Ministry of Higher Education and Scientific Research Southern
Southern Technical University
Engineering Technical College / Basrah
Department of Electrical Power Techniques Engineering
Induction Motor/3
Third Year
Lecturer : M.Sc. 𝐖𝐚𝐟𝐞𝐞𝐪𝐚 𝐀𝐛𝐝𝐮𝐥𝐫𝐚𝐳𝐚𝐤 𝐇𝐚𝐬𝐚𝐧
Lec.8
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Southern Technical University Stage: Third Year
Engineering Technical College / Basrah Sub. :Induction Motor
Department of Electrical Power Techniques Engineering Lecturer :M.Sc. Wafeeqa A. Hasan
Induction Generator:-
• An induction generator does not differ in its construction from an induction motor. An induction machine can act
as generator or motor depending upon its slip. Below NS, it can operate as motor, above NS it's operate as
generator.
• The power factor at which an induction generator operates is fixed by its slip and its equivalent circuit constants
and not by the load. It is necessary to operate induction generator in parallel with the main supply.
• The main supply not only supply the lagging current demand by the load but also supply sufficient lagging
current to cancel the leading component of the current delivered by the induction generator. The induction
generator depends upon its leading current for excitation, and unless the connected load takes this leading
component of current, the induction generator loses its excitation. The frequency and voltage of the induction
generator are the same as for the main supply. The induction generator output is fixed by the slip.
Changes in Power Produced by a Change in Slip:-
The following changes in power occur as the slip of an induction machine changes:-
1. Below NS there is rotor current and the motor action occurs.
2. At NS the rotor current is zero. The current in the stator comes from the main supply. It consists
from the exciting current Im and the core loss component Ip. The mechanical power (taken from
prime mover) required to drive the rotor at NS is equal to the friction and windage.
3. Above NS the current in the rotor reverses in the direction as does also the component current in the
stator required to balance the demagnetizing action of the rotor current. At a speed above NS , generator
action occurs, but the power is not delivered to the external circuit until the current in the stator
has a component equal and opposite to the current Ip required to supply the coreless. Therefore,
the induction generator at this slip will supply its core loss only. Its external output is zero. At larger
slip, power is delivered to the load.
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Southern Technical University Stage: Third Year
Engineering Technical College / Basrah Sub. :Induction Motor
Department of Electrical Power Techniques Engineering Lecturer :M.Sc. Wafeeqa A. Hasan
Power Factor of the Induction Generator:-
The only current to produce generating power in an induction generator is that component of
stator current which is equal and opposite of the referred current. The power factor of
this component with respect to the primary induced voltage is fixed by the rotor constants
and by the slip. The power factor of an induction generator is leading. The power factor in
large machines is over 0.9 at full load, but at no load or small loads it may be very low.
Phase Relation Between Rotor Current Referred to the Stator and Rotor Induced Voltage E2:-
Below NS, S is +ve and I2 takes the form 𝐼2 = 𝐴 − 𝑗𝐵 (lagging current with respect to SE2).
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Southern Technical University Stage: Third Year
Engineering Technical College / Basrah Sub. :Induction Motor
Department of Electrical Power Techniques Engineering Lecturer :M.Sc. Wafeeqa A. Hasan
Above NS, S is -ve and I2 takes the form 𝐼2 = −𝐴 − 𝑗𝐵 (leading current with respect to
SE2).
•
The current I2 in the rotor can not actually lead the rotor voltage which causes it since the
rotor circuit is inductive. It is only when this current is considered with respect to the
stator. This can be seen from the following figure:-
• Consider the voltage induced in conductor "a" on the rotor. This voltage has its
maximum value when the conductor is at maximum flux density of the stator field, in
position "a". Below NS, the rotor moves to the right relatively to the field, and since
the rotor circuit is inductive, the conductor moves to position "b" before the current in it
reaches maximum. 3
Southern Technical University Stage: Third Year
Engineering Technical College / Basrah Sub. :Induction Motor
Department of Electrical Power Techniques Engineering Lecturer :M.Sc. Wafeeqa A. Hasan
• Above NS, the rotor moves faster than the field, i.e., it moves to the left with respect to
the field. In this case the conductor "a" moves to position "b’ " before the current in it
reaches its maximum value.
• In both cases the rotor when considered with respect to the stator moves in the same
direction as the field. Therefore, if the voltage and current in the rotor are observed from any
fixed point on the stator, the voltage is seen to pass through its maximum (lagging) when the
rotor is below NS and after the current passes through its maximum value (leading) when the
rotor is above NS.
Phasor Diagram of the Induction Generator:
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Southern Technical University Stage: Third Year
Engineering Technical College / Basrah Sub. :Induction Motor
Department of Electrical Power Techniques Engineering Lecturer :M.Sc. Wafeeqa A. Hasan
Use of Capacitors in Parallel with the Induction Generator:-
• It is possible to operate an induction generator without connecting it to the main
supply by connecting suitable capacitors across its terminals. This method of operation
has no practical importance because of the big size and cost of the capacitors and due
to the dropping voltage characteristics of such system.
• It is often stated that an induction generator is not self-exciting, this statement is not
true. If suitable capacitance is placed across the terminals of an induction generator, it
generally builds up as a synchronous generator because of the residual magnetism in
the rotor from previous operation.
• From the figure, "a" is the operating point and XC is the value of the capacitive reactance.
𝑋́𝐶 = 1/𝜔𝐶́ is the value of the critical capacitive reactance. This is similar to the
magnetizing characteristics of dc shunt generator.
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Southern Technical University Stage: Third Year
Engineering Technical College / Basrah Sub. :Induction Motor
Department of Electrical Power Techniques Engineering Lecturer :M.Sc. Wafeeqa A. Hasan
Application of Induction Generator:-
• Induction generators are useful when the prime mover does not run at
constant speed; for example, hydro-electric power stations, and wind power
stations because the frequency depends on that of the main supply. Also, they
are used in the braking action of the induction motor.
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Thank You
