Handbook of Small Electric Motors Episode 11 pdf

CHAPTER 9 TESTING Barry Landers Rockwell Automation 9.1 9.1 SPEED-TORQUE CURVE A typical computerized test station consists of a power analyzer, dynamometer con- troller, digital indicator, and multimeter (see Figs. 9.1 and 9.2). This kind of test sta- tion will quickly generate speed-torque curves and summary sheets, as shown in Fig. 9.3, or full sets of curves for speed versus torque,efficiency, current, watts,and power factor, as shown in Fig. 9.4. Different inertial compensations can easily result in an entire family of curves and can greatly affect results. By using a dynamometer con- troller to take the motor from idle to a predetermined minimum speed and back to idle at a controlled rate, inertial effects will nearly disappear simply by interpolating for the same speed points and averaging the two sets of data. Points from the result- ing curves will then compare very well with static measurements made at the same performance point. Finally, an accurate stall point should consist of an average of at least three sets of locked rotor data at different rotor positions. In order to obtain accurate test results, the maximum motor torque should equal at least 30 percent of the dynamometer rating. Voltage drops during the test will drastically affect results and require heavier wiring than specified by the National Electrical Code. A good rule of thumb would increase wiring in conduit by six wire gauges and bench wiring by four wire gauges.Voltage control devices such as power stats must also minimize voltage drops, either by voltage regulation or by selecting a rating of at least four times the maximum motor current. The power supply fre- quency should not vary more than 0.5 percent from the rated value. In addition, polyphase systems should not exceed 0.5 percent voltage unbalance between phases. 9.1.1 Acceleration Test This test determines the acceleration characteristics of a motor for a given amount of inertia and load torque. The inertia test stand shown in Fig. 9.5 includes a set of inertia wheels that will provide a range of inertias from 50 to 4000 lb⋅in 2 in 50-lb⋅in 2 increments and an analog tachometer to monitor speed. In order to minimize oscil- lations, the setup uses low-backlash couplings between the motor, the inertia wheel shaft, and the dynamometer. An oscilloscope monitors the voltage from the analog tachometer and provides voltage and time data to a PC through the general-purpose interface bus (GPIB) connection. At this point, the PC calculates the speeds based on the tachometer voltage constant and plots the acceleration curve (see Fig. 9.6). 9.2 CHAPTER NINE FIGURE 9.1 Typical motor test bench diagram. FIGURE 9.2 Typical test station. TESTING 9.3 FIGURE 9.3 Speed-torque curve and summary sheet generated by a computerized test station. FIGURE 9.4 Speed-torque, efficiency, current, watts, and power factor curves generated by a com- puterized test station. 9.4 CHAPTER NINE FIGURE 9.5 Inertia test stand. FIGURE 9.6 Acceleration plot. AC motors and open-loop dc motors could use the bench shown in Fig. 9.1 to acquire and document additional data such as torque, efficiency, current, and watts. However, closed-loop motors require a different approach, as outlined in the dc motor test section on speed profiles. 9.1.2 Good Test Practices 1. Review test requirements for consistency with UL and other requirements to avoid wasted test time. Obtain clarification as needed. 2. Measure the resistance of the motor windings using a precision multimeter or bridge and record ambient temperature. Ensure that resistance meets winding specifications prior to any tests. 3. Perform high-potential tests per UL requirements prior to energizing the motor. In the absence of other information, a good rule of thumb would apply a voltage equal to (1000 plus twice the rated voltage) times 1.2 for at least 1 s. 4. Select stable capacitors within at least 1 percent of specified values and check for drift rather than relying on old measurements marked on the capacitor. Always discharge the capacitors prior to checking the value to avoid damage to the meter. Switch between all selections on capacitor decade boxes when bleed- ing charge. 5. Confirm that the dynamometer has appropriate cooling operating. 6. Use brass-tipped setscrews in the coupling and check tightness periodically to avoid bad data. 7. Select appropriate ranges for equipment that does not autorange. 8. Minimize coupling backlash and torsional deflection. 9. Monitor display to confirm at least the initial printout. 10. If the motor has a ground wire, connect it to the workstation ground. 11. Recheck motor connections against specifications prior to test. 12. Adjust speed-torque test time and minimum speed to minimize motor oscilla- tion below breakdown. 9.2 AC MOTOR THERMAL TESTS Thermal tests for ac motors determine the thermal protector trip and reset temper- atures under locked and running condition, as well as the leveling temperatures for running conditions. See Fig. 9.7 for test setup. These tests usually run at nameplate- rated voltage, frequency, and current, but UL may require tests at different voltages, as listed in Table 9.1. Running conditions include full load, running overload, full-voltage idle, and reduced-voltage idle heat runs.The running test definitions follow. Full-load heat run: A dynamometer or other load device maintains the motor at the rated torque or current, as determined by customer and agency requirements. The test continues until the winding temperatures reach equilibrium for 60 min. TESTING 9.5 Running overload: A dynamometer holds the motor current per the nameplate rating until the motor windings attain thermal equilibrium for 60 min or until the protector trips.The dynamometer controller then increases the torque to provide a current increment per Table 9.1, and maintains this current until the winding tem- perature reaches equilibrium. This sequence repeats until the thermal protector opens or the motor stalls. (Note: For running-overload tests, UL requires perform- ing the first test at nameplate current. If the no-load current of the motor at rated voltage equals or exceeds the nameplate current, the first running-overload test occurs at idle. If the thermal protector opens at the nameplate current—above the no-load current—then the motor test must also occur under no-load condition at the nominal test voltage. If the protector opens at idle, then the idle test repeats for 9.6 CHAPTER NINE TABLE 9.1 Nominal Voltages and Currents for Running-Overload Test Nameplate current Current increment Nameplate voltage Nominal voltage 0.1–10 A 0.2 A 110, 115, or 120 120 10–15 A 0.3 A 200 or 208 208 15–20 A 0.4 A 208-220 or 208-230 240 220, 230, or 240 240 265 or 277 277 440, 460, or 480 480 550, 575, or 600 600 FIGURE 9.7 Typical motor test bench diagram. voltages reduced by 10-V increments until the winding temperature stabilizes with- out opening the protector. See Fig. 9.8 and Table 9.1 for these test requirements.) Idle heat run: The motor operates without load at nameplate voltage until the windings reach thermal equilibrium for 60 min. Reduced-voltage idle heat run: The motor operates without load at voltages reduced by 10-V increments below rated voltage until the windings reach ther- mal equilibrium for 60 min without tripping the thermal protector. TESTING 9.7 FIGURE 9.8 Logic diagram for idle thermal test. 9.2.1 Test Conditions UL 2111 allows the following maximum temperatures for Class B insulation with the various thermal tests: Locked rotor 225°C in first hour 200°C in the second hour Full-load heat run 165°C Running overload 165°C if protector opens 175°C if protector does not open IEC 34-1 requires a maximum temperature rise of 85°C above room temperature as measured by thermocouple or 90°C as measured by resistance. For Class F insu- lation, the maximum temperatures increase to 110 and 115°C, respectively. For full-load, running-overload, and idle heat runs, UL requires a temperature rise variation at temperature equilibrium of no more than ±1.0°C for 2 h. 9.2.2 Equipment Required Dynamometer sufficient to handle the long-term dissipation Dynamometer controller Power analyzer Speed-torque indicator Correct size and type of coupling between motor and dynamometer Motor test stand and means of securing stand to stationary mount Type J thermocouples and accompanying chart equipment Printer Computer hardware and software Locking bar for locked-rotor test (optional) Powerstat Fan for quick cooling of the test motor 9.2.3 Test Setup Motor Setup Requirements 1. Mount the motor in a test stand with the protector at the six o’clock down posi- tion with the motor connected to the dynamometer. 2. Couple the motor to a dynamometer with a thermally insulating coupling. For locked-rotor tests, either apply enough torque with a dynamometer to overcome the motor starting torque or use a thermally insulated lock bar to stall the motor. 3. Align and secure test stand to dynamometer. 4. Connect thermocouples from the motor to chart equipment with electrical isola- tion. 5. Connect motor to capacitor or relay (if specified) and to power source per out- line. 9.8 CHAPTER NINE 6. Confirm that the dynamometer cooling and dissipation rating meets the test needs before operating the dynamometer. For locked-rotor tests or tests on intermittent-duty motors, let the test run with the motor off for a short time to clearly document a room temperature start. Other tests may start without delay. All locked-rotor tests, regardless of thermal rating, must start with winding temperatures within 5°C of room temperature. For other thermal tests, only intermittent-duty motors must begin test with windings within 5°C of room temperature. Computer-controlled tests can produce many times as much information as the old manual tests,while providing greater accuracy, drastically reduced test times,and at least an order-of-magnitude productivity increase. 9.3 DC MOTOR TESTING Figures 9.9 and 9.10 show a typical computerized dc motor test bench. Everything except thermal tests should begin with a motor temperature of 25 ± 5°C. DC motor thermal tests determine safe operating area and thermal resistance values. Both tests determine the operating conditions for an 85°C winding temperature rise above room temperature by thermocouple or 90°C by resistance for Class B insula- tion. For Class F insulation, the maximum temperatures increase to 110 and 115°C, respectively. TESTING 9.9 FIGURE 9.9 Typical computerized dc motor test bench diagram. 9.3.1 Voltage Constant Test The K e test checks the voltage constant in volts per thousand revolutions per minute (V/krpm) for a backdriven dc test motor. Any motor capable of maintaining an exact speed under a varying load can serve as a backdrive motor. For brush dc motors, measure the dc voltage generated by the test motor with a multimeter (gen- erally in both rotations). For brushless dc motors, acquire the peak-to-peak voltage with an oscilloscope and divide by twice the drive speed in krpm to obtain the K e . 9.3.2 Terminal Resistance Test While multimeters can accurately measure brushless dc motor resistance, brush dc motors experience variations in brush contact drop as well as resistance changes based on the relative position of the brush to the commutator bars. Brush dc motors therefore require averaging several locked-rotor measurements to provide a stable reading,or,preferably,a dynamic measurement,while backdriven at low revolutions per minute. For either test, attach the motor terminals to a dc power supply and set the current limit high enough to reduce contact drop fluctuations and low enough to minimize heating during the test. In the absence of a specification,use a current limit of 25 percent of the rated motor current.Acquire the voltage necessary to drive the current through the motor for calculation of resistance (R = V/I). For dynamic mea- surements, reverse either the polarity of the voltage or the rotation of the drive motor and average the two measurements to remove the counter-emf contribution. Backdriven speeds of 30 to 100 rpm work reasonably well and will generally provide better repeatability than a locked test. 9.10 CHAPTER NINE FIGURE 9.10 Typical computerized dc motor test bench. [...]... section provides examples of the value of sound, vibration, and current spectral analysis for electric motors Since spectral analysis encompasses a tremendous range of diverse technical areas, requiring many books to adequately cover, this section provides primarily hands-on snapshots of a few uses for motors The Vibration Institute in Willowbrook, Illinois, offers a wide range of literature and training... suitable control Since this type of noise problem occurs seemingly at random and varies radically from motor to motor, field noise complaints often seem unpredictable and, in some motors, defy verification Modal analysis offers a method of experimentally determining the type of movement, or mode of vibration, associated with a natural frequency Figure 9.39 includes an animation of the mode previously described,... signal distortion and not higher-frequency bending resonances This type of resonance often does not show up after assembly into the customer unit because of the mass and stiffness additions While increased shaft stiffness will usually resolve this problem, added damping often provides a more practical solution in small motors Motors with tachometer armatures assembled to the same shaft have a torsional... measurement The apparatus illustrated in Fig 9.13 can accurately measure torque ripple, as long as the moment of inertia of the measuring device remains much smaller than the motor moment of inertia (otherwise inertia filtering invalidates the ripple measurement) * Adapted from the Electro-Craft Handbook: DC Motors, Speed Controls, Servo Systems (1980) 9.14 CHAPTER NINE FIGURE 9.12 FIGURE 9.13 Torque ripple... NINE FIGURE 9.18 FIGURE 9.19 False brinell on inner race of bearing Shaker table nature of the defect will vary the contribution of each component and often will cause modulations that produce families of sidebands mingled with harmonics These sidebands can greatly complicate bearing spectral analysis, particularly when combined with large numbers of harmonics and sidebands from gears or other components... 1.925781 Hz tion of methods to reduce noise and predictions of effectiveness For example, the spectra in Fig 9.31 consist of 80 percent gear pump pinion frequencies and less than 20 percent rotor magnetic frequencies Even completely eliminating the motor noise would not achieve the required customer unit noise reductions These data allowed an objective means of negotiating the best methods of meeting the... provides this type of display After a fast Fourier transform (FFT), the horizontal axis changes to frequency, as shown in Fig 9.15 However, the basic analysis techniques remain the same, regardless of whether the electrical signal derives from sound, vibration, current, flux, surface finish, roundness, or any other parameter containing data of a periodic nature For variable-speed motors, the waterfall... handle to avoid contaminating data with handle natural frequencies An accelerometer (often triax- FIGURE 9.36 Amplification of vibration at resonance (From Ronald Eshelman, Vibration Control, courtesy of the Vibration Institute.) 9.30 CHAPTER NINE FIGURE 9.37 FIGURE 9.38 Spectrum of sirenlike tone, ∆x = 264 Hz Spectrum of sporadic pure tone, ∆x = 24 Hz ial) measures the response at predetermined locations... shown in Fig 9.41 At this point, modal analysis software uses the FRF and geometry information to determine the various modes of vibration, including their frequency, damping, amplitude, and phase This information allows empirical confirmation of finite element analysis predictions of natural frequencies made during the design process and minimizes the number of prototype iterations required to optimize... defect frequency calculations in Fig 9.27 appear straightforward, the presence of hundreds of intermingled harmonics, sidebands with unrelated carriers, and difference frequencies can greatly complicate analysis In addition, field returns often generate bearing frequencies at variance with new bearings because of the effects of wear and slippage The spectra in Fig 9.28 further demonstrate unexpected harmonics . section provides examples of the value of sound, vibration, and current spectral analysis for electric motors. Since spectral analysis encompasses a tremendous range of diverse technical areas,. power spectrum of bearing showing inner race and ball defects. nature of the defect will vary the contribution of each component and often will cause modulations that produce families of sidebands. requiring a means of measurement. The apparatus illustrated in Fig. 9.13 can accurately measure torque ripple, as long as the moment of inertia of the measuring device remains much smaller than the