Lecture 5
AC Machines and Rotating
magnetic field
AC generator and motor
Rotating magnetic field in a 3-phase coil stator
If two magnetic fields are present in a machine, .then a torque will be created which will tend to line up the two
magnetic fields. If one magnetic field is produced by the stator of an ac machine and the other one is produced by
the rotor of the machine, then a torque will be induced in the rotor which will cause the rotor to turn and align
itself with the stator magnetic field. If there were some way to make the stator magnetic field rotate, then the
induced torque in the rotor would cause it to constantly "chase" the stator magnetic field around in a circle.
How to produce a rotating magnetic field in a three-phase stator?
How can the stator magnetic field be made to rotate?
The fundamental principle of ac machine operation is that if “a three-phase set of currents, each of equal magnitude
and differing in phase by 120°, flows in a three-phase winding, then it will produce a rotating magnetic field of
constant magnitude.”
Three-phase stator
A set of currents are applied to the 3-phase stator coils
Flux densities resulting from these currents are;
The direction of the magnetic field vector is given by the
right-hand rule: If the fingers of the right hand curl in the
direction of the current flow in the coil, then the resulting
magnetic field is in the direction that the thumb points
Electrical Angle
Mechanical Angle
Electrical Cycle = 360o
The Relationship between Electrical Frequency and the Speed of Magnetic Field Rotation
Өse : synchronous electrical angle, (rad)
Өsm : synchronous mechanical angle, (rad)
fse : synchronous electrical frequency, (Hz)
fsm : synchronous mechanical frequency, (Hz)
ωse : synchronous electrical frequency, (rad/sec)
ω sm : synchronous mechanical frequency, (rad/sec)
In general, if the number of magnetic poles on an ac machine stator is P, then there are P/2 repetitions of the winding
sequence a-c' -b-a' -c-b' around its inner surface, and the electrical and mechanical quantities on the stator are related by
The resulting mechanical speed of the stator magnetic fields in revolutions per minute (nsm)is given by
𝟏𝟐𝟎𝒇𝒔𝒆
𝒏𝒔𝒎 = 𝟔𝟎𝒇𝒔𝒎 =
𝑷
Reversing the Direction of Magnetic Field Rotation
If the current in any two of the three coils is swapped, the direction of the magnetic field 's rotation will be reversed.
This means that it is possible to reverse the direction of rotation of an ac motor just by switching the supply connections on any
two of the three coils.
Synchronous
and Induction Machines
In both asynchronous (induction) and synchronous machines, the stator has axial slots which consist of 3-phase stator winding
wound for a specific number of poles. 3-phase AC power is supplied to the stator to create a magnetic field in the stator rotating with
synchronous speed (nsm =
𝟏𝟐𝟎𝒇𝒔𝒆
)
𝑷
Induction (Asynchronous) Machine
Short-circuited rotor
Synchronous Machine
•
•
In this machine, the rotor winding is fed with an
external DC supply with the help of slip rings. A rotor
with permanent magnets can also be used. Thus, the
rotor has a constant magnetic field. The N pole of the
rotor will move toward the S pole of the magnetic field
of the stator, and vice versa. This magneto-mechanical
attraction creates a force which will drive the rotor to
follow the stator rotating magnetic field in a
synchronous manner
Synchronous machine rotor always runs at a speed (nrm)
equal to its synchronous speed. nrm = nsm= 120f/P
•
•
•
In this machine, the rotor is short-circuited (squirrel cage rotor type) or
closed through an external impedance (wound rotor type). The stator
rotating magnetic flux goes to the rotor inducing currents in the rotor
windings; which in turn create magnetic fields in the rotor. Hence, the
rotor will start to rotate in the direction of the rotating stator magnetic
field, however, with a speed (nrm) slower than synchronous speed(nsm).
The difference, or "slip," between the rotor speed and synchronous speed
varies from about 0.5 to 5.0% .
The induction machine's essential character is that it is created solely by
induction without the need of external sources.
They involve a degree of 'slip' in order that the current may be produced
due to the relative movement of the stator field and the rotor ( nrm < nsm )
• How to produce a rotating magnetic field in a machine stator?
• How to reverse direction of rotation of an AC motor
• What are the main differences between a synchronous and an induction machine