Inspection and Testing of Motors

Inspection and Testing of Motors

Motor is a vital component of any drive system and its quality has to be ensured to avoid instability and breakdown. This can be achieved by evolving a well coordinated system of inspection and testing of the motors. This involves the following aspects:-

1. Visual inspection of the motor.
2. Testing the technical parameters
3. Quality control system adopted by the manufacturer.
4. Name plate.

We shall now discuss the above aspects in detail.

1. Visual Inspection :

This involves checking of the following aspects:

1) The motor should be physically inspected for any welding defects. 2) The terminal box should be epoxy molded or as per the given specification and size. 3) The connecting leads are of adequate size and are provided with sleeves of the specified make. 4) The electrical connections of the leads are properly tightened .5) Provision of arrow mark to indicate the direction of rotation 6) Bearings of specified make are used. 7) Provision for lubrication (if not self lubricated). 8) Cooling system like the fan blades and their proper mounting.

One or two motors, among the batch of motors offered for inspection, may be dismantled to check the quality of winding, slot insulation, the soundness of soldering of the terminal leads and the bearings used for both the driving and non-driving end.
The certification of welders and other artisan staff should also be checked.

1. Testing of technical parameters:

This involves testing of the following parameters:

1. Resistance of Armature and field, if it is a DC motor. In case of AC motor, the resistance of each phase should be measured. The difference in the resistances should be within specified limits. Otherwise, unequal currents will flow in each phase, causing overloading and consequent burning out of the motor. Sometimes, it may cause frequent tripping of the overload relay. In all cases, the results obtained should be tallied with the prototype test results to ascertain any significant deviations. Normally more than 10% deviation is not allowed, as a general rule, unless otherwise specified.
2. No load test on the motor. The no load currents and losses calculated should conform to prototype test results. Excessive no load current not only decreases the efficiency of the motor but also will cause overheating or low power factor. It will also increase the energy bill.
3. Rotor Blocked test. This test is conducted to ascertain the load losses and also to check for any mechanical damages.
4. No load running: The motor may be run for half an hour and observe the temperature rise of the body as well as the winding temperatures. Resistance method may be adopted to check the winding temperature. The bearing noise may be checked for their soundness.

5) In case of DC motor, sparking at the brushes should be checked during running. After running for considerable time, the carbon deposition on the commutator should be checked. In case of DC motors, no load and load test (either by back to back connection or by application of resistance load) should be conducted. The carbon brushes used must have the mark for discarding and must be of proper grade and quality.

6) The direction of rotation should be checked and be tallied with the arrow mark. In case of wrong direction of rotation, it should be set right by correcting the terminal connection sequence.

3) Quality control system adopted by the manufacturer: This is the most important aspect. It should be checked from the documents and if desired a few components may be tested and verified with the documents. Physical checking, if possible by conducting surprise check, of the manufacturing process, like how the windings are done (that is, whether proper lapping of insulation or proper quality tapes are used and how they are varnished and cured, etc) how the impregnation of the motor is done, whether correct curing cycles are followed or not and whether the varnishes being used conform to the specified brand and within the shelf life period. Whether vacuum levels are correctly maintained in the vacuum chamber should be checked.

Dynamic balancing of the rotor should be done for every rotor by the manufacturer. The records and the working condition of the balancing equipment should be verified by the inspector during inspection. The documented results may be cross checked, if deemed necessary.

The stage inspection records should be checked to ascertain whether the standard quality practices are being followed by the manufacturer or not. It should also be checked whether the defective materials and components are properly disposed off and are not mixed with the healthy components.

Any anomalies observed should be brought to the notice of the manufacturer through the sales manager and stringent corrective action should be taken to avoid their recurrence.

All these aspects should be checked and noted. The following documents should be checked:

1. The sources of the components of the motors like the bearings, winding wires, sleeves, terminal boxes, varnishes, insulating tapes, hardware, etc should be from the approved vendors.
2. Each component must be given batch number and identification to ensure traceability.
3. Test report should be furnished for each component by the vendor. The manufacturer of the motors should also possess basic equipment for testing the raw materials.
4. The calibration of the instruments and testing equipment shall be up to date.
5. Chemical composition and orientation of punchings and of the copper segments, their conductivity should be checked by taking a few samples.
6. A surprise check may also be conducted at the premises of the sub-vendors with the help of the manufacturer to check that standard quality practices are followed by them too and also to check their genuine existence vis-à-vis the approved vendor list.
7. The storage of raw materials is done at proper temperature and dust free environment, wherever so required to be followed. For example, the insulating varnishes and tapes should be stored at the temperature specified by the sub vendor.
8. There should be separate storage space earmarked for the incoming materials and the finished products.
9. There should be proper monitoring and storage of shelf life items.
10. Packing of the finished product is done properly as per the specification given to the vendor to avoid damage in transit.

4) Name Plate: Finally, the name plate on the motor should be checked. It should be properly fixed by riveting. It should contain vital information like the name and emblem of the manufacturer, the serial number and type number, the type of connection, the month and year of manufacture, the rated current and voltage, the number of phases ( in case of AC motor) the frequency ( in case of AC motor) the class of insulation adopted and any other stipulated information.

The best practice would be to maintain a check list consisting of the above observations and monitor the performance of the vendor for future assessment and review. The same practice can be extended to all other products.