Electrical Machines Flashcards
A generator machine converts mechanical energy into electrical energy by the principle of what?
Faraday’s Law
What is called the source of mechanical power or energy used to turn the rotor of the generator
Prime mover
“Whenever conductor is moved within a magnetic field in such a way that the conductor cuts across magnetic lines of flux, voltage is generated in the conductor” this statement is also known as?
Electromagnetic Induction by Michael faraday
The magnitude of voltage generated depends upon what 4 factors?
- ) The strength of magnetic field
- ) The angle at which the conductor cuts the magnetic field
- ) The speed at which the conductor is moved,
- ) the length of the conductor within the magnetic field
What method is used to determine the direction of current in a generator?
Right Hand Rule(Conventional Current)
Center - Current
Fore - Field
Thumb - Thrust/motion
The basic or elementary generator is an alternator, Also Known as ______
An AC Generator
It is the process of changing the generated voltage in the armature to a pulsating dc voltage
Commutation Process
Basic Equation of the generated voltage in DC generator
Sipon Ago
E = (ZPϕN) / (a*60)
“Sipon Ago”
P=number of poles
N=speed of the armature core rotation (rpm)
Z=total number of elements or conductors in the armature
ϕ=number of flux per pole (Wb)
a=number of armature paths
In a self-excited shunt wound generator, what is the configuration of the field winding to the armature and load?
The field coil is connected in parallel
Relevant formulas for Self-Excited shunt generator
(Basically just remember the loop analysis lol)
I(load)=P(load)/V(load)
Ish = V(load)/R(shunt)
Ia = Ish + I(load)
Total generated voltage=V(load) + Ia*Ra
Power generated by generator = Ea*Ia
What are the different classifications of generators?
Series-wound
Shunt-Wound
Compound
In a ______ generator, the current that flows in the armature is the same with the one flowing through the field winding and external circuit(load circuit)
Series-wound Generator
Relevant Formulas for Series-wound generator
Ia = I(se) = I(load)
I(load)=P(load)/V(load)
(just use KVL and you can do it :) )
In a ______ generator, the field consists of many turns of small wires that are connected in parallel with the load
Shunt-wound Generator
What is the difference between long shunt and short shunt compound generator?
In long shunt, the series field coil is connected in series with the armature. In short shunt, the series field is connected in series with the load.
Total number of elements/conductors formula
Z=(elements/slot)*(total number of slots)
Elements per slot of the following types of windings:
Simplex, Duplex, Triplex, Quadruplex
Z/slot = 2*m
ex. Simplex - 2 elements per slot Duplex - 4 elements per slot Triplex - 6 elements per slot Quadruplex - 8 elements per slot
Total number of brushes if not specified:
Nlap = P Nwave = 2
number of armature current paths (for Sipon Ago)
a(lap) = mP a(wave) = 2m
m - ‘m’ - plex
Simplex: m = 1
Duplex: m = 2
Voltage Regulation Formula
%VR = (Vnl - Vfl) / Vfl *100%
no, full, full
What are the different losses in an electrical machine?
Armature Circuit loss Brush-contact loss core loss eddy current loss Hysteresis loss Field circuit loss Stray load loss
Armature Circuit loss formula
Pa=Ia^2*Ra
Brush Contact Loss
Pb = Ia*Vbrush
1V (for 1 carbon brush)
Core loss formula
P(core) = Peddy + P(hysteresis)
Eddy current loss formula
P(eddy) = Ke(NBm*t)² *W
Ke=proportionality constant/eddy current coefficient N=armature rotating speed W=Core weight Bm=Maximum flux density t=armature core lamination thickness
Hysteresis loss formula
P(hysteresis) = Kh•N•(Bm^1.6)•W
Kh=proportionality constant/hysteresis coefficient
N=armature rotating speed
W=Core weight
Bm=Maximum flux density
Field Circuit loss formula
Pf = Ish²•Rsh + Ise²•Rse
Stray load loss formula
P(stray loss) = 1% of the output for machine 150kW and over
Efficiency formula for Generators
Ratio of output power to the input
η=Pload / (Pload+2*Pₗₒₛₛ)
Pₗₒₛₛ - consists of constant/rotational losses, as well as variable losses
It is the part that rotates in a generator
Rotor
It is the part that remains stationary in a generator
Stator
Two different types of alternators
Rotating-Armature Alternator
Rotating-Field Alternator (DEFAULT)
What rpm describes if a motor is High or Low speed?
if >1200 rpm, high speed
if <1200 rpm, low speed
Examples of high speed prime mover
Steam and gas turbine
Example of low speed prime mover
internal combustion and electric motors
Voltage Equation for Generator
E=4.44⋅f⋅N⋅φ⋅kₚ⋅kd x10^-8 where: E=total voltage generated f=frequency N=number of turns φ=pitch factor kₚ=pitch factor (1 if not given) kd=distribution factor (1 if not given)
Alternator/AC Motor Frequency formula, given number of poles and frequency
f = P⋅N / 120
f=frequency, in hertz
P=number of poles
N=speed in RPM
Note: Used for either Alternators or AC Motors
A machine that converts electric energy into mechanical energy by utilizing forces exerted by magnetic fields produced by current flowing through conductor
Motor
What rule is used for definite relationship between the magnetic field, direction of current and direction which the conductor tends to move for DC motor
For Conventional Current, Use Left Hand Rule:
Center - Current
Fore - Field
Thumb - Thrust/Force
Types of DC motors
Shunt, Series, short-shunt compound motor
What kind of motor should be used for c͟o͟n͟s͟t͟a͟n͟t͟ s͟p͟e͟e͟d͟?
AC motor
What kind of motor is preferred for v̲a̲r̲i̲a̲b̲l̲e̲ s̲p̲e̲e̲d̲
DC motor
It is a familiar type of motor which is very similar to dc motor
Series AC motor
It may be considered as polyphase motors of constant speed and whose rotors are energized with dc voltage
Synchronous motors
The most commonly used ac motor that uses either single of polyphase whose rotors are energized by induction
induction motors
It is a very small induction motor with sizes from about 1/500 hp to 1/6 hp. it has low starting torque, with little overhead capacity and low-efficiency motor
Shaded-pole motor
It has sizes up to 3/4 hp and can operate nearly at constant speed. It requires fair starting torque with fair efficiency
Split-phase motor
Similar to split-phase motor but with higher starting torque due to starting capacitor
Split-phase motor (with capacitor?)
It has a stator winding connected to the source of power and the rotor winding to the commutator. It has a varying speed characteristics
Repulsion Motor
It operates as a repulsion motor during starting then as induction motor when running. It has high starting motor torque for long duration
Repulsion-start induction motor
It has a squirrel-cage winding in the rotor. It can be constant or variable speed repulsion motor
Repulsion-inductor motor
It has high starting torque which is constructed to operate on alternating current up to 60 cycles. It has good efficiency and excellent overload capacity with variable speed that can be controlled over very wide limits
Series or universal motors
It operates at synchronous speed with constant speed(its obvious advantage) that can be determined only by the supply frequency and the number of poles on the machine
Synchronous motor
It is widely used because of its all-purpose characteristics. good starting torque and good overload capacity
Squirrel-cage induction motor
It is with rotor construction distinct from squirrel-cage but with similar stator construction with easily controllable variable speed
Wound-rotor type induction motor
It has a stator similar to induction motor but its rotor consists of a set of salient-poles with constant speed even load changes
Synchronous motor
The most commonly used type of AC motor. It is simple, rugged and costs relatively little to construct
Induction Motor
Part of a Generator that serves as the supporting structure
Yoke
Part of a Generator that provides the magnetic field through electromagnetic Induction
Pole
Poles in a generator are always _____ in number
even
Part of the generator that contains windings, in which produces the output current
Armature
The Armature (spins/does not spin) along with the ________
Spins along with the commutator
The part of the generator that allows the basic alternator(AC) output a DC Current
Commutator
the Commutator (spins/does not spin) along with the _____
Spins along with the Armature
The part of the generator that keeps contact with the commutator
Brush
An optional part of the generator that will cancel the magnetic field produced by the armature windings, since it is not desired
Interpoles
Formula for Generated Voltage across a moving conductor, in the presence of a uniform magnetic field
E = BL[Vsin(θ]
B - Flux Density (Tesla)
L - Length of Conductor (meter)
V - Tangential Velocity (m/s)
θ - Angle Between Velocity vector and flux lines
Voltage across a moving conductor in the presence of a magnetic field is maximum when Velocity Vector is ________ to the Flux Lines
Perpendicular
Voltage across a moving conductor in the presence of a magnetic field is minimum when Velocity Vector is ________ to the Flux Lines
Parallel
Alternative Formula for Generated Voltage in a DC Generator (2Ponz/c)
E = 2PΦNZ / c*(m)
Z - # armature winding conductors
P - Number of PAIRS of poles (Actual #poles / 2 )
N - Revolutions per second (R/s)
Φ - Flux per Pole
Formula for ‘c’ in (2Ponz/c)
Cwave = 2 —— “Kaway” (w/ 2 hands)
Clap = 2P ——“Clap”“Palakpak”
P - # PAIRS of poles
When not mentioned in a problem, the DEFAULT assumption is to use a _______ Wound Generator
Shunt Wound
Both Rse and Rsh are ____________
Field Windings
A Compound Wound Generator where the H-Fields produced by Rse and Rsh Aid each other
Cumulative Compound
A Compound Wound Generator where the H-Fields produced by Rse and Rsh Oppose each other
Differential Compound
A Compound Wound Generator Classification where The No-Load Voltage is equal to the Full-Load Voltage
Flat Compounded
0% Voltage Regulation
A Compound Wound Generator Classification where The No-Load Voltage is Less than the Full-Load Voltage
Under Compounded
Positive Voltage Regulation
A Compound Wound Generator Classification where The No-Load Voltage is Greater than the Full-Load Voltage
Over Compounded
Negative Voltage Regulation
Type of Winding that forms loops as it expands around the armature core
Lap Winding
Lap winding is used for High _______ Applications
Current
The term used to describe the coil span at the back end of the armature (opposite side of the commutator)
Back Pitch
The term used to describe the coil span at the Front end of the armature (The side of the commutator)
Front Pitch
Formula for Back Pitch(Yb) in Lap Winding
Yb = Yf +- 2m
Yf - Front Pitch
m - ‘m’-plex
+ if Progressive
(- if Retrogressive)
When the Lap Winding expands from Left to Right, it is considered as __________
Progressive
When the Lap Winding expands from Right to Left, it is considered as __________
Retrogressive
Type of Winding that forms Waves as it expands around the armature core (Hmmm… ano kaya yun?)
Wave Winding :v
Wave winding is used for High _______ Applications
Voltage
Formula for Average Pitch in a Wave Winding
Y = ( Z +- 2*m)/P
Z - # of elements on armature
m - ‘m’-plex
P- #Poles
Alternate Formula for Average Pitch in a Wave Winding
Y = (Yb + Yf) / 2
Yb - Back Pitch
Yf - Front Pitch
Formula for Coil Pitch(Ys)
Ys = Coil span (unit is in #slots) / Slots per Pole
For Alternators (AC OUTPUT), ________ are used on the rotor instead of Commutators (since Commutators are responsible for AC»>DC)
Slip Rings
Alternators can either be ________ or _______
Single Phase, 3-Phase
Alternators are best used for _____
Electrical Power, connected parallel to the load
Rotor type that is high-speed, and uses 2 or 4 poles
Turbine Driven
Rotor type that is low-speed, and uses several poles
Salient-Pole
Formula for Alternator EMF per Phase
E(per Phase) = 2.22kpkdfΦ*Z
kp - pitch factor (1 by default) kd - distribution factor (1 by default) f - frequency (Hz) Φ - Flux PER POLE (Wb) Z - #conductors PER PHASE
Formula for Force on a Conductor(for motors)
F = BIL*sinθ
B - Flux Density (T)
I - Current (A)
L - Length (m)
θ - Angle Between Current Vector and Flux Line
DC Motor Back EMF
Same formula used for Generator EMF (either sipon ago or 2ponz/c )
Why the need for the formula: E = KΦN
E = ZPΦN/a*60
BUT Z,P,and a are parameters that are not easily configured on the fly because it is part of the motor/generator’s physical construction
therefore, we set these variables as a constant: K = ZP/60a
Finally: E = KΦN
This formula is used for problems with Φinitial, Φfinal, Ninitial, Nfinal, etc.
Formula for DC motor Speed
Nrpm = 60a(Eback)/PZΦ
Derived from “sipon ago”
or
reverse engineer motor speed ‘N’ from 2ponz/c
Motors Operate by the principle of _________
Lenz’s Law
Mathematical Expression for Powers involved in DC Motor
[Vin * Ia] = [Ea * Ia] + [Ia² * Ra]
[Vin * Ia] - Electrical Input
[Ea * Ia] - Mechanical Output
[Ia² * Ra] - Armature Losses
Formula for Torque of a DC Motor
EaIa = τω
τ = Ea*Ia / ω τ = Ea*Ia / 2πn
n - Rev/s
Unit for Torque (τ)
N*m or Joules
Alternative Formula for Torque (Using 2ponz/c variables)
τ = PIaZΦ / πc
“Piso Spicy”
Note: use 2ponz/c variables
Why the need for the formula: τ = KΦIa
τ = PIaZΦ / πc
But P, Z, π, and c are constant (not easily changed)
so: K = PZ/π*c
Therefore: τ = KΦIa
This formula is used for problems with Φinitial, Φfinal, Ia(initial), Ia(final), etc.
When a motor initially starts, the armature draws ______________ at t = 0, due to ____________
High Current due to Back EMF
What solution can be implemented for the initial high current in a motor?
Add a Rheostat in series to the armature winding, initially at high resistance to prevent high current overdraw, and then it gradually decreases in resistance for normal operation
Formula for Starting Armature Current when starting the DC motor
Assuming Back EMF = 0:
Iastart = Vs / (Ra - Rrheo)
Vs - Supply Voltage
Ra - Armature Resistance
Rrheo - Rheostat Resistance
Formula for Speed Regulation(%SR)
%SR = (Snl - Sfl) / Sfl x100%
The three configurations for Speed Control in a DC Motor
- ) Rheostat in Series to Armatire
- )Rheostat in Series to Shunt Winding
- )Rheostat in Parallel Series Winding
Which Speed Control Configurations have the Control Speed Inversely Proportional to the Rheostat Resistance?
Rheostat in Series to Armature, and
Rheostat in Parallel Series Winding
Which Speed Control Configurations have the Control Speed Directly Proportional to the Rheostat Resistance?
Rheostat in Series to Shunt Winding
What Determines the speed of an AC Motor
Frequency of Supply Voltage
3 Types of AC Motors
- ) Series AC (Similar to DC Motor)
- ) Synchronous Motors
- ) Induction Motors
Another Term for Series AC Motor
Universal Motor
Since it can operate on either AC or DC
AC Motors are either _________ or ________
Single Phase or Polyphase
Synchronous Motor VS Induction Motor:
Which one requires a Separate DC Exciter?
Synchronous Motors Require a separate DC Exciter
Induction Motors are Self Excited
The Speed of the Synchronous motor is _______ to the load Resistance
Not related to the load resistance (Constant Speed)
The Speed of the induction motor is _______ to the load Resistance
Directly Proportional
The Speed of the induction motor is (Controllable/not controllable)
Controllable
The Speed of the Synchronous motor is (Controllable/not controllable)
Not Controllable (No Questions will appear regarding the speed of a synchronous Motor)
The Power Factor Required to operate a Synchronous Motor is __________
Any Power Factor can operate a synchronous motor
The Power Factor Required to operate an Induction Motor is __________
A Lagging Power Factor
The Synchronous motor is relatively (Cheap/Expensive) Compared to Induction Motors
Expensive
Another use for synchronous motors
can be used to improve the Power Factor of an electrical power distribution
Can induction motors be used to improve Power Factor of a line?
No.
Only used for mechanical loads
________ Motors are often used to drive DC Generators
Synchronous
Induction Motors have a ________ Construction
Simple and Rugged
Two Types of Induction Motors
- Squirrel Cage
- Wound Rotor
The speed of rotation of a rotating Magnetic Field, when the Field is the rotor
Synchronous Speed (Ns)
The Actual Speed of the Rotor
Rotor Speed (Nr)
Formula for Slip
Slip = Ns - Nr
Ns - Synchronous Speed
Nr - Rotor Speed
The Synchronous speed is (>,
Greater than
