If the motor isn’t open or grounded and the comparison test is even, the motor can be started. Check the amperes of all three lines, and compare the results with the motor’s nameplate. (There’s always a small difference in ampere readings between lines.) To verify that the motor has no problem 
use the procedure described in the following section.
If the amperes are normal, it’s important to find out what caused the disconnect to open. If the cause of the problem isn’t found, the incident should be recorded for future reference. Possible causes are covered under “When Motors Overheat” in
Line/Lead Interchange Test for Uneven Amperes
Most three-phase motors will have different ampere readings on all three lines, especially when the motor has no load. If the amperes are quite uneven, the problem lies in the power supply or in the motor winding.
This can be determined by first recording the amperes of each line and then interchanging all three lines or all three motor leads, whichever is easier. (Changing all three keeps the rotation the same.)
If all three lines keep the same readings, the problem lies in the line. If all three motor leads keep the same reading, the problem lies in the motor.
Recording Motor Faults
A history of motor breakdowns is valuable in maintaining production. Future problems are then easily recognized and possibly prevented. Ideally, a plant’s motor problems are all recorded on a good software program.
A well-designed report sheet makes recording problems easier. It should include a sketch of a stator showing the location (top, bottom, front, back) of the breakdown. For example, a breakdown that occurs frequently at the bottom of the motor indicates a moisture problem. (Simply drilling a drain hole may be the solution for this particular problem.)
Most failures are visible, but the fault should be recorded, with reference to a lead number.
Schematics of the wye and delta nine-lead connections should be used to reference the location of all breakdowns (according to lead location).
Line conditions should be recorded. These include voltage and amperes of each line (no load and full load). If possible, the readings should be taken both at the control and at the motor
Locating Motor Faults
Test Equipment for Locating Motor Faults
Locating and identifying motor faults can pinpoint failure causes. This is an important part of preventive maintenance. A motor service center should identify, whenever possible, the cause of failure. Identifying a failure is especially important with motors that are replaced rather than repaired.
Limited single-phase current works very well for testing three-phase stators. Figure 7.11 is a diagram of a test panel that can be constructed with stove elements. The test panel can be designed for higher current. Lowresistance grids can be used if more amperes are needed.
FIGURE 7.11 Circuitry of a test panel, using resistors.
This test panel can also be used with a higher hertz power supply than 60 Hz. (Locate the ammeter on the 60-Hz side of the power supply.) Higher hertz creates more inductance and lowers the amperes and torque of a motor being tested.
High-hertz test instruments create more inductive reactance than do 60-Hz test instruments. A few shorted turns are much more apparent with induction-based test instruments than with an ohmmeter.
An ohmmeter uses low-voltage DC and measures only the resistance of the wire. The short length of wire contained in shorted turns (that are eliminated from the phase) has very little resistance. Identifying a faulty phase by comparing its resistance to two normal phases is harder to do with an ohmmeter.
This type of motor testing should be done in a dry location, because if the winding is grounded to the frame, the test can be dangerous. The motor may have to be disassembled to accurately diagnose and locate the winding fault.
The test results that follow are described as high or low resistance (when using an ohmmeter), or low or high amperes (using limited single-phase current). The term high resistance on an ohmmeter is the same as low current with limited single-phase current.