Rooting around in the fascinating stuff at the bottom of a draw labelled 'Engineering - Junk Miscellaneous'. Delving amongst the delightful...
An Engineer in Wonderland – Contactor conundrums
As part of the great lathe re-wire, I have had to learn a bit about contactors.
The starter that came with the lathe had been bolted to the front of the machine, then the front cover had been removed exposing the contactor to the world and allowing it to be operated directly by a finger. And allowing access to ground leakage paths, like fingers.
It is a 12A one from Telemecanique and looks rather well made.
A lot of reading and googling has taught me:
a) The power guys use different circuit symbols to the semiconductor guys – one Alice’s capacitor is another Alice’s normally-open contact.
b) A 20A contactor is good for a 2.2kW three-phase motor, but a 2.2kW single-phase motor needs a 40A one.
c) A 480V coil contactor buzzes a lot on 240V, but this does not seem to strike some people as odd.
d) There are never quite enough auxiliary contacts for you to make a nice reversing contactor from two normal ones.
e) It is easier to remotely-mount the current limit elsewhere with a three-phase contactor if you want to use it for a single-phase motor with a soft start that mysteriously needs a two-pole live switch (K1 in the data sheet, what is that for?)
f) Not all inductive loads are equal, which is why there is an ‘ACxx’ rating scheme for loads.
g) Stepping back into relay logic is great fun.
h) DIN mounting rails are a neat idea.
j) The Colchester lathe starting switch is an engineering marvel.
k) Colchester’s switch wiring diagram is an engineering disaster.
And lastly, buying a single-phase to three-phase inverter and re-wiring the original three-phase motor from star to delta is probably easier in the long run.
Apologies to all those who have been doing this for years, for whom none of this is a revelation.
And don’t get the idea that I have lost my tiny mind in the excitement and forgotten about safety - but if you have spotted some dangerous beginners error above, do tell as I value my friend!
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