Test Instruments for Testing in Place
The following test instruments can be used to troubleshoot the ninelead three-phase motor in place: clamp-on volt/ ammeter, ohmmeter, megohmmeter, tachometer, induction-based test instruments, and infrared gun.
The digital clamp-on type of volt/ammeter is capable of capturing instantaneous lockout amperes. The lockout feature records the locked rotor ampere reading.
An ohmmeter and a megohmmeter are used to test slot insulation (winding lead to frame).
The tachometer is used on a loaded motor to see if it’s below its nameplate RPM.
Induction-based test instruments, described in Chapter 8, are highfrequency types of testers. They work on motors of any size.
The infrared gun locates hot spots such as poor connections. This instrument has many other uses; some are described in Chapter 8.
Core and Rotor Damage
If a motor has tripped its protection or blown a fuse, the cause of the disconnect should be identified before the motor is reenergized. (A failed motor that has had arcing in its slots would be unnecessarily damaged if the motor is restarted.) If a ground or short occurs in the stator slots, the motor’s core and rotor will be severely damaged. Sometimes a large number of slot teeth will fuse together with melted copper. The copper must be ground, filed, or chiseled out of the core before rewinding the motor. In some cases, so many teeth are removed that the core’s magnetic balance is thrown off.
If there is extensive core damage, there are three options: If repair is not economical, replace the motor. Restack the laminations, staggering the damaged laminations evenly around the stator. Replace the damaged laminations (Fig. 7.7).
If an arc forms in the slot near the air gap, it’s directed at the rotor. This can melt aluminum rotor bars in just a few seconds. Motors up to 100 horsepower with this type of damage may not be (economically) repairable and therefore may require replacement.
FIGURE 7.7 New replacement laminations. Jasper Electric.
A motor connected to an ungrounded system is less likely to suffer severe core damage. It may run normally until another part of its winding, another device, or another motor on the system develops a ground.
Troubleshooting the Nine-Lead
Motor (Wye or Delta) in Place
If a motor is smoking or has any other obvious major problems, it has to be replaced. The cause of the problem, however, should be identified. If the motor’s history is recorded on computer, the data should be checked before troubleshooting the motor.
If someone operates the motor, get that person’s input. (This may help identify the problem more quickly.) Factors of concern are load change, modifications in the machine, sound change, and history of past problems. Load change or machine modification may require a larger motor.
Visual Check of the Nine-Lead
Motor (Wye or Delta)
If the motor has an open frame, visually check for burned windings. If you can’t see the windings, check the paint on the shell of the stator. If the windings have heated excessively, the paint will look discolored or scorched
where the core meets the shell. An overheated winding or bearing journal can be easily identified by the discolored paint. (Special paints are available that will change color when they reach a predetermined high temperature.) Check for a burned-winding smell in the motor’s connection box if the motor is totally enclosed. Burned windings require rewinding or replacing the motor.
The preceding checks should be made before doing more extensive testing.
Check for open protection (fuse breaker or control protection). If fuses are blown or other protection is open, don’t restart the motor. Restarting the motor can damage the service line components and the motor’s major components (core or rotor).
Shut off and lock the motor’s main disconnect switch so it can’t be reenergized. Check the incoming voltage of all three lines with a voltmeter. The supply voltage should be within 10 percent of the motor’s voltage rating. (If the motor can run loaded, the voltage should be tested while the motor is running.)
Check for voltage unbalance. An unbalance greater than 1 percent can overheat a motor and cause failure. This problem doesn’t cause immediate failure. When the motor does fail, its winding looks like those in Fig. 7.8.
FIGURE 7.8 A winding that failed from unbalanced voltage. EASA.
Testing from Control (Wye or Delta)
If none of the visual observations shows a problem, the motor can be tested from its control (or disconnect). The following tests are done from the control.
A standard ohmmeter (multimeter) should be used to test for grounds and opens. It can also be used on the windings of smaller motors (up to 10 horsepower). A microhmmeter is used to test larger motors. Inductiontype test instruments will test motors of any size. These instruments are all covered in Chapter 8.
After the power is off and locked, use an ohmmeter to test (at the T [motor] terminals in the control) for the following:
• Ground test from the control
• Open winding test from the control
• Open winding test (wye)
• Open winding test (delta)
• Comparison test from the control
• Rotor test from the control
Testing from the control also tests the lines to the motor. A motor that fails any of these tests must be rewound or replaced.
Ground Test from the Control
Disconnect and lock out the power. The ground tests start from the motor side (T terminals) of the control. The first test should be with a multimeter, set on ohms. (Use it first to check for a grounded winding, because it wouldn’t have enough power to damage contaminated insulation.)
Test from any motor terminal to the conduit. A short (or pegged) reading means that there’s a ground—in the line or in the motor.
Open the motor’s connection box and carefully check the motor lead connections for a possible ground to the box or to its cover. Disconnect the motor and test from the leads to the motor’s nameplate—or to a clean spot on its frame. If the ground is in the motor, it has to be replaced.
Open Winding Test from the Control
Disconnect and lock out the power. Test from T terminal to T terminal with an ohmmeter. If an open circuit is found, it will be in the line or in the motor winding.