are various methods of starting of an induction motor:
(a) Direct on line starting
(b) Star – Delta starting
(c) Auto transformer starting
(e) Part-winding starting
most of the squirrel-cage induction motors draw 6 to 7 times Full-Load current
while starting direct on line, it is desirable to have some mechanism to bring
down this starting current. Y- Δ starting reduces the starting line current as
well as torque to one-third of the DOL value. Soft-start employs back to back
connected thyristor pairs in between the supply lines and the motor so that
reduced voltage is applied depending upon the firing angle chosen. This
experiment observes first the starting performance of the induction motor at
various loads on DOL starting and using a soft-start (at different firing
direct on line starting is done as follows: Rated supply voltage is applied to
the induction motor with the specified R1, x1, R2, x2 values. (R1=0.294 Ohms;
L1 =1.39mH R2 =0.156 Ohms; L2 =0.74 mH Lm = 41 mH Moment of inertia =0.4 kgm2
Vrated =400 V I rated = 250 A Speed =970
DIAGRAM FOR DOL
DIAGRAM FOR SOFT-START
Speed and current transients are observed for both
types of starters by using the tacho and current sensors. Record these
transients, observe how long it takes for the machine to come up to its rated
speed using each of these methods when it is running on no-load. Repeat this
for different loads and also for different firing angles for the soft-start.
The equation governing induction motor dynamics are as follows:
where Va = Va - ½ Vb - ½ Vc
Va = 0 + Ö3/2Vb - Ö3/2 Vc
Vq = 0
where Va , Vb,
Vc are the stator applied
voltages( per phase) instantaneous values
& Vq,, Vd are rotor voltages.
is obtained by transforming 3-phase stator and rotor to their 2-phase
equivalents and then transforming all the rotor quantities to stationary
reference frame. (Please refer to Unified Theory of Electrical Machines by C.
L1 = M + l1
Mutual + leakage inductance of stator
L2 = M + l2
Mutual + leakage inductance of rotor
wr = instantaneous rotor speed
These four simultaneous differential equations allow
us to solve for currents for the given m/c parameters and i/p voltages.
T = -M ia iq + M ib id =
Te - TL
J dwr / dt
From which wr (instantaneous rotor speed)
can be solved for.
Thus all these five simultaneous differential
equations can be solved and hence the response of the m/c can be obtained.
The waveforms of line-to-line voltage, three-phase
currents and the speed are shown at a firing angle of 110 degrees on no-load
condition of the induction motor are shown below.
Induction Motor Braking
motor braking can be done by various methods like regenerative braking,
plugging and dynamic braking (AC dynamic and DC dynamic brakings and capacitor
braking). In this experiment capacitor braking will be explored.
capacitor braking, the induction machine will be disconnected from the supply
and connected to a capacitor bank connected in either Star or Delta. So, now
the excitation would be provided by the capacitor and the machine will start
acting like a generator (Why?). The generated electric power will be dissipated
in the inherent resistance of the windings and thus braking is accomplished.