Rockwell Automation 20G PowerFlex 750-Series AC Drives - Manual (Page 236)

236 Rockwell Automation Publication 750-RM002B-EN-P - September 2013
Chapter 4 Motor Control
AC Induction Motors
P35 [Motor Ctrl Mode] induction motor options.
• 0 = “Induction VHz”
• 1 = “Induction SV”
• 2 = “Induction Econ”
• 3 = “Induction FV”
AC Induction Motors (ACIMs) are the simplest and most rugged electric motor
and consist of two basic electrical assemblies: the wound stator and the rotor
assembly. The induction AC motor derives its name from currents flowing in the
secondary member (rotor) that are induced by alternating currents flowing in the
primary member (stator). The combined electromagnetic effects of the stator and
rotor currents produce the force to create rotation.
ACIMs typically feature rotors, which consist of a laminated, cylindrical iron
core with slots for receiving the conductors. The most common type of rotor has
cast-aluminum conductors and short-circuiting end rings. This AC motor
“squirrel-cage” rotates when the moving magnetic field induces a current in the
shorted conductors. The speed at which the AC motor magnetic field rotates is
the synchronous speed of the AC motor and is determined by the number of
poles in the stator and the frequency of the power supply: ns = 120f/p, where ns
= synchronous speed, f = frequency, and p = the number of poles (that is 120*60
Hz] / 4 = 1800 RPM). To control motor speed other than the fixed utility
frequency requires a VFD.
Synchronous speed is the absolute upper limit of AC motor speed. If the AC
motor’s rotor turns exactly as fast as the rotating magnetic field, then no lines of
force are cut by the rotor conductors, and torque is zero. When AC induction
motors are running, the rotor always rotates slower than the magnetic field. The
AC motor’s rotor speed is just slow enough to cause the proper amount of rotor
current to flow, so that the resulting torque is sufficient to overcome windage and
friction losses, and drive the load. The speed difference between the AC motor’s
rotor and magnetic field, called slip, is normally referred to as a percentage of
synchronous speed: s = 100 (ns - na)/ns, where s = slip, ns = synchronous speed,
and na = actual speed. Or it is listed on the nameplate as a base speed (1780
RPM) at rated FLA, frequency, and based on the number of poles.