Inverter - Maytag MFR Series Service Manual

The inverter in Maytag commercial washers is
used mainly as a motor speed control device.
By altering the frequency of the current to the
motor the inverter can safely vary the speed
at which the drive motor runs. This allows the
washer to have the multiple speed ranges it
needs with only one, simple design, 3 phase
AC motor. Some other benefits of using
the inverter are: reduced need for complex
pulley and drive systems, more dynamic
control, lower power consumption, and better
protection of the motor under virtually all
operating conditions.
The inverter is powered by the AC line current
supplied to the washer, which is rectified
(changed into) DC current inside the inverter.
This DC current can be sent to the motor in
a series of high speed positive and negative
DC pulses. The generation of those pulses,
control of them and their frequency, amplitude,
etc. is done via the onboard computer located
inside the inverter.
By changing the pattern, amplitude, and
frequency of the pulses, the inverter mimics
3 phase AC current and the AC motor
connected to the inverter responds as if
connected to "pure" 3 phase AC current.
The inverter communicates with the main
control board in the washer via a RS485
communication cable that connects between
them.
The inverter does many other things in the
course of supplying power to the motor that
are beyond the scope of this text. They,
however, are not necessary to know in order
to be proficient in troubleshooting inverter
driven Maytag commercial washers.

inveRTeR

Input power to the inverter is supplied via a
contactor (high current relay) that is controlled
by the main control board at the front of the
washer. This contactor is typically located in
the upper rear area of the cabinet. Powering
the washer does NOT automatically power the
inverter drive; control of power to the inverter
drive is a function of the main control board at
the front of the washer via the CM1 contactor.
The input power is at line voltage potential
(208- 240VAC) with the exception of the
MFR18 and MFR25 models which are
powered with 120VAC line voltage current and
do not use Mitsubishi inverters.
Line voltage input power is also connected
to the input side of the contactor. (CM1 on
schematic (See page 9-1)). It is switched out
through that contactor's output side when
the main control board sends current to the
contactor's solenoid (coil).
From the output side of the contactor, line
voltage power is supplied to the inverter's
terminals "L1" and "L2". Those terminals are
the inverter's AC input, which powers the
inverter's electronic circuits.
With power established at the input of the
inverter, the washer control board located at
the front of the washer can then communicate
with the inverter and instruct it, when desired,
to power the motor. If so instructed the
inverter sends current out to the motor -- on
terminals "U", "V", & "W" (the inverter's output)
-- which are connected to the motor via a
shielded motor wiring harness.
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