Rockwell Automation 20G PowerFlex 750-Series AC Drives - Manual (Page 238)

238 Rockwell Automation Publication 750-RM002B-EN-P - September 2013
Chapter 4 Motor Control
particularly suited to handle hard-to-start loads. Another useful characteristic of
this motor is the sloping shape of its speed-torque curve. This lets the motor slow
down during periods of peak loads, enabling any flywheel energy that has been
stored by the load to be released. Typical applications include punch presses and
press brakes.
AC Motors - Design F exhibit low starting torque, low starting current, and low
slip. These AC motors are built to obtain low locked-rotor current. Both locked-
rotor and breakdown torque are low. Normally these AC motors are used where
starting torque is low and where high overloads are not imposed after running
speed is reached.
In summary, we see that when matching an AC motor to the requirements of a
specific load it is important to check the torque requirements of the load and the
torque capabilities of the motor in addition to speed and horsepower.
At least three torque values are important:
• Starting torque
• Breakdown torque
• Full-load torque.
Wound-rotor AC Motors
P35 [Motor Ctrl Mode] induction motor options.
• 0 = “Induction VHz”
• 1 = “Induction SV”
• 3 = “Induction FV”
Squirrel-cage AC motors are relatively inflexible with regard to speed and torque
characteristics, but a special wound-rotor AC motor has controllable speed and
torque. Application of wound-rotor AC motors is markedly different from
squirrel-cage AC motors because of the accessibility of the rotor circuit. AC
motor performance characteristics are obtained by inserting different values of
resistance in the rotor circuit.
Wound-rotor AC motors are generally started with secondary resistance in the
rotor circuit. The AC motor resistance is sequentially reduced to permit the
motor to come up to speed. Thus, AC motors can develop substantial torque
while limiting locked-rotor current. This secondary AC motor resistance can be
designed for continuous service to dissipate heat produced by continuous
operation at reduced speed, frequent acceleration, or acceleration with a large
inertia load. External resistance gives AC motors a characteristic that results in a
large drop in rpm for a fairly small change in load. Reduced AC motor speed is
provided down to about 50% rated speed, but efficiency is low.
Retrofitting a Wound-rotor motor with a VFD is possible by eliminating the
switching and resistor control infrastructure and shorting the slip rings