Troubleshooting ac Motors
To detect defects in electric motors, the windings are normally tested for ground faults, opens, shorts, and reverses. The exact method of perform ing these tests will depend on the type of motor being serviced. However, regardless of the motor type, a knowledge of some important terms is necessary before maintenance personnel can approach their work satis factorily.
Ground: A winding becomes grounded when it makes an electrical contact with the iron of the motor. The usual causes of grounds include the following: Bolts securing the end plates come into contact with the winding; the wires press against the laminations at the corners of the slots, which is likely to occur if the slot insulation tears or cracks during winding; and the centrifugal switch may be grounded to the end plate.
Open circuit: Loose or dirty connections as well as a broken wire can cause an open circuit in an electric motor.
Shorts: Two or more turns of the coil that contact each other elec trically will cause a short circuit. This condition may develop in a new winding if the winding is tight and much pounding is necessary to place the wires in position. In other cases, excessive heat developed from over loads will make the insulation defective and will cause shorts. A short cir cuit is usually detected by observing smoke from the windings as the motor operates or when the motor draws excessive current at no load.
GROUNDED COILS
The usual effect of one grounded coil in a winding is the repeated blowing of a fuse, or tripping of the circuit breaker, when the line switch is closed, that is, providing the machine frame and the line are both grounded. Two or more grounds will give the same result and will also short out part of the winding in that phase in which the grounds occur. A quick and simple test to determine whether or not a ground exists in the winding can be made with a conventional continuity tester. In testing with such an in strument, first make certain that the line switch is open, causing the motor leads to be dead. Place one test lead on the frame of the motor and the other in turn on each of the line wires leading from the motor. If there is a grounded coil at any point in the winding, the lamp of the continuity tester will light, or in the case of a meter, the dial will swing toward infinity.
To locate the phase that is grounded, test each phase separately. In a three-phase winding it will be necessary to disconnect the star or delta connections. After the grounded phase is located the polegroup connec tions in that phase can be disconnected and each group tested separately. When the leads are placed one on the frame and the other on the grounded coil group, the lamp will indicate the ground in this group by again lighting. The stub connections between the coils and this group may then be disconnected and each coil tested separately until the exact coil that is grounded is located.
Sometimes moisture in the insulation around the coils or old and defective insulation will cause a high-resistance ground that is difficult to detect with a test lamp. A megger can be used to detect such faults, but in many cases a megger may not be available. If not, use a test outfit con sisting of a headphone set (telephone receiver) and several dry cell bat teries connected in series as shown in Fig. 15-1. Such a test set will detect a ground of very high resistance, and this set will often be found very ef fective when the ordinary test lamp fails to locate the trouble.
Armature windings and the commutator of a motor may be tested for grounds in a similar manner. On some motors, the brush holders are grounded to the end plate. Consequently before the armature is tested for grounds, the brushes must be lifted away from the commutator.
When a grounded coil is located, it should be either removed and reinsulated or cut out of the circuit. At times, however, it may be inconve nient to stop a motor long enough for a complete rewinding or permanent repairs. In such cases, when trouble develops, it is often necessary to make a temporary repair until a later time when the motor may be taken out of service long enough for rewinding or permanent repairs.
To temporarily repair a defective coil, a jumper wire of the same size as that used in the coils is connected to the bottom lead of the coil imme diately adjacent to the defective coil and run across to the top lead of the coil on the other side of the defective coil, leaving the defective coil entire ly out of the circuit. The defective coil should then be cut at the back of the winding and the leads taped so as not to function when the motor is started again. If the defective coil is grounded, it should also be discon nected from the other coils.
SHORTED COIL
Shorted turns within coils are usually the result of failure of the insula tion on the wires. This is frequently caused by the wires being crossed and having excessive pressure applied on the crossed conductors when the coils are being inserted in the slot. Quite often it is caused by using too much force in driving the coils down in the slots. In the case of wind ings that have been in service for several years, failure of the insulation may be caused by oil, moisture, etc. If a shorted coil is left in a winding, it will usually burn out in a short time and if it is not located and repaired promptly will probably cause a ground and the burning out of a number of other coils.
One inexpensive way of locating a shorted coil is by the use of a growler and a thin piece of steel. Figure 15-2 shows a sketch of a growler in use in a stator. Note that the poles are shaped to fit the curvature of the teeth inside the stator core. The growler should be placed in the core as shown, and the thin piece of steel should be placed the distance of one coil span away from the center of the growler. Then, by moving the growler around the bore of the stator and always keeping the steel strip the same distance away from it, all of the coils can be tested.
If any of the coils has one or more shorted turns, the piece of steel will vibrate very rapidly and cause a loud humming noise. By locating the two slots over which the steel vibrates, both sides of the shorted coil can be found. If more than two slots cause the steel to vibrate, they should all be marked, and all shorted coils should be removed and replaced with new ones or cut out of the circuit as previously described.
Sometimes one coil or a complete coil group becomes short-circuited at the end connections. The test for this fault is the same as that for a shorted coil. If all the coils in one group are shorted, it will generally be indicated by the vibration of the steel strip over several consecutive slots, corresponding to the number of coils in the group.
The end connections should be carefully examined, and those that appear to have poor insulation should be moved during the time that the test is being made. It will often be found that when the shorted end connections are moved during the test the vibration of the steel will stop. If these ends are reinsulated, the trouble should be eliminated.
OPEN COILS
When one or more coils become open-circuited by a break in the turns or a poor connection at the end, they can be tested with a continuity tester as previously explained. If this test is made at the ends of each winding, an open can be detected by the lamp failing to light. The insulation should be removed from the pole-group connections, and each group should be tested separately.
An open circuit in the starting winding may be difficult to locate, since the problem may be in the centrifugal switch as well as the winding itself. If fact, the centrifugal switch is probably more apt to cause trouble than the winding since parts become worn, defective and, more likely, dirty. Insufficient pressure of the rotating part of centrifugal switches against the stationary part will prevent the contacts from closing and thereby produce an open circuit.
If the trouble is a loose connection at the coil ends, it can be repaired by resoldering the splices, but if it is within the coil, the coil should either be replaced or a jumper should be connected around it until a better repair can be made.
REVERSED CONNECTIONS
Reversed coils cause the current to flow through them in the wrong direc tion. This fault usually manifests itself- as do most irregularities in winding connections- by a disturbance of the magnetic circuit, which results in excessive noise and vibration. The fault can be located by the use of a magnetic compass and some source of low-voltage direct current. This voltage should be adjusted so it will send about one fourth to one sixth of the full-load current through the winding, and the de leads should be placed on the start and finish of one phase. If the winding is three phase, star-connected, this would be at the start of one phase and the star point. If the winding is delta-connected, the delta must be disconnected and each phase tested separately.
Place a compass on the inside of the stator and test each of the coil groups in that phase. If the phase is connected correctly, the needle of the compass will reverse definitely as it is moved from one coil group to another. However, if any one of the coils is reversed, the reversed coil will build up a field in the direction opposite to the others, thus causing a neu tralizing effect which will be indicated by the compass needle refusing to point definitely to that group. If there are only two coils per group, there will be no indication if one of them is reversed, as that group will be com pletely neutralized.
When an entire coil group is reversed, it causes the current to flow in the wrong direction in the whole group. The test for this fault is the same as that for reversed coils. The winding should be magnetized with direct current, and when the compass needle is passed around the coil groups, they should indicate alternately N.S., N.S., etc. If one of the groups is reversed, three consecutive groups will be of the same polarity. The remedy for either reversed coil groups or reversed coils is to make a visual check of the connections at that part of the winding, locate the wrong connection, and reconnect it properly.
When the wrong number of coils are connected in two or more groups, the trouble can be located by counting the number of ends on each group. If any mistakes are found, they should be remedied by recon necting properly.
REVERSED PHASE
Sometimes in a three-phase winding a complete phase is reversed by either having taken the starts from the wrong coils or by connecting one of the windings in the wrong relation to the others when making the star or delta connections. If the winding is connected delta, disconnect any one of the points where the phases are connected together and pass cur rent through the three windings in series. Place a compass on the inside of the stator and test each coil group by slowly moving the compass one complete revolution around the stator.
The reversals of the needle in moving the compass one revolution around the stator should be three times the number of poles in the winding.
In testing a star-connected winding, connect the three starts together and place them on one de lead. Then connect the other de lead and star point, thus passing the current through all three windings in parallel. Test with a compass as explained for the delta winding. The result should then be the same, or the reversals of the needle in making one revolution around the stator should again be three times the number of poles in the winding.
These tests for reversed phases apply to full-pitch windings only. If the winding is fractional pitch, a careful visual check should be made to determine whether there is a reversed phase or mistake in connecting the star or delta connections.
The following troubleshooting chart may be used by qualified per sonnel who have the proper tools and equipment. These instructions do not cover all details or variations in equipment, nor do they provide for every possible condition to be met in actual practice.
General Troubleshooting Chart
Motor Fails to Start:
1. Blown fuses. Replace fuses with proper type and rating.
2. Overload trips. Check and reset overload in starter.
3. Improper power supply. Check to see that power supplied agrees with motor nameplate and load factor.
4. Improper line connections. Check connections with diagram supplied with motor.
5. Open circuit in winding or control switch. Indicated by humming sound when switch is closed. Check for loose wiring connections. Also see that all control contacts are closing.
General Troubleshooting Chart (continued)
5. Mechanical failure. Check to see if motor and drive turn freely. Check bearings and lubrication.
6. Short-circuited stator. Indicated by blown fuses. Motor must be re wound.
7. Poor stator coil connection. Remove end bells and locate with test lamp.
9. Rotor defective. Look for broken bars or end rings.
10. Motor may be overloaded. Reduce load.
Motor Stalls:
1. One phase may be open. Check lines for open phase.
2. Wrong application. Change type or size. Consult manufacturer.
3. Overload motor. Reduce load.
4. Low motor voltage. See that nameplate voltage is maintained. Check connection.
5. Open circuit. Fuses blown; check overload relay, stator, and push but tons.
Motor Runs and Then Dies Down:
1. Power failure. Check for loose connections to line, to fuses, and to control.
Motor Does Not Come Up to Speed:
1. Not applied properly. Consult supplier for proper type.
2. Voltage too low at motor terminals because of line drop. Use higher voltage on transformer terminals or reduce load. Check connections. Check conductors for proper size.
3. Starting load too high. Check the load that the motor is supposed to carry at start.
4. Broken rotor bars or loose rotor. Look for cracks near the rings. A new rotor may be required as repairs are usually temporary.
5. Open primary circuit. Locate fault with testing device and repair.
Motor Takes Too Long to Accelerate:
1. Excess loading. Reduce load.
2. Poor circuit. Check for high resistance.
3. Defective squirrel cage rotor. Replace with new rotor.
4. Applied voltage too low. Get power company to increase power tap.
Wrong Rotation:
1. Wrong sequence of phases. Reverse connections at motor or at switchboards.
Motor Overheats While Running Under Load:
1. Overloaded. Reduce load.
Frame or bracket vents may be clogged with dirt and prevent proper ventilation of motor. Open vent holes and check for a continuous stream of air from the motor.
General Troubleshooting Chart (continued)
3. Motor may have one phase open. Check to make sure that all leads are well connected.
4. Grounded coil. Locate and repair.
5. Unbalanced terminal voltage. Check for faulty leads, connections, and transformers.
Motor Vibrates After Corrections Have Been Made:
1. Motor misaligned. Realign.
2. Weak support. Strengthen base.
3. Coupling out of balance. Balance coupling.
4. Driven equipment unbalanced. Rebalance driven equipment.
5. Defective ball bearing. Replace bearing.
6. Bearings not in line. Line up properly.
7. Balancing weights shifted. Rebalance motor.
8. Polyphase motor running single phase. Check for open circuit.
9. Excessive end play. Adjust bearing or add washer.
Unbalanced Line Current on Polyphase Motors During Normal Operation:
1. Unequal terminal volts. Check leads and connections.
2. Single-phase operation. Check for open contacts.
Scraping Noise:
1. Fan rubbing air shield. Remove interference.
2. Fan striking insulation. Clear fan.
3. Loose on bedplate. Tighten holding bolts.
Noisy Operation:
1. Air gap not uniform. Check and correct bracket fits or bearing.
2. Rotor unbalance. Rebalance.
Hot Bearings, General:
1. Bent or sprung shaft. Straighten or replace shaft.
2. Excessive belt pull Decrease belt tension.
3. Pulleys too far away. Move pulley closer to motor bearing.
4. Pulley diameter too small. Use larger pulleys.
5. Misalignment. Correct by realignment of drive.
Hot Bearings, Ball :
1. Insufficient grease. Maintain proper quantity of grease in bearing.
2. Deterioration of grease or lubricant contaminated. Remove old grease, wash bearings thoroughly in kerosene, and replace with new grease.
3. Excess lubricant. Reduce quantity of grease; bearing should not be more than half filled.
4. Overloaded bearing. Check alignment, side, and end thrust.
Broken ball or rough races. Replace bearing; first clean housing thor
TROUBLESHOOTING SPLIT-PHASE MOTORS
If a split-phase motor fails to start, the trouble may be due to one or more of the following faults:
1. Tight or “frozen” bearings
2. Worn bearings, allowing the rotor to drag on the stator
3. Bent rotor shaft
4. One or both bearings out of alignment
5. Open circuit in either starting or running windings
6. Defective centrifugal switch
7. Improper connections in either winding
8. Grounds in either winding or both
9. Shorts between the two windings
TIGHT OR WORN BEARINGS. Tight or worn bearings may be due to the lubricating system failing, or when new bearings are installed, they may run hot if the shaft is not kept well oiled.
If the bearings are worn to such an extent that they allow the rotor to drag on the stator, this will usually prevent the rotor from starting. The inside of the stator laminations will be worn bright where they are rubbed by the rotor. When this condition exists, it can generally be easily detected by close observation of the stator field and rotor surface when the rotor is removed.
BENT SHAFT AND BEARINGS OUT OF LINE. A bent rotor shaft will usually cause the rotor to bind when in a certain position and then run freely until it comes back to,the same position again. An accurate test for a bent shaft can be made by placing the rotor between centers on a lathe and turning the rotor slowly while a tool or marker is held in the tool post close to the surface of the rotor. If the rotor wobbles, it is an indication of a bent shaft.
Bearings out of alignment are usually caused by uneven tightening of the end-shield plates. When placing end shields or brackets on a motor, the bolts should be tightened alternately, first drawing up two bolts which are diametrically opposite. These two should be drawn up only a few turns and the other kept tightened an equal amount all the way around. When the end shields are drawn up as far as possible with the bolts, they should be tapped tightly against the frame with a mallet and the bolts tightened again.
OPEN CIRCUITS AND DEFECTIVE CENTRIFUGAL SWITCHES. Open circuits in either the starting or running winding will cause the motor to fail to start. This fault can be detected by testing in series with the start and finish of each winding with a test lamp.
A defective centrifugal switch will often cause considerable trouble that is difficult to locate unless one has good knowledge of the operating characteristics of these switches. If the switch fails to close when the rotor stops, the motor will not start when the line switch is closed. Fail ure of the switch to close is generally caused by dirt, grit, or some other foreign matter getting into the switch. The switch should be thoroughly cleaned with a degreasing solution such as AWA 1,1,1and then inspected for weak or broken springs.
If the winding is on the rotor, the brushes sometimes stick in the holders and fail to make good contact with the slip rings. This causes sparking at the brushes. There will probably also be a certain place where the rotor will not start until it is moved far enough for the brush to make contact on the ring. The brush holders should be cleaned and the brushes carefully fitted so they move more freely with a minimum of friction be tween the brush and the holders. If a centrifugal switch fails to open when the motor is started, the motor will probably growl and continue to run slowly, causing the starting winding to burn out if not promptly dis connected from the line. In most cases, however, the “heaters” in the motor control will take care of this before any serious damage occurs. This fault is likely to be caused by dirt or hardened grease in the switch.
REVERSED CONNECTIONS AND GROUNDS. Reversed connections are caused by improperly connecting a coil or group of coils. The wrong connections can be found and corrected by making a careful check on the connections and reconnecting those that are found at fault. The test with a de power source and a compass can also be used for locating reversed coils. Test the starting and running windings separately, ex citing only one winding at a time, with direct current. The compass should show alternate poles around the winding.
The operation of a motor that has a ground in the windings will de pend on where the ground is and whether or not the frame is grounded. If the frame is grounded, then when the ground occurs in the winding, it will usually blow a fuse or trip the overcurrent device.
A test for grounds can be made with a test lamp or continuity tester. One test lead should be placed on the frame and the other on a lead to the winding. If there is no ground, the lamp will not light, nor will any deflec tion be present when a meter is used. If the light does light, it indicates a ground due to a defect somewhere in the insulation.
SHORT CIRCUITS. Short circuits between any two windings can be detected by the use of a test lamp or continuity tester. Place one of the test leads on one wire of the starting winding and the other test lead on the wire of the running winding. If these windings are properly insulated from each other, the lamp should not light. If it does, it is a certain indication that a short exists between the windings. Such a short will usually cause part of the starting winding to burn out. The starting winding is always wound on top of the running winding, so if it becomes burned out due to a defective centrifugal switch or a short circuit, the starting wind ing can be conveniently removed and replaced without disturbing the running winding.