25-08-2017, 09:32 PM
Pulse-width modulation
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Pulse-width modulation (PWM), as it applies to motor
control, is a way of delivering energy through a
succession of pulses rather than a continuously
varying (analog) signal. By increasing or decreasing pulse
width, the controller regulates energy flow to the motor
shaft. The motor’s own inductance acts like a filter, storing
energy during the “on” cycle while releasing it at a rate
corresponding to the input or reference signal. In other
words, energy flows into the load not so much the
switching frequency, but at the reference frequency.
PWM is somewhat like pushing a playground-style
merry-go-round. The energy of each push is stored in the
inertia of the heavy platform, which accelerates gradually
with harder, more frequent, or longer-lasting pushes. The
riders receive the kinetic energy in a very different manner
than how it’s applied.
How PWM works
Command
(modulating) signal
Output stays high as
long as the command
is greater than the carrier.
High
High
Low
Low
Chopping
(carrier) signal
H
H
H H
H
H
H
L
L L
L
PWM signal
Comparator
+
-
Linear
Pass element
R
Driver
Command
input
Supply
voltage
RL
Load
Command
input
PWM
PWM
control
Duty
95% 50% 5% cycle
Supply
voltage
Load
A simple
comparator with a
sawtooth carrier
can turn a
sinusoidal
command into a
pulse-width
modulated output.
In general, the
larger the
command signal,
the wider the
pulse.
Pulse-width modulation
The output of a PWM amplifier is either zero or tied to the supply
voltage, holding losses to a minimum. As the duty cycle changes to
deliver more or less power, efficiency remains essentially constant.