Page 682 - Mechatronics with Experiments

668 MECHATRONICS

PWM circuit
Triangular
Current wave gen. DC power source
feedback loop (Converter: AC to DC)
+
_ Driver DC + Inverter (DC to AC)
Comparator
Driver Tr 3 Tr 5
Current Tr 1
comand Commutation +
algorithm _ Driver Tr Tr 4 Tr 6
Comparator 2
Driver
+ DC –
_ Driver
Comparator
Driver
A S

+ T
Sensor
+
Stator windings
K
phase
adv
Enable phase Squirrel-cage
advance option rotor
FIGURE 8.42: Current commutation and regulation in AC motor phases by a field oriented
vector control (FOVC) drive.

method. It is classified as one of the scalar control methods as oppose to the vector control
methods discussed next.
The power stage of a Volts/Hertz drive for a three-phase AC induction motor is a
three-phase inverter. The most common power switching devices are power MOSFETs
and IGBTs. Power MOSFETs are voltage controlled transistors with low power loss, but
they are sensitive to temperature. IGBTs (insulated gate bipolar transistors) are essentially
bipolar transistors where the base is controlled by MOSFETs. IGBTs have higher switching
frequency, but are less efficient than MOSFETs.
For a three-phase AC induction motor control (Figure 8.42), a three-phase drive
(“inverter”) is needed. In each leg of the bridge, power transistors operate in opposite state
to each other, that is Tr1 = ON and Tr2 = OFF or Tr1 = OFF and Tr2 = ON. If both are ON
at the same time, the supply voltage potential would be effectively shorted and most likely
damage the power transistors due to excessive current. Furthermore, it should be kept in
mind that power transistors have a longer turn-OFF time than turn-ON time. Hence, when
the state in the power transistors change, there should be some “dead-time” (delay or wait
time) of the transistor to be commanded to go from an OFF to ON state so that the other
one which is going from ON to OFF state has sufficient time to do so, in order to avoid the
short circuit. PWM peripherals of modern microcontrollers and digital signal processors
automatically handle this issue and dead-time insertion is programmable.

Vector Control Drive: Field Oriented Vector Control Algorithm The
roles of the drive are the commutation and the amplification of current in the stator windings
1
(Figure 8.42) . The only difference is that a DC brushless motor has a permanent magnet
rotor, whereas an AC induction motor has a squirrel-cage rotor with no magnets. Therefore
the magnetic field of the rotor of an AC motor is not physically locked to the rotor, unlike the

1 This section can be skipped without loss of continuity. See [17] for more details.