We've updated our privacy policy. Click here to review the details. Tap here to review the details.

Successfully reported this slideshow.

Your SlideShare is downloading.
×

Activate your 30 day free trial to unlock unlimited reading.

Activate your 30 day free trial to continue reading.

Top clipped slide

1 of 25
Ad

- 1. Induction Motor Review By Mr.M.Kaliamoorthy Department of Electrical & Electronics Engineering PSNA College of Engineering and Technology 1
- 2. Outline Introduction Construction Concept Per-Phase Equivalent Circuit Power Flow Torque Equation T- Characteristics Starting and Braking References 2
- 3. Introduction Induction motors (IM) most widely used IM (particularly squirrel-cage type) compared to DC motors Rugged Lower maintenance More reliable Lower cost, weight, volume Higher efficiency Able to operate in dirty and explosive environments 3
- 4. Introduction • IM mainly used in applications requiring constant speed – Conventional speed control of IM expensive or highly inefficient • IM drives replacing DC drives in a number of variable speed applications due to – Improvement in power devices capabilities – Reduction in cost of power devices 4
- 5. Induction Motor – Construction Stator balanced 3-phase winding distributed winding – coils distributed in several slots produces a rotating magnetic field Rotor usually squirrel cage conductors shorted by end rings Rotating magnetic field induces voltages in the rotor Induced rotor voltages have same number of phases and poles as in stator winding 5 a b b’ c’ c a’ 120o 120o 120o
- 6. Induction Motor – Construction 6
- 7. Induction Motor – Concept • Stator supplied by balanced 3-phase AC source (frequency f Hz or rads/sec ) – field produced rotates at synchronous speed s rad/sec (1) where P = number of poles • Rotor rotates at speed m rad/sec (electrical speed r = (P/2) m) • Slip speed, sl – relative speed (2) between rotating field and rotor • Slip, s – ratio between slip speed and synchronous speed (3) 7 f P P s 4 2 f P ns 120 m s sl s m s s
- 8. Induction Motor – Concept • Relative speed between stator rotating field and rotor induces: – emf in stator winding (known as back emf), E1 – emf in rotor winding, Er • Frequency of rotor voltages and currents: (4) • Torque produced due to interaction between induced rotor currents and stator field • Stator voltage equation: • Rotor voltage equation: 8 1 2 E I L πf j I R V s ls s s s sf fr r lr r r r r lr r r r I L πf j I s R E I L πf js I R sE 2 2
- 9. Induction Motor – Concept • E1 and Er related by turns ratio aeff • Rotor parameters can be referred to the stator side : 9 r eff r r eff r eff r r r eff L a L R a R a I I E a E 2 ' 2 ' ' 1 Rr/s + Vs – Rs Lls Llr + E1 – Is Ir Im Lm + Er –
- 10. Induction Motor – Per Phase Equivalent Circuit • Rs – stator winding resistance • Rr’ – referred rotor winding resistance • Lls – stator leakage inductance • Llr’ – referred rotor leakage inductance • Lm – mutual inductance • Ir’ –referred rotor current 10 Rr’/s + Vs – Rs Lls Llr’ + E1 – Is Ir’ Im Lm
- 11. Induction Motor – Power Flow 11 cos 3 L T in I V P m L out T P Stator Copper Loss (SCL) s s SCL R I P 2 3 Rotor Copper Loss (RCL) ' 2 ' 3 r r RCL R I P Airgap Power Pag Converted Power Pconv Rotational losses Prot (Friction and windage, core and stray losses) s R I P r r ag ' 2 ' 3 s s R I P r r conv 1 3 ' 2 ' Electrical Power Mechanical Power ag RCL sP P Note: ag conv P s P 1
- 12. Induction Motor – Torque Equation • Motor induced torque is related to converted power by: (5) • Since and , hence (6) • Substituting for Ir’ from the equivalent circuit: (7) 12 ag conv P s P 1 s r s 1 2 2 ' 2 ' 3 lr ls r s s s r e X X s R R V s R T m conv e P T s r r s ag e s R I P T ' 2 ' 3
- 13. Induction Motor – T- Characteristic • T- characteristic of IM during generating, motoring and braking 13
- 14. Induction Motor – T- Characteristic Maximum torque or pullout torque occurs when slip is: (8) The pullout torque can be calculated using: (9) 14 2 2 2 max 2 3 lr ls s s s s X X R R V T 2 2 ' max lr ls s r X X R R s r s Trated Pull out Torque (Tmax) Te 0 rated smax s 1 0
- 15. Induction Motor – T- Characteristic Linear region of operation (small s) Te s High efficiency Pout = Pconv – Prot Pconv = (1- s )Pag Stable motor operation 15 r s Trated Pull out Torque (Tmax) Te 0 rated smax s 1 0
- 16. Induction Motor – NEMA Classification of IM NEMA = National Electrical Manufacturers Association Classification based on T- characteristics Class A & B – general purpose Class C – higher Tstart (eg: driving compressor pumps) Class D – provide high Tstart and wide stable speed range but low efficiency 16 s
- 17. Induction Motor – Starting • Small motors can be started ‘direct-on-line’ • Large motors require assisted starting • Starting arrangement chosen based on: – Load requirements – Nature of supply (weak or stiff) • Some features of starting mechanism: – Motor Tstart must overcome friction, load torque and inertia of motor- load system within a prescribed time limit – Istart magnitude ( 5-7 times I rated) must not cause • machine overheating • Dip in source voltage beyond permissible value 17
- 18. Induction Motor – Starting • Methods for starting: – Stat-delta starter – Autotransformer starter – Reactor starter – Soft Start 18
- 19. Induction Motor – Starting • Star-delta starter – Special switch used – Starting: connect as ‘star’ (Y) • Stator voltages and currents reduced by 1/√3 • Te VT 2 Te reduced by 1/3 – When reach steady state speed • Operate with ‘delta’ ( ) connection – Switch controlled manually or automatically 19
- 20. Induction Motor – Starting • Autotransformer starter – Controlled using time relays – Autotransformer turns ratio aT • Stator voltages and currents reduced by aT • Te VT 2 Te reduced by aT 2 – Starting: contacts 1 & 2 closed – After preset time (full speed reached): • Contact 2 opened • Contact 3 closed • Then open contact 1 20
- 21. Induction Motor – Starting • Reactor starter – Series impedance (reactor) added between power line and motor – Limits starting current – When full speed reached, reactors shorted out in stages 21
- 22. Induction Motor – Starting • Soft Start – For applications which require stepless control of Tstart – Semiconductor power switches (e.g. thyristor voltage controller scheme) employed • Part of voltage waveform applied • Distorted voltage and current waveforms (creates harmonics) – When full speed reached, motor connected directly to line 22
- 23. Induction Motor – Braking • Regenerative Braking: – Motor supplies power back to line • Provided enough loads connected to line to absorb power – Normal IM equations can be used, except s is negative – Only possible for > s when fed from fixed frequency source • Plugging: – Occurs when phase sequence of supply voltage reversed • by interchanging any two supply leads – Magnetic field rotation reverses s > 1 – Developed torque tries to rotate motor in opposite direction – If only stopping is required, disconnect motor from line when = 0 – Can cause thermal damage to motor (large power dissipation in rotor) 23
- 24. Induction Motor – Braking • Dynamic Braking: – Step-down transformer and rectifier provides dc supply – Normal: contacts 1 closed, 2 & 3 opened – During braking: Contacts 1 opened, contacts 2 & 3 closed – Two motor phases connected to dc supply - produces stationary field – Rotor voltages induced – Energy dissipated in rotor resistance – dynamic braking 24
- 25. References • Chapman, S. J., Electric Machinery Fundamentals, McGraw Hill, New York, 2005. • Rashid, M.H, Power Electronics: Circuit, Devices and Applictions, 3rd ed., Pearson, New-Jersey, 2004. • Trzynadlowski, Andrzej M. , Control of Induction Motors, Academic Press, 2001. • Nik Idris, N. R., Short Course Notes on Electrical Drives, UNITEN/UTM, 2008. • Ahmad Azli, N., Short Course Notes on Electrical Drives, UNITEN/UTM, 2008. 25

No public clipboards found for this slide

You just clipped your first slide!

Clipping is a handy way to collect important slides you want to go back to later. Now customize the name of a clipboard to store your clips.Hate ads?

Enjoy access to millions of presentations, documents, ebooks, audiobooks, magazines, and more **ad-free.**

The SlideShare family just got bigger. Enjoy access to millions of ebooks, audiobooks, magazines, and more from Scribd.

Cancel anytime.
Be the first to like this

Total views

8

On SlideShare

0

From Embeds

0

Number of Embeds

1

Unlimited Reading

Learn faster and smarter from top experts

Unlimited Downloading

Download to take your learnings offline and on the go

You also get free access to Scribd!

Instant access to millions of ebooks, audiobooks, magazines, podcasts and more.

Read and listen offline with any device.

Free access to premium services like Tuneln, Mubi and more.

We’ve updated our privacy policy so that we are compliant with changing global privacy regulations and to provide you with insight into the limited ways in which we use your data.

You can read the details below. By accepting, you agree to the updated privacy policy.

Thank you!

We've encountered a problem, please try again.