TCM FB-8 Series, Service Manual

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Operating principle of 3-phase induction motor
As shown in Figure 2.3, when the magnetic
poles N and S are rotated outside the rectangular
oil, the coil side cuts the magnetic flux to induce
electromotive force in the direction indicated in the
figure, thus flowing circulating current inside the
coil. This causes rotating force in the same direction
as in which the magnetic poles rotates so that the
coil also rotates, following the magnetic poles.
Motors which operate in such a principle is called
an induction motor.
Three-phase alternating current is suitable for
rotating the magnetic coil. When 3-phase current is
passed into a 3-phase winding, a revolving magnetic
field which rotates a certain speed is created. It
is thus possible to perform the same action as the
purpose of rotating the magnetic poles N and S in
Figure 2.3. The 3-phase winding consists of an iron
core and three coils A, B, and C, spaced 120° apart,
installed in the iron core slots and connected in Y
or delta connection. The sketch “b” in Figure 2.4
shows a typical Y connection which is widely used
for the connection of induction motors.
When the directions (A
→
A’, B
→
B’, C
→
C’)
in which current flows into the coil are regarded
as the positive direction, the current of the coil in
each phase at the moment
①
at (a), (b) and (c) in Figure 2.5, +Im flows into the coil A and –Im/2 flows into
both coils B and C. Therefore, the synthetic magnetic flux in the space becomes (c) to create a magnetic
field moving from right to left. This means that the magnetic pole is located at the right side and the S at the
left side. As such, each moment from
②
-
⑥
is followed in order, the synthetic magnetic field created by
synthetic magnetic flux is a magnetic field which rotates by 60° clockwise. With time, it rotates to cause a
rotating magnetic field which makes a complete turn per one cycle of alternating current. The poles formed
at each moment are N and S, and thus this type of motor is called a 2-pole motor.
Fig. 2.5 Rotating Magnetic Field due to 3-phase Alternating Current
Fig. 2.3 Operating Principle of Induction Motor
Fig. 2.4 3-phase Winding
(Rotating
direction of coil)
(Rotating direction
of magnetic pole)
2. MOTORS