Page 679 - Mechatronics with Experiments

ELECTRIC ACTUATORS: MOTOR AND DRIVE TECHNOLOGY 665
T m
B s
T max

N B r θ rs
–180° –90° 90° 180°
S
θ rs

–T max
(a) (b)
FIGURE 8.40: Torque production as a result of the interaction between two magnetic fields.
(a) A permanent magnet in an external magnetic field, (b) torque as a function of the angle
between the two flux vectors. In the case of AC induction motors, the external field flux density
(B s ) is setup by the stator current and the equivalent field of the magnet (B r ) is setup by the
rotor conductors as a result of the induced voltage.

◦
90 angle between the stator and rotor flux density vectors in order to maximize torque
production per unit current.
Open loop steady-state torque-speed characteristics of an AC induction motor are
shown in Figure 8.41 for the case of constant AC voltage magnitude and frequency supplied
to the stator windings. Note that the shape of that curve can be changed by using different
stator winding and slotting configurations. In fact, different motor designs are used to meet
different steady-state torque-speed curve shapes needed by different applications. Most AC
induction motors have maximum slip in the range of 5–20% of the synchronous speed.
Maximum slip is defined as the slip speed at which maximum torque is reached. Clearly,
the smaller the maximum slip is, the smaller the speed variations of the motor under varying
loads. Likewise, if large speed variation is desired as the load varies, a motor with large
maximum slip should be used.

Torque
Motor action Generator Action
Max torque
(C)
Linear torque -speed
(B)
range: between zero slip
and maximum slip
(A)
100% Rated point: ( , T) rated
Rated torque Starting torque syn
s,max Speed
(Max slip)

FIGURE 8.41: Open loop torque speed curve of an AC induction motor. Under a given
constant AC voltage magnitude and frequency, the steady-state torque-speed relationship is
nonlinear. This curve can be given different shapes by different electromagnetic designs.
Curves (A), (B), and (C) show different AC motor torque-speed characteristics that can be
achieved by varying designs.