Share here your strategies for taking the CBT PE Power Exam

[BebeshKing PE]

Now that we are transitioning from Pencil & Paper to Computer Based Test for the PE Power, what is you strategy in taking this from open book to a closed book exam?

Here are my personal strategies:

1. Get familiar with the PE power reference handbook 

2. As much as possible, don't memorize much formulas, and yet, learn to derive them. 

3. Specialty formulas that can be easily plug and play during the exam are the ones to be memorized.

4. Try to use PDFs for the codes and PE power reference handbook and use the search lookup to get hang of it. 

5. Try to do practice exams by just using the PE power reference handbook, your common knowledge, and personal experiences.  

What could you share?  

 

[akyip]

I'm doubling down reviewing and studying some of my more complicated conceptual subjects - electrical machines, fault analysis, batteries, etc. I'm making notes on some not-so-common things to know on these subjects that maybe I should try to memorize (e.g. temperature effects on battery and ground resistance testing).

Also, silly question... how do you make your appointments for the CBT exam? I was trying to register for it on the NCEES dashboard, and I don't see it up to the screen where I actually have to make a payment for registering for the exam. Does making an appointment time slot come after you pay for registering for the exam?

 

[Dothracki PE]

I'm doubling down reviewing and studying some of my more complicated conceptual subjects - electrical machines, fault analysis, batteries, etc. I'm making notes on some not-so-common things to know on these subjects that maybe I should try to memorize (e.g. temperature effects on battery and ground resistance testing).

Also, silly question... how do you make your appointments for the CBT exam? I was trying to register for it on the NCEES dashboard, and I don't see it up to the screen where I actually have to make a payment for registering for the exam. Does making an appointment time slot come after you pay for registering for the exam?

Yes, you have to register, pay for the exam, and if you are approved from your state board it will have a link to the pearson vue page where you can schedule the exam.

 

[akyip]

Yes, you have to register, pay for the exam, and if you are approved from your state board it will have a link to the pearson vue page where you can schedule the exam.

Thanks for this info!

Later on, I'll compile a list of some of the conceptual things to know (and maybe have to know off the top of head) by subject. I'll try to share later this week, and others can add on to this list...

 

[akyip]

So a few posts earlier, I mentioned I was putting together a list of conceptual things to know...

For example, this is my compilation of useful (albeit maybe random) things to know specifically for synchronous generators.

·        If the internal stator voltage Ea begins the lead the terminal voltage Vt by a factor of N while the generator is synced to an infinite bus, then the poles in the rotor will lead the poles in the stator field by an increase of a factor of N times the previous mechanical angle αold.

·        To connect & synchronize a synchronous generator to the electrical grid or bus, using a synchronizing relay (ANSI #25 relay) is required. The following parameters must match:

o   Same frequency

o   Same voltage magnitudes

o   Same phase rotation sequences

·        Overexcited synchronous machine: Re(Ea) > |Vt| à delivers reactive power Q.

·        Ideally excited synchronous machine: Re(Ea) = |Vt| à neither delivers nor absorbs reactive power Q.

·        Underexcited synchronous machine: Re(Ea) < |Vt| à absorbs reactive power Q.

·        Interconnected synchronous machines in a network:

o   Increase in power transfer --> increased system stability

o   Decrease in power transfer --> decreased system stability

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·        Synchronous generator connected to an infinite bus:

o   For real and reactive power to flow from the generator to the bus, the generator’s internal voltage Ea must lead the infinite bus terminal voltage Vt.

o   If the rotor rotates at constant synchronous speed, the generator torque equals the load torque.

o   Generator torque > Load torque à Rotor will increase in speed.

o   Generator torque < Load torque à Rotor will decrease in speed.

·        Increase the mechanical angle α between a generator’s rotor and stator fields can lead to instability if this angle exceeds 90° (corresponding to maximum power).

·        Methods to increase power system stability:

o   Increase internal voltages of generators.

o   Increase generator excitation current.

o   Decrease impedance of transmission/distribution system.

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·        Synchronous reactance Xs or Xd: for steady-state or sustained short-circuit current

·        Transient reactance Xs‘ or Xd’: for short-circuit current a few cycles after the fault first occurs

·        Sub-transient reactance Xs‘’ or Xd’’: for short-circuit current at exact moment of fault (the lowest value of synchronous reactance)

o   The reactance of the generator at initiation of a fault

o   The lowest reactance value of a synchronous generator (Xs‘’ < Xs‘ < Xs)

o   Lasts from about 1 to 3 cycles

o   A function of sub-transient inductance

·        When a synchronous generator loses its prime mover (mechanical input), it becomes a synchronous motor and absorbs real power.

·        Localized heating in the end of the armature is caused by an under-excited synchronous generator.

o   When a generator is under-excited, there are low field currents and the armature’s end region has localized heating.

·        High field currents correspond to an over-excited synchronous generator.

·        A synchronous machine has no slip.

o   Its rotor speed and speed of stator magnetic field are always equal.

·        In a 3-phase synchronous machine with balanced circuits of equal magnitude and 120° apart, the sum of stator flux vectors φa, φb, and φc produce a resultant flux of constant magnitude that rotates around the machine at a frequency equal to the frequency of the current.

·        If 2 generators are mounted on the same shaft, then they have the same speed.

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o   For highest speed, the number of poles must be minimized.

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·        If a stator of more than 2 poles makes a single full mechanical rotation of the magnetic field, then more than a single full electrical cycle is produced.

o   Each pole pair (each 2 poles) represents 1 electrical cycle.

·        The insulation class of a synchronous generator’s windings dictates the highest temperature that the synchronous generator can withstand during operation.

·        Hysteresis losses exist in AC machines.

o   DC machine losses include stray load losses, core losses, and brush losses.

·        Iron cores allow many times more flux than air cores.

o   Flux is needed to generate the MMF required for voltage torque in electric machines.

·        A generator in droop mode at greater than 0% will decrease speed as load increases.

o   Droop slope is never positive.

·        A generator in isochronous mode at greater capacity than full load will have its speed reduced as additional load is connected.

o   As the amount of load applied to a system is increased, the frequency sag/drop is greater.

·        For load sharing, a generator is either in droop or isochronous mode.

o   Isochronous = 0% droop setting (constant droop slope) à frequency is constant regardless of load

·        Prime movers for a generator

o   Steam turbine

o   Gas turbine

o   Hydro turbine

o   Diesel engine

·        Magnetic pickup (MPU) on a generator’s prime mover acts as a sensor that provides speed input from the prime mover.

·        The overspeed trip device on a generator’s prime mover will shut down the prime mover to prevent overspeeding.

·        When the generator frequency is constant and additional load is placed on the generator, more fuel is needed in the prime mover to carry the additional load.

FFeel free to comment on this, make any corrections as you see necessary. By no means do I recommend trying to memorize all this. Rather, these concepts just stuck into my brain the more I redid practice exams and problems...

 

[akyip]

I also made a compilation of useful albeit maybe random things to know specifically for synchronous motors.

Synchronous Motors – PE Exam Things To Know

·        Overexcited synchronous machine: Re(Ea) > |Vt| à delivers reactive power Q.

·        Ideally excited synchronous machine: Re(Ea) = |Vt| à neither delivers nor absorbs reactive power Q.

·        Underexcited synchronous machine: Re(Ea) < |Vt| à absorbs reactive power Q.

·        Interconnected synchronous machines in a network:

o   Increase in power transfer à increased system stability

o   Decrease in power transfer à decreased system stability

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·        Synchronous motors always run at synchronous speed ns regardless of loading.

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·        A synchronous motor with no load or with its load removed will have a 0° torque angle.

·        A brushless exciter is used to start a synchronous (not induction) motor.

·        A synchronous machine has no slip.

o   Its rotor speed and speed of stator magnetic field are always equal.

·        In a 3-phase synchronous machine with balanced circuits of equal magnitude and 120° apart, the sum of stator flux vectors φa, φb, and φc produce a resultant flux of constant magnitude that rotates around the machine at a frequency equal to the frequency of the current.

·        Synchronous motors must be excited by an external DC source. They are not self-starting.

o   The external DC source provides the necessary power for the rotor.

o   This power is sent through slip rings and brushes.

·        Synchronous motors are often used to improve power factor because they can be overexcited (delivering reactive power) or run at unity power factor.

·        If a stator of more than 2 poles makes a single full mechanical rotation of the magnetic field, then more than a single full electrical cycle is produced.

o   Each pole pair (each 2 poles) represents 1 electrical cycle.

·        Hysteresis losses exist in AC machines.

o   DC machine losses include stray load losses, core losses, and brush losses.

·        Iron cores allow many times more flux than air cores.

o   Flux is needed to generate the MMF required for voltage torque in electric machines.

·        If a synchronous motor’s excitation current increases while power & speed are constant, its power factor improves.

·        If a synchronous motor’s load is reduced while field current is constant, its power factor will become leading.

Again, feel free to comment on this. If you think you see any corrections that need to be made, let me know. I should also mention that I copied and pasted some items my synch gen list because they overlap.

 

[akyip]

Also one more list, this time for induction motors:

Induction Motors – PE Exam Things To Know

·        Various starting methods for induction motors

o   Soft-Start or Reduced-Voltage Controller: used to slowly ramp up motor speed and torque using a reduced voltage for a short period during initial start-up. Best suited for speed & torque control only during initial starting (not during run time).

o   Variable Frequency Drive (VFD): used to modify/control a motor’s speed, depending on changing mechanical loading conditions while running. More $$$ than soft starter.

o   Across-The-Line Starting: most common starting method for most induction motors. Energizes the motor will full voltage as soon as motor circuit is energized. Can lead to tripping issues.

o   Remote Starting: just the used of hard-wired push buttons to energize the motor control circuit from a remote/distant location away from the motor. Does not affect starting current or torque.

·        Motor torque is proportional to square of voltage (the basis for reduced-voltage or soft starting).

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·           Slip: [SIZE=11pt][/SIZE]

o   Motor operation: n < nsso 0 < s < 1

o   Braking operation: n is negative (rotor spins in opposite direction of stator field) so s > 1

o   Generator operation: n > nsso s is negative, s < 0

·        In a locked-rotor test, the parallel magnetizing branch is neglected.

·        Flux φ is proportional to V/f.

o   Too much flux means saturation and overheating

o   Too little flux means not enough torque produced for the load

o   An increase in flux φ means motor torque increases.

o   Power is proportional to torque and speed.

o   If frequency f decreases, then:

§  Speed n decreases

§  V/f ratio will increase

§  Torque will increase

·        Speed vs. load curve:

o   As load increases, motor speed n decreases.

o   As load decreases, motor speed n increases and approaches synchronous speed ns.

·        Induction motor with high rotor resistance:

o   Lower starting current

o   Greater starting torque

o   Better starting and lower-speed performance

·        To reverse the direction of an induction motor, just switch/swap only any 2 lines.

o   When 2 leads are reversed, the rotating magnetic field becomes reversed. Speed ns becomes -ns.

o   Power flow will still be in the same direction.

o   Torque is reversed because the rotating magnetic field is now reversed.

·        Change in slip is proportional to change in square of voltage: [SIZE=11pt][/SIZE]

·        Single-phasing of a 3-phase induction motor:

o   Occurs when one of the 3 phases is disconnected, leaving only 2 phases intact.

o   The current to the motor will drastically increase. Can lead to overheating.

o   The motor will still run, but once it stops it cannot self-start.

o   It becomes less efficient, and can overheat.

o   Produces more vibrations than a regular 3-phase motor.

·        Vector control (not V/f control) allows for motor speed and torque to be controlled.

o   With an encoder feedback in closed loop control, high starting torque at 0 RPM is possible.

·        Reducing the stator voltage of an induction motor:

o   The rotor current must increase to maintain the same torque. This increases rotor copper (I2R) losses.

o   The input power factor will improve, since load component increases & magnetizing component decreases.

o   Air gap flux will be less, since air gap flux density is proportional to voltage.

·        Low-resistant rotor induction motor has steeper torque-speed curve.

·        At synchronous speed, both low- and high-resistance rotor induction motor have the same performance.

·        An induction motor operated at lower than rated frequency:

o   Lower frequency à lower synchronous speed ns

o   An induction motor still maintains its max torque value for f < 60 Hz

o   An induction motor at lower frequency still has a starting torque.

·        An asynchronous machine (e.g. induction motor) has a slip and thus a difference in speed between the rotor and stator magnetic field.

·        An induction motor running at synchronous speed ns does not develop any torque.

o   The different in speed is needed to induce the magnetic field that causes torque.

·        An induction motor uses electromagnetic induction instead of a DC excitation field to be run.

·        The amount of torque an induction motor is dependent on stator voltage.

·        Slip is required to create torque.

·        The rotor resistance affects speed-torque characteristics.

·        The slip at which maximum torque occurs is directly proportional to rotor resistance, but the maximum torque itself is independent of rotor resistance.

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·        Induction motor versus transformer: excitation current as percentage of rated current
At full load, the excitation current for an induction motor will be significantly larger than the excitation current of a transformer.

o   The induction motor has an air gap. The transformer has an iron core.

o   Magnetizing current in an induction current is much larger (30% of rated current) than in a transformer (2% of rated current).

·           Motor speed regulation: [SIZE=11pt][/SIZE]

·        The optimal zone of operation for an induction motor is where the shaft rotation speed range is above peak torque.

o   If the motor speed falls behind this peak torque (to the left of the max torque point on the torque-speed curve graph), the motor will be unstable.

·        Hysteresis losses exist in AC machines.

o   DC machine losses include stray load losses, core losses, and brush losses.

·        Iron cores allow many times more flux than air cores.

o   Flux is needed to generate the MMF required for voltage torque in electric machines.

·        When a capacitor is connected to the load side (outgoing side) of a motor’s overload relay, the relay will see less current and its settings should be reduced.

·        When an induction motor’s mechanical load is suddenly reduced to zero, the motor will accelerate to synchronous speed and torque will become zero.

·        An induction motor will stall when motor pull-out torque is less than load torque.

·        Breakdown torque = Pull-out torque = Maximum torque for an induction motor

·