EEE1012 Introduction to Electrical & Electronics Engineering Chapter 9: Introduction to Electric Machines by Muhazam Mustapha, October 2010

Learning Outcome Be able to theoretically explain the various types of electric motors Be able to theoretically explain the various types of electric generators Be able to mathematically solve some parameters of DC motors By the end of this chapter students are expected to:

Chapter Content Electric Machines in General DC Machines Synchronous Machines Induction Machines

Electric Machines

Rotating Machines Electromechanical machines are commonly rotational in nature The machines require one to be static and the other one to be rotating –Stator: stationary –Rotor: rotating Both stator and rotor produce magnetic winding whose field will try to align each other – this produces mechanical motion

Rotor and Stator × · · × Rotor Stator Rotor winding Stator winding Current going in Current coming out Stator Field Rotor Field

Commutator Action Commutator reverses current in coil every half cycle There can be more than 1 pair of commutators

Windings Two types of magnetic windings: –Armature: the winding connects to load –Field: the winding only to produce field Either armature or field winding can be located as rotor or stator The location of field and armature determines the type of the machine

Machine Types (Generator & Motor) Type Winding TypeLocationCurrent Type DC ArmatureRotor DC FieldStator DC Synchronous ArmatureStator AC FieldRotor DC Induction PrimaryStator AC SecondaryRotor AC

DC Machines

DC Machines are hard to construct, but easiest to discuss and analyze Hence all our mathematical discussion on machines will be on DC machines Other machine type will be covered as theory

Configurations Separately Excited LaLa RaRa VaVa VfVf RfRf LfLf IaIa IfIf DC Machines can be constructed in a few configurations depending on series or parallel structure or the availability of a second power source

Configurations Shunt Connected LaLa RaRa VaVa VfVf RfRf LfLf IaIa IfIf Series Connected LaLa RaRa VaVa VfVf LfLf

Configurations Short-Shunt Compound LaLa RaRa VaVa IaIa Shunt Winding Long-Shunt Compound LaLa RaRa VaVa Series Winding IaIa Shunt Winding Series Winding

Steady State Equations LaLa RaRa V L or V s IaIa E b, ω m LfLf RxRx RfRf IfIf Referring to the following DC machine model, we can deduce some formulas for motor and generator at constant speed RSRS LSLS IsIs

Steady State Equations Generator

Steady State Equations Motor

Machine Constant The armature constant of k a p = number of magnetic poles N = number of conductors per coil M = number of parallel paths in armature winding

Conversions n = round per minute, r/min ω m = radian per second, rad/s 1 horse power = 746 watts

Synchronous Machines

Alternator Just another word for AC generator Normally a permanent magnet or a DC powered electromagnet will be placed at rotor to generate AC current Stator would be wound with solenoid that carries the generated energy – there can be more than one windings hence it can generate more than 1 phase of electricity

Alternator · × × × · · N S Three Phase Single Phase Coils at stator

Synchronous Motor Virtually identical to alternator Needs a DC voltage exciter at rotor to start Called synchronous because it spins at the same rate as the AC frequency used to drive it

Induction Machines

Induction Motor The stator part is almost identical to synchronous motor AC current (single or multi-phase) will be fed into stator – produces spinning field There is no power or permanent magnet placed in the rotor Rotor and stator are electrically separated Then how mechanical force is applied to the rotor?

Induction Motor Mechanical motion is possible by the induction process that is identical to the one in transformer The changes in the magnetic flux from stator will induce current into the rotor winding and causes magnetic attraction or repel between stator and rotor poles

Induction Motor The changes of the magnetic field need to involve the cutting of the rotor coils (Faraday’s Law) This cutting is what called ‘slip’ between the rate of stator’s field rotation and the rate of rotor’s spin Without the slip induction machine couldn’t work

Induction Motor The ‘slippings’ also means that the rotation of rotor is not in-sync with the stator field rotation rate This is the main electrical difference between synchronous machine and induction machine

Induction Generator Makes use of the same induction concept in induction motor – slipping process It requires a starting power at rotor to produce magnetic field for the induction process to start After that, the power generated by the generator itself will be used to produce the needed rotor magnetic field