August 2019 59
Switch Repair
L3 Out (C) 1Three Phase Machines
ForwardReverse
37111559132481121661014L2 Out (B)L3 InL1 InL2 InL1 Out (A)L2 Out (C) 137111559132481121661014L3 Out (B)L3 InL1 InL2 InL1 Out (A)2
New switch fi tted inside old casingto its operating speed. To reverse the
direction of the motor, the live and neutral
connections of (conventionally) the start
winding need to be swapped. The Moeller
contact arrangement diff ers from the
Dewhurst switch and it took some head
scratching to fi gure out how I could make
it work for my single phase motor. I have
an uneasy feeling that the solution I came
up with is more complicated than it needs
to be, but it works so I’m happy.
As always, make sure everything is
isolated before starting work and if you
do not feel competent to work with
mains switching, find someone who is to
assist you.
For my single phase lathe, this is how
I wired it. There are sixteen terminals
on the switch body, four pairs of which
were already linked by Moeller. I added
additional links to the following terminal
numbers: 1-7, 3-5, 9-11, 13-15, 4-10 and
8-14. The switch wiring is then supply
live to 2, supply neutral to 6, motor start
winding Z1 to 1, motor start winding Z2 to
5, motor run winding A1 to 9 and motor
run winding A2 to 13. If I had found that
the motor ran backwards when the switch
was set to forwards, I would have swapped
over the connections to terminals 1 and 5.
Removing the switch front plate allowed
the switch to be mounted into the old
Dewhurst switch casing and with all the
wiring connected, the front cover fi tted
without problem, as shown in photo 2.
To make understanding the wiring
solution that I used easier, I made a
schematic drawing, shown below. The
drawing also shows how the Moeller
switch could be used to repair a Dewhurst
switch fi tted to a three phase lathe. This
is simpler in that the only additional links
needed are 1-7, 3-5 and 9-11. ■