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I have a piece of machinery that was moved from a building with 240 V to a building with 208 V and all is well but the contactors don't seem to pull in all the way. Could I remove enough turns from the coils to make them work on 208 V or is it not that simple?

If magnetic field is proportional to current, it would seem that removing turns to lower resistance/inductance would increase current and thus increase the magnetic field, pulling the contactor in all the way.

I could buy a transformer as the coils are AC but the only 208 V to 240 V transformers I found were very expensive, even for the small ones.

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  • \$\begingroup\$ May not be that simple - reducing the number of turns will indeed increase the current flowing. But - because there are now a smaller number of turns it will need more current to energize the relay! On DC the two things exactly cancel out. On AC the inductance going as the square of the number of turns will increase current more so it may work. \$\endgroup\$
    – Kevin White
    yesterday
  • \$\begingroup\$ It sounds like the contactor has marginal performance. In principle you could put a capacitor in series with the coil to 'cancel' some of the inductance, but it would require an extremely large value. If you remove turns, you'd really have rewind the coil with thicker wire -- basically fill the same volume with winding. \$\endgroup\$
    – jp314
    yesterday
  • \$\begingroup\$ Reducing the amount of copper makes the coil less efficient, which means it gets hotter for the same field. How much thermal margin do you have in hand? \$\endgroup\$
    – Neil_UK
    yesterday
  • \$\begingroup\$ I mostly would want to figure out why its only 208V, assuming both is the same country something seems to be wrong here \$\endgroup\$
    – PlasmaHH
    17 hours ago
  • \$\begingroup\$ The machine originally came from Czechoslovakia. \$\endgroup\$
    – ElectronicsNoob
    14 hours ago

2 Answers 2

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I have a piece of machinery that was moved from a building with 240 V to a building with 208 V and all is well but the contactors don't seem to pull in all the way.

The risks of lowering the turns on a coil are: -

  • Current will increase and may jeopardize the copper wire
  • Altering bought in goods may not be highly sensible
  • If you went back to a building with 240 volts you would have to add back the turns

Based on all of that (and given that you appear to have more than one contactor to rewind), I'd simply buy a step up transformer for the contactor circuits thus restoring the 240 volts to the control panel. There are several ways you can add a transformer of course.

For instance, consider a transformer that produces 36 volts from 240 volts. If, it were run from 208 volts, the secondary would be 31 volts. If you wire that secondary (observing phase polarity) in series with the panel feed to your contactors, you can "manufacture" near enough the right panel voltage from 208 volts. In other words, if you add 31 volts to 208 volts you get 239 volts.

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  • 2
    \$\begingroup\$ That is a very inventive idea, not one I would have thought of \$\endgroup\$
    – Simon Fitch
    yesterday
  • 1
    \$\begingroup\$ Using the secondary of a step down transformer in series with the primary’s voltage, essentially turning it into a step up autotransformer. Genius! \$\endgroup\$
    – ElectronicsNoob
    yesterday
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    \$\begingroup\$ It might also be easy/cheap to pick up replacement coils of a different voltage. Many contactors are designed with replacement coils. \$\endgroup\$
    – SomeoneSomewhereSupportsMonica
    yesterday
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    \$\begingroup\$ I actually implemented the autotransformer solution this morning and it worked beautifully only after finding out that the winding directions only work one way. I suppose that can be used to drop the voltage in differential mode. \$\endgroup\$
    – ElectronicsNoob
    19 hours ago
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    \$\begingroup\$ It did work, but accidentally discovered that it can be used to lower the voltage too. \$\endgroup\$
    – ElectronicsNoob
    14 hours ago
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We have 110v dc relays that we use with 133 volts. We add a 2700 ohm resistor in series. Makes them pick up at about 90 to 95 volts thereby preserving the relay. We have some that are piked up nearly constantly for 20 years or more. Maybe use this approach. Just add resistance in series so the relay gets 208 volts ac of course making sure the resistor is of proper wattage to hamdle the current. This will work with ac or dc. Good luck.

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  • \$\begingroup\$ Robert K - Thanks for trying to help. However the OP is reporting that the contactors originally working on 240 V, seem to be marginal and don't pull-in completely on a lower voltage of 208 V. || Your situation (110 V relays powered from 133 V) is the opposite of the OP's situation, since you have a supply voltage which is higher than the coil voltage. The OPs new supply voltage is lower than the previous one. || How would your suggestion of adding resistors (to lower the voltage even more) help the OP's situation? TY \$\endgroup\$
    – SamGibson ♦
    12 hours ago
  • \$\begingroup\$ Also, the OP's contactors are AC coils. These have low inductance when the armature is released which gives strong initial pull-in, When the armature closes the inductance increases significantly and the current reduces thereby allowing the coil to run cooler. Adding series resistance may mess up this characteristic and cause problems. In any case, as has been pointed out, the OP's voltage is too low so a resistor solution can't work. \$\endgroup\$
    – Transistor
    10 hours ago

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