Download AC motor - induction2

ABD RAHIM 2008
simply means of an electro-mechanical switch which is opened and closed to
stop and start the motor
ABD RAHIM 2008
ABD RAHIM 2008
- Low cost and simple
-Torque too high – causes snatch
- Torque too low – motor stalls
- Motor can stall in transition
ABD RAHIM 2008
ABD RAHIM 2008
ABD RAHIM 2008
ABD RAHIM 2008
ABD RAHIM 2008
ABD RAHIM 2008
Wound rotor induction motor.
Breakdown torque peak is shifted to zero speed by increasing
rotor resistance.
ABD RAHIM 2008
ABD RAHIM 2008
ABD RAHIM 2008
(to apply an adjustable voltage to the motor and increase this voltage
gradually over a user-selectable acceleration period)
Advantages :
•Reduced starting current
•Reduced starting torque
•Less mechanical stress
•Improved control of
acceleration and deceleration
ABD RAHIM 2008
ABD RAHIM 2008
INDUCTION MOTORS
Equivalent circuits …
stator
rotor
-Similar circuit with Transformer’s
-There are no internal voltage source EA because it doesn’t have any
independent field circuit.
-The only different is that the secondary (rotor part) is moveable thus
it will affect the ER, and impedance RR and jXR
ABD RAHIM 2008
INDUCTION MOTORS
Rotor circuit model…
stator
-The voltage magnitude and
frequency of induced voltage on
the rotor ER directly proportional
to the slip of the motor.
rotor
-ELR - voltage at Locked Rotor
condition
-So does the frequency of the induced
voltage :
Rotor reactance :
But,
ABD RAHIM 2008
INDUCTION MOTORS
Rotor circuit model..
-With above circuit, rotor current IR is:
Z R, eq
ABD RAHIM 2008
INDUCTION MOTORS
Final Rotor equivalent circuit ….
ABD RAHIM 2008
INDUCTION MOTORS
Per-phase equivalent circuit ….
(Chapman )
or
(Rashid)
ABD RAHIM 2008
INDUCTION MOTORS
Torque & Power-flow diagram ….
PAGor Pg=3Ir2Rr /s
Pconvor Pdev=PAG-PRCL =(1-s)PAG
τind or τdev=Pconv/ωm
Pout=Pconv-PF&W -Pmisc
3 phase
Source – Y
connected
or
PRCL=3Ir2Rr
Pcore=3Vm2/Rm≈3Vs2/Rm
PSCL = 3Is2Rs
ABD RAHIM 2008
Power Loss Area
Efficiency Improvement
1. Fixed loss (iron)
Use of thinner gauge, lower loss core steel reduces eddy
current losses. Longer core adds more steel to the
design, which reduces losses due to lower operating flux
densities.
2. Stator I2R
Use of more copper & larger conductors increases cross
sectional area of stator windings. This lower resistance
(R) of the windings & reduces losses due to current flow
(I)
3 Rotor I2R
Use of larger rotor conductor bars increases size of cross
section, lowering conductor resistance (R) & losses due
to current flow (I)
4 Friction & Winding
Use of low loss fan design reduces losses due to air
movement
5. Stray Load Loss
Use of optimized design & strict quality control
procedures minimizes stray load losses
ABD RAHIM 2008
INDUCTION MOTORS
Torque & Power-flow diagram ….
Stated that, τind =Pconv/ωm
Substitution …
Thus,
While ,
=(1 - s)PAG / (1 - s)ωsync
τind =PAG / ωsync
τload =Pout / ωm
ABD RAHIM 2008
INDUCTION MOTORS
Example 7.2:
Ans: a)38.6kW
b)37.9kW
c)37.3kW
d)88%
ABD RAHIM 2008
INDUCTION MOTORS
Example 7.3:
a) ns:1800rpm,ws:188.5rad/s,nm:1760rpm, wm:184.4rpm
b) Is:18.88L-33.6A
c)0.833lagging
d) Pconv:11585W,Pout:10.485W
e)Tind:62.8Nm, Tload:56.9Nm
f) n:83.7%
ABD RAHIM 2008
INDUCTION MOTORS
Per-phase equivalent circuit ….
(Rashid)
In real world:
•Value of Xm is very large (Rm
can be omitted)
•Xm2 >> (Rs2 + Xs2)  Vs ≈Vm
•Thus, for circuit simplification,
the magnetizing reactance Xm
can be moved-out to the stator
winding.
ABD RAHIM 2008
INDUCTION MOTORS
The input impedance of the motor becomes:
Power factor angle:
ABD RAHIM 2008
INDUCTION MOTORS
Rotor current (rms):
Previously noted,
PAG or Pg=3Ir2Rr /s
And developed torque τdev or,
Thus ,
τind =Pdev / ωm = PAG / ωsync
--(15-18)
ABD RAHIM 2008
INDUCTION MOTORS
ABD RAHIM 2008
ABD RAHIM 2008
Extended Torque-speed characteristic ..
Pullout torque or
breakdown torque =
max possible
handling torque.
The curve is nearly linear
up to full load (Rr >> Xr,
thus Ir,Br and Tind rise
∞ increasing slip).
s
Plugging = reversing of magnetic
field rotation by switching any two
stator phases. The reversed Tind
will stop the motor rapidly and
rotate it to reverse direction.
If speed driven
faster than ns the
Tind reverses and
motor becomes
generator.
ABD RAHIM 2008
At start:
start:high
high
At
current
currentand
andlow
“pull-up”
torque
low “pull-up”
torque
INDUCTION MOTORS
Torque-speed characteristic ..
At 80% of full
speed: highest
“pull-out”
torque and
current drops
At full or synchronous
speed: torque and
stator current are zero
ABD RAHIM 2008
•LRT is higher than 100% of the FLT
•Starting current at LRC) may reach 1000% of FLC
•Once rotor starts to rotate the torque may decrease a bit for certain
classes of motors to a value known as the pull up torque.
•breakdown torque is due to the larger than normal 20% slip.
•Slip will be only a few percent during normal operation.
•Any motor torque load above the breakdown torque will stall the motor.
• The torque, slip, and current will approach zero for a “no mechanical
torque” load condition. This condition is analogous to an open
secondary transformer.
ABD RAHIM 2008
INDUCTION MOTORS
Characteristics for NEMA designs….
All motors, except class D, operate at %5 slip
or less at full load.
• Class B (IEC Class N) motors are the
default motor to use in most applications.
With a starting torque of LRT = 150% to
170% of FLT, it can start most loads,
without excessive starting current (LRT).
Efficiency and power factor are high. It
typically drives pumps, fans, and machine
tools.
• Class A starting torque is the same as
class B. Drop out torque and starting
current (LRT)are higher. This motor
handles
transient
overloads
as
encountered
in
injection
molding
machines.
as encountered in injection molding
machines.
ABD RAHIM 2008
•
•
•
•
INDUCTION MOTORS
Characteristics for NEMA
designs….
Class C (IEC Class H) has higher
starting torque than class A and B at LRT
= 200% of FLT. This motor is applied to
hard-starting loads which need to be
driven at constant speed like conveyors,
crushers, and reciprocating pumps and
compressors.
Class D motors have the highest starting
torque (LRT) coupled with low starting
current due to high slip ( 5% to 13% at
FLT). The high slip results in lower speed.
Speed regulation is poor. However, the
motor excels at driving highly variable
speed loads like those requiring an
energy storage flywheel. Applications
include punch presses, shears, and
elevators.
Class E motors are a higher efficiency
version of class B.
Class F motors have much lower LRC,
ABD RAHIM 2008
INDUCTION MOTORS
1)
1.67%
2)
48.6Nm
3)
2900rpm
4)
29.5kW
ABD RAHIM 2008