An Engineer in Wonderland – Contactor conundrums

As part of the great lathe re-wire, I have had to learn a bit about contactors.

motorcontactor-thumb.jpgAs 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:

motorcontactor-web.jpga) 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!


Should you feel the need, DON’T RESPOND BELOW as we are having a spam crisis.

Instead, email
No email addresses are collected for marketing purposes from responses to this blog. I will keep it that way for as long as possible.



  1. Good day to you Mr Kurt.
    The crossed-out capacitor symbol in the world of power engineers is a normally-closed contact.
    Lots of the symbols are on this website:
    – including the entirely non-intuative (to my eyes) mechanical operation symbols.
    And thank you for emailing your comment.
    We at Electronics Weekly have thought up a 100% reliable way to block all spam comments – which is to auto-delete anything with a URL in it.
    Unfortunately, those that are in charge of the IT system say this is impossible, although not why.
    And so the Spam Wars continue

  2. Dear Alice,
    Interesting comment about symbology. I had a workmate recently ask about some symbols on a schematic that looked like capacitors, but one had a slash through it, and they were located in a position that would have have more appropriate for a switch.
    Do you have examples of those symbols, or the standards where they are defined??
    I’m used to the variations between electronic symbology differences between the USA and parts of Europe, but this was completely unknown to me.
    thanks much, and good luck with that Spam(tm) problem.

Leave a Reply

Your email address will not be published. Required fields are marked *