Page 664 - Mechatronics with Experiments

650 MECHATRONICS
Power
supply
AC

Rectifier
bridge

Current Current V
command amplifier Power
Driver Q
Q transistor
PWM circuit H-bridge
Driver
Q M Q
Driver

Driver
Current
feedback
FIGURE 8.30: Block diagram of the brush-type DC motor drive: PWM amplifier with current
feedback control.

of the rotor position. Hence, the brushless motor requires a rotor position sensor for its
power stage.

8.4.2 Drives for DC Brush-type and Brushless Motors
Drive is considered as the power amplification stage of an electric motor. It is the drive
that defines the performance of an electric motor. The most common type of power stage
amplifier used for DC brush-type motors is an H-bridge amplifier (Figure 8.30). The H-
bridge uses four power transistors. When controlled in pairs (Q1 & Q4 and Q2 & Q3), it
changes the direction of the current, and hence the direction of generated torque. Notice
that the pair Q1 & Q3 or the pair Q2 & Q4 should never be turned ON at the same
time since it would form a short-circuit path between supply and ground. The diodes
across each transistor serve the purpose of suppressing voltage spikes and provide a free-
wheeling path for the current to follow. Large voltage spikes occur across the transistor
in the reverse direction due to the inductance of the coils. If a current flow path is not
provided, the transistors may be damaged. The diodes provide the alternative current path
for inductive loads and let current pass through the coil. When a diode is ON, the current
flows from the negative end of the power supply to the positive end due to the fact the
inductive voltage raises the negative side of the potential temporarily (transient) higher
than the positive side. The use of diodes in all power amplifiers for different motor types
serves the same purpose.
Transistors take a longer time to change from the ON to OFF state than they do to
change from OFF to ON state. In other words, transistors turn OFF slower than they turn
ON. When transistor states in one leg of the H-bridge are changed (i.e., Q1 is ON, and
Q2 is OFF, then we command Q1 to be OFF and Q2 to be ON, and visa versa), Q1 will
stay partially turned ON when Q2 is fully turned ON. This difference is in the tens of
microseconds range. As a result, this would result in a short circuit between supply and