Elec467 Power Machines & Transformers

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# Elec467 Power Machines & Transformers - PowerPoint PPT Presentation

Elec467 Power Machines &amp; Transformers . Electric Machines by Hubert, Chapter 1 Topics: M agnetics, electromagnetic forces, generated voltage and energy conversion. Magnetic Fields.

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### Elec467 Power Machines & Transformers

Electric Machines by Hubert, Chapter 1

Topics: Magnetics, electromagnetic forces, generated voltage and energy conversion

Magnetic Fields

Right hand rule: current generated fields have the flux in the same direction as your fingers with the thumb of your right-hand pointed in the direction of current flow. Current flow is for conventions current flow with the positive source supplying the current.

Group fields are the combined effect of individual wires with the flux lines emanating from the north pole and entering the south pole.

Magnetic Circuits

l

(a) Transformer action captures magnetic flux within the circular path of the metallic core over length l passing totally within the cross-sectional area A.

(b) Application of transformer application combined with motor action.

Magnetic formulas

Magnetomotive force (mmf)

F = N*IN is # of turns, I is current in amps. units: ampere-turns

Magnetic Field Intensity

H = F/l lis magnetic length in meters. units: A-t/m

Magnetic FluxReluctance

Φ=F /Rin Webers R =l/µA in A-t/Wb

Flux DensityPermeability

B = Φ/A in teslas µ = B/H in Wb/A-t m

A is square metersµ = µr µo µo= 4π·10-7 Wb/A-t m

Labeling realms

Saturation occurs when all the magnetic domains in a material are aligned together. Since you are changing the material on the molecular level by reorienting the domains, this absorbs energy heating up the material.

Magnetization & the relative permeability curves

The magnetization is H, the magnetic field intensity…given that N and l are constant for any one electromagnet, it’s directly related to the current flow.

Hystersis occurs when a material is driven by an AC current

b-O is the residual magnetism

c-O is the coercive

force

needed to remove the residual magnetism

Hysteresis Power Loss calculation: Ph = kh·f·Bnmax

Ph is Hysteresis Power loss Kn is a constant

f is frequency n is Steinmetz exponent

Bmax is maximum value of the flux density wave in Teslas

Flux bunching

Flux polarity rule: flux flowing in the same direction are the same polarity and repel each other, flux flowing in opposite direction are of opposite polarity and attract each other.

Application of flux bunching: motor action

Counterclockwise rotational movement is generated from flux bunching with the application of current in the rotor winding.

Calculating the mechanical force: BlI

Fmechanical= B · leff · I (Newtons) where leff is l·sinα or l·cosβ

B is flux density and I is current

Torque using a simplified diagram

Distance d is the moment arm measured from the center of the shaft to the center of the conductor.

The force F is the mechanical force calculated using the BLI formula from the previous slide.

TORQUE (TD) is the product of the mechanical force times the radius d (measured in meters) times the number of conductors: TD = 2 · Fmech · d (N·m)

Generator Action

Lenz’s Law is a special case of Newton’s 3rd law of physics: for every action there is an equal and opposite reaction. In this case the action is the movement of conductor X to the right thru the flux. The movement in this direction if caused by motor action would have current flowing from X to X’ and flux bunching occurring on the left side of the conductor. Assume there is zero current flow at the start. Since the movement source was mechanical, the reaction of the conductor is opposite and seeks to oppose the movement to the right. The conductor’s flux field appears as seen in (b) and the current is drawn from the conductor when previously there was none. The load the mechanical source (aka prime mover) feels is the opposing force created by the movement.

Generator Action
• Speed voltage and BLV rule follow Faraday’s Law: the voltage induced is proportional to the number of turns and rate of change of flux.
• The direction that the voltage is induced is in a direction to oppose the action that caused it according to Lenz’s Law.
• The potential voltage e induced by a conductor moving through a magnetic field is:

e = B leffv

where B is flux density in Teslas and v is the velocity of the conductor in meters/second.

Sinusoidal current flow from motion

When a closed coil as seen to the left in a magnetic field is driven continuously, the current is created by generator action and for every 360º of rotation, the current changes direction twice. The flux field seen by the conductors varies in a sinusoidal fashion from zero when moving horizontally to maximum Φ.

Φ= Φmax sin(wt)

The rate of rotation is w in radians

The voltage created is

Emax = 2πf NΦmax

and is a sinusoidal waveform.

Erms = 4.44f NΦmax

Flux power losses

While a coil generates flux flowing through an iron core, it also creates circular currents flowing in the material in the same pattern as the winding. These are called eddy currents. Naturally, a flowing current uses power. A solution to reduce eddy losses is to used insulated sheet metal for the core.

This power loss is calculated using

Pe = kef2B2max (Watts/unit mass)

ke = constant

f = frequency of flux wave in Hertz

Bmax= maximum value of flux density wave in Teslas

Force multiplier: multiple poles

With multiple poles additional cycles are created by generator action provided the windings are properly positioned. The frequency created in one rotation is ½ the number of poles.

cycles = Pn/2

Multiply this value by the number of rotations in a second to get the frequency generated in Hertz.

The position of the poles around the rotor are known as space degrees. The cycles are known as electrical degrees and can exceed 360º.