Here is a youtube video covering all these details. Must watch for understanding.
Electric Motors are the running horses of any industry today. The are used anywhere from a house hold to a heavy industry. Everything from electric shaver to a car and an aeroplane contains electric motor. The definition of electric motor can be a device converting electrical energy to mechanical energy. A motor is taking electrical power and gives us a rotational torque which is further used to do a useful work. The motor is using electrical energy to do some useful work. A motor is used in a vehicle to start the engine from stand still to start condition. A motor is used to convey material in an industry. A motor is used to rotate a huge mill along with material. In other words electric motor is so important that it is used everywhere in day to day life.
Motor are pron to wear and tear with time. Motor are overhauled after certain interval. Motor needs alignment when are remove for overhauling. Motor alignment is a very important step after a motor is overhauled or repaired and to be fixed back on its foundation. This guide will not illustrate motor connections, diagrams, starting, running and repairing. Big motors if stopped should not be started immediately. Instead time should be taken to give relaxation to winding heat generated. Otherwise if started all of a sudden, the winding will fail or burn. This phenomenon of motor is expressed in motor duty rating and is denoted with symbol S. There are S1, S2, S3, S4 and S5 duty class motors. The motor insulation is defined as motor insulation class. There are A class, B class, C class, F class and H class motors available. The F class motor is very common. Where is the H class motor has very good insulation which can withstand much more heat. There is another important parameter for motor is its power factor. The power factor is the cosin of the angle between current and voltage. The power factor rating is between 0 and 1. There is no unit of power factor. The better the power factor of a motor, the better the motor. Normal power factor for motor is closed to 0.88. Other parameter for a motor is its voltage rating. The voltage rating of a motor is mentioned on its name plate. There are single phase or three phase type motors. The three phase induction motors are motor efficient than single phase motor. The three phase motor are connected either Star or Delta. The big motors are first connected in star and then they are running in delta. This method is known as start delta starter or starting. There is a timer used in the star delta starter circuit. The function of timer is to start the motor in star and then run it in delta. The motors were tested in old days by maggar or megger. The meger was a high resistance measurement test equipment with a DC generator fitted inside that. The testing personnel was rotating the handle of the mager to a constant speed of 120rpm so it was producing a voltage of 500 volts DC inside which where appearing on the magger probes. The probes then were touched to the motor terminals and motor body under test. Then by this way we were testing motor winding and winding insulation. Sometimes in this method motors were breaking down or insulation breakdown was occurring. Now a days more safe ways like partial discharge testing or PD testing and PdMa type testing is common. These tests and non destructive type or NDT. This is not good that you are testing a motor and then making is defective during testing. The owner of the motor will sure hold your collar if you destroy his or her motor during testing. Motor are permanent grease lubricated or external lubricated type or oil lubricated type. The permanent lubricated type motors have anti friction bearings which are factory lubricated and then sealed using metal or plastic seals. They are supposed to run till the life of a motor. There is only sometimes need arises to replace the motor bearings and seals after two, four, five, six , eight or ten years. This depends upon many parameters. Some are speed of the motor, load on the motor and the surrounding of a motor. Induction motors are very reliable and very simple in construction. They have very less wearing out parts. As in the DC motor the story is changed. There is commutator, brushes and other parts inside. There is commutator reaction in DC motor. Whereas such things are not there in an AC induction motor. AC induction motor is an asynchronous motor. The induction motor has two types. First is squirrel cage induction motor and the other is the slip ring motor. The squirrel cage induction motor is very simple and have rotor bars inside. Whereas the slip ring motor has wound rotor and have slip rings on the shaft. There are carbon brush touching to the slip ring which is bringing out the rotor current to the outside starter. In some motors there is a slip ring shortening and brush lifting device installed. Such motor have long brush life and little wear on brushes and slip rings. The brushes and slip rings are only used during motor starting. The rest of the time the brush lifting device is short circuiting the slip rings.
A motor have two very common defects. There is mechanical defect in which bearing of a motor fails. There are many reasons for bearing failure. Lack of lubrication is the key reason of bearing failure. Bearings are the parts separating motor moving part rotor to the stater part. Bearings are made fraction free and oil is used to lubricate them. If they are well lubricated, they run without failure and damage may not occur to them. If they are operated without lubrication, they are destroyed. They lubrication should be provided to a motor at certain interval otherwise a motor may fail. There is sometimes special lubrication unit installed with motors having plan journal bearings. These units must be kept in good order to ensure proper lubrication to the motor. The other defect can be pitting in the bearing. Pitting is due to many reasons. Pitting can be dust and dirt entrance in the bearing or it can be due to electric current passed through a bearing. The dust can be entered due to defective seal. They current is passed in the bearing due to hysteresis currents produced in the motor core and then that current is passed through the bearings from the motor shaft. This phenomenon is common in variable frequency inverter driven motors. There other reason for the current pass through motor bearing is sometimes arc welding is performed close to motor which causes high current passage through the motor bearing. If current is passed through a bearing it produces pitting in the bearing races. There is insulated bearing used to prevent such a condition. Insulated bearings are not very cheap. They are expensive compared to normal bearings. The other cause of bearing failure is the material fatigue. The metal of the bearing fails with passage of time. So if become vital to replace those bearings. The bearing replacement of a motor is known as motor overhauling.
The other motor defect is its winding failure. The motor winding is carrying current and current produced heat when it is flowing in the motor winding. Also for the current flow voltage is applied. The voltage gets spikes some times the spikes are very high value transients for very short time. They break down the insulation provided to winding. So if the conductor of the motor cuts under high current or the insulation provided to winding breaks down under high voltage, the motor is known as burnt. In both cases the motor will not work and it may need a rewinding. In rewinding of a motor new conductors and insulation is provided. The rewinding of a motor is a full technical job and is carried out under the specialists supervision. If a motor is rewind in a proper way it may serve like a new motor again but if it is not rewind properly, a motor will not performed properly and it will be a defective motor and will not fulfill the function. The normal people who are rewinding a motor have such defects as they are not enough careful or they save small money during rewinding process. The result is a bad repair and the repaired motor then show different defects like overheating, more current consumption or less torque.
Here is an image of industrial motor.
Here is an image of motor alignment job after new or overhauled motor installed to its foundation.
Here is the youtube video covering all this detail in video.
0 Comments: