induction machines

Question 1

typical effiecncy

Answer 1

> 90%

Question 2

key components

Answer 2

soft iron core,
3 phase winding,
soft iron rotor
squirrel cage

Question 3

setup of staor winding

Answer 3

iron stator core in which a three-phase stator winding is incorporated into a series of slots in the core

Question 4

airgap field produced by phase A

Answer 4

Ba = magnetic flux density in the airgap
K = a constant which relates to the dimensions of the iron circuit in the machine
N×Ia = phase Ampere-turn

Question 5

current in each coil a,b ,c

Answer 5

Ia=isin(wt)
Ia=isin(wt-120)
Ia=isin(wt-240)

Question 6

synchronous speed

Answer 6

Ns=f*60/pole pairs

Question 7

induction speed in comparison to synchronous speed

Answer 7

an induction machine runs at a speed which is close to, but always
slightly lower than the synchronous speed

Question 8

equivalent cicuit for one phase

Answer 8

a resisistor and inductor then in parallel an inductor/resistor/back emf

Question 9

what does each element of the equivalent cicuit represent

Answer 9

R1- stator winding resistance per phase
X1- stator leakage resistance
Rm- eddy cuurent and hysteress(iron losses)

Question 10

Vp and Ip when star connected

Answer 10

Vp=Vl/sqrt(3)

Ip=Il

Question 11

Vp and Ip when delta connected

Answer 11

Ip=Il/sqrt(3)

Vp=Vl

Question 12

slip

Answer 12

S=(Ns-N)/Ns

Question 13

what happens when the speed reaches synchronous speed

Answer 13

The rotor and stator field move at the same speed. No change in flux linkage, no induced voltage , zero current and therefore zero torque

Question 14

Power delivered by the stator

Answer 14

Ps=(2piNs/60)*T

Question 15

Mechanical output from the rotor

Answer 15

Pr=(2piN/60)*T

Question 16

rotor loss

Answer 16

the difference between stator power and power from the rotor

Question 17

rotor loss in teerms of power delivered by stator and slip

Answer 17

stator power*slip

Question 18

rotor equivalent circuit

Answer 18

An inductor and a resistor in series

Question 19

what do the resistor and inductor represent

Answer 19

R-rotor resistance

L- rotor leakage and inductance (fields that do not couple to the stator)

Question 20

impedance of the inductor

Answer 20

j2pifL

Question 21

torque and power

Answer 21

P=Tw

Question 22

referring voltage uses

Answer 22

E’2=(N1/N2)*E2

turns ratio

Question 23

referring current and voltage

Answer 23

I’^2R’=I^2R

Question 24

for referring current

Answer 24

I’2=(N2/N1)*I2

Question 25

for referring resistance and inductance

Answer 25

R’2=(N1/N2)^2*R2

Question 26

what is the referred circuit multiplied by

Answer 26

f1/f2

Question 27

aswell as the referred components what is in equivalent circuit

Answer 27

in parralel and and in parralel to eachother and inductor and resistor

Question 28

what is the star connected phase voltage

Answer 28

Va=V/sqrt(3)

Question 29

what side of pull out torque is a machine normally operated

Answer 29

Above the pull out torque

Question 30

what speed is when current at a maximum and why can’t this be maintained

Answer 30

at 0 speed - it will overheat

Question 31

slip is an indicator of

Answer 31

rotor thermal losses

Question 32

what is lagging/power factor

Answer 32

power with a phasor

cos(theta)=x lagging

Question 33

copper loss equation

Answer 33

I^2*R

Question 34

output power using torque

Answer 34

P=Tw

Question 35

f1 to f2 using slip

Answer 35

f1=s*f2

Question 36

how does a reluctance stepper motor work

Answer 36

the rotor alligns with energised teeth

Question 37

how can the number of steps in a reluctance stepper be increased

Answer 37

half stepper mode

Question 38

what is 2 benefits of half stepper mode

Answer 38

the overshoot of the rotor is lower and greater resolution of load positioning

Question 39

At a high step rate what happens to pul out torque

Answer 39

it goes down