Basics + Magnetism Flashcards by Anthony Royster

explain why measuring the speed at the edge of the wheel is a poor way to represent its motion

depending on the distance to the COR the object it’s velocity changes .

Angular velocity is constant everywhere on a wheel - better way of representing motion

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how to you represent the total angle (radians) that a wheel has turned in terms of :

Angular Velocity
Angular acceleration

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How do you calculate angular velocity in terms of:

Radians
Frequency
RPM

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What is Force

What is Torque

When a force is applied to an object, an acceleration is induced
F=Ma

Torque is produced when a force exerts a twisting moment on an object
T=Fr
(r= distance from force to axis of rotation - perpendicular distance)

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What is Mechanical Work

How do you calculate Mechanical work

What is power

How do you calculate Power

Mechanical work is done when a force moves an object a distance

W=Fd or W = 2piTorque (d=2pir & F=T/R )

Power is the rate of which work is done

P= W/t or P = 2piT/t

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A car has an engine power of 200kw and is moving at 20m/s, the rolling radius is 40cm. What is the torque produced by the engine

Assume 100% efficiency

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explain how to calculate a gear ratio

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describe the moment of interia

resistance to change in rotational motion

also called angular mass

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Question

the torque calculated is the net torque

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A Charged particle in a magnetic field will experience ______________________________________________________________________________

What’s this law Called

What’s the equation

a force perpendicular to both the direction of motion, and the direction to the magnetic field

Lorentz Force

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Explain the Laplace Force

What’s the equation.

Current carrying wire in a magnetic field will experience a force perpendicular to the direction of current and magnetic field direction.

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Explain the Biot-Savart Law
Whats the equation

If a current is passed through a wire, a magnetic field that loops around the wire will be produced.

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What are the 4 rules associated with magnetic field plots

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how do you calculate change in angular velocity in terms of torque.

write the general formula, and the one for a vehicle

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what is ampere’s law

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how do you calculate magnetic flux density?

what is the unit

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find the expressions for
magnetic field intensity
magnetomotive force
flux density

for a transformer, specify the units

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derive the equivalent of ohms law for for magnetic circuits

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which one of the following materials is the best for a transformer core

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draw a labelled hysteresis loop for the magnetisation of a transformer core

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What is the Coercive Force

intensity of the magnetic field required to reduce the magnetic flix density of a magnetised material to zero after reaching saturation

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What is remanence

the residual magnetic field density that remains when the magnetic field intensity drops to zero after it has reached saturation

what is magnetic saturation

where magnetic flux density reaches a maximum for increasing values of magnetic field intensity H

Evaluate the performance of a tranformers for the following two materials using their magnetisation curves

Hard materials are difficult to magnetise and remagnetise Soft materials are easy to magnetise and remagnetise A transformer core needs to magnetise and demagnetise rapidly Hence a soft material would be better suited with a skinnier hysteresis , less hysteresis loss

What is the difference between the Lorenz and Laplace force

multiply force by 10 for the windings

Answer part A

answer part b

Draw the Circuit schematic for a sep ex DC motor Derive the Equation for the : steady state current through the armature steady state torque produced

For a Sep Ex DC Motor draw a graph of Torque against angular velocity , Assume the DC Motor is operating in a steady state at all times. Demonstrate how the Graph changes when : Voltage is Increased Voltage is Decreased Field Current is Increased Field Current is Decreased

Draw a graph of torque against angular velocity for a Sep Ex DC Motor, assume the dc motor is not operating in a steady state.

Draw the Circuit schematic for a Shunt DC motor Derive the Equation for the : Steady state voltage through the armature and Field inductor Angular velocity

Draw a velocity agaisnt torque graph for a shunt DC Motor in ideal and real conditions

Draw the Circuit schematic for a SeriesDC motor Derive the Equation for the : Steady state voltage Torque Produced

what's the difference between a thyristor and a diode

Diode: Allows one-way current flow. Two terminals: anode and cathode. Commonly used in rectification and protection. Thyristor: Acts as a switch controlling high voltage/current. Three terminals: anode, cathode, gate. Current pulse applied to gate, acts like a switch -unlike a transistor it the size of the base current doesn't regulate the conductivity between the collector and emitter. Used in power control and switching applications.

draw a single diode phase rectifier, and single phase thyristor rectifier draw the amplitude response produced by both

What is faradays law

change in magnetic flux through a circuit induces an electromotive force (EMF) in the circuit, proportional to the rate of change of the magnetic flux.

For a Sep Ex motor used as a generator, what is the equation for the induced voltage. In terms of Change in Flux and Current

What is the Efficiency of a DC Motor

Compare a Brushed to a Brushless DC Motor

Brushed Less Complex (More mature) technology Cheaper Easier Control Higher Maintenance required on the brushes as they wear out over time Brushes produce sparks -> fire hazard & signal interference Lower Efficiency and Speed - More mechanical limits Lower Power density Brushless More Complex (Modern) Technology Expensive Harder Control - microcontroller are required Lower Maintenance required as there are no brushes No sparking Higher Efficiency and Speed - Less mechanical limits Higher Power Density

Draw a torque agaisnt speed graph for a series DC Motor in ideal conditions

how do you calculate power factor

apparent. /real power

What are the applications for series, shunt and sep-ex dc motors

what are the advantages and disadvantages of , shunt , series and sep ex dc motors

How do you calculate the Peak induced Voltage in a rotation generated

what are the main losses in dc motors

Electrical resistive losses within the armature Hysteresis and Eddy Current losses induced in the magnetic core Mechanical losses due to friction Brush contact loss( brushed machines _> voltage drop at the interface) Magnetic leakage currents (magnetic flux not contributing to torque)

How do inductions motors work and how do they gen4erate torque

- Stator creates a rotating magnetic field when supplied with AC power. - Rotor experiences induced current due to the changing magnetic field. - Interaction between the stator's field and the rotor's induced current generates torque. - Rotor spins slower than the magnetic field, creating "slip." - Resultant magnetic field is the sum of the phases, constant magnitude and at the same frequenc6y of the 3 phase supply.

How do you determine the synchronous speed of an ac motor?

-> number of poles = pole pairs x 2

How many pole pairs does this induction motor have? What is the relationship between electrical and mechanical angles In a power plant would you use a generator with a high amount of pole pairs or low?, why do we need to adjust the pole pairs for different scenarios?

2 Nuclear, fossil fuels -> low number of pole pairs, high speed turbine Hydroelectric -> high number of pole pairs, low speed turbine. Need to maintain grid frequency of 50hz

How does a synchronous AC motor Work?

- Synchronous motors rotate at the same speed as the AC supply frequency. - The stator generates a rotating magnetic field. - The rotor's magnetic field locks with the stator's field. - No "slip" occurs between the rotor and the stator fields. - Requires permanent magnets or an exciter (DC SOURCE) for synchronization of the rotor. (-> uses DC voltage (no current so very little power loss) from a battery ie. car battery to energise the rotor, then uses some of the power generated to recharge the battery.)

draw the equivalent circuit (per phase) of a synchronous generator, ignore resistance

draw the phasor diagramv(per phase) of a synchronous generator, ignore resistance

feta = power factor angle delta = load angle (angle between rotor and stator fields)

What power factor modes can synchronous generators operate in? Draw the phasor diagrams for each

over excited -> rotor B strength > Stator B field. (determined by grid) Reactive (inductive) Power is introduced over excited -> rotor B strength < Stator B field. (determined by grid) Reactive (capacitive ) Power is introduced

power factor =1 : unity power factor power per phase = apparent power

What considerations have to be made when connecting a power generator to the infinite bus.

what happens when you misallign the Voltage, Phase sequence, Frequency and phasor phase. whilst connecting to an infinite bus

Highlight the steps required to connect a generator to the national grid (infinite bus)

What does the load angle signify in electrical machines? , and what is the formula that relates load angle to mechanical angle

E0 = Rotor , E = stator

what occurs when the load angle exceed +-180 degrees

Synchronism is lost , no net force between stator and rotor

derive the reactive/real power of a synchronous generator

P = per phase = Line to line voltage 3P = 3 phase voltage power swap sun with cos

explain why increasing the size of a cable does the efficiency increase

compare the high speed vs low speed applications of synchronous and induction machines

why can a rotor not turn at the synchronous speed of the magnetic field Define slip for an induction motor

if both turn at the same speed, relative speed =0, so not flux lines are cut and no force/current is induced. (no slip)

what are the equations for the Mechanical Power output and rotor loss of a n induction motor

Draw the equivalent circuit for a induction motor

Find expressions for current, rotor impedance and load angle within an induction motor's equivalent circuit

-> assume i0 is very small compared to i1/i1

Find expressions for mechanical angle , and impedance seen from the stator side of the circuit within an induction motor's equivalent circuit

Draw the torque-speed and current-speed characteristic of an induction motor while motoring

Draw the two simplified models for an induction machine, separate the rotor resistance and the load.

-> can also turn the xm and Rm into an open circuit

What is the equation for torque of an induction motor

Draw the torque-speed characteristic of an induction motor while motoring + regenerating

prove power transfer theory

at what point is the maximum power delivered to an induction motor. calculate an expression for the slip and speed of the motor when this occurs

Breakdown Torque Breakdown Speed

find an expression for the breakdown torque in terms of Supply voltage, Impedance of the stator , synchronous speed and mechanical angle

Determine expressions for Power supplied to the Rotor and load , power lost by the rotor Reactive Power , Active power and Apparent Power for an induction motor

Why cant you use variable resistors to control a squirrel cage. Suggest another induction motor you can use this control method on

Explain stator voltage control of a induction machine

explain variable frequency control

if a load has 150 KVA of apparent power with 0.7pf lagging what's the real and reactive power. When calculating the apparent power what must you do

ensure the voltage and current have the same polarity of phase (both positive or both negative)

For a star connected motor of V(LL) = 400v Calculate the Torque, Slip and current drawn at breakdown

Basics + Magnetism Flashcards

(100 cards)