Field Windings
The amortisseur winding imparts squirrel-cage
motor starting and accelerating characteristics to
the synchronous motor. The field winding
develops a torque similar to that of a wound rotor
motor with a single-phase secondary circuit.
The single-phase characteristic results in two
torque components, one rotating in the same
direction as the rotor and one rotating in the
opposite direction. The second component is
positive in value to 50 percent speed and negative
from that point to pull-in. Figure 18 illustrates the
torque components and the resultant torque
derived from the field windings. If a motor was
built with a relatively weak cage, the dip in torque
due to the effect of the negative rotating component
could result in the motor not being able to
accelerate much past half-speed.
During starting and accelerating, the field
winding is short-circuited through a field discharge
resistor to limit the induced field voltage. The ratio
of this resistance to the field resistance has a
significant effect on the starting torque, the torque
at pull-in and, to a lesser degree, on starting kVA.
Figure 19 shows the effect of varying the field
discharge resistance on one specific motor. It will
affect the negative rotating component of the field
winding torque, and a high value of resistance may
reduce it considerably. However, a high value of
field discharge resistance results in a high induced
voltage across the field winding. In extreme cases,
as in the case of open-circuit field starting (infinite
resistance) the induced field voltage may exceed
100 times the normal excitation voltage.
Field Windings
The amortisseur winding imparts squirrel-cage
motor starting and accelerating characteristics to
the synchronous motor. The field winding
develops a torque similar to that of a wound rotor
motor with a single-phase secondary circuit.
The single-phase characteristic results in two
torque components, one rotating in the same
direction as the rotor and one rotating in the
opposite direction. The second component is
positive in value to 50 percent speed and negative
from that point to pull-in. Figure 18 illustrates the
torque components and the resultant torque
derived from the field windings. If a motor was
built with a relatively weak cage, the dip in torque
due to the effect of the negative rotating component
could result in the motor not being able to
accelerate much past half-speed.
During starting and accelerating, the field
winding is short-circuited through a field discharge
resistor to limit the induced field voltage. The ratio
of this resistance to the field resistance has a
significant effect on the starting torque, the torque
at pull-in and, to a lesser degree, on starting kVA.
Figure 19 shows the effect of varying the field
discharge resistance on one specific motor. It will
affect the negative rotating component of the field
winding torque, and a high value of resistance may
reduce it considerably. However, a high value of
field discharge resistance results in a high induced
voltage across the field winding. In extreme cases,
as in the case of open-circuit field starting (infinite
resistance) the induced field voltage may exceed
100 times the normal excitation voltage.
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