Magnetic Circuits + Electromagnetic induction (3, 4)

What is the difference between magnetising force and magnetomotive force? (A, At/m, m) (At/m, A, m)

Magnetomotive force = current * coil windings amount (Fm=IN).

Magnetising force = magnetomotive force / length of magnetic circuit (H=IN/l)

Magnetising force = magnetomotive force / length of magnetic circuit (H=IN/l)

Difference between total flux and flux density?

Total flux (webers) is total flux. Flux density (teslas) is how tightly packed lines of flux are. Teslas equivalent to webers per square metre

How is flux density calculated? (T, Wb, m2)

Flux density = total flux / area of magnetic circuit (B=Φ/A)

What is permeability?

A material's ability to allow passage of magnetic flux

What is relative permeability?

The permeability of a material relative to free space

How is permeability calculated?

actual permeability = permeability of free space (12.57*10-7) * relative permeability (μ=μ0*μr

What is magnetic reluctance?

A material opposing the formation of magnetic flux (resisting being magnetised), determines amount of magnetising force required to create given amount of flux

How does the length of a magnetic circuit affect its reluctance?

(Like electrical wire resistance) reluctance is proportional to length

How does cross-sectional area of magnetic circuit affect reluctance?

(Like electrical wire resistance) reluctance is inversely proportional to cross-sectional area (reduces when larger)

How does permeability affect reluctance?

reluctance is inversely proportional to permeability (reduces when more permeable)

How is reluctance of a material calculated?

Reluctance in At/Wb = length of magnetic circuit / (permeability of free space * relative permeability * cross-sectional area of magnetic circuit) (Rm = l / (μ0μrA))

Compare Ohms Law to Hopkinson's Law

Ohms Law = V=IR, Hopkinsons Law = mmf = Φ*Rm. Very similar, except one's for electrical circuits and one's for magnetic circuits

How do you calculate total flux? (Wb, A, At/Wb)

Total flux = (Current * Number of turns) / reluctance (Φ=IN/Rm)

How do you calculate total reluctance in a magnetic circuit with an air gap? (At/Wb)

Total reluctance = reluctance of core + reluctance of air gap

What are the names of the two magnetic quantities shown on a hysteresis loop?

B = Flux Density, H = Magnetising Force

What does it mean when a magnetic material is saturated?

When the flux density has almost reached its maximum for the material. Attempting to apply more mmf will have negligible effects (like capacitor)

What are the saturation regions of a hysteresis loop?

Pre-saturation: from start where magnetising force increases in near-linear fashion

Saturation region: where curve turns to form "knee", increase in flux density starts having negligible effects

Post-saturation region: negligible increase in saturation from higher mmf

What happens to permeability of a magnetic material as flux density increases?

Permeability effectively increases at high flux densities

What is coercive force?

Magnetising force required to reduce flux to zero

What axis on a hysteresis loop is for coercivity?

X axis

What is remanence?

(residual magnetism) A measure of the remaining magnetisation when driving field is zero

What is the relationship between area of a hysteresis loop and energy loss due to hysteresis?

Greater area in loop means more energy required to magnetise material in opposite direction

For a transformer on AC with flux direction being reversed every 20ms, what size hysteresis loop area is better?

Small, less energy to switch

For a material that needs to stay magnetised after magnetising force has been removed, what size hysteresis loop would be better?

Large, needs to retain more magnetising force

What is hysteresis/iron loss?

Energy lost due to the oscillation of magnetic saturation on a hysteresis loop, larger area means more loss

Magnetic circuits on AC have voltages and therefore unwanted currents induced in the iron circuit called eddy currents, which heat up the iron core. How can this be reduced?

Laminating the iron core to produce electrical resistance

What is magnetic leakage?

Useless magnetic force leaking into alternate paths from intended design

What is magnetic fringing?

Flux that spreads out and becomes less compact when passing through high reluctance air gaps

How does hysteresis contribute to energy losses in magnetic circuit?

Hysteresis is energy required to overcome residual magnetism in an iron circuit, creating heat loss from constantly realigning iron molecules

How does magnetic leakage contribute to energy losses in a magnetic circuit?

Magnetic leakage causing stray eddy current losses in surrounding materials

What are eddy currents?

Currents induced in the core material by alternating flux which can generate waste heat energy

With pickup and dropout voltages of a relay, why is dropout voltage lower than pickup voltage?

Dropout voltage is required to de-energise the relay, but is lower because an energised circuit has lower reluctance (easier to keep energised than to initially energise)

(Faraday's Law) What are 3 things required for electromagnetic induction to occur?

Conductor, magnetic field, relative motion between the two

(Faraday's Law) what is the relationship between the number of turns being cut by a changing magnetic field and the value of EMF produced?

EMF produced is proportional to number of turns being cut (more turns cut, more emf produced)

(Faraday's Law) What is the relationship between change in value of flux and value of EMF produced?

EMF produced is proportional to change in flux (more flux change, more EMF)

(Faraday's Law) What is the relationship between rate turns are cut by changing flux and value of EMF produced?

EMF produced proportional to rate turns are cut by changing flux (more cuts, more EMF)

How to calculate induced EMF via Faraday's Law? (V, Wb, s)

Induced EMF in volts = number of conductors/turns * (flux change in webers / time change in seconds) (E=N(ΔΦ/Δt))

What three parameters determine magnitude of a generated EMF in a magnetic field?

Flux density of magnetic field, length of conductor cutting through magnetic field, velocity of conductor cutting relative to magnetic field

What is the relationship between flux density of the field and value of EMF produced?

Higher flux density, more lines of flux cut (higher flux density = more EMF)

What is the relationship between length of conductor and value of EMF?

Longer conductor, more EMF

What is the relationship between velocity of relative motion and EMF produced?

Relative velocity between conductor/turns and field is proportional to EMF produced (higher velocity, greater EMF)

How many lines of force are cut in each pic and how do they affect the voltage generated?

In first diagram, conductor cuts 6 lines of force, and the maximum amount of voltage is generated. In the second, 4 lines cut, which creates less voltage (sin part of e=Blvsin(θ)), third cuts no lines of force so no voltage generated

How do you calculate EMF generated?

EMF generated can be calculated using the formula e = B l v sin(θ), where B is the magnetic field strength, l is the length of the conductor, v is the velocity, and θ is the angle between the field and the conductor.

Using Fleming’s Right Hand Rule for Generators, what direction are the magnetic dipoles?

Using Fleming’s Right Hand Rule for Generators, what direction is the motion?

Using Fleming’s Right Hand Rule for Generators, is the current running into the page or out?

What is relationship between current through the conductor and the magnetic field around the conductor? Also how does changing amount of current affect magnetic field?

As current increases, conductor’s magnetic field’s range increases. This flux cuts through conductor material inducing emf in conductor. Using Flemings’s right hand rule, induced emf opposes change in current

Using right-hand grip rule and fleming’s right hand rule, show how the magnetic field is distorted by the field around the conductor and what force is exerted on the conductor

When conductor is moved down through the field, induced EMF produces flux opposing downward force (drives conductor upwards)

How is force on a conductor due to Lenz’s Law calculated?

How does an AC current clamp meter work?

Changing conductor flux cuts clamp conductors, inducing EMF measured by meter

How does an AC transformer work?

AC Current supplied to primary winding, changing flux induces EMF in second winding, EMF generates changing magnetic field in secondary winding, generating current