Product Manual

PWM types have three elements. The first converts AC to DC, the
second filters and regulates the fixed DC voltage, and the third con-
trols average voltage by creating a stream of variable width DC pulses.
The filtering section and higher level of control modulation account
for the PWM drive’s improved performance compared with a common
SCR drive.
AC Drives
AC drive operation begins in much the same fashion as a DC drive.
Alternating line voltage is first rectified to produce DC. But because an
AC motor is used, this DC voltage must be changed back, or inverted,
to an adjustable-frequency alternating voltage. The drive’s inverter
section accomplishes this. In years past, this was accomplished using
SCRs. However, modern AC drives use a series of transistors to invert
DC to adjustable-frequency AC.
This synthesized alternating current is then fed to the AC motor at the
frequency and voltage required to produce the desired motor speed.
For example, a 60 hz synthesized frequency, the same as standard line
frequency in the United States, produces 100% of rated motor speed.
A lower frequency produces a lower speed, and a higher frequency a
higher speed. In this way, an AC drive can produce motor speeds from,
approximately, 15 to 200% of a motor’s normally rated RPM – by
delivering frequencies of 9 hz to 120 hz, respectively.
Today, AC drives are becoming the systems of choice in many industries.
Their use of simple and rugged three-phase induction motors means that
AC drive systems are the most reliable and least maintenance prone of all.
Plus, microprocessor advancements have enabled the creation of so-called
vector drives, which provide greatly enhance response, operation down
to zero speed and positioning accuracy. Vector drives, especially when
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With advances in power electronics,
even so-called “micro” drives can be
used with motors 40 HP or higher.
Full-featured unit shown includes
keypad programming and alphanu-
meric display.