Unit-3-DC Motor electrical machine I.pdf

dc motor

1. B = Flux density due to the flux produced by the field winding.

L = Active length of the conductor
T = Magnitude of the current passing through the conductor.

left hand rule can be diagramatically shown as in the Fig -2.1

Direction of

fux

hand
Direction of

Fig-2.1 Cure

Apply the rule to crosscheck the direction of force experienced by a single
2.2 (a), (b),(c) and (d).

conductor, placed in the magnetic field, shown in the Fig

2. xperienced by conductor

It can be seen from the Fig. 2.2. that if the direction of the main field in which

current carrying conductor is placed, is reversed, force experienced by the
conductor reverses its direction. Similarly keeping main flux direction unchanged,
the direction of current passing through the conductor is reversed, the force
experienced by the conductor reverses its direction. However if both 1
(directions are reversed, the direction of the force experienced remains the

same.

3. voltage across armature circuit = V — E,

TER, is the armature circuit resistance, then, I

4. This is known as power equation of the d.c. motor
:ctric power supplied to armature (armature input)

Enl, = power developed by armature (armature output)

sted in armature (armature Cu loss)

the armature.

Since, V and R, are fixed, p eloped by the motor depends upon

armature current. For maximum power. dP./dl, should be zero.

CA

a,” LR

Hence mechanical power developed by the motor is maximum v

is equal to half the applied voltage.

5. Now Eyl, = power dev

E,

Output in watts = I,

Output in watts
o Ty (put in watts.

Output in watts

or T, N-m

6. ‘or a shunt motor, flux q is practically constant.

Tel,

8. upto magnetic saturation

10. Fig.2.16

11. n

V-LR)
or a om

for shunt motor
R,+Re. for series motor

Spe
eld rheosiat
in fol ereut

L . a
i

Fig. 2.18 Fig.2.19

12. WITHOUT
CONTROLLER

TH
NTROLLER

13. Motoe-gonerator Sat

21. Efficiency = Quput
Tnput
Output =Input — Losses and Input = Output + Lo

Therefore, the efficiency of a de. machine can also be exp
following forms:

Input — Losse
Input

Efficiency

Output

Eftcieney = Sar La

25. Instead of just stating the applications, the behaviour of the various

characteristics like speed, starting torque etc, which makes the motor
more suitable for the applications, is also stated in the Table