Three-Phase AC machines

1. Three-Phase AC machines Resource 1 Introduction to Motors and Generators

2. Introduction to Motors and Generators Three-Phase AC Machines Resource 1 Aims • To provide an understanding of the motor and generator effect that links electricity to magnetism • To provide an understanding of how to apply Fleming’s left and right hand rules.

3. Introduction to Motors and Generators Three-Phase AC Machines Resource 1 Objectives At the end of this lesson you should be able to: • Describe the effects of placing a current carrying conductor in a magnetic field • Perform simple calculations for the force on a conductor in a magnetic field • Apply Fleming’s Left Hand Motor rule • Describe the effects of moving a conductor through a magnetic field • Perform simple calculations for the induced EMF across a conductor moving through a magnetic field • Apply Fleming’s Right Hand Generator Rule • Describe the effects of passing a current through a coil of wire to form an electromagnet

4. North pole Force South pole The Motor Effect F=BIL[Newtons] B= Density of the magnetic flux in Teslas I= Induced current in Amps L L= Length of conductor in field in metres F Example 1 If a conductor of length 0.4m carrying a current of 10.6A is placed in a magnetic field with a flux density of 0.03T, determine the force experienced by this conductor in newtons. B I F=0.03x10.6x0.4 =0.1272N

5. North pole Force L South pole The Motor Effect Fleming’s Left Hand Rule Each digit of your hand must be at right angles to both of the other two thumb motion second finger current firstfinger B field I If thecurrentis reversed, the direction of motion will change

6. North pole second finger current thumb motion first finger Force field South pole The Motor Effect Fleming’s Left Hand Rule Each digit of your hand must be at right angles to both of the other two F B I If thecurrentis reversed, the direction of motion will change

7. North pole second finger current thumb motion first finger field South pole The Motor Effect Fleming’s Left Hand Rule Each digit of your hand must be at right angles to both of the other two F Force B I If thefieldis reversed, the motion will be in the opposite direction

8. North pole Force South pole The Motor Effect Fleming’s Left Hand Rule Each digit of your hand must be at right angles to both of the other two secondfinger field F firstfinger current thumb B motion I If thefieldis reversed, the motion will be in the opposite direction

9. The Motor Effect Using the following convention, we can show why Fleming’s left hand rule works Currentinto page Currentout of page fieldis clockwise fieldis anticlockwise

10. North Pole North Pole South Pole South Pole The Motor Effect Field lines in the same direction cause repulsion, field lines in opposite directions cause attraction Force attraction repulsion Force repulsion attraction

11. North Pole South Pole The Motor Effect The force on a conductor can be increased by forming a single turn coil Blue spot represents the central pivot point

12. North Pole South Pole The Motor Effect The force on a conductor can be increased by forming a single turn coil Top conductor experiences force to left Force

13. North Pole Force South Pole The Motor Effect The force on a conductor can be increased by forming a single turn coil Top conductor experiences force to left Force Bottom conductor experiences force to right

14. North Pole Force South Pole The Motor Effect The force on a conductor can be increased by forming a single turn coil Top conductor experiences force to left Force Bottom conductor experiences force to right Combined action causes rotation

15. North Pole T T South Pole The Motor Effect Forces add up to a rotational force called Torque (T) in Newtons per metre

16. North Pole T T South Pole The Motor Effect For a multi-turn coil Torque produced T= 2nFr n= number of coil turns F= force on single conductor r= radius of coil

17. North Pole T T South Pole The Motor Effect For a multi-turn coil Torque produced T= 2nFr Example 2 A 100 turn coil has a radius of 0.1m and a length of 0.15m. It is placed at right angles in a magnetic field of flux density 0.08T and carries 12A, calculate the force on each conductor and the total torque produced by the coil. F=BIL=0.08x12x0.15 =0.144N T=2nFr= 2 x100x0.144x0.1 =2.88Nm

18. North pole South pole The Generator Effect e=BLv[Volts] B= Density of the magnetic flux in Teslas L= Length of conductor in field in metres B - v= velocity in metres per second e + Example 3 Calculate the EMF induced across the ends of a wire of length 0.3m when it is moved through a magnetic field of flux density 0.015T at a speed of 50m/s.. Velocity L I v e=0.015x0.3x50 =0.225Volts

19. Fleming’s Right Hand Rule North pole Each digit of your hand must be at right angles to both of the other two second finger thumb current motion first finger field South pole Velocity The Generator Effect B - e + I L v If themotionis reversed, the polarity of EMF will change and the current will be reversed

20. North pole South pole Velocity The Generator Effect Fleming’s Right Hand Rule Each digit of your hand must be at right angles to both of the other two B secondfinger + thumb e motion current - L v first finger field I If themotionis reversed, the polarity of EMF will change and the current will be reversed

21. North pole South pole Velocity The Generator Effect Fleming’s Right Hand Rule Each digit of your hand must be at right angles to both of the other two B secondfinger + thumb e motion current - L v first finger field I If thefieldis reversed, the polarity of EMF will change again and the current will be reversed again

22. North pole South pole Velocity Fleming’s Right Hand Rule The Generator Effect Each digit of your hand must be at right angles to both of the other two B - first finger e thumb field motion + L second finger current I If the field is reversed, the polarity of EMF will change again and the current will be reversed again

23. North Pole South Pole The Generator Effect An EMF can be generated in a rotational motion by forming a coil Motion Motion EMF generated in both sides of the coil add up

24. North Pole South Pole The Generator Effect An EMF can be generated in a rotational motion by forming a coil Linear velocity v of each conductor can be worked out from the rotational speed N and the radius r v v = 2 π rN m/s 60 The total EMF E of a coil having n turns moving at right angles to a magnetic field is as follows v E = 2 ne Volts

25. An EMF can be generated in a rotational motion by forming a coil Example 4 A 200 turn coil has a radius of 0.12m and a length of 0.23m. It is placed in a magnetic field of flux density 0.06T and rotated at 3000rpm. When the coil is in its vertical position at right angles to the field, calculate (a) the EMF on each conductor (b) the total EMF produced by the coil. The Generator Effect =37.7m/s v=2 π x 0.12x 3000 60 e=0.06 x 0.23x37.7 e=BLv Volts E= 2 x200x0.52 v= 2 πrNm/s 60 =0.52Volts E= 2neVolts =208.1Volts

26. Electromagnetism When a coil is formed of many wire turns, the magnetic fields around each wire add up to produce a strong electromagnet. One side of this magnet will be a North Pole while the other side will be a South Pole If the current in the electromagnet is reversed, the magnetic poles will swap sides.

27. Electromagnetism If the coil is wrapped around a soft iron core, the electromagnetic field becomes much stronger. Electromagnets are used in motors and generators so that the strength of the field can be varied. In a motor, this affects the speed and torque produced. In a generator, it affects the voltage generated.

28. Further Study – Types of motor DC motors Series Field Shunt Field Compound Field AC induction Squirrel Cage Slip ring – wound rotor AC synchronous Salient Pole Cylindrical

29. Further Study - DC Motor Performance Shunt Field Series Field Compound Field Speed Torque

30. Synchronous Cage Induction Wound induction Further Study - AC Motor Performance Speed Speed