The amortisseur (squirrel cage) winding is designed to bring the rotor up to 95 to 98 percent of synchronous speed. At this time DC is applied, bringing the rotor up to synchronous speed.
The amortisseur winding is not designed for continuous duty. Each time the motor starts, the amortisseur winding expands and contracts. This will eventually cause the bars to crack and become open. Open bars cause a loss of torque, and the rotor can’t get up enough speed to complete the start cycle. (See “Assorted Rotor Problems” in Chapter 6.)
The amortisseur winding can be checked with an infrared gun (if the motor has an open frame). Start the motor and run it long enough to heat the amortisseur winding. Check the bars and end ring connections immediately. A bar that is cooler than the rest is open. An end ring connection that is hotter than the rest has a bad connection.
Stator Winding
The stator winding is connected in the same way as any three-phase stator. It will be connected either wye or delta—usually multiple wyes or deltas. It’s subject to the same problems explained in Chapter 6.
Large synchronous motors are form-wound, with each coil wrapped individually. They are designed to operate on high voltage.
The same test procedures are used on both large and small motors. Instruments for testing them include an ohmmeter, a megohmmeter, a microhmmeter, and a surge tester.
The first test should be done with an ohmmeter, from a lead to the frame of the motor. A low reading (less than 50 megohms) indicates that conducting contaminants may be on the windings. The megohmmeter can be used next to verify the ohmmeter readout. Be sure to ground a lead to the frame after the megohmmeter test.
The comparison test for shorted windings can now be done with a microhmmeter at the motor terminals in the control box. (Breakdowns such as shorted coils are usually very visible.)
If there is a difference in the readings of the comparison test (compare leads 1 to 2 with 2 to 3, and 3 to 1), use the surge tester to confirm the microhmmeter results.
Corona
High voltage will cause a phenomenon called corona. Corona is the blue light that surrounds the coils while the motor runs. (The air next to an energized winding becomes ionized and glows.) It will also make a hissing or buzzing sound. Like any electrically caused spark, the corona gives off a radio frequency.
A minute amount of damage to the insulation is done each time the corona discharges. It erodes a tiny particle of insulation which—over time—will produce a fine white dust. (This can be minimized by coating the coils with a conducting paint in the area where they contact the slot iron.) Damage occurs slowly—over years in most cases.
More corona damage is caused by the ozone it produces. An arc changes oxygen to ozone gas. This gas attacks insulation by chemically degrading it. Ozone does the greatest damage in voids in the slot, or between turns. Vacuum/pressure impregnation (VPI) application of varnish will reduce the number of voids. (The varnish must have the right level of viscosity to successfully eliminate voids.)
BearingS
Worn sleeve bearings reduce the air gap on one side of the rotor. The result is extreme mechanical stress to the shaft and bearings. There should be about 35 pounds per square inch of magnetic pull, equally distributed around the rotor (if it is centered). If the air gap is reduced by one-half on one side, the magnetic pull on the wide side will drop to approximately 10 pounds per square inch, while the narrow side will go up to nearly 150 pounds per square inch. Bearing wear is accelerated by the increased stress.
Air gap should be measured yearly on pedestal bearing motors.
Temperature change can cause movement of the supporting components. (There have been cases of concrete swelling when reinforcement mesh rusts.)
Step Voltage Test
The step voltage test (explained in the Chapter 8 section, “Instruments for In-depth Testing and Scheduled Maintenance”) is a reliable method of testing large synchronous motors and generators. It’s a maintenance procedure, done with an instrument called a winding analyzer (Fig. 7.2). The test should be scheduled on a regular basis.