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Question:

Will a 110V transformer work with a 100V contactor?

I fried a transformer in my lathe. I went to the electrical supply near my house and the got me a new transformer and said it will work. The old transformer is 3 times bigger. they are both 100VA. the voltage on the original is 230v in and 100v out. It powers the on/off switches for the forward/ reverse micro switches. from the micro switch it powers the magnetic puill in for the contactors. The contactor say 100V.will using a 110v transformer cause and problems or issues?Should I return the one I purchased and order a 100V ?I have work piled up and waiting and really need this lathe operational ASAP.Thank You in advance for any information.Earle

Answer:

No, contactors and other electromechanical switches should be rated for at least twice the supply voltage, because of inductive voltage spikes during circuit opening.
I assume the new transformer is 230/110v? If the contactor coil is rated 110v, it should be fine. Most contactor coils, as with most electrical equipment these days have a large voltage tolerance. However, you need to check that the contactor coil is rated for 110v. You may find it's rated for 100v to 120v.
It should be fine. A 10 increase in voltage will create about a 10 % increase in current through your contactor coils and control switches. This is generally well within the safe limits of electrical design.
You ALSO need to be concerned with the POWER RATING. The bigger transformer is capable of handling more power (current) than the smaller one. Therefore, even though the VOLTAGES are correct, the smaller transformer will over heat and burn out, POSSIBLY CAUSING A FIRE! There should be a CURRENT RATING on the old transformer (like 5 amps or 10 watts), and you need to get a new transformer that has BOTH the same VOLTAGE AND CURRENT capabilities.
Since they are both 100 VA units, capacity seems to be OK. The voltage however should be checked further. For example, you say the unit you removed was 230 volts in and 100 volts out. This is a ratio of 230/100 2.3 to 1. Thus if you have 230 volts on the high side of the transformer, you will get 100 volts on the low side of the transformer. If the new unit you purchased is 230 volts in and 110 volts out then the ratio is 230/110 2.09 to one. So if you have 230 volts on the high side you get 110 volts on the low side. However, I suspect the new transformer is rated as 240 volts high side and 110 volts low side for a ratio of 240/110 2.18 to one. So f the input is 230 volts you will get 105.4 volts on the output. Thus the increase of the output may only be 5 volts, not 10 volts. Bottom line is that the new transformer will work just fine either way. Over-voltages due to switching surges may be ignored as most designs have already taken that into account with voltage variations of +/- 10% normally. Hope this helps, Newton1Law

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