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Electrical Machines, Drives and Power Systems > Fundamentals of Electricity, Magnetism, and Circuits > Flashcards

Fundamentals of Electricity, Magnetism, and Circuits Flashcards

1
Q

What is the convectional current flow?

A

It’s the theoretical flow of electrons from the positive terminal to the negative terminal

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2
Q

How does current actually flow?

A

It flows from the negative terminal to the positive terminal because the electrons are negatively charged

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3
Q

What is a source?

A

A source is something that delivers electrical power

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4
Q

What is a load?

A

A load is something that absorbs electrical power

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5
Q

What do the arithmetic signs of + and - represent?

A

The direction of electrical current

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6
Q

What is the effective value of an ac voltage?

A

Effective = Peak value / Square root of 2
The effective value is sometimes also called the Root Mean Square value of the voltage

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7
Q

What does the effective value of an ac voltage measure?

A

It’s a measure of the heating effect compared to that of an equivalent dc voltage

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8
Q

How can we represent phasors?

A

We have two phasors, the voltage phasor E and the current phasor I. These can be drawn like 2D vectors where their size represents their relative magnitude and the angle between them represents the phase difference between them.
Rotating clockwise tells you the angle that the respective phasor leads the other one by

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9
Q

Why may we see a distortion in a voltage waveshape?

A
  • Magnetic Saturation in the cores of transformers
  • Switching action of thyristors
  • IGBT’s in electronic drives (Insulated-Gate Bipolar Transistor)
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10
Q

What is a Harmonic?

A

Any voltage or current whose frequencies are integral multiples of f where f is the lowest frequency of a set of sine waves

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11
Q

What is a sine wave with the lowest possible frequency?

A

The Fundamental Frequency

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12
Q

What is a square wave composed of?

A

A fundamental wave and an infinite number of harmonics. The higher harmonics have smaller amplitudes so they are less important. However, the high frequency harmonics produce steep sides and sharp courners

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13
Q

How are harmonics created?

A
  • Nonlinear loads such as electric arcs and saturated magnetic circuits
  • Whenever voltages and currents are periodically switched, such as in power electronic circuits
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14
Q

What is Fundamental Power?

A

The useful power that causes a motor to rotate and an arc furnace to heat up

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15
Q

What is the Harmonic Power?

A

The harmonic voltage multiplied by the harmonic current

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16
Q

What usually happens to the harmonic power?

A

Dissipated into heat into the ac circuit as heat, so it doesn’t do useful work

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17
Q

What is the product of a harmonic current and fundamental voltage?

A

Always 0

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18
Q

What is the energy in an inductor?

A

W = 1/2 * L * I^2

  • W = Energy stored in the coil
  • L = Inductance of the coil
  • I = Current
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19
Q

What is the energy in a capacitor?

A

W = 1/2 * C * E^2

  • W = Energy stored in the capacitor
  • C = Capacitance of the capacitor
  • E = Voltage
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20
Q

What is the equation for the magnetic field intensity?

A

H = U/I

  • H = Magnetic Field Intensity
  • U = Magnetomotive force acting on the component
  • I = Length of the component
21
Q

What is the eqation of the magnetic flux density?

A

B = phi / A

  • B = Flux Density
  • phi = Flux in the component
  • A = Cross-section of the component
22
Q

In a vacuum, what is the relationship between the magnetic flux density and the magnetic field intensity?

A

B = mew0 * H

  • B = Flux Density
  • mew0 = Magnetic Constant [4 * pi * 10^-7] N/A^2
  • H = Magnetic Field Intensity

Approximately, H = 800000 B

23
Q

What materials have the same B-H curves as vacuum’s?

A

Non-Magnetic Materials such as paper, copper, rubber and air

24
Q

What is the B-H curve for a magnetic material?

A

B = mew0 * mewT * H

  • B = Flux Density
  • mew0 = Magnetic Constant [4 * pi * 10^-7] N/A^2
  • H = Magnetic Field Intensity
  • mewT = Relative Permeability of the material

mewT varies with the flux density in the material

25
Q

What are the two key points stated in Faraday’s Law of Electromagnetic Induction?

A
  1. If the flux linking a loop (or turn) varies as a function of time, a voltage is induced between its terminals
  2. The value of the induced voltage is proportional to the rate of change of flux
26
Q

What is the equation for the induced voltage using Faraday’s principles?

A

E = N * Rate of change of flux linkage

  • E = Induced Voltage
  • N = Number of turns in the coil
27
Q

What is the value of the voltage induced in a conductor?

A

E = Blv

  • E = Induced voltage
  • B = Flux density
  • l = Active length of the conductor in the magnetic field
  • v = Relative speed of the conductor
28
Q

What is the Electromagnetic (Lorentz) force on a conductor?

A

F = BIl

  • F = Force acting on the conductor
  • B = Flux density of the field
  • I = Current in the conductor
  • l = Active length of the conductor

Thie force is greatest when the conductor is perpendicular to the field, and zero when the conductor is parallel

29
Q

What is the direction of the force acting on a straight conductor?

A

Whenever a conductor carries a current, it’s surrounded by a magnetic field
The direction of the field depends on the direction of the current
If current flows into the page, then the field acts clockwise

30
Q

If we create a coil using a magnetic material, with a current source, and flow current through it, we create a magnetic field.
We can then increase the current, which will then cause the values of the flux density (B) and the magnetic field intensity(H).
What happens as we then decrease the current to zero?

We can plot a graph of B on the y axis and H on the x axis to help

A

The flux density doesn’t follow the original curve, but moves along a curve above the original
As we reduce the magnetic field intensity, the magnetic domains that were lined up under the influence of field H tend to retain their original orientation
This is known as hysteresis
When H is reduced to 0, we can see that substancial flux density remains, which is called residual flux density, or residual induction

31
Q

How can we remove residual flux?

A

The current needs to flow in the opposite direction, and then we need to increase H in the opposite direction.

32
Q

What is the name of the field intensity required to reduce the flux to zero?

A

Coercive Force

33
Q

What must happen to reduce the flux density to zero?

A

We need to furnish energy to overrcome the frictional resistance of the magnetic domains

34
Q

What type of current is used in transformers?

A

Alternating Current

35
Q

What is a hystersis loop?

A

When we have an alternating current, we have an alternating direction for the magnetic domains.
This then leads to having maximum peak flux densities (+B and -B) and peak magnetic field intensities (+H and -H)
If we plot B as a function of H, we will see a closed curve which is called a hysteresis loop

36
Q

What is Hysteresis loss?

A

As the flux moves between the positive and negative maximums, the magnetic material absorbs energy during the cycle. As this happens, the energy will be dissipated by heat

37
Q

What is the Hysteresis loss equal to?

`

A

The amount of heat released per cycle [J/m^3] is equal to the area [in T-A/m] of the hysteresis loop

38
Q

How can we reduce Hysteresis loss?

A

We can select magnetic materials with a narrow hysteresis loop, such as the grain-orientated silicon steel used in the cores of alternating-current transformers

39
Q

What is an Eddy Current?

A

Loops of electrical current induced with conductors by a changing magnetic field inside the conductor. These currents come from the principles listed under Faraday’s Laws

40
Q

How can we reduce the losses from Eddy currents in a stationary iron core?

A

We can split the core into two along it’s length. We then need to insulate between the two halves. The total losses of the two halves is considerably less than the losses of the whole. We can repeat this process to reduce the losses further.

41
Q

What is the instantaneous voltage induced relative to the inductance of the circuit?

A

e = L * (di/dt)

  • e = Instantaneous voltage induced in the circuit
  • L = Inductance of the circuit
  • (di/dt) = Rate of change of the current
42
Q

How can we find how the current increases and decreases with time for an inductor?

A

We can use the volt-second method which allows us to create a graph

I at time (Initial time + dt) = Initial Current + di
di = 1/L * (e * dt)
di = 1/L (Volt-seconds across the inductance during the interval dt)

43
Q

What is Kirchhoff’s Voltage Law?

A

The sum of the voltages around a closed loop is zero. Thus in a closed circuit, the sum of the voltage rises and drops equal zero

44
Q

What is Kirchhoff’s Current Law?

A

The sum of the current that arrive into a point equal the sum of the current that leaves the same point

45
Q

If we have an impedance of Z, carrying a current of I, what will the magnitude of the voltage E be?

A

E = IZ

46
Q

What is the polarity of E when moving across an impedance Z in the same direction as the current flow I?

A

E is positive

47
Q

What is the polarity of E when moving across an impedance Z in the opposite direction as the current flow I?

A

E is negative

48
Q

What are the three types of Impedance?

A
  1. Resistive (R)
  2. Inductiive (jX)
  3. Capacitive (-jX)
49
Q

What must we do first to solve Kirchhoff’s Laws with ac circuits?

A

We need to convert all resistive elements into either resistive, inductive, capacitive elements, or a combonation of all three

Electrical Machines, Drives and Power Systems (3 decks)

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