DC Machine_session 3 out of four sessions

DC Machine_session 3 out of four sessions

1. Electrical Machines-1
Unit 3: DCMachines
Dr.Mrs. Haripriya H. Kulkarni.

2. Review of previous Lecture







•Transformer-1 phase and 3 phase

3. Learning Objectives
Comparison between Motor and Generator
Study of D.C. Machine
Basics of Machine
Operating principle of Motor and Generator
Different parts and its function

4. Learning Outcomes
Students will be able to :
Compare Motor and Generator
Study of D.C. Machine
Explain basics of Machine
Demonstrate operating principle of Motor and Generator
Different parts and its function

5. DC Generator Construction
Commutator
Brushes

7. Derivation of EMF Equation of a DC Machine – Generator and Motor
Let,
P – number of poles of the machine
ϕ – Flux per pole in Weber.
Z – Total number of armature conductors.
N – Speed of armature in revolution per minute (r.p.m).
A – number of parallel paths in the armature winding.
In one revolution of the armature,
the flux cut by one conductor is given as:

8. Therefore, the average induced e.m.f in one conductor will be:

9. Back emf in Motor
When the current-carrying conductor placed in a magnetic field, the torque induces on the conductor, the
torque rotates the conductor which cuts the fl,c, of the magnetic field.

According to the Electromagnetic Induction Phenomenon "when the conductor cuts the magnetic field,
EMF induces in theconductor".

The Fleming light-hand rule determines the direction of the induced EMF.

It isseen that thedirection of the induced emf is opposite tothe appliedvoltage. '111ereby theemfis
known as the counter emf or back emf.

The back emf is developed in sedes with the applied volti,ge, but OppOSite in direction, i.e., the back emf
opp05es the current which causes it.

10. What actually Back emf in Motor does?(Significance)
Back EMF represents that portion of tl1e supply voltage which multiplied by the
current equals tl1e mechanical workproduced (roughlyspeaking).
(Therestofthesupplyvoltage is used upas1-Rclrop and produces heat - it is wasted
energy. So you really want your back EMF to be as close to the supply voltage as
possible).

The back EMF plays a self-regulating role by limiting current and energy flow
through the motor. In a parallel motor, it is only due to back EMF that if you apply a
good voltage source toan armature with a verylow resistance, whether no-load or ft
l-load, the motor would run nicelyat a near-constant speed.