1. Electro Mechanical System
Direction of Rotation
Positive crests of the currents in a positive ABC phase sequence follow each other in the order A  B  C  A
This phase sequence produces a magnetic field that rotates clockwise for windings arranged ABC in a clockwise layout around the stator.
Changing either two phase sequence or the winding layout will cause the magnetic field to rotate in the opposite direction.
Early machines are build with salient poles, the stator of modern motors is smooth with slots cut into the stator to hold the windings.
In practice the use of a single coil per pole is subdivided into 2, 3,or more coils lodged in adjacent slots, constituting a phase group.
When there are two or more coils, they are distributed in successive stator slots and connected in series.
Lecture 20
Electro Mechanical System

2. Electro Mechanical System
Direction of Rotation
Lecture 20
Electro Mechanical System

3. Electro Mechanical System
Direction of Rotation
In the following diagram the upper group of phase A is composed of five coils distributed in five succes­sive slots.
The five coils together produce one upper broad pole, associated with phase A.
Current Ia in the two groups produces 2-pole flux pattern.
Lecture 20
Electro Mechanical System

4. No. of poles - Synchronous Speed
The speed of the revolving flux could be reduced by increasing the number of poles
To construct a 4-pole stator, the coils are distributed so that the four identical groups of phase A now span only 90° of the stator circumference.
Groups are connected in series so that adjacent groups produce mmf’s acting in opposite directions. Ia flows in the stator winding of phase A, it creates four alternate N-S poles.
Windings of other two phases are identical but are displaced from each other by a mechanical angle of 60°.
Lecture 20
Electro Mechanical System

5. No. of poles - Synchronous Speed
Number of poles can be increased as much as we like provided there are enough slots.
we can take a 3-phase, 8-pole stator. Each phase consists of 8 groups, and the groups of all the phases together produce an 8-pole rotating field.
Each phase covers mechanical angle of 360/8 = 45°.
Suppose the current in phase A is at its maximum positive value. The magnetic flux is then centered on phase A, and the N-S poles are located.
One-half cycle later, the current in phase A will reach its maximum negative value.
The flux pattern will be the same, except that all the N poles will have become S poles and vice versa.
Entire magnetic field has shifted by a angle of 45° and this gives us the clue to finding the speed of rotation.
Lecture 20
Electro Mechanical System

6. Electro Mechanical System
Synchronous Speed
Flux moves 45° & takes 8 half-cycles or 4 full cycles. On 60 Hz system time of one turn is 4 X 1/60 = 1/15 s. The flux turns at the rate of 15 r/s or 900 r/min.
Rotating speed depends upon the frequency & no. of poles on the stator.
Synchronous speed ns =120f/p
where : f = frequency of source [Hz]
p = number of poles
Lecture 20
Electro Mechanical System

7. Starting of a Squirrel-Cage motor.
Let the stator of an induction motor connected to a 3-phase & rotor be locked in a stationary position.
Revolving magnetic field produced by the stator cuts across the rotor bars & induces a voltage in all the conducting bars.
The induced voltage is ac because of the rapid succession of time varying flux from N to S to N, etc. (transformer action).
The frequency of the induced voltage depends on the number of N and S poles that sweep across a conductor per second.
At rest (zero speed) it is always equal to the source frequency.
End-rings form a 3-phase short circuit and the induced voltage drives a large current through all the bars (100’s of amps).
Large currents react with the stator magnetic field creating strong forces.
The sum of all mechanical forces produce a torque on the rotor.
Lecture 20
Electro Mechanical System

8. Electro Mechanical System
Rotor Acceleration
As soon as the rotor is released;
The torque causes rapid acceleration in the direction of the rotating flux field.
The rotor speed increases and the relative velocity of the magnetic field with respect to the rotor diminishes.
Frequency and the magnitude of induced voltage decreases since, rotor bars are being cut more slowly.
As a result, the very large rotor current decreases rapidly with increased rotor speed.
The mechanical forces and torque weakens.
The speed continues to increase, but it will never catch up with the revolving field (synchronous speed).
At synchronous speed, rotor would no longer cut any flux and induced voltage and current would fall to zero.
The rotor speed is slightly less than synchronous speed so as to produce a current in the rotor bars.
Lecture 20
Electro Mechanical System

9. Electro Mechanical System
Applying Loads
Consider a motor initially running at no-load
A mechanical load is connected to the rotor shaft
A counter-torque causes a decrease in rotor kinetic energy and a slow down of the rotor speed
The relative speed between the rotor and the rotating flux (at synchronous speed) increases
Flux cuts the conducting bars at a higher & higher rate
Frequency & magnitude of induced voltage increases
Larger induced voltage drive more current in the rotor
Current and flux react to produce greater drive torque
The rotor speed and the mechanical load will reach a state of equilibrium
The motor torque will equal the load counter-torque
The speed will be stable
Lecture 20
Electro Mechanical System