Electric Machines Part 2a Introduction AC Machines
This is the first part of topic 2 in the series of "Electric Machines”
In this part we will give an introduction to AC
machines which includes 3 phase AC motors
and 3 phase AC generator.
There are 4 type of AC machines
Induction motor, Induction generator
Synchronous motor, Synchronous generator
Specific details about these machines will be discussed in later videos
Now let us discuss the general construction of an AC machine.
Each 3 phase AC machine consist of two parts
We have the Stator and we have the Rotor
Stator: which is the stationary part of the machine.
It consists of a frame, core and 3 phase
winding
Rotor: the rotating part of the machine. There are two types of rotor in induction machines.
One is the squirrel cage rotor, it consists of conductor bars that are shorted with end rings
and a core in which the conductor bars are placed.
The other one is the phase wound rotor, which consist of 3 phase winding similar to stator winding
and has an option of adding external resistance through carbon brush and slip rings.
Also there are two types of rotor in synchronous machine
named as salient and non salient poles…
the salient poles protrude out of the rotor and
non salient poles do not protrude out of the rotor face.
Now let’s move on to the next area of construction and
talk about the field and armature winding.
This concept of armature and field is related
only to synchronous machines, which is
An armature in ac machine is the stator winding.
While field winding is the constant dc supply provided at rotor. please watch the quick 30 second sponsorship video. [[ Sponsor 30 second video ]] And now we’re back...
let’s now have a quick idea about principle on which the AC machines work.
Whenever a current carrying conductor is placed with in a magnetic field,
a force acts upon it.
The reason behind this force is due to the
interaction of the magnetic field that is supplied, and the magnetic field
produced due to the current carrying conductor.
if this conductor is arranged in order to form a loop,
a torque is developed due to the change in direction of current.
This torque will then move the coil in a
circular path, hence creating the motor effect.
Now let us connect this with motional EMF.
A Motional EMF is whenever a conductor is placed in a changing magnetic field,
it tends to have a voltage induced on it
and if it is formed in a close loop and
a current flows through it,
a force acts upon it.
The opposite is also possible that is a
conductor moving in a constant magnetic field
would experience a voltage induced on it.
After having a brief idea about motional EMF now
let’s move on to our discussion about how motors works using thiese concepts.
in an AC machine, 3 phase stator windings are place in a rotor conductor.
The voltage provided to the stator windings produces the required rotating flux.
The rotating flux (magnetic field) cuts the rotor bar and induces an EMF in the rotor.
The current that is set up in the rotor bars tends to produce its own magnetic field
that tries to catch up the rotating magnetic field produced by the winding .
The rotor gets the torque needed to rotate can be seen through the torque formula for
A.C machine which links the magnetic field of the rotor and stator with torque
now this is the equation T is the torque
Br is the rotor magnetic field and Bs is the stator magnetic field
The formula explains that in order to have a torque set up
it is required to have both magnetic fields in the rotor and the stator to be present,
as we stated before, or else there is no rotation of rotor.
This phenomenon is used in AC induction motors.
And for synchronous motors there is a separate
field winding on the rotor to create the
magnetic field that follows the rotor.
We will look at synchronous motors
in more detail in later parts.
That’s all from this part. In the next video we will look at induction motor in detail..
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