Page 666 - Mechatronics with Experiments

652 MECHATRONICS
Triangular wave
Analog input signal
carrier signal

PWM output signal (PWM equivalent version of the analog input signal)
FIGURE 8.31: PWM circuit function: the carrier signal is a high frequency triangular signal.
The input signal is an analog signal value. The output pulse has a fixed frequency which is the
carrier frequency. The ON/OFF pulse width is varied as a function of the value of the input
signal relative to the carrier signal.

loop bandwidth (i.e., at least 10 to 25 times larger) in order to minimize the effect of PWM
frequency on the overall performance of the control system.
The PWM circuit converts an analog input signal (i.e., amplified error signal from the
current loop which is an analog signal) to a fixed frequency but variable pulse width signal.
By modulating the ON-OFF time of the pulse width at a high switching frequency, hence
the name “pulse width modulation,” a desired average voltage can be controlled. The PWM
circuit (Figure 8.31) uses a triangular carrier signal with a high frequency (also called the
switching frequency). When the analog input signal is larger than the carrier signal, the
pulse output is ON, when it is smaller, the pulse output is OFF.
PWM is another way of transmitting an analog signal which takes on a value
between a minimum value and a maximum value, that is a value between 0–10 VDC or
−10–10 VDC. Instead of transmitting the signal by an analog voltage level, PWM transmits
the information as a percentage of the ON cycle of a fixed frequency signal. When the signal
value is to have the minimum value, the duty cycle (ON cycle) of the signal is set to zero
percent. When the signal value is to have the maximum value, the duty cycle (ON cycle)
is set to 100 percent. In other words, the analog value of the signal is conveyed as the
percentage of duty cycle of the signal.
Figure 8.32 shows the block diagram of a current controlled drive for a three-phase
brushless DC motor. The switch set is based on the familiar H-bridge, but uses three bridge
legs instead of two legs as is the case for an H-bridge drive for brush-type DC motors.
Since each leg of the H-bridge has two power transistors, the brushless motor drive has
six power transistors. The stator windings are connected between the three bridge legs as
shown in Figure 8.32. The so-called Y-connection shown is the most common type of phase
winding connection, while Δ-connection is used in rare cases. At any given time, three of
the transistors are ON and three are OFF. Furthermore, two of the windings are connected
between the DC bus voltage potential and have current passing through them in a positive
or negative direction, whereas the third winding terminals are both connected to the same
voltage potential (either V DC or 0 V) and act as the balance circuit. The combination of
the ON/OFF transistors determines the current pattern on the stator, hence the flux field