Exam 1

What does the field established by the stator windings of a three-phase induction machine consist of?

even number of magnetic poles. The field rotates at a speed known as synchronous speed

What is the fundamental cause of magnetic fields?

forcing of moving electrically charged particles in circular or helical path

If a machine acts as a ________________, the electrical power is __________ from the armature

generator taken

If a coil is grasped with the fingers pointing in the current direction, the thumb...

points in the direction of the magnetic field inside the coil

What does it mean by Stepper Motor?

power is applied to one stator winding at a time

Inductance is a measure of an element's ability to...

resist a change in current (or ability to store flux)

Torque versus speed characteristics for series-connected dc motor or universal motor?

decreases exponentially

Torque versus speed characteristics for shunt-connected or permanent-magnet dc motor?

decreasing linearly

What is reversed periodically on a DC motor during rotation? Why?

-the current direction -Due to mechanical switch composed of brushes mounted on the stator and a commutator mounted on the shaft

If two waveforms x1 and x2 have the same frequency we can define a ....

"phase shift" which gives us a measure of the difference between their zero crossings. | θ1 -θ2 |is the phase shift. -If θ1 > θ2, we say that x1 leads x2 or x2 lags x1 by θ1 -θ2.

What is Norton's Theorem?

-Any collection of batteries and resistances with two terminals is electrically equivalent to an ideal current source I in parallel with a single resistor R. -The value of r is the same as that in the Thevenin equivalent and the current i can be found by dividing the open circuit voltage by r.

What does a commutation for a single armature winding on a 2-pole machine do?

-Conductor moves through the magnetic field produced by the stator -Conductors, the field, and direction of motion are mutually perpendicular, thus a linear machine -The induced voltage is Constant in each conductor as it moves under pole. -As conductors move between poles, the field direction reverses. Hence, the induced voltage falls to zero and build up with opposite polarity. -A Commutator known as Mechanical Switch reverses the connections to the conductors as they move between poles, thus the polarity of the induced voltage seen from the external machine is constant.

On a BLDC Motor, a ___________ _____________ indicates that the rotor has approached alignment with the stator field, the controller switches power to the next ________ ___________

-Position Sensor -stator winding

Power factor value does not indicate whether the ________ leads or lags the __________

-current -voltage

The rotor conductors of a squirrel-cage induction machine are aluminum bars connected to rings that _____ the ends together. These conductors are formed by casting molten aluminum into slots in the ___________ _____ _______ .

-short -laminated iron rotor

A 3-phase AC Induction motor will not start from a _____________ because the motor cannot supply the starting ____________ by the load.

-standstill -torque

What is the most common polyphase system?

3 phase

The magnetic field around a long straight wire carrying a current can be determined with ________________ law aided by considerations of symmetry

Ampere's

State Ampere's Law

Ampere's Law states that for any closed loop path, the sum of the length elements times the magnetic field in the direction of the length element is equal to the permeability times the electric current enclosed in the loop.

Thevenin's Theorem

Any two-terminal passive dc network can be replaced by an equivalent circuit consisting of a voltage source and a series resistor. -short voltage source -open current sources

What is one issue with DC motors?

Because all electrical energy is distributed in AC form, we need a full-wave rectifier or an inverter on some sort to convert AC to DC

How can speed be adjusted on a DC Motor?

By varying a rheostat that is in series with the armature

For a toroidal coil, the R is from the _________ ________ to the center and 2r is from the outside perimeter to the _____ perimeter

Core centerline inside

Purely reactive(inductor/capacitor) loads have a power factor equal to ...

zero

Dc Motors are powered from a ____ source

DC

BLDC can operate from a ____ ________ with characteristic similar to those of a Shunt DC Motor.

DC Source

State Faraday's law of electromagnetic induction

Electromagnetism predicts how a magnetic field will interact with an electrical circuit to produce a electromotive force (EMF)

What is included in the rotor assembly of a dc machine?

External armature terminal, brush, commutator bars, shaft, laminated iron, and armature conductors in slots on rotor.

Determine the inductance L of a 500-turn coil assuming the Reluctance R = 4.6x10^6 A.turns/Wb

L = (500^2) / 4.6x10^6 = 54.35mH

BLDC Motors are essentially _____ ______________ ________equipped with positioning sensors; etc Hall effect.

PM Stepping Motors

How does power flow to and from an induction motor?

Power flows left to right from a three-phase electrical source into an induction motor and then to a mechanical load. Some of the power is lost along the way due to various causes.

What is the gap field?

The field produced by the current in winding a varies sinusoidally in space around the circumference of the gap

Lenz's Law

The induced voltages have positive and negative polarity at both doted terminals when Ф increases and decreases, respectively.

The closer the load is to being purely resistive, the more power it dissipates for the same

V effective and I effective

What changes the torque-speed characteristic of the wound-rotor machine?

Variation of resistance in series with the rotor windings

State Lenz's Law

When an EMF is generated by a change in magnetic flux, it produces a current where magnetic fields opposes the changes which produces it

When is no field winding needed?

When the field is produced by permanent magnets (PMs)

What is a phasor?

a radius vector with a constant magnitude in the complex plane with one end fixed at the origin

How are stator winding represented?

a single turn, but in a real machine, each winding has many turns distributed around the circumference of the stator such that the air-gap flux varies sinusoidally with θ

According to convention, currents entering dotted terminals produce...

aiding fluxes

What is a transformer?

an apparatus for reducing or increasing the voltage of an alternating current.

What do Armature Windings do?

carry a current that depends on the mechanical power produced. Typically, the armature current amplitude is small if the load is light and larger for heavier loads

What does a commutator consist of? What does it do?

conducting segments and each Commutator segment is connected to the armature conductors on the rotor. The Brushes are in sliding contact with the commutator. As the Commutator rotates, switching action caused by brushes moving from one segment to another changes the current direction in the armature conductors.

What do DC generator do?

convert kinetic energy from a prime mover, such as Steam Turbine or a diesel Engine into Electrical energy. VF is external DC source supplying IF current for field coils.

Assume two coils are wound on the same core: -Flux produced by one coil is _________ to the second coil and Vice Versa -Flux linkages of coil 2 caused by current of coil 1 is denoted as _____ -Flux linkages of coil 1 caused by current of coil 2 is denoted as _____ -Flux linkages of coil 2 caused by its own current is denoted as ____ -Flux linkages of coil 1 caused by its own current is denoted as ____

coupled λ21 λ12 λ22 λ11

Depending on the type of machine, either stator or rotor or (both), contains _________ _________ ___________configured into coils. Slots are cut into the stator and rotor to contain the ________ and their______________.

current carrying conductors windings insulations

An electrical motor consists of a ____________ rotor that spins inside a ________ connected to a shaft that couples the machine into its ___________ load

cylindrical stator mechanical

Torque versus speed characteristics for AC Induction motor?

increases and suddenly decreases

A lagging power factor means the voltage is _________ the current

leading

A leading power factor means the current is _______ the voltage

leading

For a rectangular prism, the reluctance, R, of a magnetic path depends on the mean ________, the ____ (A), and the __________ (mu) of the material

length Area permeability

Magnetic fields can be visualized as...

lines of flux that form closed paths

A voltage is induced in a conductor ___________ so as to cut through magnetic flux lines

moving

Purely resistive loads have a power factor equal to ...

one

A charge moving through a magnetic field experience a force, f, _____________ to both velocity, u, and flux density, B.

perpendicular

If a wire is grasped with the thumb pointing in the current direction, the fingers....

point in the direction of the magnetic field

What is an advantage of DC Motors?

speed and direction could be controlled more easily than those of AC counterparts. However, advancement of electronic systems particularly Frequency Variable Source Drive made AC induction motor attractive to achieve speed control

In Y-Y configuration, the magnitude of line-to-line voltage is equal to

sqrt(3) times the phase voltage magnitude

Inductance depends on the ________ of the number of turns, ________ material, and ______________

square core dimensions

Capacitance is a measure of an elements ability to...

store charge

Flux density vector B is ________ to the lines of flux

tangent

In Y configuration, the magnitude of line current is equal to

that of the phase current

Ampere's law

the line integral of magnetic field intensity around a closed path is equal to the sum of the currents flowing through the surface bounded by the path.

The stator of a two-pole machine contains ....

three identical windings spaced 120˚ apart

What is the primary purpose of Field Winding?

to set up the magnetic field in the machine. The current in the Field Winding is independent of the mechanical load imposed on the load ( except in Series Connected Motors).

Currents in coils windings set up magnetic fields that interact with fields to produce ________, T

torque

How is speed control achieved on a BLDC Motor?

variying the amplitude and duration of the pulses applied to the stator Winding

Torque versus speed characteristics for synchronous motor?

vertical line

When the flux linking a coil changes, a _____________ is induced in the coil. The polarity of the voltage is such that if a circuit is formed by placing a resistance across the coil terminals, the resulting current produces a field that tends to __________ the original change in the field.

voltage oppose