week 2 (chapter 2) Flashcards
changing any two of the three phases in a three phase motor will effect it how?
direction of rotation
what are three advantages of three phase over single phase systems?
- 73% more power for 50% conductor increase
- less pulsating force
- smaller and less expensive equipment
what is the degree of separation for the coils of a three phase motor?
120 deg
there is a ________ _______ between any two phases at any time throughout the cycle of a three phase sine wave.
potential difference
in a three phase sine wave, one phase is at ______ and the other two are?
zero/providing power
in actual practice, within a three phase generator the ____ are stationary and the ________ ____ is rotating?
coils/magnetic field
what two configurations can three phase generators be connected in?
delta and wye
what is the square root of 3? why does it matter?
1.732/used to calculate voltage difference between phases and the neutral
in a _____ system, coil and phase current are the same.
WYE
in a _______ system, system voltage is the same as phase voltage.
DELTA
in a delta system, phase current is higher than coil current by how much?
a value multiplied by 1.732
on historical _____ systems, it was thought a lineman could barehand a phase safely with no electrocution risk.
DELTA
which system is the one of choice for today’s utilities?
WYE
WYE systems can serve a given load with less or more current than a DELTA system?
less
where do DELTA systems exist today?
isolated areas
what two systems can look alike but are not the same?
DELTA and three wire WYE (no neutral)
to verify a system is in fact DELTA or three wire WYE, you must check the?
secondary connections of the substation transformer bank.
four wire WYE is more reliable in regards to?
fault protection
voltage of each phase is dependent on the?
degree of rotation
in order for generators to be connected via the grid, they ALL must have?
exactly the same angle of rotation
if all generators on a transmission system are not in unison, their _____ will be different.
voltages
the direction a motor spins depends on _____________________ and __________________?
direction the generator that powers it spins
how the electricians hooked up the motor
ABCABCABCABC is a representation of what?
phase sequence
if a source supplies ABC phase sequence to a motor that requires right hand rotation, but the phases are connected BCA, will the motor still turn to the right?
yes
what are all the possible connections of ABC phase sequence that would result in clockwise rotation?
ABC….BCA……CAB
if BCA results in right hand rotation of a motor, then what would CBA result in?
reverse rotation
if a change in phase sequence occurs at a point on the power line, where is the rotation effected?
downline from the sequence (farther from source)
what can be done to prevent a rotation problem when removing and re-installing jumpers?
draw the connections on the job print
phase sequence has to be _______ before the worksite can be left
proven
to prove a corner is reconstructed properly, many companies require the line be phased where?
at the open switch
high voltage meter used for proving phase sequence?
phasing meter
if voltage is read across two corresponding conductors of an open point, what has happened?
phase sequence has been changed.
if you read voltage across an open point of a single phase you SHOULD get a reading of?
0 volts
three phase induction motors have both a ___ and ____.
rotor/stator
in an induction motor, as the magnetic field peaks between the stator poles, _____ is induced in the rods of the rotor
current
to get peak voltage from RMS, use?
RMS x 1.414 = peak
to get RMS voltage from peak use?
peak x .707 = RMS
“WYE is current the same”, is a trick to remember?
that current stays the same in a WYE circuit
