AN2388 Datasheet, PDF (6/54 Pages) STMicroelectronics – Sensor field oriented control

Background
AN2388
causes the rotor to slow down (or slip increasing). On the contrary, a decreasing of the load
causes the rotor to speed up (or slip decreasing). The slip is expressed as a percentage and
can be determined with the following formula:
% Slip = ^{N s − N r} ×100
N_s
where;
N_s = synchronous speed in RPM
N_r = rotor speed in RPM
Figure 2. Rotor structure
Rings
Shaft
Conductors
Skewed slots
Slots in the inner periphery of the stator accommodate 3-phase winding a, b, c. The turns in
each winding produces an approximately sinusoidally-distributed flux density around the
periphery of the air gap. When three currents that are sinusoidally varying in time, but
displaced in phase by 120° from each other, flow through the three symmetrically placed
windings, a radially directed air gap flux density is produced that is also sinusoidally
distributed around the gap and rotates at an angular velocity equal to the angular frequency
ω_s of the stator currents.
The flux produced by the stator current is a sinusoidally-distributed wave. This flux revolves
and collides with the rotor bars, generating rotor current in the short-circuited rotor bars.
Because of the low resistance of these shorted bars, only a small relative angular velocity ω_r
between the angular velocity ω_s of the flux wave and the mechanical angular velocity ω of
the two pole rotor is required to produce the necessary rotor current.
The relative angular velocity ω_r is called the slip velocity. The interaction of the sinusoidally
distributed air gap flux density and induced rotor currents produces a torque on the rotor.
1.2
Three-phase induction motor and classical AC drives
Three-phase AC induction motor are widely used in many fields. They are classified in two
categories:
● Squirrel cage motor
● Wound-rotor motor
90% of the three-phase AC Induction motors are squirrel cage motors because of their lower
cost and the possibility of starting heavier loads with respect to wound-rotor motors. The
range of power ratings goes from one-third to hundred horsepower.
The wound-rotor motor is a variation of the squirrel cage induction motor. While the stator is
the same as that the squirrel cage, it has a set of windings on the rotor which are not short-
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