MOTOR ACCELERATION STUDY Flashcards
TORQUE FORMULA
TORQUE=(VOLTAGE)^2
WHAT IS SLIP?
DIFFERENCE BETWEEN ROTATING MAGNETIC FIELD SPEED AND ROTOR MAGNETIC FIELD SPEED
WHY MAGNETIZING CURRENT OF MOTOR IS GREATER THAN MAGNETIZING CURRENT OF TRAFO?
BECAUSE PERMEABILITY OF MOTOR IS LESS WHILE PERMEABILITY OF TRAFO IS HIGH
WHICH HAS HIGHER PERMEABILTY ,TRAFO OR MOTOR?
TRAFO
WHICH NEEDS HIGHER MAGNETIZING CURRENT MOTOR OR TRAFO?WHY
MOTOR BECAUSE PERMEABILTY OF MOTOR IS LESS
LOAD TORQUE CHARECTERISTICS SHOULD BE HIGER OR LESS THAN MOTOR LOAD CHARACTERISTIC
LESS THAN MOTOR LOAD CHARECTERISTIC
MOTOR WONT START IF
MECHANICAL TORQUE IS GREATER THAN ELECTRICAL TORQUE
MOTOR ACCELERATES AND START IF
ELECTRICAL TORQUE IS GREATER THAN MECHANICAL TORQUE
MOTOR MAY STALL IF
ELECTRICAL TORQUE EQUAL TO MECHANICAL TORQUE
WHAT ARE THE THINGS ARE PROVIDED BY STATIC MOTOR STARTING STUDY
1) MOTOR’S STARTING CURRENT
2) VOLTAGE AT MOTOR TERMINAL DURING STARTING
3) STARTING POWER FACTOR OF MOTOR
IF MOTOR STARTS IN STATIC MOTOR STARTING STUDIES
IT WILL DEFINITELY START INDYNAMIC MOTOR STARTING STUDIES
FORMULA FOR MOTOR EFFICIENCY?
EFFICIENCY= OUTPUT POWER(MECHANICAL)/INPUT
POWER(ELECTICAL)
EFFICIENCY OF MOTOR IS DETERMINED BY
2 TYPES OF LOSSES
1) FIXED LOSSES-INDEPENDENT OF LOAD
2) VARIABLE LOSSES-DEPENDENT ON LOAD
FACTORS BEHIND FIXED LOSSES
1) LOSSES DUE TO CORE MAGNETIZATION(HYSTERISIS AND EDDY CURRENT LOSSES)\
2) FRICTION LOSSES
3) WINDAGE LOSSES
FACTORS BEHIND VARIABLE LOSSES
1) STATOR i^2R LOSS(DEPENDS ON MOTOR LOAD AND MAGNETIZING CURRENT)
2) ROTOR i^2R LOSS(DEPENDS ON MOTOR LOAD)
3) STRAY LOSSES( IS AND IEC STANDARDS TAKES FIXED VALUE AS 0.5% OF INPUT FOR THIS LOSS)
ELECTRICAL POWER (P) FORMULA?
P=SQR ROOT OF 3 VI* COS (PI) AND P= APPARENT POWER * COS(PI)
APPARENT POWER FORMULA?
APPARENT POWER =SQR ROOT OF 3VOLTAGELINE CURRENT(IL) OR POWER TRIANGLE
(KVA AMPS) i FORMULA?
I= RATED MECHANICAL POWER / EFFICIENCY*PF
WHAT IS SERVICE FACTOR?
;;NEED A PROPER EXPLANATION
SLIP FORMULA?
SYNCHRONOUS SPEED-(MINUS) ROTOR SPEED/SYCHRONOUS SPEED
SLIP=(Ns - ROTOR SPEED)/Ns
IF MOTOR LOADING INCREASES WHAT HAPPENS TO LOSSES AND EFFICIENCY?
LOSSES INCREASES AND EFFICIENCY DECREASES
FAULT CURRENT FORMULA?(LRA)
LRA =FLA/%Z WHEREAS %Z = X’’
X’’=%Z AND X’’ FORMULA
X’’= (FLA/LRC) * (100 IF NECCESARRY)
(LRC FAULT CURRENT’S KVA) FORMULA?
LRC’S APPAERNT POWER =1.732VLRC
1 HP IS EQUAL TO
1 HP=0.746 TIMES OF KW(MECHANICAL OUTPUT)
HOW WILL YOU CONVERT MECHANICAL OUTPUT POWER 34 KW OF MOTOR INTO HP HORSE POWER?
HP=34(KW)/0.746
since 1 HP = 0.746 TIMES OF KW(MECHANICAL OUTPUT)
TORQUE FORMULA ?
T = (9.55* MECHANICAL RATING )/RATED SPEED OF ROTOR
UNIT CAN BE IN N-M OR KN-M
MAXIMUM BREAK DOWN TORQUE IS SAME AS
PULL OUT TORQUE
AT MOTOR STARTING CONDITION MOTOR ACT AS WHAT TYPE OF LOAD?
CONSTANT IMPEDANCE LOAD
NOTE: (VOLTAGE DECREASES STARTING CURRENT DECREASES)
WHEN WE NEED STARTER FOR MOTOR?
IF MOTOR BUS VOLTAGE IS NOT WITHIN LIMITS
WORST CASE SCENERIO FOR MOTOR STARTING STUDIES
CONSIDER BASE LOAD AT TIME OF MOTOR STARTING
CONNECTED TRAFO MUST BE FULLY LOADED UNDER THIS CONDITION
PERFORM MOTOR STARTING STUDIES FOR (WORST CASE SCENERIO)
1) FOR HIGHEST RATED MOTOR SINCE HIGH FLA RESULTS IN HIGH LRC WHICH CAUSES MAXIMUM VOLTAGE DIP(THIS CONDITION AFFECTS OTHER RUNNING LOADS COZ OF VOLTAGE DIP AT COMMON BUS CONNECTED TO LOAD)
2) SMALL MOTOR WITH LONGEST CABLE LENGTH(IT WILL NOT AFFECT OTHER RUNNING LOADS SINCE COMMON BUS VOLTAGE REMAINS UNAFFECTED BUT IT WILL AFFECT SMALL MOTOR ITSELF COZ OF VOLTAGE DIP AT MOTOR TERMINAL)
What motor is this which
1) Draws high starting current
2) low rotor resistance
3) has low starting torque
4) rotor heating is low at start
5) Leakage reactance is low
Single cage induction motor
What motor is this which
1) Draws low starting current
2) high rotor resistance
3) has high starting torque
4) rotor heating is high at start
5) Leakage reactance is high
Double cage induction motor
what are the purpose of motor starting studies?
1) to know voltage dip
2) to know starting current
3) helps us to provide input for protection
4) to know current drawn by motor
what is static motor study?
it is nothing but load flow study at motor starting, before motor starting, after motor starting.
it is performed when motor data is not available
what are the inputs required for static motor stating study?
1) grid short circuit current
2) motor parameters(kw, kva,v)
3) normal load flow parameters
what are the inputs required for dynamic motor stating study?
1) motor model
2) motor type
3) what kind of load
3) inertia of the system
4) resistance and reactance of motor
5) load characteristics of motor
why inertia of system is so important?
because it determines the acceleration time of motor
what happens if inertia is higher?
motor will take long time to start
what happens if motor takes long time to start?
larger the current(starting current) is needed to start a motor.it causes temp rise which cause to exceed thermal limit of motor, thermal damage, insulation damage.
what is a breakdown torque?
maximum torque a motor able to produce
what is an advantage of double cage induction motor?
1) it provides high starting torque and low starting current during starting
2) it provides good operating efficiency and speed regulation during running condition
what is the criteria for motor starting study?
As per std we should perform motor starting study if
1) motor kva rating(HP) exceeds 30% of base kva rating of supply trafo
2) motor kva rating(HP) exceeds 10% to 15% of generator kva rating
what is the other name for static motor study?
voltage drop snap shot study
what are the output given by dynamic motor starting study?
1) accelaration time
2) accelaration torque
3) voltage
4) load torque
5) motor torque
6) speed of motor
7) slip of motor
8) current
9) frequency
10) regulator response
12) exciter operation
13) governor action
if there is high voltage drop at pcc what happens to short circuit current?
short circuit current decrease
motor starting study should be performed at which s.c. current?
in both max and min s.c.current
what is a weak system?
it has high source impedance , low s.c. current, high voltage drop
what is a stong system?
it has low source impedance , high s.c. current, low voltage drop
if voltage drop beyond required limit during motor starting study what is remedy?
starters
FLA FORMULA
KW/1.732VPF*efficiency
kw= mechanical output
Torque formula
9550*kw/rotor speed
what other methods you can use to decrease voltage drop
1) increasing cable size(only in lv not mv)
2) adjusting transformer impedance(but effect will be minimum)
3) adjusting OLTC level
what is W*R^2?
inertia
w- weight of motor
R- radius
H-inertia constant
what is the relationship between speed and slip
speed is inversely proportional to slip
what is acceleration torque?
difference between motor torque and load torque
motor should be chosen based on what?
load inertia
how will reduce acceleration time of motor?
by giving lower inertia of
if load is 30kw how we should model motor?
we should oversize the motor to make it run at 80% of its full load
what will you do if you don’t have R and X parameters for dynamic motor starting study?
perform dynamic motor starting study by going for characteristic modelling by obtaining data for torque-slip characteristic data(by knowing torque, PF, current and slip)
what are types of motor connected loads?
1) constant torque
2) constant horse power
3) variable torque
what is constant torque load and eg?
it has same torque at all speeds. but horse power varies with speed
about 90% of all application except pump
eg: conveyors, hoisting load, surface winding machines ,positive displacement pumps, piston, screw compressor
what is constant horsepower load and eg?
it has high torque at lower speed and low torque at higher speed
eg: lathes, milling machine, drill press and center winders
what is variable torque load and eg?
torque varies to the square of speed and horse power requirements increase as the cube of speed.
eg: centrifugal pump, turbine pump, centrifugal blowers, fans, and centrifugal compressors
objective of motor starting study?
1) to check whether motor starts or not during load condition
2) TO find whether there is disturbance to other loads which are connected, because of voltage dip during starting
why sometimes motor cannot start?
1while starting motor takes huge starting current which results in voltage drop in network due to transformer impedance and cable impedance. since Torque=V^2.while starting if voltage drop increase then electrical torque is decreased and if electric torque is less than mechanical torque required by motor then motor wont able to accelerate with connected load( Te
from where you will get load model parameters?
from mechanical team
what is the output of motor starting study?
1) voltage dip during motor starting
2) motor starting current
3) motor starting power factor
advantages of slip ring motor?
1 )external resistance circuit so speed control is possible
2) starting torque is high
3) less starting current and PF
disadvantages of slip ring motor?
1) copper loss is high
2) efficiency is low
3) requires much maintainence
application of slip ring motor?
in high torque applications
advantages of squirrel cage motor?
1) construction is simple
2) efficiency is high
3) less maintainence
disadvantages of squirrel cage motor?
1) starting torque is low
2) high starting current and pf
3) no external resistance
Application of squirrel cage motor?
in low torque application
