversed, rpm will be reversed. Consequently, it is necessary to roll the connections (A to B, B to C, C to A).
Harmonic currents are introduced when the line voltages applied to the motor include components at frequencies other than the fundamental This condition results in a higher operating temperature of the affected motor at a given load. Accordingly, the maximum power applied to the motor must be de-rated. A better solution is to mitigate the harmonics, for example by means of frequency filters.