Page 683 - Mechatronics with Experiments

ELECTRIC ACTUATORS: MOTOR AND DRIVE TECHNOLOGY 669
Torque (% T ) Torque (% T )
r
r
100%Vr, Vr is rated voltage 50%
75%
75%Vr
100%
50%Vr 125% of rated frequency
25%Vr
100 50 75 100 125
Speed (% w ) Speed (% w )
syn syn
(a) (b)
Torque (% T ) Torque (% T )
r
r
V , w V , w e2 V , w e3
02
03
01 e1 T
max
Intermediate
duty zone
T
rms
V > V > V Continuous
03 02 01
w > w > w duty zone
e3 e2 e1
Speed (% w ) Speed (% w )
syn
syn
(c) (d)
FIGURE 8.43: AC induction motor torque-speed performance in steady-state under various
control methods for varying voltage, frequency, and current in the motor stator phase
windings: (a) variable voltage amplitude, fixed frequency, (b) variable frequency, fixed voltage,
(c) variable voltage and variable freqency, while keeping the voltage to frequency ratio constant
for different frequency ranges (Volt/Hertz method), (d) field oriented vector control.
DC brushless motor. In order to commutate the current in the windings so that two magnetic
fields are perpendicular for maximum torque generation per current unit, measuring rotor
angle is not sufficient to know the relative angle between the magnetic field of the stator and
the magnetic field of the rotor. The “field oriented vector control algorithm” is the name
used for AC motor current commutation where the angle between the magnetic fields (the
magnetic field of the stator and the induced magnetic field of the rotor) is estimated based
on the dynamic model of the motor.
Assuming that this angle between the two magnetic fields is known, an AC motor
can be commutated to provide essentially the same torque-speed characteristics of a DC
brushless motor (Figure 8.43d). The only difference may be in the transient response.
The field oriented vector control algorithm are a current commutation algorithm for AC
induction motors. This current commutation algorithm attempts to make an AC induction
motor behave like a DC motor, that is to have a linear relationship between the torque
and commutated current. The hardware components of a drive for AC motors which can
implement the vector control commutation algorithm are identical to that of a drive for
DC brushless motors. Both drives attempt the same thing: to maintain a perpendicular
relationship between the field magnetic flux vector and the controlled current vector.
The AC induction motor differs from the DC brushless motors in two ways. First,
the controlled current is on the stator which induces current in the conductors of the
rotor. That induced current generates its own magnetic field in the rotor. The induced
magnetic field is not locked to the rotor. There is a slip between the rotor and the induced
field. Second, there are two components of the controlled current that are of interest: the
component that is parallel to the rotor field and the component that is perpendicular to it.
It can be shown mathematically [17] that the parallel component (magnetization current)
determines the torque gain of the motor, whereas the perpendicular component determines
the current multiplier for torque generation. Let us express the torque–current relationship