Transcript Speed control 3-phase induction motors

Prepared by:•Chandan
pathak (140413117008)
•Utpal rathod(140413117010)
•Hitendra vyas(140413117012)
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The expression for the actual speed of a 3-phase
induction motor is given by:-
This show that the actual speed depend on to
factors
Synchronous speed Ns & Slip s.
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If we change any one of them so we can vary
the speed of an induction motor
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Another way to change the speed is to change the
torque produce by the motor which is given by :-
If the motor torque is less than the load torque
then the motor speed will be decrease & if the
torque is higher then the load torque the motor
speed will increase.
Mainly two methods control the speed of
induction motor by stator parameters and rotor
parameters.
The parameter like frequency, supply voltage, no
of poles, external resistance can be control the
speed.
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In this type of control the frequency is constant
and stator voltage varied to vary the speed of
induction motor.
Torque is proportional to the square of stator
supply voltage V1 follows :-
But E₂ is proportional to the supply voltage V₁
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Torque slip characteristic up the value S is low
so that (s X₂)² value is neglect
R₂ and K is also constant hence ,
Stator supply voltage V₁ is reduced from to
bV₁, where b < 1.
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Actual speed N is given :-
The actual speed can be changed by the
changing synchronous speed. But synchronous
speed Ns can be change by the changing the
stator frequency F₁ so theoretically speed can
be control by only F₁.
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Only changing in F₁ , keeping V₁ is constant has an
adverse effect on air gap flux because air gap flux
given by,
So air gap flux ᶲₐg is proportional to ratio of (V₁/F₁).
Hence the ratio (V₁/F₁) is kept constant by changing
the stator voltage V₁ and frequency F₁ simultaneously.
This is necessary to keep the air gap flux constant .
Hence this mathod is called constant (V/F) control.
• The ac input is constant voltage and constant frequency is
apply to ac to dc converter which is rectifier.
• At the out put of ac to dc convector a dc voltage is obtain a
capacitor bank is used to reduce the ripple contents in the dc
voltage.
• The dc voltage is applied of an inverter. The inverter is an
electronic circuit which convert the dc voltage in to the 3-face
variable voltage and frequency ac voltage .
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The synchronous speed is given by,
So it is possible to change the synchronous
speed by changing the number of poles.
The no of poles can be changed by changing
the connection of stator winding with the help
of simple switching .
The no of the poles in the ratio of 2:1 .so two
different speeds are obtained.
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This control is basically the stator voltage
control because when the rheostat change
which is connected to the stator circuit ,a part
of supply voltage will drop across the rheostat.
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The actual voltage applied to the stator is
reduced the speed change due to variable
stator voltage.
In the stator position a minimum stator voltage
is applied to the speed of the induction motor
is minimum stator voltage is applied an speed
of the induction motor is minimum and the
reduced towards the run position the stator
voltage is increased and the speed also
increases.
The set up speed control Using external rotor resistance Is
show in figure.
• The variable rotor resistance Is connected through The
slipring and brushes.
• The values of critical slip Sm Corresponding to the
maximum torque increases with the increases In the value
of R₂ and the torque slip characteristics.
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If assume that the load torque Tl is constant
then the operating
Point of the introduction motor shifts from
point A to point B.
So the motor speed changes from N₁ to N₂
where N₂ is less than N₁.
Thus the speed decreases with increase in the
rotor resistance R₂.