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MOTOR ACCELERATION STUDY Flashcards

(75 cards)

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1
Q

TORQUE FORMULA

A

TORQUE=(VOLTAGE)^2

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2
Q

WHAT IS SLIP?

A

DIFFERENCE BETWEEN ROTATING MAGNETIC FIELD SPEED AND ROTOR MAGNETIC FIELD SPEED

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3
Q

WHY MAGNETIZING CURRENT OF MOTOR IS GREATER THAN MAGNETIZING CURRENT OF TRAFO?

A

BECAUSE PERMEABILITY OF MOTOR IS LESS WHILE PERMEABILITY OF TRAFO IS HIGH

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4
Q

WHICH HAS HIGHER PERMEABILTY ,TRAFO OR MOTOR?

A

TRAFO

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5
Q

WHICH NEEDS HIGHER MAGNETIZING CURRENT MOTOR OR TRAFO?WHY

A

MOTOR BECAUSE PERMEABILTY OF MOTOR IS LESS

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6
Q

LOAD TORQUE CHARECTERISTICS SHOULD BE HIGER OR LESS THAN MOTOR LOAD CHARACTERISTIC

A

LESS THAN MOTOR LOAD CHARECTERISTIC

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7
Q

MOTOR WONT START IF

A

MECHANICAL TORQUE IS GREATER THAN ELECTRICAL TORQUE

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8
Q

MOTOR ACCELERATES AND START IF

A

ELECTRICAL TORQUE IS GREATER THAN MECHANICAL TORQUE

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9
Q

MOTOR MAY STALL IF

A

ELECTRICAL TORQUE EQUAL TO MECHANICAL TORQUE

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10
Q

WHAT ARE THE THINGS ARE PROVIDED BY STATIC MOTOR STARTING STUDY

A

1) MOTOR’S STARTING CURRENT
2) VOLTAGE AT MOTOR TERMINAL DURING STARTING
3) STARTING POWER FACTOR OF MOTOR

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11
Q

IF MOTOR STARTS IN STATIC MOTOR STARTING STUDIES

A

IT WILL DEFINITELY START INDYNAMIC MOTOR STARTING STUDIES

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12
Q

FORMULA FOR MOTOR EFFICIENCY?

A

EFFICIENCY= OUTPUT POWER(MECHANICAL)/INPUT

POWER(ELECTICAL)

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13
Q

EFFICIENCY OF MOTOR IS DETERMINED BY

A

2 TYPES OF LOSSES

1) FIXED LOSSES-INDEPENDENT OF LOAD
2) VARIABLE LOSSES-DEPENDENT ON LOAD

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14
Q

FACTORS BEHIND FIXED LOSSES

A

1) LOSSES DUE TO CORE MAGNETIZATION(HYSTERISIS AND EDDY CURRENT LOSSES)\
2) FRICTION LOSSES
3) WINDAGE LOSSES

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15
Q

FACTORS BEHIND VARIABLE LOSSES

A

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)

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16
Q

ELECTRICAL POWER (P) FORMULA?

A

P=SQR ROOT OF 3 VI* COS (PI) AND P= APPARENT POWER * COS(PI)

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17
Q

APPARENT POWER FORMULA?

A

APPARENT POWER =SQR ROOT OF 3VOLTAGELINE CURRENT(IL) OR POWER TRIANGLE

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18
Q

(KVA AMPS) i FORMULA?

A

I= RATED MECHANICAL POWER / EFFICIENCY*PF

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19
Q

WHAT IS SERVICE FACTOR?

A

;;NEED A PROPER EXPLANATION

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20
Q

SLIP FORMULA?

A

SYNCHRONOUS SPEED-(MINUS) ROTOR SPEED/SYCHRONOUS SPEED

SLIP=(Ns - ROTOR SPEED)/Ns

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21
Q

IF MOTOR LOADING INCREASES WHAT HAPPENS TO LOSSES AND EFFICIENCY?

A

LOSSES INCREASES AND EFFICIENCY DECREASES

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22
Q

FAULT CURRENT FORMULA?(LRA)

A

LRA =FLA/%Z WHEREAS %Z = X’’

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23
Q

X’’=%Z AND X’’ FORMULA

A

X’’= (FLA/LRC) * (100 IF NECCESARRY)

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24
Q

(LRC FAULT CURRENT’S KVA) FORMULA?

A

LRC’S APPAERNT POWER =1.732VLRC

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25
1 HP IS EQUAL TO
1 HP=0.746 TIMES OF KW(MECHANICAL OUTPUT)
26
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)
27
TORQUE FORMULA ?
T = (9.55* MECHANICAL RATING )/RATED SPEED OF ROTOR UNIT CAN BE IN N-M OR KN-M
28
MAXIMUM BREAK DOWN TORQUE IS SAME AS
PULL OUT TORQUE
29
AT MOTOR STARTING CONDITION MOTOR ACT AS WHAT TYPE OF LOAD?
CONSTANT IMPEDANCE LOAD | NOTE: (VOLTAGE DECREASES STARTING CURRENT DECREASES)
30
WHEN WE NEED STARTER FOR MOTOR?
IF MOTOR BUS VOLTAGE IS NOT WITHIN LIMITS
31
WORST CASE SCENERIO FOR MOTOR STARTING STUDIES
CONSIDER BASE LOAD AT TIME OF MOTOR STARTING | CONNECTED TRAFO MUST BE FULLY LOADED UNDER THIS CONDITION
32
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)
33
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
34
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
35
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
36
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
37
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
38
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
39
why inertia of system is so important?
because it determines the acceleration time of motor
40
what happens if inertia is higher?
motor will take long time to start
41
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.
42
what is a breakdown torque?
maximum torque a motor able to produce
43
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
44
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
45
what is the other name for static motor study?
voltage drop snap shot study
46
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
47
if there is high voltage drop at pcc what happens to short circuit current?
short circuit current decrease
48
motor starting study should be performed at which s.c. current?
in both max and min s.c.current
49
what is a weak system?
it has high source impedance , low s.c. current, high voltage drop
50
what is a stong system?
it has low source impedance , high s.c. current, low voltage drop
51
if voltage drop beyond required limit during motor starting study what is remedy?
starters
52
FLA FORMULA
KW/1.732*V*PF*efficiency kw= mechanical output
53
Torque formula
9550*kw/rotor speed
54
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
55
what is W*R^2?
inertia w- weight of motor R- radius H-inertia constant
56
what is the relationship between speed and slip
speed is inversely proportional to slip
57
what is acceleration torque?
difference between motor torque and load torque
58
motor should be chosen based on what?
load inertia
59
how will reduce acceleration time of motor?
by giving lower inertia of
60
if load is 30kw how we should model motor?
we should oversize the motor to make it run at 80% of its full load
61
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)
62
what are types of motor connected loads?
1) constant torque 2) constant horse power 3) variable torque
63
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
64
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
65
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
66
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
67
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
68
from where you will get load model parameters?
from mechanical team
69
what is the output of motor starting study?
1) voltage dip during motor starting 2) motor starting current 3) motor starting power factor
70
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
71
disadvantages of slip ring motor?
1) copper loss is high 2) efficiency is low 3) requires much maintainence
72
application of slip ring motor?
in high torque applications
73
advantages of squirrel cage motor?
1) construction is simple 2) efficiency is high 3) less maintainence
74
disadvantages of squirrel cage motor?
1) starting torque is low 2) high starting current and pf 3) no external resistance
75
Application of squirrel cage motor?
in low torque application

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