Work contents
1. To make a scheme from laboratory work 3.3 (see figure 6.1, a ). Stator winding must be connected in star.
2. To set up the induction motor startup, and load using direct current generator, to measure currents and power of the induction motor by the phases and rotation frequency.
3. To investigate induction motor work in the single-phase mode, at opened knife-switch "Р".
4. The winding of stator is connected in delta; to startup and load of induction motor in the symmetric mode. To measure currents and power of induction motor windings phases and rotation frequency for load current quantity.
5. To investigate the work of induction motor having stator windings opened delta connection, and the load current according to item 4.
6. To investigate the work of induction motor with a phase rotor at symmetric and unbalanced resistance of rotor windings.
7. To carry out startup of induction motor with a phase rotor at the open state of one rotor phase.
Work explanations
The unbalanced modes of induction motors operation appeared in the following cases: 1) during distortion of symmetry of network voltage;
2) during unbalanced resistances in the stator and rotor circuits;
3) during the asymmetrical scheme of motor winding connection. Such modes can appear because of disrepairs and breakdowns.
1. The cutting out of network linear wire or burning out of one of safety devices (figure 6.1) is one of the most widespread abnormal modes of induction motors work. Motor is moved into the one-phase mode independent on the stator connection scheme. Field of stator is immovable, is pulsates and it can be decomposed into two circular revolved fields. These fields have identical frequencies of rotation n1 but they revolve in different directions.
Field, which direction of rotation coincides with the rotor rotation is named direct one. Rotor slip relatively to this field is determined by the expression
The field, which direction of rotation is opposite to the rotation of rotor, is named reverse. Rotor slip relatively to this field
The magnetic fields of positive and negative sequence are induced in the rotor winding corresponding EMF, which create the currents of positive and negative sequences in the reserved winding. These currents are interacted with the fields and create the electromagnetic torques of positive and negative sequence:
,
where R21=R22
A resulting torque influences to the machine rotor
The
analysis of mechanical characteristics (fig. 6.2) shows that if
Sdir=1
we
have
T=0,
i.e. starting torque is absent. For the motor rotation it must be
rotated by the external force in any direction. If linear wire is
cut rotor continues to work, rotating in the same direction and
consuming bigger current from the network. Motor having a switch-off
phase can work protractedly if load is in
times less, than rated load in the symmetric mode.
Induction motor work if one phase of the stator winding is cut is connected in star similar to above-mentioned.
2. The scheme of the opened delta is possible at the cutting of one of phases of the stator winding, connected in delta (fig. 3.1,b).
Magnetic field, created by the stator winding is revolved on the ellipse (fig. 6.3). It creates torque at the steady part of mechanical characteristic. Torque is insignificantly different from a torque during the normal work. A starting moment is small; it worsened starting conditions, in separate cases motor can not rotate.
Load on motor axle must be decreased to 70% of rated one during the prolonged the motor operation.
К-50
К-50
V
A
W
V
A
W
В1
С1
С2
С3
С1
С2
С3
В
В2
С4
С5
С6
С4
С5
С6
а)
b)
Figure 6.1 – Schemes for
investigation of motors operation if the phase is cut
3. Induction motor having the unbalanced resistance in the rotor circuit.
Unbalanced rotor occurs in the following cases:
1) cutting of the pins of short-circuited rotor winding;
2) cutting of phase rotor winding;
3) inequality of additional resistances in the phase rotor circuit.
The currents of positive I21 and negative I22 sequences appear in the rotor winding having frequency Sf1. The currents of rotor and stator positive sequence create common direct field. This field in interaction with a current I21 creates the of positive sequence torque Tdir (curve 1 in the fig. 6.5).
Currents I22 create the field, revolved in opposite direction with frequency n2Р= Sn1 relatively to the rotor, and relatively to stator with frequency
n2C= n2- n2P= n1(1- S1)- n1S=(1-2S1) n1
and they induce the current in stator I12 having frequency
f2C = (1-2S1) f1,
which are passed through a network and at small S1 current beatings are resulted. Superposition of the currents is harmful for the other consumers of this network.
Current beating having the frequency f1-f1(1-2S1)=2Sf1 causes of ammeter pointer oscillation plugged in the stator circuit.
Currents of negative sequence I12 of stator create the negative sequence field, which interacted with I22 creates the electromagnetic moment of negative sequence (curve 2 in figure 6.5).
Resulting electromagnetic torque on machine axle is
presented by a curve 3 in figure 6.5.
Motor
has two regions of steady work: at S=Srated
and at
as it is seem from fig. 6.5. If the cutting of rotor phase happened
during the operation of the motor having load less, then rated, rotor
frequency of rotation will be almost
invariable (point
"а" in fig. 6.5). If load exceeds rated one, a motor will
be rotated having half of previous frequency - on the second steady
part of mechanical characteristic (point
«b»).
Motor can be sent to the basic steady part of characteristic (point “a”), if large resistance is put into the rotor circuit and this resistance is decreased slowly at the motor startup (when small load is on the axle). A motor is “got stuck” having half frequency of rotation and small additional resistance or if this resistance is decreased sharply.