Rockwell Automation 20G PowerFlex 750-Series AC Drives - Manual (Page 239)

Rockwell Automation Publication 750-RM002B-EN-P - September 2013 239
Motor Control Chapter 4
connected to the rotor windings. CAUTION! Because wound-rotor motors
were not originally designed for use with inverters, the dielectric strength of the
motor construction cannot withstand the reflected wave voltages that can get
subjected at the motor connections (1.5 to 2.5 times drive’s bus voltage).
Appropriate mitigation must be considered. General rule of thumb, size the VFD
so that it is capable of providing continuous current at 125 to 135% of FLA of the
motor, due to elimination of resistors and its design for higher starting torque.
Multispeed AC Motors
P35 [Motor Ctrl Mode] induction motor options.
• 0 = “Induction VHz”
• 1 = “Induction SV”
• 3 = “Induction FV”
Consequent-pole AC motors are designed for one speed. By physically
reconnecting the leads, a 2:1 speed ratio can be obtained. Typical synchronous
speeds for 60 Hz AC motors are: 3,600/1,800 rpm (2/4 pole), 1,800/900 rpm
(4/8 pole), and 1,200/600 rpm (6/12 pole).
Two-winding AC motors have two separate windings that can be wound for any
number of poles so that other speed ratios can be obtained. However, ratios
greater than 4:1 are impractical because of AC motor size and weight.
Power output of multispeed AC motors can be proportioned to each different
speed. These AC motors are designed with output horsepower capacity in
accordance with one of the load characteristics.
When retrofitted with a VFD, the motor is generally wired for the speed range
intended to be optimized. Autotuned per representative nameplate information
and operated as a single winding single speed induction motor.
Synchronous AC Motors
P35 [Motor Ctrl Mode] induction motor options.
• 0 = “Induction VHz”
Synchronous AC motors are inherently constant-speed electric motors and they
operate in absolute synchronism with line frequency. As with squirrel-cage
induction AC motors, speed is determined by the number of pairs of poles and is
always a ratio of the line frequency.
Synchronous AC motors are made in sizes ranging from sub-fractional self-
excited units to large-horsepower, direct-current-excited AC motors. In the
fractional-horsepower range, synchronous AC motors are used primarily where
precise constant speed is required.