Electrical machines

What is the unit of H ?

Ampere per meter

What is Reluctance ?

The core opposition to the magnetic flux flow

What does the right hand rule determines ?

The direction of the magnetic field produced by current

What are the reasons behind (calculations of the flux are always an approximation ) accurate within 5% of the actual answer ?

leakage flux, reluctance calculations, air gaps, amount of flux in the material

What do we call the region where the flux changes rapidly?

Unsaturated region

When mmf is removed the flux becomes 0 ? T or f

False

The power supplied to a dc load is given by ?

= V x I

Describe the principle that is used in ( motor action ) ?

A current carrying conductor will experience a force when placed within a magnetic field

Describe the principle that is used in ( transformer action ) ?

A time varying magnetic field induces voltage in from one coil to another

What is permeability ?

Ability of the material to produce magnetic field

What is fringing ?

Air gap between the core

How do we determine the behavior of magnetic permeability?

Applying dc to an iron core and gradually increasing the current

Explain why does a magnet has a North Pole and a South Pole ?

Because of the interaction of fields, magnets align with earth's field creating the North Pole and south pole.

Inductive load consumes ?

Both Real and reactive power from the source

What is Hysteresis loss ?

Bringing flux to zero by applying coercive force leads to energy loss

How to force the flux to zero ?

Coercive mmf in the opposite direction

What are the two paths magnetic field can take ?

Core and air around it

Magnetic flux produced by the current in the coil leaves the core and it isn't confined within ? T or F

False, does not leave and it's confined

A time changing flux induced ————— within a ferromagnetic core in the same manner it would in a wire wrapped around that core?

Voltage

When is the leakage flux is important ?

Determining flux linkages between coils and self inductance of coils in transformers and motors.

eddy currents are flowing in a resistive material so energy is ?

Dissipated

What is the magnetomotive force of an N turn current carrying coil is given by ?

F=Ni

Hysteresis means ?

Failing to follow the previous flux paths

Relative permeability is not a convenient way to compare the magnetizability of materials ? T or F

False

When mmf is higher flux decreases ? T or F

False

In what direction the air gap is in ?

Flow of magnetic flux

What prevents the flux from passing through each turn with same amount such as faraday's law assumed ?

Flux leakage

Faraday's law

Flux passes through a single turn of a coil a voltage will be induced in that turn directly proportional to the rate of change in the flux

If the magnetic flux density B is constant throughout and the area is constant then magnetic flux formula is ?

Flux=BxA

When is the permeability constant?

Free space

What is the effort that the current is putting in to produce magnetic field ?

H

How is the energy lost in eddy current ?

Heat in the core

At first a small increase in the mmf produce what happens to the graph (flux)?

Huge increase

What are the types of energy losses ?

Hysteresis loss and eddy current losses

What causes magnets to align with the earth's field ?

Interactions of these fields

What is the magnetic flux ?

Is the amount of magnetic field passing through a surface at certain amount of time.

What are magnetic fields ?

It's a fundamental mechanism that converts energy from one form to another

In which part of the graph most machines operate near to....?

Knee of the curve

What do we call the transition between the unsaturated and saturated region ?

Knee of the curve

Inductive load produces ?

Lagging current and voltage

What happens to the area in the air gap when fringing occurs?

Larger than the area of the core

What is ampere's law ?

Law that determines the production of magnetic field by a current

Capacitive load produces ?

Leading current and voltage

What do we call the small amount of flux that leaves the iron ?

Leakage flux

Explain the magnetic flux lines by convention ?

Magnetic flux lines leaves the North Pole and enters South Pole.

What are the methods used to decrease eddy current losses

Many strips and lamination and insulating oxide or resin between strips to decrease the area

Which end does the flux re-enters ?

Negative

What is the sign of the capacitive load ?

Negative impedance angle

Does the flux lines behave in the same manner when fringing occurs ?

No

Is permeability constant in iron and other ferromagnetic materials ?

No

Later when mmf produces increases what happens to the graph?

No change at all

How do we ensure that the flux is produced is proportional to mmf ?

Operate the core in the unsaturated region

What is the inverse of reluctance?

Permeance

The feet of lenz's flux in transformers is to predict ?

Polarity of the voltages induced in the transformer

Which end does the flux exits ?

Positive end of the mmf source

What is the sign of Inductive load impedance angle ?

Positive impedance angle

Describe the principle how magnetic field is used in current carrying wire ?

Produces a magnetic field in the area around it

Capacitive load supplies ?

Reactive power to the source

Capacitive load consumes ?

Real power

What do we call the region where the curve flattens out ?

Saturated region

The amount of energy lost to eddy current is proportional to the ?

Size of the paths they follow in the core

What does the strength of the magnetic field depends on ?

The material of the core

What does the value of permeability depends on ?

The type of material used

Earth has a magnetic field that is relatively weak compared to

Transformers , motors and generators

Actual eddy current losses are proportional to the square of the lamination thickness? T or F

True

Amount of permeability is proportional to the flux induced in the core ? T or F

True

Does the mmf in the Magnetic circuit has polarity associated with it ? T or f

True

Faraday's Law assumes that the same flux is present in each turn of the coil ? T or F

True

H Is known as the effort required to induce a magnetic field ? T or F

True

Magnetic field will be proportional to the amount of current ? T or F

True

Majority of the flux in an iron core remains inside the core instead of traveling through the surrounding air which has lower permeability ? T or F

True

Mmf and hysteresis loss are directly proportional? T or f

True

Mmf is equal to F=Hl ? T or F

True

Resistive load consumes real power from the source ? T or f

True

Resistive load has Zero impedance angle ? T or F

True

Resistive load has current and voltage in phase ? T or F

True

The magnetic flux density B is the amount of flux passing through a unit area ? T or F

True

The permeability of iron is much higher compared to air ? T or F

True

The mmf of a magnetic circuit is equal to the total current applied to the core ? T or F and why ?

True, because mmf drives the flow of flux

Describe the principle that is used in ( generator action ) ?

Whenever a conductor is placed in a varying magnetic field an EMF gets induced in the conductor

If the windings are tightly coupled will the flux be the same in each turn ?

Yes

eddy currents

spread out whirlpools of induced current in a solid conductor

Reactive power represents

the energy that is first stored and then released by the load ( inductor or capacitor )