Q1/ what do you know about D.C machine?The same machine can be operated as motor and generator and it's require more maintenance.Q2/ List the main parts of D.C. machine and describe the functions of each?Frame : provide the protection to whole machine Magnetic Poles and Field coils : the poles are surrounded by field coils supplied with currentArmature: to rotate the coils in the magnetic field.Comutator: use to change AC to DC in armatureBrushes : made of high grade carbon or combination , carry the load current from rotor coils to external circuit.Q3/ If a D.C. motor is given to you for troubleshooting, how will you start this work ?1- Check branch circuit fuses , circuit breakers , thermal overload relay and small fuses on motor control panel2- Disconnect the armature leads (A1,A2), field leads (F1,F2)or (S1,S2).3- Ground test by connecting one terminal of megger to shaft and the other to each lead in turn {low resistance means ground fault}.4- Open circuit test by connecting 2 megger terminals to each corresponding circuit leads {High resistance means open circuit fault}5- Short circuit test by connecting one terminal of megger to Series/Shunt lead and the other to armature lead {low resistance means short circuit fault}Q4/ Describe the procedure to test and troubleshooting a Shorted Shunt Field coil using a voltmeter?1- Apply rated AC voltage to shunt field (F1-F2).2- Voltage drop is measured across each field coil.3- Voltage measured should be same cause each coil has the same number of turns.4- If voltage across the coils are equal it can assumed all coils are not shorted.5- If voltage across the coils is 10% less than average one or more turns are shorted.

Q5/ Describe the procedure to test and troubleshoot an Open Shunt Field coil using : Voltmeter :-1- Apply rated voltage to shunt field (F1-F2).2- Measure one terminal (F1 or F2 ) to successive coil leads.3- If voltage measured is Zero the coil is not open.4- If voltage measured is full line the coil is opened.

Ohmmeter :-1- Measure resistance across each coil using megger or ohmmeter.2- Low resistance means closed coil.3- High resistance (Mega ) means opened coilQ6/ Describe the procedure to test and troubleshoot a Grounded Shunt Field coil using :Voltmeter:-1- Apply rated voltage AC/DC to the shunt field (F1-F2).2- Measure voltage between each coil and ground (frame, iron, pole)3- Zero or low voltage measured means grounded coil

Ohmmeter:-1- Measure resistance across each coil and ground .2- Zero or low resistance means grounded coil.

Q7/ Describe the procedure to test a Reversed Shunt Field coil ?Magnetic compass :1- Apply DC voltage to shunt field (F1-F2).2- Test polarity of each coil.3- If 2 consecutive pole indicate same polarity the poles are reversed.4- The test pole should alternate N,S

Two Screw Drivers :1- Use 2 screw drivers bridged across the gap between adjacent field pole .2- Correct polarity will cause attraction between screw drivers but reversed will cause them repel.

Q8\ List the troubles of series field winding and describes the procedure to locate them? Grounded frame open circuit short circuit.Q9/ Describe the procedure to test and troubleshoot a Grounded Series Field coil using :Clamp on meter:-1- Limit AC voltage to 5V or under using transformer and current limiting resistor.2- Connect one transformer lead to the frame and another to S1.3- Test with clamp on meter around the coil to coil connection until amperes show on the meter (mean grounded coil).Ohmmeter:-1- Secure one probe of Megger to the frame.2- Fasten S1,S2 to the other probe , low resistance measured means ground fault.

Q10/ Describe the procedure to test and troubleshoot a Shorted and Open Series Field coil using :Short :-Voltmeter :1- Apply 2-5V AC to S1 & S2.2- Measure voltage across each coil.3- If measured voltage across coils are equal it means all coils are free from shorted.4- Lower reading will be found across shorted coils.Ohmmeter :1- Use micro ohmmeter to measure resistance of each coil.2- Lower resistance means shorted coil.Open :-Voltmeter:1- Apply 5V to field terminals (S1,S2).2- Measure from terminals (S1,S2) to successive coil leads.3- If voltage measured is Zero , the coil is not opened , if its full so the coil is opened.Ohmmeter :1- Measure resistance across each coil using megger.2- Low resistance means not opened coil , high resistance means opened coilQ12\ Describe the procedure to test and troubleshoot an Open Armature coil using Growler? 1- Set up the armature on the growler.2- Test 2 adjacent bars with AC milli voltage.3- Rotate the armature and continue testing adjacent bars.4- If the 2 bars connected to open coils, the meter will not be deflection (indicate open fault)5- All other bars will give deflection (means no fault). It can be tested without meter by shorting adjacent commutator bars with screw drivers . spark means good coil , no sparking mean opened coil.

Q13/ How will you perform emergency repair of an open armature?Parallel wound armature :1- Will have only one burned spot on commutator for each open coil.2- Emergency repair can be made by bridging over affected bars with a jumper.Series wound armature :1- Will have half of No. of poles burned on commutator .2- Bridge only on the burned spot.Q14/ Describe the procedure to test and troubleshoot a Shorted Armature coil using :Growler:1- Set up the armature on the growler.2- Winding act like primary of transformer and growler act like secondary.3- If current indicate then it's shorted.4- A feeler is wipod from slot to slot.5- The feeler is vibrate with growling noise when the coil shorted.Test lamp :1- To test fractional HP machines , drills ,vacuum cleaner, mixer,etc.2- The choice of lamp tester is trial and error with rating of 25-100 W (120V).3- The lamp will be more brightly when it's connected across shorted coil.Q15/ How will you perform emergency repair of a shorted armature ?1- By cutting the shorted coil in half at the back end of the armature.2- Care to prevent damage to the adjacent coil.3- The open end should be taped and secured to the armature.4- The commutator bars must be bridged over with a jumper.

Q16/ Explain Ground test to troubleshoot an Armature Circuit using :External Growler :-1- Place the armature in the growler's jaw.2- Connect one test lead to the shaft3- Connect each commutator bar individually with the other test lead.4- If there is spark when contact with the commutator's bar there a ground fault.Megohmmeter :-1- One test probe of megger is attached to shaft and another to armature coils at the commutator riser where armature coils are connected.2- Low resistance means ground fault.3- Mega resistance mean there's no ground fault.Test lamp :-1- Use small wattage lamp 10-25W.2- Connect one probe to the shaft and the other probe to commutator leads.3- If the lamp lights , the coil is grounded.

Q18/ How will you perform emergency repair of a grounded armature ?1- Test the wires of coils individually for ground fault using insulation resistance tester.2- If the coil wires clean of ground fault then the commutator is grounded.3- Most ground fault happened on string band that hold mica in position.4- The ground fault may be cleared by digging out the carbonized mica and filling with commutator cement.Q19/ Explain the maintenance and troubles of Brushes and Commutator ?Commutator maintenance :1- Good film is very important for stationary commution and minimum brush and commutator wear.2- Good film (conduct current) & (provides lubrication).Brushes :1- Brush dust has to be wiped out otherwise it can cause short circuit.2- Brush seating to fit commutator should done carefully.3- Brush face must make good contact with all segments under it.

Q20/ Outline the procedure to identify the leads of a D.C. compound motor?1- The shunt field leads (F1,F2)are usually smaller than armature and series leads , it's also have higher resistance than armature and series.2- Connect one probe to commutator and another to the leads, the small resistance is the armature leads the remaining leads are series leads.3- Apply enough voltage to rotate the rotor if it's counter clockwise (CCW) it's ok, if the direction of rotation is clockwise (CW) interchange the armature leads (A1,A2)4- Disconnect the shunt field and apply the enough voltage and see the rotation , if its (CCW) it's ok , if it's (CW) interchange the series leads (S1,S2).5- The standards of rotation by NEMA : Counter clock wise facing the commutator end A1 is positive , A2 is negative.

Q21,Q22/ Prepare the troubleshooting chart for a D.C. motor in tabular form as shown on next page with at least three/four causes & remedies if the motor has following troubles :S. NoTroubleCauseRemedy

1Motor does not startNo supply voltageCheck circuit breakers ,supply fuses, repair any damaged winding.

Mechanical overloadCheck motor rating; reduce load if necessary ; replace worn bearings on armature.

Improper brush-armature contactAdjust spring tension on brush holders.

Field coil openRepair/replace field coil.

Armature circuit openRepair/replace defective part of the circuit.

2Motor runs hotMechanical overloadCheck motor rating; reduce load if necessary.

Defective armature coilsRepair/replace defective coils.

Brushes positioned off the neutral planeReposition brushes.

Incorrect line voltageAdjust voltage to proper level.

Defective field coilsRepair/replace defective coils.

Improper ventilationClean motor vents; provide additional ventilation if necessary.

3

Motor runs fastLine voltage too highAdjust supply voltage. as required ; chance connection as required.

Defective shunt fieldReplace shorted or open coils.

Improper shunt field rheostat settingReadjust as required.

4Motor runs slowLow supply voltageAdjust supply voltage as required.

Mechanical overloadCheck motor rating ; reduce load if necessary.

Incorrect current in armature circuitCheck for short of opens in the armature circuit.

Starting resistance not removed from circuitCheck the control circuit.

Brushes positioned off the neutral planeAdjust the brushes as necessary.

5Motor stops after running for a short period of timeSupply failsCheck with test lamp.

Overload relay tripsCheck the relay and replace it or set the contacts.

Open circuit in no volt coilCheck and replace.

6Field coils overheatedShort circuit in field windingCheck the coil with megger. If burnt get the coils rewound.

overloadReduce the load.

7Armature coils overheatedPoor ventilationMaintain proper ventilation.

OverloadReduce the load.

8Sparking at the motor's brushesContamination between brushes and commutatorClean with solvent; polish and seat as required.

Wrong brush gradeCheck for manufacturer's recommended grade.

Incorrect brush tensionAdjust brush tension as required.

Brushes positioned off the neutral planeAdjust the brushes as required.

Open in armature circuitRepair/replace defective component.

Eccentric armatureGrind if possible ; otherwise replace armature.

High mica and/or commutator barsUndercut mica and recut bars ; otherwise replace. armature.

External vibrationEliminate vibration source; replace defective bearings; align and tighten motor.

Reverses connection on commutating coil or short in armature circuitProperly connect commutating coil; repair/replace defective components.

9Marking , itching and flag spot on commutatorExcessive loadsCheck motor rating; reduce loads as necessary.

Brushes positioned off the neutral planeAdjust brushes to proper position.

Wrong grade of brushesCheck manufacturer's recommendations for correct brush grade.

Rough or uneven commutator surfaceStone or smooth commutator surface.

Contaminated environmentFilter all air flow into the motor.

10Threading of commutator barsLow current density of brushesIncrease load or change brush grade.

Abrasive materials imbedded in bottom of brushChange of brushes; remove source of abrasive material.

Wrong grade of brushCheck manufacturer's recommendation for proper brush grade.

11Copper dragging over mica in commutatorEnvironmental contaminationFilter air circulating through motor.

Copper imbedded in brushChange brushes; remove source of copper.

Hard spot in brushChange brushes.

Wrong grade or brushCheck manufacturer's recommended brush grade.

Excessive vibrationRemove source of vibration; check bearing wear ; check motor base bolts for proper torque.

12Banding of commutator surfaceExcessive brush filmChange brush grade.

One brush of set is wrong gradeConsult manufacturer's recommendations for correct brush grade.

Hard spot in brushReplace brush.

13Commutator out of roundBent armature shaftChange armature shaft.

Worn or defective bearingsChange bearings.

Commutator machined incorrectly

Wide temperature variations and high speedStabilize environmental temperatures.

14Breaks in commutator surfaceHandling damageUse proper hanfling procedures

Commutator struck by foreign objectRepair damage; guard against further damage

Brush holder in contact with armatureRepair damage; rise rush holder to proper position

15High Commutator barsLoosened or hot barsPrevent motor sitting still while energized; prevent handling damage or incorrect installation of bars

16High micaNormal wear of commutator barsUndercut mica