Sebelum mnelakukan troubleshooting pada motor listrik, perlu dipahami sifat-sifat motor terhadap perubahan tegangan, frekuensi dan beban.
Pengaruh umum variasi tegangan dan frekuensi pada karakteristik motor induksi
Oct 21, 2015 - Motor Bolak Balik ini adalah salah satu kerja motor induksi 3 phasa yang sering digunakan pada mesin mesin produksi oleh banyak kalangan.
Pada dasarnya motor induksi mempunyai karakteristik secara eksak sulit ditentukan antara lain karena :
- Rugi motor tidak tetap
- Kurva magnetisasi tidak linier
- Adanya cacat harmonisa
- Kerugian gesekan tidak sebanding lurus dengan kecepatan
Dengan demikian sulit untuk menentukan secara pasti perubahan karakteristik disebabkan oleh perubahan tegangan dan frekuensi.
Namun secara umum perubahan karakteristik motor terhadap perubahan tegangan dan frekuensi dapat diperkirakan seperti dalam tabel dibawah ini :
Karakteristik | Tegangan | frekuensi | ||
110% | 90% | 105% | 90% | |
Torka Start | Naik 21% | Turun 19% | Turun 10% | Naik 11% |
Torka Maksimum | Naik 21% | Turun 19% | Turun 10% | Naik 11% |
Persentase Slip | Turun 15 – 20% | Naik 20 – 30% | Naik 10 – 15% | Turun 5 – 10% |
Efisiensi – Beban Penuh – Beban 75% – Beban 50% | Turun 0 – 3% Turun Sedikit Turun 0 – 5% | Turun 0 – 2% Sedikit Berubah Naik 0 -1% | Naik Sedikit Naik Sedikit Naik Sedikit | Turun Sedikit Turun Sedikit Turun Sedikit |
Faktor Daya – Beban Penuh – Beban 75% – Beban 50% | Turun 5 – 15% Turun 5 – 15% Turun 10 – 20% | Naik 1 – 7% Naik 2 – 7% Naik 3 – 10% | Naik Sedikit Naik Sedikit Naik Sedikit | Turun Sedikit Turun Sedikit Turun Sedikit |
Arus Beban Penuh | Turun 0 – 5% | Turun 5 – 10% | Turun Sedikit | Naik Sedikit |
Arus Start | Naik 10% | Turun 10% | Turun 5% | Naik 5% |
Kenaikan Temperatur Beban Penuh | Naik 10% | Turun 10– 15% | Turun Sedikit | Naik Sedikit |
Kapasitas Beban Lebih Maksimum | Naik 21% | Turun 19% | Turun Sedikit | Naik Sedikit |
Pengaruh Tegangan tidak seimbang pada Kinerja Motor Listrik
Bila tegangan line yang dipakai oleh motor induksi 3-fasa tidak sama, akan menghasilkan arus yang mengalir didalam gulungan stator tidak seimbang. Sedikit perbedaan tegangan fasa satu sama lainnya menimbulkan perbedaan arus yang besar sekali. Konsekuensinya temperatur akan lebih tinggi dibandingkan dengan bila motor beroperasi pada beban yang sama dengan tegangan seimbang.
Bila tegangan tidak seimbang yang dipakai, tenaga kuda rated motor harus diturunkan sebanding dengan tabel berikut untuk menghindari kerusakan pada motor listrik :
% Tegangan Tak Seimbang | 1 | 2 | 3 | 4 | 5 |
Operating Factor | 0,99 | 0,95 | 0,9 | 0,83 | 0,76 |
Tidak direkomendasikan mengoperasikan motor listrik pada tegangan tidak seimbang tidak lebih dari 5%.
Untuk menghitung persentase ketidakseimbangan tegangan digunakan rumus :
Penyimpangan Maksimum dari
Persentase Tegangan rata-rata
Tegangan = 100 x ————————————————--
Tidak seimbang Tegangan rata-rata
Contoh :
Dengan tegangan Line 3,45 , 3,4 , dan 3,3 kV.
Tegangan rata-ratanya 3,383 kV dan Deviasi Maksimum = 3,383 – 3,3 =
0,083 kV
% unbalance = 100 x (0,083)/93,383) = 2,45%
Bila kapasitas motor listrik 100 HP akan diperoleh kapasitas motor listrik yang baru ( dari tabel diatas), yaitu :
100 HP x 0,93 = 93 HP
Perbandingan Metoda Starting untuk Motor Induksi Sangkat Tupai
Tabel berikut menunjukkan persentase Arus dan Torka dari berbagai Starter dan Tegangan Terminal Motor.
Tipe Starter | % Harga Tegangan Penuh | ||
Tegangan motor | Arus Line | Torka Poros Motor | |
Tegangan Penuh | 100 | 100 | 100 |
Auto Trafo 80% tap 65% tap 50% tap | 80 65 50 | 64 42 25 | 64 42 25 |
Start Y, Jalan Delta | 100 | 33 | 33 |
Summary of Motor Failures, and possible causes and remedies
Failure due to | Possible causes of failure Condition | Description of results of failure | Possible remedies |
Overspeed | Turbining due to reverse fluid flow through pump or compressor | Damaged rotor core, end-winding and any rotating part due to centrifugal forces, damaged bearing surfaces Damaged stator due to debris from rotor failuer | Rewind rotor Replace overstressed component Check shaft for straightness Check outer diameter of core for concentricity Check stator/rotor for rubbing Check bearing condition Fit overspeed and/or reverse rotation protection |
Stall | Seizure of bearing / gearbox / coupling, etc Jammed mechanism, overload condition, combined with failure or incorrect setting of overcurrent protection devices | Motor will absorb locked rotor current and generally have only minimal cooling at zero speed resulting in one or a combination of : 1.Heat-damaged stator winding 2.Heat-damaged rotor Winding 3.Damaged leads or terminations | Check stator and rotor windings and rewind as necessary. Check leads and terminations and replace where necessary. Install or correctly set overcurrent protection devices. Rectify cause of failure condition |
Overload | Increase demand on motor due to abnormal load conditions combined with failure / incorrect setting of overcurrent devices and failure of thermal cutout devices when fitted | Depending on degree of overload increased losses, therefore accelerated thermal ageing of stator / rotor insulation systems and slipring / commutator assemblies due to elevated temperature rise leading to premature failure. Accelerated brush shedding and slipring /commutator wear, possibly leading to flash over. | Rewind rotor / stator Clean or replace slipring / commutator assemblies and brushes Fit thermal cutout devices and / or correctly set overcurrentn protection devices |
Overvoltage | Wrongly tapped transformer Wrongly connected three-phase motor (delta-connected instead of star) Voltage surges due to vacuum contactors, lightning strikes, system faults, etc | Increased losses resulting in same result as for Overload Electrical breakdown of insulation resulting in inter-turn and earth faults. | As for Overload Fit overvoltage protection Identify and repair damaged coils Rewind stator / rotor, possibly with higher system insulation level Fit surge protection capacitor |
Undervoltage | Wrongly tapped transformer Wrongly connected three-phase motor (star-connected instead of delta) | Increased losses resulting in same result as for Overload | As for Overload Fit undervoltage voltage protection |
Bearing seizure | Insufficient or excessive lubricant Contaminated lubricant or bearing surfaces Old lubricant Misalignment ciombined with inadequate monitoring of bearing temperature / vibration levels Brinelling from local vibration source, bearing capacity overload, eg excessive tension in drive belt Offloading of ball / roller bearing on vertical motors Corrosion due to ingress of moisture Electrical pitting due to shaft currents caused possibly by breakdown of bearing insulation | Motor stops rapidly resulting in possible damage to bearing supports / end-shields, shaft, etc See also stall | Replace bearing and seals Metal spray, or plate and regrind shaft. Check bearing supports / end-shields and shaft Fit bearing temperature / vibration monitors See also stall Remove cause of overload Clean or replace bearing insulation |
Single-phasing (three-phase ac motor) | Loss of one supply line due to system fault Failure of protection scheme | Loss of torque and asymmetric fault currents in stator and rotor windings, resulting in severe Overload condition | See Overload Fit or set protection scheme |
Ingress of abrasive dust | Wrong enclosure specified for working environment | Abrasion of exposed insulating surfaces leading to premature failure Overheating due to blocking of iar passages resulting in same conditions as failure of cooling system | Rewind as necessary Change motor enclosure Resite motor See Overload |
Ingress of moisture | Wrong enclosure specied duty Failure of enclosure of bearing seals | Electrical breakdown of insulation, resulting in interturn and earth faults Bearing seizure due to corrosion | Identify and repair damaged coils Rewind stator / rotor, possibly using sealed winding system Identify and repair damaged seals Check compatibility of enclosyre for duty Dry out motor thoroughly and reseal winding with varnish before reintroducing into service Replace bearing, bearing seals and possibly lubricant |
Failure of Ventilating / cooling system | Burn-out of auxiliary cooling motor Loss of coolant Blocked cooler tubes or air passages | Increased temperature rise, resulting in increased thermal ageing of stator / rotor insulation systems and slipring commutator assemblies, premature failure | See Overload Rectify cause of fault |
Frequent start / stop duty cycle ( Cage induction motors ) | Inadequately design motor for duty cycle Wrong type of motor being used Wrong type of starting system being used | Fractured rotor bars / end rings Thermally aged stator winding Dameged stator caused by debris from rotor failure Stator end-winding movement resulting in stator winding failure Coreplate burning at rotor tooth tip | Rewind rotor using an engineered design Rewind stator with a designed end-wiring bracing system Replace starting scheme ( fluid coupling, soft starter ) or type of motor |
Failure of slipring assembly ( Wound rotor induction motors ) | Failure to replace brushes at specified interval leading to metal-to-metal contact Rapid brush wear Severe overload Contaminants in surrounding atmosphere Incorrect brushes Worn brush holder or brush pressure incorrect Wrong grade fitted | Metal from slipring worn on melted away See Overload | Replace sliprings and brush assembly Identify and rectify original cause of failure |
Failure commutator assembly (DC motors) | As for Failure of slipring assembly Severe sparking at brushes due to bad commutator condition, brushes set off-neutral, motor overload or interpoles not properly adjusted, resulting in rapid brush wear and overheating | Metal from commutator worn or melted away See Overload | Replace commutator and brush assembly Identify and rectify original cause of failure |
Identifying problems with the motor’s individual component parts
Component | Visible signs, test or meaurements | Cause and remedial action required |
Windings | Mechanical damage to insulation | Carelessness in assembly or disassembly – local repairs, eg by retaping and scaling with varnish, will generally suffice if the damage is not extensive |
Bracing, intercoil packers and winding supports | Looseness | Poor design/manufacture – general ageing Any movement must be eliminated to prevent insulation failure due to fretting fatigue |
Wedges | Looseness | Poor design/manufacture – general wear – possibility accelerated by high vibration levels Looseness must be eliminated ( possibly by rewedging and/or redipping and curing) On vertical motors loose wedge can drop |
Corepacks | Signs of localized overheating on surface Excessive tooth movement | Breakdown of inter-laminar insulation Increased losses and overheating will occur leading to winding failure Corepack and winding will generally require replacing Axial movement in corepack teeth may lead to high frequency tooth vibration (caused by magnetic fields) leading to fatigue failure at narrowest part of tooth Increase pressure on the tooth tips at the extremities of the core – if any of the teeth have broken off, the corepack will generally require replacing |
Bearings | Pitting, scratching, scoring, denting, corrosion of bearing surfaces, balls, rollers or cages Contaminated lubricant | A number of causes can results in the bearing damage These can range from damage caused : 1. During fitting 2. Brinelling during transit or due to operation of adjacent machinery on site 3. Inadequate or contaminated lubricant 4. Shaft currents 5. Cummulative errors in tolerances; and 6. metal fatigue due to to operating bearing in excess of design life If any damage is apparent it will be necessary to replace complete bearings in the case of ball and roller bearings and possible bearing shells in the case of white-metal bearings Shaft seatings should examined for damaged and if any is apparent it will be necessary to remachine |
Rotor cage | Broken bars Bars detached from short circuit ring Loose bars Damaged or cracked short-circuit ring Hairline cracks in short-circuit rings and exposed bars can be identified by using suitable dye pen techniques Ultrasonic techniques can be used to check integrity of bar / short-circuit ring joints | Rotor design inadequate for motor duty Motor being misused in service If any damage to the rotor cage is apparent, fitting new bars and/or short-circuit rings will be necessary Care must be taken in selecting the appropriate materials For instance, high-speed motor may be fitted with chrome-copper short-circuit rings, which have been selected for their superior mechanical strength Simple rebrazing of thes will diminish the mechanical strength and could possibly lead to latermechanical failure |
Sliprings and commutators | Rapid brush wear Wear-scoring or pitting of rings or commutator Excess temperature | Incorrect brush pressure-correct pressure Incorrect brush grade-fit correct grade Damaged commutator or rings, eg scoreor out-of-round Repair damage or replace Defective commutation-trace reason and rectify Brush sparking-trace reason and rectify Chemical attack of commutator rings due to corrosive environment Review degree of enclosure of commutator and rings-fit rings of different composition immune to attack from particular corrosive elements Sparking of brushes-trace fault and rectify Over temperature-trace fault and rectify Blocked ventilating passages Incorrect brush pressure Badly-bedded brushes Motor overloaded Incorrect brush grade fitted-diagnoseand rectify condition |