FUNDAMENTALS OF MOTOR TECHNOLOGY

1. FUNDAMENTALS OF MOTOR TECHNOLOGY

2. Electromechanical Energy Conversion • Electromechanical energy conversion device is a link between electrical and mechanical systems • When mechanical system delivers energy through the device to electrical system, device is called generator • When electrical system delivers energy through the device to mechanical system, device is called a motor

3. Basic principles of rotating electric machines • Electric machines utilize the concepts of electromagnetism • A magnetic field interacts with either electric field or mechanical force to produce the other • A current carrying conductor produces a magnetic field in its vicinity

4. Right-hand Rule • Hold the conductor in right hand with fingers closed around conductor and thumb pointing towards the direction of the current • The fingers will point towards the direction of the magnetic lines of the flux produced around the conductor

5. Cork Screw Rule • Direction and travel in which it has to be rotated are related to each other the same way as direction of current in a conductor and direction of the field produced due to current • Magnetic field exists in the plane perpendicular to the conductor

6. Magnetic field lines around a current carrying conductor

7. Flux produced by Current Carrying Coil • Flux can be produced by causing the current to flow through a coil instead of a conductor • The direction of the magnetic flux in the coil is given by the Right-hand rule

8. Force on a Conductor in a Magnetic Field If a conductor carries current in a magnetic field, then a mechanical force is exerted on it. The force exerted on the conductor is given as : F = B L I

10. Motoring Action • As voltage is applied to stationary conductors, magnetic field is produced • This magnetic field in turn induces voltage in the rotor conductors, in case of some motors (induction motors) or voltage is externally applied to the rotor conductors • This voltage also produces magnetic field • Magnetic field of stator and rotor, together, put the rotor in running condition

11. Depiction of a motoring action

12. Electric machine concepts • Electrical machines (either a motor or a generator) can be broadly classified as DC Machines and AC Machines – depending on the nature of supply given to them • An electrical machine can be interchangeably associated with either a motor or a generator • Fundamentals for a particular category (like DC or Synchronous) are identical for both the generator and motor

13. Revolving magnetic field • 3-phase AC power supply is connected to the stator terminals of an induction motor • 3-phase alternating currents flow in the stator windings • These currents set up a changing magnetic field (flux pattern) and rotates around the inside of the stator • The speed of rotation is in synchronism with the electric power frequency and is called the synchronous speed

14. Principles of operation • 3-phase AC Voltage connected to the Stator windings • Currents establish magnetic field (flux pattern) • Rotates around the inside of the stator • Rotation Speed in synchronism with the power frequency

15. Principles of operation • Flux distribution in a 4 pole motor at any one moment • Shows the 2-North and 2-South poles

16. Principles of operation • In its simplest form ... • 3-phase Stator windings connected to power supply • flux completes one rotation for every cycle of mains • On 50Hz, the stator flux rotates at 50 revs per second • Rotor turns at 50 x 60 = 3,000 revs per minute. • Called a 2 pole motor (2 poles 1-North, 1-South) • The design of the Stator windings can be changed to be suitable for 4-pole operation ... • Therefore rotates at half the speed ... 1,500 rev/min • Called a 4 pole motor (4 poles 2-North, 2-South)

17. Motor Armature • The rotational or speed e.m.f is produced in opposition to the applied voltage. This is known as counter or back e.m.f. • Mechanical torque is produced as required by the load driven by the motor. For more torque and mechanical power output, there must be more input to the motor from the mains. • The motor draws current according to the requirement of the load.

18. Idealized Machine • All rotating electrical machines have common features • There is a stationary member called a stator and a rotating member called a rotor

19. Types of Electrical Machines • An Induction Machine • A Synchronous Machine • A D.C. Machine

20. 3-Phase AC induction motor • AC Induction Motors are one of the most successful inventions …… consume > 50% of all electrical energy generated • They are very popular for Industrial Applications • Simplicity … easy to manufacture • Reliability … very little maintenance • Relatively low cost … more kW per Rupee • Work well even in a bad environment • Dust-proof • Water-proof • Can be used for Variable Speed Control • Speed proportional to frequency

21. Simple depiction of a motor

22. Basic construction • Two types of Rotor Construction • Wound Rotor type, which comprises 3 sets of windings with connections to 3 sliprings on the shaft • Squirrel Cage Rotor type, which comprises a set of copper or aluminium bars installed into the slots, which are connected to an end-ring at each end

23. Squirrel Cage Induction Motor • Most common type of AC motor • Known as workhorse of Modern Industry • Most cost effective motor • Can be designed for any kind of environment • Construction of rotor gives this name • Rotor consists of a series of conducting bars laid into slots carved in the face of the rotor and shorted at either end by large shorting rings

25. Assembly details of a typical AC induction motor

26. Assembly details of a typical AC induction motor • A Motor (whether DC or AC) comprises of 2 electromagnetic parts: • Stationary part called the Stator consists of the frame, which provides the physical support. • Rotating part called the Rotor, supported at each end on bearings.

27. Assembly details of a typical AC induction motor • The other parts, which are required to complete a motor are: • Two end-flanges to support the two bearings • Two Bearings to support the rotating shaft • Steel shaft for transmitting the torque to the load • Cooling fan located at the NDE to provide forced cooling for the stator and rotor • Terminal box on top or either side to receive the external electrical connections

28. Basic construction • Basic Design unchanged in over 50 years … but now have smaller physical size and lower cost per kW due to • Modern insulation materials • Computer based design optimisation techniques • Automated manufacturing methods • International standardisation ... physical dimensions • Both Stator and the Rotor are made up of : • Magnetic circuit … laminated grain oriented steel • Electric circuit … insulated copper or aluminium

29. Stator & Rotor

30. Stator and rotor laminations • The magnetic path of a motor comprises a set of slotted steel laminations • These silicon steel laminations are pressed into the cylindrical space inside the outer frame • The magnetic path is laminated to reduce eddy currents.... lower losses and lower heating • A set of insulated electrical windings, which are placed inside the slots of the laminated magnetic path • Rotor laminations are varied based on the type of torque characteristic to be realized

31. Simple view of squirrel cage induction rotor

32. Squirrel Cage Rotor

33. Stator core lamination The basic stator structure, called as core, is composed of steel laminations (or stampings) Silicon steel is used for making the laminations These are shaped in such a fashion as to form poles around which are wound the copper wire coils Set of coils put together and grouped into various patterns form a winding These primary windings connect to, and are energized by, the voltage source to produce a rotating magnetic field

34. Stator core lamination

35. Typical Stator of a Motor

36. Winding coils at the time of insertion into Stator

37. Single phase induction motor • This motor is used mostly in small sizes, where polyphase current is not available • Characteristics are not as good as the polyphase motor and for size larger that 10 HP, the line disturbance is likely to be objectionable • These motors are commonly used for light starting and for running loads up to 1/3 HP Capacitor and repulsion types provide greater torque and are built in sizes up to 10 HP. • Mostly finds application in domestic sector

38. Slip Ring Induction Motor • The motor starts with a full resistance bank • As speed of the motor increases, the resistances are shorted, one by one • As the motor reaches full speed, the whole bank of resistance is shorted out and the motor now runs alike a squirrel cage induction motor • Very much useful to develop high starting torque • Reduced starting current puts less burden on the power system

39. Slip Ring Motor • Wound Rotor type, comprises 3 sets of insulated windings • Connections brought out to 3 slip rings mounted on the shaft • The external connections to the rotating part are made via brushes onto the slip rings • Consequently, this type of motor is often referred to as a slip ring motor

40. Simple view of slip ring induction rotor

41. Simple view of a Slip Ring Rotor

42. Rotor core laminations

43. Synchronous motor • Synchronous motor is a constant speed motor • Can be used to correct the power factor of the 3-phase system. • Like the Induction motor in terms of the stator, the synchronous machine has either a permanent magnet arrangement or an electromagnet (with current supplied via slip rings) rotor. • In simple terms, the rotor will keep locking with the rotating magnetic field in the stator. • In many synchronous machines, a squirrel cage is in incorporated into the rotor for starting.

44. Constructional features of a Synchronous Motor

45. Speed of AC induction motor • AC Induction motors can be designed and manufactured with the number of stator windings to suit speed requirements • 2 pole motors .... stator flux rotates at 3,000 rev/min • 4 pole motors .... stator flux rotates at 1,500 rev/min • 6 pole motors .... stator flux rotates at 1,000 rev/min • 8 pole motors .... stator flux rotates at 750 rev/min etc • Speed of Stator Flux is called Synchronous Speed

46. Actual rotor speed • When Rotor Speed approaches synchronous speed ..... • Magnitude and frequency of rotor voltage becomes small • If rotor reached synchronous speed, the rotor windings would be moving at the same speed as the rotating flux • Induced voltage (and current) in the rotor would be zero • Without rotor current .... no rotor field and no Torque • To produce Torque ..... • Rotor must rotate at a slower (or faster) speed • So, the rotor settles at a speed less than rotating flux called the Slip Speed • The difference in actual speed to synchronous speed is called the Slip

47. Rotor slip • The slip will vary according to Load Torque • As load torque increases, the slip increases • More flux lines cut the rotor windings • Increases rotor current and magnetic field • Consequently increases rotor torque • Typical slip between 1% (no-load) to 6% (full-load) • Slip (in per unit) is given by : • Actual rotational speed is

48. Equivalent circuit of AC motor • Electrical circuit can be represented by an equivalent circuit • Sketch shows ... motor does not have separate field windings • Stator current therefore serves a double purpose .... • Carries Magnetising current for rotating magnetic field ... IM • Carries Rotor current that provides shaft torque .……….. IR

49. SIMPLIFIED EQUIVALENT CIRCUIT • Equivalent circuit simplified by taking out 'transformer' • adjusting XR and RR values by the turns ratio N = NS/NR ie. 'transferring' them to the stator side • So, must also adjust for frequency ... which depends on slip

50. More simplified circuit • Rotor Resistance is Variable .... • Rotor current IR …. depends primarily on the slip (s) • Magnetising Inductance is roughly Constant .... • Magnetising Current IM ...... depends on voltage (V)