Mechanical characteristics of electric motor

mechanical features of electric drives
noise
bearings
transmission

1. Mechanical characteristics of electric motor

2. introduction While selecting a motor for a particular drive, the mechanical characteristics are also taken into account. The following features determine the suitability of the motor.   Types of enclosures Bearings Noise Transmission of drive

3. Types of enclosures

4. All the major parts of the motors such as windings, bearings, insulation etc are to be protected from the surroundings contaminated air. In an industry the air surrounding the motors may contain metal, dust, oil, mist, water, dust inflammable fumes etc. also accidents may occur to persons coming in contact with the moving parts. Therefore it is necessary to provide proper enclosures. The different types of enclosures are as follows.

5. Open type

6. This type can only be used where the atmosphere and surroundings are free from all contaminations and surrounding air completely dry. The advantage of this type of motor is that the cost of cooling is very low. But this type is rarely used since there is no protection to the motor parts.

7. Screen protected type

8. In this type of machines openings provided for ventilation are covered with wire mesh screen. This type of enclosures does not protect the motor against dirt and dust. But larger bodies and big insects cannot enter into the machine.

9. Drip proof type

10. This motor has ventilating opening provided in such a way that drops of liquid or solid falling on it vertically are prevented to enter inside. This type of motor cannot be used where inflammable dust particles are present in the surrounding air. Such motors are used in damp atmosphere. E.g Pumpsets .

11. Totally enclosed type

12. This type of motors has solid frames and end shields but no opening for ventilation. They are cooled by surface radiation only. In this type machines no dirt or foreign matter can enter and block the air passage. These machines are used for very dusty atmosphere. E.g saw mills, coal handling plants and stone crushing quarries.

13. Splash proof type

14. In this type, the ventilation ducts are provided in such a way that drops of liquid or solid particles reaching the machine at any angle between vertical and 100˚ from it cannot enter the machine.

15. Flame proof type

16. Hence these motors are used in coal mines, gas plants, oil refineries etc., where the risk of fire is more These enclosures do not communicate an internal fire to the external environment.

17. Pipe ventilated type

18. Large sizes of totally enclosed motor employ pipe ventilation. Air is drawn through pipe from outside the building, where clean air is available and forced to cool the motor.

19. Bearings

20. Bearings are the parts of machines which house and support the main shaft. It provides free rotation of the moving parts with minimum friction. There are two types of bearings usually employed in motors. Ball or Roller bearing Sleeve or brush bearing

21. Ball or Roller bearing

22. Ball or roller bearing consist of an inner and outer race and cage containing steel roller or balls. The outer race is attached to the housing(end cover) and the inner race is attached to the shaft. When the shaft rotates, the steel ball also rotates. Hence the friction of the shaft is minimized. It has a longer life and maintenance costs are low. It occupy less space. But the initial cost of ball and roller bearings is high. It is used in three phase induction motor where smaller air gap is possible. It is used for chain, belt and gear drives.

23. Sleeve or brush bearing

24. Sleeve or brush bearings are normally made of bronze. The rotating shaft is supported by bearing component and is rigidly fixed to the frame of the machine. It has self lubricating properties due to capillary action. It is lubricated by a metal ring freely rotating on the shaft carrying oil to the bearings. It is mainly used in direct coupled drive such as fan and universal motor. It gives noiseless operation and their life is long. Because of larger wear of bearings, this type of bearing is used in larger air gap induction motor.

25. Transmission of drives

26. Various methods employed for transmission of mechanical power are described below. Direct drive Belt drive Rope drive Chain drive and Gear drive

27. Direct drive

28. In direct drive, motor is coupled directly to the driven machine with the help of solid or flexible coupling. Flexible coupling protects the motor from jerks. It is more efficient and requires minimum space and it is the simplest method. It can be used where driven and driving machine speed are same.

29. Belt drive

30. In belt drive, belt is used to transmit the power from motor to driven machine through pulley system. The mechanical power wasted due to slip is about 3 to 4 percentage. Maximum power of 300 H.P can be transmitted through this drive.

31. Advantages Greater flexibility in the original design of a plant is possible. It gives convenient speed ratio thereby high speed motors can be utilized. The tendency of slipping especially under heavy loads is reduced because it will absorb a portion of the shock of suddenly applied loads.

32. Rope drive

33. This method for transmission for power is used, when it is not possible to employ belt drive. A number of ropes run in V-grooves over pulleys. The advantages of rope drive are negligible slip and ability of taking sudden loads. It is mainly used in lift and cranes.

34. Chain drive

35. Chain drive is very costly in comparison to belt and rope drive. It can be used for high speed ratio ( upto 6:1). It is more efficient and transmits large amount of power. It is noiseless, sliplesss and smooth in operation

36. Gear drive

37. Gear drive is used when high speed motor is to drive a low speed machine. The coupling between the two is through a suitable ratio gear box.

38. Noise Noise is the another important features to be considered while making the selection of a motor. It should be kept as low as possible in the workshops, hospitals and other domestic purposes. The noise may be due to bearing, vibrations, magnetic pulsations and faulty foundations. To reduce noise, journal bearing may be used in place of ball bearings. The motor should be mounted on a heavy concrete or cast iron block. The electrical connections should be made through flexible conduits.

39. The end