Electric Motor Maintenance and Troubleshooting

Electric Motor Maintenance and Troubleshooting, 2nd Edition
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Streaking or grooving under only some brushes or flat and burned spots can result from a load mismatch and cause motor electrical problems. Grooved commutators should be removed from service. A brassy appearance shows excessive wear on the surface resulting from low humidity or wrong brush grade. High mica or high or low commutator bars make the brushes jump, causing sparking. Carbon dust, copper foil or other conductive dust in the slots between commutator bars causes shorting and sometimes sparking between bars.

If correcting any obvious deficiencies does not eliminate sparking or noise, look to the less obvious possibilities: 1. If brushes were changed before the problem became apparent, check the grade of brushes. Weak brushes may chip. Soft, low abrasive brushes may allow a thick film to form. High friction or high abrasion brushes wear away the brown film, producing a brassy surface. If the problem appears only under one or more of the brushes, two different grades of brushes may have been installed.

Generally, use only the brushes recommended by the motor manufacturer or a qualified brush expert. The brush holder may have been reset improperly. Setting the brush holder off neutral causes sparking. Normally the brushes must be equally spaced around the commutator and must be parallel to the bars so all make contact with each bar at the same time. An eccentric commutator causes sparking and may cause vibration. Normally, concentricity should be within. Various electrical failures in the motor windings or connections manifest themselves in sparking and poor commutation. Look for shorts or opens in the - 12 - A weak interpole circuit or large air gap also generate brush sparking.

Is there power on all lines? Three-phase motors won't start on one-phase. Restore power on all lines A2: Check starter. Is overload protection device opened?

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Replace or reset device. Does it open again when starting? A3: Is there power on all lines to motor? Restore proper voltage. A5: Check motor terminal connections. Are any loose or broken? Repair connections. A6: May be wrong motor for application. Is starting load too high? Install Design C or Design D motor. Install larger motor. A7: Is driven machine jammed or overloaded? Remove jam or overload. A8: Are misalignments, bad bearings or damaged components causing excessive friction in driven machine or power transmission system? Repair or replace component.

A9: Are bad bearings, bent shaft, damaged end bells, rubbing fan or rotor or other problem causing excessive friction in the motor? Repair or replace motor.

A Check stator. Are any coils open, shored or grounded? Repair coil or replace motor. A Check commutator. Are any bars or rings broken? Replace rotor. Problem B - Motor runs noisy B1: Are vibrations and noise from driven machine or power transmission system being transmitted to motor? Locate source of noise and reduce.

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Motor bearings can last virtually forever by simply providing an ideal contamination free, well lubricated bearing environment. Applications of electrical motor can be found in industries, electric transportation, robotics, process control and energy conservation. Brush grade selection requires specialized knowledge of the carbon materials and fabrication methods used. Textbook too pricey? Replace if some are too abrasive. Reduce the load or install a larger motor. Grind commutator.

Isolate motor with belt drive or elastomeric coupling. B2: Is a hollow motor foundation acting as a sounding board? Redesign mounting. Coat foundation underside with sound dampening material. B3: Check motor mounting. Is it loose? Be sure shaft is aligned. B4: Is motor mounting even and shaft properly aligned? Shim feet for even mounting and align shaft.

B5: Is fan hitting or rubbing on stationary part or is object caught in fan housing? Repair damaged fan, end bell or part causing contact. Remove trash from fan housing. B6: Is air gap nonuniform or rotor rubbing on stator? Recenter rotor rubbing on worn bearings or relocate pedestal bearings. B7: Listen to bearings. Are they noisy? Lubricate bearings. If still noisy, replace. B8: Is voltage between phases three-phase motors unbalanced?

Balance voltages. B9: Is three-phase motor operating on one- phase? Won't start on single-phase. Restore power on three- phases. Problem C - Motor overheats C1: Is ambient temperature too high? Reduce ambient, increase ventilation or install larger motor. C2: Is motor too small for present operating conditions? C3: Is motor started too frequently? Reduce starting cycle or use larger motor. C4: Check external frame. Is it covered with dirt which acts as insulation and prevents proper cooling? Wipe, scrape or vacuum accumulated dirt from frame. C5: Feel output from air exhaust openings.

Is flow light or inconsistent indicating poor ventilation? Remove obstructions or dirt preventing free circulation of air flow. If needed, clean internal air passages. C6: Check input current while driving load. Is it excessive indicating an overload? Go to Step C C7: Is the driven equipment overload? Reduce load or install larger - 16 - C8: Are misalignments, bad bearings or damaged component causing excessive friction in driven machine or power transmission system? Repair or replace bad components. C9: Are motor bearings dry? Does motor still draw excessive current?

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C Are damaged end bells, rubbing fan, bent shaft or rubbing rotor causing excessive internal friction? C Are bad bearings causing excessive friction? Determine cause of bad bearings See Problem D. C Check phase voltage. Does it vary between phases? Restore equal voltage on all phases. Restore proper voltage or install motor built for the voltage.

C Check stator. Are any coils grounded or shorted? Repair coils or replace motor. Problem D - Motor bearings run hot or noisy D1: Check loading. Is excessive side pressure, end loading or vibration overloading bearings? Reduce overloading. D3: Is bent or misaligned shaft overloading bearings? Replace bent shaft or align shaft.

Tighten or replace end bell. Are bearings misaligned, worn or damaged? If motor still runs noisy or hot, replace bearings. Is there power on the lines? Are contacts closed? Restore input power. E2: Check controller. Is the overload protective device open? Reset or replace device. Does it open again when starting motor? E3: Check controller.

Is there voltage available at output terminals? Check controller for open starting resistor, broken leads and connection s or other malfunction s. E4: Set the controller for full speed. Is the voltage for field or armature circuits too low? Check voltage from power source. Correct if too low. Check controller for malfunction. E5: Check for weak or non- existent field. Is motor field open? Has one field coil shorted?

Repair broken leads or connection s.

Rewind or replace open or shorted coil. E6: Check for open armature circuit. Is voltage at motor armature terminals zero when starting? Repair damaged armature circuit. E7: Is driven machine jammed or overloaded? Remove jam or overload or install larger motor. E8: Are misalignments, bad bearings or worn components causing excessive friction in driven machine or power transmission system? Correct misalignme nt or repair or replace worn component. E9: Are bad bearings, bent shaft, rubbing fan or rotor, damaged end bells, or other mechanical problems causing excessive friction in motor?

Repair or replace damaged motor component s or install new motor. Did it reverse? F2: Shunt and series field may be bucking each other. To check and correct: Reconnect the shunt or series field to correct polarity. Connect armature for desired rotation direction. Try fields separately to determine rotation direction and connect so both give the same rotation. Problem G - Motor runs but overload protective device trips too often.

G1: Is motor too small for load? Have loading conditions changed? Reduce load or install larger motor. G2: Check controller. Is overload device set too low for application? Increase overload setting. NEVER exceed safe limits specified by codes or equipment maker. G3: Is motor overheating? See Problem H.

Problem H - Motor overheats H1: Is ambient temperature too high? H2: Check external frame. Is it covered with layer of dirt which acts as insulation and prevents proper cooling? H3: Feel output from air exhaust openings. Remove obstructions or dirt preventing free of air flow.

H4: High load speed consumes extra horsepower overloading motor. Is motor operating above normal speed? See Problem J.

H5: Check for overload. See Steps E7 thru E9. Problem I - Motor runs too slowly. I1: Is motor overloaded? I2: Is the field resistance too low? Add proper resistance.

I3: Check for shorts in armature or between commutator bars. Are armature coils or wedges burned? Are any commutator bars burned? Replace or replace coils or bars. I4: Check brush holders. Are brushes set ahead of neutral? Reset brushes to neutral. I5: Voltage to armature too low. Set controller for full speed. Is voltage at output terminals below nameplate voltage? Check power source output voltage.

Raise if too low. Is motor operating cold? Increase load or reduce ventilation to increase heating. Install new motor. Problem J - Motor runs too fast. J1: Is driven load too light allowing Increase load or install - 23 - J2: Check for a weak field per Steps J3 through J6. J3: Are shunt or series coils reversed? Reconnect reversed coils for proper polarity. J4: Is there excessive resistance in shunt field circuit? Remove excessive resistance. J5: Is excessive heat causing higher resistance in shunt field circuit? Increase ventilation or correct other cause of overheating. J6: No field causes unbalanced shunt motor to race.

Is field circuit open? Repair broken lead or connection. Replace open coil. J7: Set controller for full speed. Is voltage at output terminals of controller above nameplate voltage? Reduce output voltage. J8: Check brush holders. Are brushes set behind Reset brushes to neutral. Problem K - Motor runs noisy K1: Are vibrations and noise from driven machine or power transmission system being transmitted to motor? K2: Is a hollow motor foundation acting as a sounding board? K3: Check motor mounting. K4: Is motor mounting even and shaft properly aligned?

K5: Is fan hitting or rubbing on stationary part or is object caught in fan housing? K6: Is air gap nonuniform or armature rotor rubbing on pole pieces? Tighten loose pole piece. Recenter armature by replacing worn bearings or relocating pedestal bearings. K7: Listen to bearings. K8: Are bearings noisy or running hot? See Problem L. K9: Are the brushes developing high or low frequency chatter? See Problem M. Problem L - Motor bearings run hot or noisy L1: Check loading.

L2: Is sleeve bearing motor mounted on a slant causing end thrust? L3: Is bent or misaligned shaft overloading bearings? Clean bearings and lubricate with proper grade. M1: Is motor overloaded? Reduce overload or install larger motor. M2: Is vibration from driven machine or motor Locate source of vibration and reduce. M3: Check brushes and brush holders. Are brushes worn too short? Replace brushes.

M4: Does each brush fit commutator as indicated by polished surface over entire brush face. Refit brushes to commutator. M5: Are brushes hanging up in holders? Clean brushes and holders. Remove rough surfaces that cause extra friction. M6: Are brush springs broken or is spring pressure too light? Replace spring or increase pressure. Be sure pressure is equal on all brushes. M7: Is spring pressure to high? May also cause brush chipping Reduce pressure or replace with lighter spring. M8: Are brush holders set off neutral? May also cause brush Reset holders at neutral.

May also cause brush chipping Reset holders for brush angle recommended by motor manufacturer. M Chipping brushes may also indicate wrong brush material. Are brushes too weak for duty? Consult motor manufacturer for recommendations. M Check commutator. Is commutator surface under brushes polished brown color? Normal condition.

Go to Step M M Is commutator surface black generally caused by sparking? Check for overloads, low spring tension, poorly undercut mica, loose commutator bars, etc. Correct sparking. Dress commutator. M Is there thick film on commutator - may appear black? Use more abrasive brushes.

M Is commutator surface bright and brassy looking? If humidity is below 2 grams per cu. M Is commutator surface contaminated from paint spray, oil or chemical fumes? Is there excessive moisture in air? Clean commutator and brushes and protect motor from contamination. Install motor with proper enclosure to protect commutator. M Is commutator streaked or grooved under one or more brushes?

Be sure all brushes same grade. Replace if some are too abrasive. Check for faulty shunt connections causing unbalanced load; repair. M Is commutator rough or eccentric? Grind commutator round Undercut mica - 30 - M Is mica above bar surface? Undercut mica. M Are some commutator bars too high, too low or loose? Replace commutator or tighten V-ring bolts to tension recommended by manufacturer and grind commutator.

M Are there flat or burned spots on commutator bars caused by unbalanced load in armature circuit? Balanced load. Grind commutator. M Is conductive film carbon dust or copper flaking causing shorts between armature bars? M Are there any shorts or opens in armature circuits? Locate and repair. M Are there any grounds, shorts or opens in the field wiring Locate and repair.

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M Are connections to brush holder poor or broken? M Is the interpole current weak or the air gap too great? Increase interpole current or reduce gap. Lock out and tag out all electrical circuits. Test for voltage before touching any components. Operating successfully for a period of time before failing; or if it was 2. Installed recently. Before proceeding, Record pertinent motor nameplate data. HP RPM. Rated voltages Armature and field Rated currents Armature and field Inspect the motor for any obvious defects that would prevent safe testing.

Damaged windings Smoke, copper particles Loose connections melted wire nuts, burned insulation Broken or missing parts Pulleys, belts, covers, etc. Defective brushes or brush holders. If the main fuses are OK, press the start button and measure the armature and shunt field voltages at the controller using a DC voltmeter. All output voltages must be in accordance with the motor nameplate If rated voltage is measured, the problem is in the motor or motor wiring. A zero or a very low reading indicates that something is wrong with the controller or control wiring. Test and inspect controller If no output is read from the controller, determine if the problem is in the control circuit and correct it.

Is the controller tripped? If tripped determine cause and correct problem. Over current Excessive load over a period of time - 35 - No tach signal Attempt a reset. Test and inspect motor Inspect electrical connections to the motor. Correct any loose or broken connections. Are there any loose brush leads replace any brushes that are too short or damaged If the motor still does not operate, disconnect the power supply from the motor and use the ohmmeter to check the armature circuit for continuity.

An open connection in the armature circuit could be caused by: 1. Worn and hung up brushes 2. Blown brush shunts 3. Open Interpole circuit 4. Open Series Field if so equipped 5. Open armature Comm. Grounded windings. Test all windings for ground failure using the megohmmeter. Any grounded windings must be repaired before power is applied to the motor Mechanical problems with the motor or driven equipment.

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Mechanical problems such as worn bearings or a broken pinion could cause a mechanical overload. Determine if the problem is in the motor itself or in the driven equipment. Uncouple the motor and turn the armature by hand. If the armature moves freely and the motor starts without tripping the overload relay or blowing fuses when uncoupled, the problem is most likely in the driven equipment and not in the motor. Shorted armature winding You can check the armature for shorts while the motor is uncoupled.

After removing all brushes from the commutator, apply rated voltage to the shunt field and rotate the armature by hand. If the motor will run for a short while before the overload trips or the fuse blows, shut down the motor and then feel the armature coils with your hand. Shorted coils will feel hotter than the others because they will have had heavy circulating currents induced into the shorted turns. Defective field winding. Reduced field strength will cause high armature currents. Test the shunt field continuity by measuring and recording the resistance of the shunt with an ohmmeter.

A reading of infinity indicates an open circuit in the winding, which requires repairs to the motor. Test the shunt field winding for shorted turns. Compare your shunt field resistance measurement to the nameplate data. The motor nameplate may tell you field resistance. If the resistance of the field is equal to or less than the resistance listed on the nameplate, your winding is probably shorted.

Any shorts between the windings will require repairs to the motor. Starting resistors that are shorted out prematurely. Timer or current sensing problems. Measure the armature and the shunt field voltages at the motor terminals to be sure they are in accordance with the nameplate data.

The speed of the motor will increase if the armature voltage is higher than shown on the nameplate. The speed may also be higher if the applied shunt field voltage is lower than the value shown on the nameplate. If applied voltages are in accordance with the motor rating, there is a defect in the winding. Test for: 1. Grounds in all windings.

Shorts in shunt field. Continuity in shunt and series coils. High resistance indicates an open circuit. Shorts between shunt and series field if it is a compound- wound motor. Reduced armature voltage will decrease motor speed. Increased field voltage will decrease motor speed.

If applied armature and field voltages are in accordance with the motor nameplate, inspect for high resistance connections in the armature circuit. Loose connections. Check for hot spots and discolored insulation around the connections. Contactors in controller making good contact. Armature resistors — OLD If the speed of the motor varies continuously with constant voltages applied, and you have eliminated excessive resistance in the armature circuit, check for shorted armature coils.

You can check the armature for shorts while the motor is uncoupled. Shut down the motor and then feel the armature coils with your hand. Mechanical problems rather than electrical ones are usually the cause. First of all, measure the Armature current with the DC ammeter to see if the motor is overloaded. Inspect the Brushes 1. Make sure no brushes are missing and that all of them are properly seated on the commutator.

Check that all brush leads are intact and that they are securely fastened to the brush holder. Check brush springs for correct pressure. Make sure the brushes fit properly and move freely in their brush boxes. They should not be too tight or too loose. Check brush holder mountings for looseness, which could be caused by burnt brush holder insulation carbonization or loose fasteners. Check the brush rigging jumpers to be certain that they are tight and securely connected. Inspect the brush-mounting ring for damage, and make sure it is securely locked in the neutral position.

Inspect the Commutator mechanical focus 1. Physical damage for example, from rubbing. Is the commutator run-out excessive. Commutator run-out should be less than. The normal operating speed of the machine will dictate how much tolerance in run-out can be allowed. Generally, if the commutator run-out exceeds. Inspect the commutator, making certain there are no raised segments.

Make sure there are no segments with flat spots. Check for high mica. Be sure there is no foreign matter between the commutator bars 7. All of the conditions above can cause the brushes to bounce at high motor speeds. The interruption of the armature current will invariably cause sparking. Severe vibrations from an unbalanced armature. Run the motor uncoupled before removing it for service to see if the imbalance is in the motor or the machine. Worn bearings. Uneven air gaps can result if bearing clearances are out of whack. The movement of the armature can also cause the brushes to bounce.

Inspect the Commutator electrical focus Inspect the commutator segments for signs of discoloration Burned segments indicate open circuits 1. Broken coil leads behind the risers. Thrown solder and loose connections at the risers. Darkened segments can indicate shorted armature coils. The armature can be tested as we mentioned earlier to determine if it is shorted. Be Advised, a definite pattern of darkened commutator segments every third or fourth segment, for instance , is often mistakenly assumed to indicate a problem. The design of the armature can produce patterns in the film that the brushes put down.

Some characteristics of armature design that can cause current fluctuations and bar patterning are: Odd turns in armature coils Commutator bar and armature core slot combinations The bottom line is, if the discoloration pattern is repeated around the entire diameter of the commutator, there is probably NOT a problem with the armature winding. Now we will discuss troubleshooting motors that fail shortly after installation. Determine if the overcurrent devices are properly sized and properly adjusted.

Check the motor lead connections to be sure they are correct and tight. Make sure the controller is functioning properly. If these checks indicate no reason for the malfunction, gather all the facts that you can and call Electrical Equipment Company for assistance. Some of the procedures outlined below require special equipment and great familiarity with the construction of DC motors. We highly recommend that they should be undertaken only with the aid and assistance of motor shop personnel.

You will especially want the motor shop personnel there to protect your interests if you feel there are any manufacturer or repair warranty considerations. If a dual voltage shunt field motor has the fields connected for high voltage and low voltage is applied to the field, the motor will run at higher than rated RPM. This condition will also produce high armature currents. To restore the speed of the motor to its normal range, reconnect the shunt field for low voltage. A note of caution: A shunt field connected in parallel for low voltage and then connected to a high voltage field supply will result in the shunt field burning out if adequate overcurrent protection is not provided.

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The manufacturer or the repair shop could NOT warrant such an overcurrent condition. Reversed Series Field polarities may cause compound-wound motors to run at above the nameplate RPM when the motor is under load. If the motor rotation is correct, go ahead and interchange the series field leads Sl and S2. Log in. Categories search. Architecture Building Interior Design. Economics - General Economics Texts. General Philosophy Philosophers Popular Philosophy. Religion Spirituality. Crime Fiction Thriller. Net and ADO. Look inside with.

Not in stock What does this mean? Talk to real people Contact us seven days a week — our staff are here to help. Electric Motor Maintenance and Troubleshooting, Second Edition covers: Troubleshooting and testing DC machines AC electric motor theory Single-phase motors Three-phase induction motors Troubleshooting less common motors, including synchronous, two-speed one-winding, and multispeed Test instruments and services. Hide preview Click here to look inside this book with Google Preview.

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