As discussed previously, the application of
polyphase power to the stator winding results in
the development of a rotating magnetic field.
Magnetic lines of force developed by this rotating
magnetic field cut across the cage bars and
generate voltages, causing currents at slip
frequency to flow in them. The interaction
between these currents and the rotating magnetic
field develops torque, tending to turn the rotor in
the same direction as the rotating magnetic field.
Recommended starting torques range from
40% to 200% of full load torque. See NEMA
recommendations on torques for various
synchronous motor applications.
Torque comes from the action of the flux
produced by the stator. Getting more torque
requires more flux and, therefore, more iron.
More torque also results in more inrush. For any
given motor, the effective resistance and reactance
of the cage winding are the principal factors in
determining motor torques. Typical starting and
accelerating torques and their associated kVA
values are shown in Figure 16.
The double cage motor was developed to give
essentially constant pull-up torque. The shallow
cage has a high resistance and low reactance, while
the deep cage has low resistance and high
reactance. At standstill, slip frequency equals line
frequency, and the high reactance of the deep cage
forces most of the induced current to flow through
the shallow cage, high resistance bars, resulting in
high starting torque.
As discussed previously, the application of
polyphase power to the stator winding results in
the development of a rotating magnetic field.
Magnetic lines of force developed by this rotating
magnetic field cut across the cage bars and
generate voltages, causing currents at slip
frequency to flow in them. The interaction
between these currents and the rotating magnetic
field develops torque, tending to turn the rotor in
the same direction as the rotating magnetic field.
Recommended starting torques range from
40% to 200% of full load torque. See NEMA
recommendations on torques for various
synchronous motor applications.
Torque comes from the action of the flux
produced by the stator. Getting more torque
requires more flux and, therefore, more iron.
More torque also results in more inrush. For any
given motor, the effective resistance and reactance
of the cage winding are the principal factors in
determining motor torques. Typical starting and
accelerating torques and their associated kVA
values are shown in Figure 16.
The double cage motor was developed to give
essentially constant pull-up torque. The shallow
cage has a high resistance and low reactance, while
the deep cage has low resistance and high
reactance. At standstill, slip frequency equals line
frequency, and the high reactance of the deep cage
forces most of the induced current to flow through
the shallow cage, high resistance bars, resulting in
high starting torque.
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