Mike Howell
EASA Technical Support Specialist
Customers sometimes send in a motor with no nameplate, or an illegible nameplate, having little knowledge of the machine’s ratings. This article will explore the process of evaluating the machine using frame size, winding data and test data to assign reasonable ratings. The general approach will be for typical NEMA or IEC foot-mounted, three-phase AC machines but could be applied to others.
Receipt Inspection
When a motor is received with a missing or illegible nameplate, have a discussion with the customer before beginning work. Typical questions to ask include:
- Was the motor in service and for how long?
- What is the application?
- Are there sister motors?
- What is the service voltage and frequency?
- Is the operating load current known?
- How is the motor started?
Additionally, if the customer knows of any previous repairs, the service center used may have records.
Initial Inspection & Test
Estimate the frame size using the shaft height and axial distance between centers of mounting feet holes. Rotate the shaft manually and perform an insulation resistance test. If the motor is in a suitable condition, perform an uncoupled dynamic test. Start the motor at a significantly reduced voltage, just high enough to achieve a reasonable acceleration time. Measure the no-load speed with a tachometer and remember that the induction motor’s uncoupled speed will be close to synchronous speed. The number of poles is 120 * frequency / synch. rpm.
With the frame size and number of poles, you can use various resources to guesstimate the output power (e.g., EASA Technical Manual, “Identifying Rating Information of Motors Without Nameplates” in the Resource Library at easa.com.)
Increase the voltage to the assumed rated voltage, monitoring the no-load amps. If the NLA begin to increase faster than the applied voltage, STOP - you are already exceeding the rated voltage, or it might be wound for a non-standard frequency. Table 1 and Table 2 can be used for determining approximate expected values. At the assumed rated voltage, the no-load amps should be close to the expected range, and the speed should now be very close to synchronous speed. If the no-load amps are extremely low compared to the expected value, it’s possible that the design operating voltage is higher than assumed, the winding is connected incorrectly for the applied voltage or the machine has a much lower power rating than typical for that frame size and number of poles. In any case, additional testing will be helpful.
Reconditioning
If the work scope requires reconditioning, the machine should be disassembled and inspected, and necessary repairs should be performed. If a rewind is performed, the stator winding data can help provide additional information to assist with assigning appropriate nameplate values.
EASA’s Motor Winding Database is a very powerful tool for evaluating as-found winding data. When nameplate data is not available, we rely on the principle that the same physical laws apply to a motor, regardless of manufacturer. The torque developed by an induction motor is proportional to D
2L (Figure 1) and the air gap flux density.
Additionally, output power is torque * speed. Thus, for a given output power, speed and size, the air gap flux densities of machines are typically similar.
In addition to comparing the air gap flux density of the as-found data to similar motors, the flux densities in the stator tooth and back iron should be checked to make sure those paths are not saturated (see Table 3). The current density in the stator winding should also be evaluated to ensure it is reasonable for the machine’s assumed duty cycle (see Table 4).
While the air gap flux density will typically fall within a reasonably small range for a given output power, speed and size, you may find a wide variance with current density. For example, a submersible pump motor will typically have less copper area per amp than a premium efficiency motor with similar ratings.
Testing in the Service Center
Testing capabilities vary widely among service centers for several reasons, including organization size, customer base, and industries served. Some of the testing described in this article is outside the scope of capabilities for many service centers. However, if needed, there are facilities where a service center could subcontract some of this work, which is a common practice among EASA members. Let’s look at some of the tests to help assign reasonable nameplate data. Note: The goal here is not to adhere to standard test procedures for evaluating machines (e.g., NEMA, IEEE, IEC). Our objective is to draw aproximate, reasonable conclusions based on the methods and data we have available. If you need help with conducting these tests or evaluating the results, contact EASA Technical Support.
No-Load Test: As previously described, the uncoupled dynamic test, or no-load test, is a useful tool. After using the machine dimensions and number of poles to estimate output power, the no-load current at the assumed rated voltage and frequency can provide reasonable assurance in the estimated ratings. Most service centers routinely perform this test. Absent a full-load test, Table 1 and Table 2 can be used to evaluate the no-load current and calculate a reasonable expected full-load current. For more information, see “No-load Current Basics: Practical Guidelines for Assessment” in the Resource Library at easa.com.
Locked-Rotor Test: The locked-rotor test is normally run by locking the motor shaft through an I-beam or load cell while coupled to a dynamometer (see Figure 2). The locked rotor current can be estimated by applying power to two line leads only. The resulting current will be approximately 86% of that obtained during a three-phase test, and mechanical locking of the rotor is unnecessary. Test equipment limitations often require that reduced voltage testing be performed. The locked-rotor torque is one variable needed when trying to determine the speed-torque characteristics (design letter) of the machine. The locked-rotor current is needed to determine the kVA code, where applicable. For more information, see “Working with Motor Locked-Rotor Test Data” in the Resource Library at easa.com.
Load Test: Load testing in service centers is typically performed using a dynamometer, and not all service centers have this capability. Manufacturer performance tests typically involve recording data at up to six load points from around 25% to 150% of rated load. Nameplate data verified or determined during this test can include voltage, current, frequency, rated speed, power factor and efficiency. Additionally, if test capabilities permit, testing at rated load for a time sufficient to achieve stable winding temperature is beneficial for estimating temperature rise. For more information, see “Load Testing of Motors: Common Methods, Procedures” in the Resource Library at easa.com.
Speed-Torque Test: Speed-torque tests performed in service centers are typically performed either with a dynamometer or with the torque determined indirectly using acceleration and inertia. This test is less common than the others described but is necessary if the design letter is to be determined. Along with the locked-rotor test data, reasonable estimates of locked-rotor torque, breakdown torque (maximum torque) and the speed at which maximum torque occurs can be achieved. For more information, see “Speed-Torque Characteristics of Three-Phase Motors” in the Resource Library at easa.com.
Example Motor
Let’s explore an example and how some of the tools described can assist with assigning reasonable nameplate data for the following motor received by a service center. It is not suggested that all of these activities would typically be performed. The goal should be to perform the activities necessary to have reasonable assurance to provide the information the customer needs.
Initial Inpection & Test Notes
- Shaft height of 8.0 inches (203 mm) and axial distance between centers of mounting feet holes of 10.5 inches (268 mm). Dimensions suggest possibly NEMA 324 frame series using the EASA Technical Manual as a resource.
- Typical frame assignments would be 50 hp (37 kW) 2 pole, 40 hp (30 kW) 4 pole, 25 hp (19 kW) 6 pole, or 20 hp (15 kW) 8 pole according to one OEM’s catalog.
- Motor determined to be ok for test run - initial 120 V 60 Hz 5.1 A 1794 rpm. Guesstimate at this point - 40 hp 4 pole 460 V 60 Hz and approximately 48 FLA. Voltage increased - 460 V 60 Hz 14.3 A 1799 rpm. No-load current is reasonable: 14.3/48 = 30%.
Reconditioning & Repair Notes
- The customer opted to have the machine reconditioned, and it was determined that a rewind was necessary.
- The winding data removed had an air gap flux density of 49,000 lines/in2 (0.76 T), similar to at least 25 other 40 hp 4 pole machines in the EASA Motor Winding Database, falling within 2% of the same stator core length (5.9 in, 150 mm) and inside diameter (9.0 in, 229 mm). Additionally, the current density, stator tooth magnetic flux density and stator back iron magnet flux density were all reasonable.
Testing
For this motor, the no-load test and stator winding data verification activities were likely sufficient to provide the customer with reasonable data related to output power, number of poles, voltage, frequency and estimated current. By performing a locked-rotor test, the kVA code could be determined as well as part of the information needed to assign a reasonable design letter. Performance test data allows for assigning a more accurate speed and current at rated load and estimates of efficiency and power factor if needed.
Performing a heat run allows for the determination of stator winding temperature at rated load and a typical service factor load (e.g., 1.15). With speed-torque test data and locked-rotor test data, there is sufficient information to determine the design letter most likely assigned by the OEM. Test data and notes for the example motor are provided in Table 5, Table 6 and Table 7. Additionally, a completed nameplate for this machine is provided in Figure 3.
Related Reference and Training Materials
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