AN2388 Datasheet, PDF (5/54 Pages) STMicroelectronics – Sensor field oriented control

AN2388
Figure 1. Stator core and windings
Background
1.1.2
Rotor
The rotor is made up of several thin steel laminations with spaced bars, which are made up
of aluminum or copper, along the periphery. In the most popular type of rotor (squirrel cage
rotor), these bars are connected mechanically at the ends and electrically by the use of
rings. The rotor consists of a cylindrical laminated core with an axially placed parallel slot for
carrying the conductors. Each slot carries a copper, aluminum or alloy bar. These rotor bars
are permanently short-circuited at both ends by means of the end rings. The rotor slots are
not exactly parallel to the shaft in order to decrease magnetic hum and slot harmonics.
Moreover this reduces the locking tendency of the rotor. In fact, the rotor teeth tend to
remain locked under the stator teeth due to direct magnetic attraction between the two. This
happens when the number of stator teeth are equal to the number of rotor teeth.
The rotor is mounted on the shaft using bearings on each end. One end of the shaft is
usually kept longer than the other for driving the load. Some motors may have
position/speed sensing devices. Between the stator and the rotor exists an air-gap, through
which, due to induction, the energy is transferred from the stator to the rotor like a
transformer. The generated torque forces the rotor and then the load to rotate.
The magnetic field created in the stator rotates at a synchronous speed (N_s).
N_s
=
60 ×
f
p_p
where:
N_s = synchronous speed in RPM
p_p = the number of pole pairs
f = the supply frequency in Hertz
The magnetic field produced in the rotor is alternating in nature because of the induced
voltage. The frequency of the induced EMF is the same as the supply frequency. Its
magnitude is proportional to the relative velocity between synchronous speed (stator
frequency) and rotor speed. Since the rotor bars are shorted at the ends, the EMF induced
produces a current in the rotor conductors.
When the magnetic field is generated the rotor starts to run in the same direction trying to
reach the same speed. The rotor revolves slower than the speed of the stator field. This
difference is called slip (s). The slip varies with the load so that an increasing of the load
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