Transcript INDUCTION MOTOR

* Induction motor speed control due to the
change in stator voltage.
* Using the closed loop feedback circuit to
control the slippery rings of AC
Study the following :
 Theory of the induction motors (single and
three phase).
 Methods of speed control of induction motors.
 Circuit diagram of equivalent of the induction
motor.
Equations of the performance and curves of it.
Examples.
Primarily , electric drives can be divided into two groups , dc drives and ac drives .
Advantages and disadvantages of ac drives with respect to dc drives are as under :
Advantages of ac drives
(i)
For the same rating, ac motor are lighter in weight as compared to dc motors .
(ii)
AC motor are less expensive as compared to equivalent dc motor .
(iii) AC motor require low maintenance as compared to dc motor .
(iv) AC motor can work in hazardous areas like chemical, petrochemical etc. whereas dc
motor are unsuitable for such environments because of commutator sparking .
Disadvantages of ac drives
(i)
Power converters for the control of ac motors are more complex .
(ii) Power converters for ac drives more expensive .
( iii) power converters for ac drive generate harmonic in the supply system and load
circuit . As result , ac motor get debated .
The advantages of ac drives outweigh their
disadvantages .(
(20% to 40%)
). AC drives are used for
several industrial applications such as in
conveyer, lift , mixer, escalator etc. In general,
there are two types of ac drives :
1. Induction motor drives .
2. Synchronous motor drives .
The stator winding are supply with balance
three – phase AC voltage , which produce voltage
in the rotor windings . It is possible to arrange the
distribution of stator winding so that there is an
effect of multiple poles, producing several cycle of
magneto motive force (mmf) or field around the air
gap .
• Three –phase induction motor are more commonly employed
in adjustable-speed drives than three–phase synchronous
motors. Three–phase induction motor are of two type ,
squirrel-cage induction motors (SCIMs) and slip-ring (or
wound-rotor)induction motors(SRIMs). Stator windings of
both types carry three-phase winding . Rotor of SCIM is
made of copper or aluminum bar short-circuited by two end
rings. Rotor of SRIM carries three-phase winding connected
to three slip rings on the rotor shaft .
• When 3-phase supply is connected to three-phase stator
winding, rotating magnetic field is produce . The speed of
this rotating field , called synchronous speed , is given by
𝝎𝒔
Also
𝑵𝒔 =
𝟏𝟐𝟎𝒇𝟏
𝒑
𝝎𝒔 =
𝟒𝝅𝒇𝟏
𝒑
rpm
=
𝒏𝒔 =
𝟐𝒇𝟏
𝒑
rps
𝟐𝝎𝟏
rad/sec
𝒑
: ωs – is synchronous speed [rad/sec] .
Ns – is the speed on the shaft [RPM].
𝝎𝟏 – is the supply angler frequency[rad/sec] .
𝒇𝟏 – is the supply frequency [ Hz ] .
p – is number of stator poles .
r1
X1
X2
E2
V1
r1
r2
X1
I1
Ie I2
I1
Xm
𝒓𝟐
𝒔
V1
X2
r1
Ie
Xm
X1
X2
I2
𝒓𝟐
𝒔
for single phase
Stator copper loss : 𝑷𝒔𝒄𝒖 = 𝑰𝒔𝟐 ∗ 𝑹𝒔
Rotor copper loss : 𝑷𝒓𝒄𝒖 = 𝑰𝒓𝟐 ∗ 𝑹𝒓
for 3-phase the loss must be multiply by (3)
Core losses : 𝑷𝒄 =
𝑽𝒎𝟐
𝟑
𝑹𝒎
≈
𝑽𝒔𝟐
𝟑
𝑹𝒎
There are many ways to controlled on the
speed of IM
1. Stator voltage control
2. Variable frequency control
3. Rotor resistance control
4. Slip energy recovery scheme
We take the third way for example (Rotor
voltage control ) .
These resistors Rx are used to control motor starting and stopping
anywhere from reduced voltage motors of low horsepower up to
large motor applications such as materials handling, mine hoists,
cranes etc.