Third-Party Inspection of Induction Motor
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In this Blog – we are providing the Third-Party Inspection of an Induction Motor – the key points to be covered during the Inspection of an Induction Motor by our Team of Competent and Expert Engineers.
About Induction Motor
Induction motors are the most frequently used type of motor used in the residential, commercial, and industrial sectors. In an induction motor, the electric current in the rotor needed to produce torque is obtained via electromagnetic induction from the rotating magnetic field of the stator winding.
There are mainly two types of induction motors on the basis of power supply, Single Phase Induction Motor and Three Phase Induction Motor.
Single Phase Induction Motor – It may be classified on the basis of its construction and starting methods.
- Split Phase Induction Motor
- Capacitor Start Induction Motor
- Capacitor Start Capacitor Run Induction Motor
- Shaded Pole Induction Motor
The single-phase induction motors are used in low-power applications. These motors are widely used in domestic and industrial applications. Some of the applications are:
- Small Fans
- High-speed vacuum cleaners
- Electric Shavers
- Drilling Machines
Three-Phase Induction Motor – It has two major components, Stator and Rotor. The stator is the stationary part whereas the Rotor is the rotating part. The load is coupled to the rotor shaft of the motor. Three-phase armature winding is wound on the stator. When balanced three-phase current flows through this winding, a constant amplitude rotating magnetic field is created in the air gap. This armature winding is connected to the 3-phase power supply and carried the load current.
Based on the rotor construction, it can be of two types: Squirrel Cage Rotor and Wound Rotor. On this ground, Induction Motor is also classified as:
- Squirrel Cage Induction Motor
- Wound Rotor or Slip Ring Induction Motor
Three-phase induction motors are used extensively in domestic and industrial appliances because these are rugged in construction requiring little to no maintenance, comparatively cheaper, and require supply only to the stator. Applications of Three-Phase Induction Motor:
- Large-capacity exhaust fans
- Driving lathe machines
- Oil extracting mills
- Textile and etc.
Electrical faults in three phase Induction Motor
Single Phasing Fault: Single phasing fault occurs when any one phase of the supply voltage is getting lost because three phases of the voltage supply are must require for the normal working of a three-phase induction motor. Due to this fault, the motor could be burned or heated up.
Reverse Phase Sequencing Fault: Reverse phase sequencing fault occurs when any one phase of the three-phase voltage supply is reversed means the phase order of supply voltages is swapped. Due to this fault, the direction of rotation of the motor has changed.
Under or Over Supply Voltages Fault: This fault occurs when supply voltages are under or over the specified limit. The limit of three phase ac supply voltages is 380 to 440 so when supply voltages have crossed this limit then the motor could be burned or heart up.
Over Load Fault: This fault occurs when the motor is overloaded means the higher load is connected at the output side of the motor and due to this load motor could be heated up or have excessive vibration.
Earth Fault: Earth fault occurs when any one phase of supply voltages is connected to the housing of the motor then the motor is totally shorted. In this condition when anyone will touch this motor then he will feel a heavy shock beside this, the motor would take over the current which could be dangerous for the induction motor.
Inter Turn Short Circuit Fault: Inter turns short circuit fault is such type of fault when two turns of the same phase or different phases are short-circuited. During this fault, the motor could be totally damaged, or coils of that specific phase could be damaged.
Crawling Fault: Crawling fault is basically an electromechanical fault in a three-phase induction motor. This fault occurs when the motor is fully loaded with full supply voltages then it does accelerate but it runs nearly one-seventh of its synchronous speed. This specific phenomenon of induction motor is called crawling.
Mechanical faults in three phase Induction Motor
Mechanical Faults: Mechanical faults are such types of faults which are generally occurred in the internal housing of a three-phase induction motor. These faults are further sub dived into three types such as,
Broken Rotor Bar Fault: The squirrel cage induction motor consists of rotor bars and short-end rings. If these bars are damaged or partially cracked, then this type of fault is called a rotor broken bar fault. There are so many reasons for occurring this fault, but it has mainly been observed that this fault occurred due to manufacturer defects. Because during the brazing process, non-uniform metallurgical stress may occur in rotor bars which may lead to the rotor bar’s failure during the rotation of the rotor
Rotor Mass Unbalance Fault: If we concentrate on the construction of the induction motor then we can easily know that the rotor of the induction motor is placed inside the stator bore and it rotates coaxially with the stator. In heavy motors, it is centrally aligned with the stator and its axis of rotation is also the same as the geometrical axis of the stator. So the air gap between the inner of the stator and the outer of the rotor is the same. Similarly, if this air gap is not the same then the situation that is referred to as eccentricity would occur. In this eccentricity situation, the rotor fault would occur which is called a rotor unbalanced fault.
Bearing Fault: In three-phase induction motor two sets of bearings are installed inside of motor housing, for supporting the motor shaft. The main purposes of these bearings are, to rotate the motor shaft freely and reduce friction. They consist of an outer and inner ring which is called races and a set of rolling elements which are called balls. The balls are fixed on the inner and outer sides of the ring and they reduce the friction of the shaft. The fiction could be further reduced by the lubrication of these balls. Sometimes the balls and outer or inner ring of the bearing is damaged due to any physical problem then the fault occurs. This fault is called a bearing fault and due to this fault, the motor totally jammed or struck.
Environmental faults in three phase Induction Motor
Environmental Fault: In a three-phase induction motor different faults could be occurred simultaneously and, in these faults, the environmental fault is also so much important. Induction motor surrounding environments such as ambient temperature and moisture etc. affect the performance of the induction motor. These factors reduce the performance of the induction motor. Besides this, the vibration in the induction motor that could be due to any reason such as, not properly installed also affects the performance of the induction motor. So during the installation of three-phase induction, this factor should be kept in mind.
Motor specifications are key to matching load and operating conditions with motor protection and efficiency. Nameplate provides important information relevant to its selection and application. Manufactured AC motors generally are hp-rated / Kw rated. Base speed is the nameplate speed given in rev/min (rpm) at which the motor develops the rated horsepower at the rated voltage and frequency. The service factor is a multiplier that may be applied to the rated power to allow a motor to be operated at higher than its rated hp. AC motor efficiency is expressed as a percentage. It is an indication of how much of the input electrical energy is converted to output mechanical energy.
All induction motor inspection and testing are done against the approved drawings, purchase order specifications, purchasers or company standards, and within the practices and rules of the country, state, or province, and any government decrees, laws, ordinances, or regulations as may apply.
The applicable codes and specifications for an induction motor that is under the construction process is:
- Design Code and Standard
- Purchase order specification
- Purchaser’s standards
- Approved Drawings
The applicable codes and standards may be based on any international standards. This content is general and can be useful for all recognized standards. Ref. some of IS Standards as:
- IS 325: 1996 AMD 3 2011
- IS 12615: 2018 ‘Line Operated Three Phase a.c. Motors
Third-Party Inspection Document Review for Induction Motor
This is the list of documents that are normally agreed upon to be presented to the inspector:
- Induction Motor Purchase Order or Contract
- Induction Motor Manufacture Proforma Invoice
- Induction Motor Manufacture Quality Control Plan
- Induction Motor Inspection and Test Plan
- Induction Motor Data Sheet
- Induction Motor Approved Drawings, (arrangements, schematics, wiring, single line)
- Induction Motor Raw Material Test Reports
- Weld Procedures and Procedure Qualification Records (if applicable)
- Induction Motor Calibration Certificates for Test Equipment
- Induction Motor Special Procedures
- Induction Motor Special Qualifications
- Induction Motor Preservation, Packing, and Shipping Procedure
- Induction Motor Packing List
Third-Party Inspection Induction Motor – Material Inspection
The review includes checks on:
- Maker’s name and location
- The certificate number, grade
- Batch Number
- Chemical composition
- Mechanical Properties
- Cast or Heat number (for metal parts)
- Heat treatment condition
- Surface finish (if required)
- Structural observations, notable defects
Ancillary items may be supported by test certification, or by certificates of conformity. It is necessary for the content of the certification to be appropriate to the required scope of the items.
It is also necessary to check material identification with the induction motor drawing datasheet, material list, and other specifications as appropriate.
When the third-party inspector carries out the material inspection, then provides the inspection visit report, the report contains the following items:
- Confirmation of satisfactory document review
- Record of the endorsement of certification reviewed/witnessed
- Record all non-conformities
- Record of any tests witnessed and the result
Third-Party Inspection Induction Motor Construction:
The inspection scope is determined in the Inspection and Test Plan. Some purchasers prefer to have stringent controls and assign the TPI for more “hold or witness points” for inspection and test activities, and some others prefer fewer “hold or witness points” and assign the TPI much more work in the “review document.” Inspection and Test Plan – The IEC, ISO, and IS standard requirements normally are applied for the inspection and test plans of induction motors in the manufacturing shop.
These are some of the inspection points that need to be witnessed, checked, monitored, and reviewed by a third-party inspector in an induction motor manufacturing shop.
- Visual and dimensional check
- Dielectrically and insulation test
- Measurement and calculation of no-load and short circuit characteristics, efficiency, power factor, current, slip, losses, torque, etc.
- Measurement and control of alignment, air gap, bearings
- Heat run test
- Noise and vibration Measurement
- No-load test
Third-Party Inspection of Induction Motor -The Inspector (TPI) checks the following in accordance with approved drawings and applicable standards:
- Paint using specified color swatch, measure thickness if specified
- Outline dimensions, interface dimensions, bus bar dimensions, clearances, and other specified dimensions
- Tightness/torque of terminals
- Degree of Protection
General Arrangement Check
The inspector checks that all instruments, compartments, control switches, etc. are correctly rated and are correctly located, with specified labeling. The inspection is carried out in accordance with approved drawings.
Winding Resistance Test
Induction motor resistance between terminals of each winding is measured by means of the double bridge method. The test would be acceptable if the imbalance value is within + or – 3%.
No Load Test
Each phase current and input power are measured in operating under rated frequency, rated voltage, and no load. The test would be acceptable if the Imbalance value of each phase current is within + or minus 5% of the mean value.
Locked Rotor Test
While locking the rotor, the voltage, current, and input power are measured under the rated frequency and the rated current. The test would be acceptable if the measured values are in specified ranges.
Low-Frequency Locked Rotor Test
While locking the rotor, the voltage, current, and input power are measured under half-rated frequency and approximately rated current. The test would be acceptable if the measured values are in the specified ranges.
Determination of Characteristics
The following characteristics are calculated by the induction motor manufacturer and are reviewed by a third-party inspector.
- Current, efficiency, power factor, and slip
- Maximum output power
- Maximum Torque
- Starting Current
- Starting Torque
The obtained values from the calculation would be acceptable if they are in the range of specified values.
Temperature Rise Test
The temperature rise is measured for the following parts operating for a rated time by means of the equivalent load method.
- Stator winding
- Stator Core
- Bearing (Coupling side and free side)
- Centre of Frame
- Inlet Air
- Outlet Air
The test would be acceptable if the measured values are in the specified ranges.
Induction Motor Insulation resistance is measured between winding and earth (using 1000V Megger) and between Accessories circuit and earth (using 500V Megger). The test would be acceptable if the measured value between winding and earth is over 10 Mega-ohm and between the Accessories circuit and earth is Over 1 Mega-ohm at the normal temperature.
It is necessary to confirm that the voltage provided in the applied specification is withstood between stator winding and earth. The test would be acceptable if withstood without any damage.
Induction motor vibration is measured in operating the motor at no load condition with rated voltage and frequency. The location of vibration measurement is at both bearing housings. Measuring directions are in horizontal, axial, and vertical positions. The test would be acceptable if measured values are within the specified values.
Induction motor noise is measured in the motor at no load condition with rated voltage and frequency. The 4 points at every 90 degrees are measured, where the height of the points is the shaft height for the horizontal motor and one-half of the motor height for the vertical motor. The test would be acceptable if measured values are within the specified values.
The induction motor is visually inspected for outside views such as rust, scratch, and bruise, coasting color, and finish, description of the nameplate, mounting direction of the terminal box, and Protection type of frame.
The induction motor is dimensionally controlled for shaft end diameter, shaft length, key and keyway, the pitch of bolt holes including bolt holes, the height of shaft center, and principal outer dimensions. The test would be acceptable if measured values confirm drawing values.
Packing, Marking, and Shipping
The following points are checked by the third-party inspector for packing inspection:
Verify that the packing materials conform to specifications and accepted practices for the mode of transport
- Verify protection against humidity, abrasion, distortion, and other damage.
- Verify shipping marks to marking instructions and record.
- Verify handling marks.
In general, it is good to conduct complete engineering tests on new designs to evaluate all motor performance parameters and commercial or routine tests on duplicate designs.
Routine Test – Testing is usually performed by the vendor to ensure the motor operates to accepted specifications. Commonly called a “commercial test” by most vendors.
Complete Test – More extensive testing which usually includes a full load heat run test, efficiency, power factor, full load and locked rotor currents, breakdown torque and percent slip, stator and bearing temperature rises, speed-torque characteristics etc.
Optional Test – Special testing specified by the customer such as surge comparison, stator winding immersion, dielectric absorption ratio, bearing housing natural frequency test, coast down test, etc.
Observed Testing – The vendor will notify the customer about the date of inspection or testing; however, the test or inspection shall be performed as scheduled, and if the customer is not present, the vendor will continue so as not to impact the production schedule.
Witnessing – Testing or inspection that places a hold on the production schedule and that the test or inspection will not be carried out until the customer is present. Requires a successful pretest of the motor in most cases.
Being a leading Third-Party Inspection Agency in India – we assured that during the inspection of the Induction motor our expert Inspection Team – prepare the prior Inspection Checklist – Considering the Inspection requirement Inputs received from the Client. Develop the Final QAP and submit it to the Client prior to Inspection. So that the Client and Supplier have a common understating of the outcome of the Third-Party Inspection of the Induction Motor. Accordingly, we provide Third-Party Inspection Services – So that by delivery of Third-Party Inspection services, our Clients feel satisfied with the outcome of the Inspection Results.
Inspection or Hold Points – A point specified by the customer to ensure various stages of the motor production meet their specifications. Such as rotor balancing, surge comparison testing, an inspection of the stator before vacuum pressure impregnation (VPI), or welding before final assembly.