DC MACHINES

Question 1

Induced emf in a dynamo

Answer 1

E=PONZ/60a

Question 2

Machine constant

Answer 2

E=kON

Question 3

Lap winding (a)

Answer 3

pm

Question 4

Wind winding (a)

Answer 4

2m

Question 5

Series motor voltage equation

Answer 5

Vt-Ia(Ra+Rf)-Vbc
Ia=I line=If=Pin/Vt

Question 6

Shunt motor voltage equation

Answer 6

Eb=Vt-IaRa-Vbc
Ia=Iline-If
If=Vt/Rf

Question 7

Torque developed

Answer 7

Pdev/2 pi N
Eb Ia/2 pi N
9.55 (pdev/N) N.m
7.04 (pdev/N) lb.ft

Question 8

Torque constant

Answer 8

t=KOI

Question 9

Motor efficiency

Answer 9

Pout/Pin
[Vt IL- (Pcu+SPL)] /Vt IL

Question 10

Maximum efficiency condition

Answer 10

Variable losses = Constant losses
Ia’= sqrt(Pk/Ra)
CONSTANT LOSSES –> Pk = SPL+ (If^2) (Rf)
VARIABLE LOSSES –> (Ia’)^2 (Ra)

I’L = Ia’ + If
Nmax = [(Vt )(I’L) - total losses] /(Vt )(I’L)

Question 11

generator conductor current

Answer 11

I = Ia/a
a = pm (lap winding)
a = 2m (wave winding)

Question 12

coil pitch

Answer 12

Ys = (Slots/Pole) - K

Question 13

back and front pitch progressive winding

Answer 13

Yb = (Z/P) +1
Yf = (Z/P) -1

Question 14

commutator pitch, Yc

Answer 14

lap winding Yc = m
wave winding Yc = (C +/- M) P/2
C- number of commutator bars/coils

Question 15

back and front pitch retrogressive winding

Answer 15

Yb = (Z/P) -1
Yf = (Z/P) + 1

Question 16

series generator voltage equation

Answer 16

Eg = Vt + Ia(Ra+Rs)+Vbc
IL=Is=Ia

Question 17

shunt generator voltage equation

Answer 17

Eg= Vt+IaRa+Vbc
Ia=Il+If
If=Vt/Rf

Question 18

dc generator maximum efficiency

Answer 18

Variable losses = Constant losses
Ia’= sqrt(Pk/Ra)
I’L = Ia’ - If
Nmax = [(Vt )(I’L)] / [(Vt )(I’L) - total losses]

Question 19

torque developed to Hp

Answer 19

HP= 2TN pi / 33,000

Question 20

mechanical power developed

Answer 20

P= 2TN pi in cps
P= 2TN pi / 60 in rpm
Td = 9.55 (Pdev/N) in N.m

Question 21

copper loss in dc machines

Answer 21

copper loss at armature and field

Question 22

mechanical loss in dc machines

Answer 22

SPL
-friction and windage
-bearing losses
-stray load losses
Core and iron loss
-hysteresis loss
-eddy current loss

Question 23

pole pitch

Answer 23

slots/ pole