Using 120VAC to actuate a 100VAC relay coil...

It might be.

There are several posibilities but most can be "evaluated" by just connecting 120 volts to the relay for a few moments while measuring the current. If the current stays at a fraction of an amp then "fire for effect" and leave power on for some minutes to see how hot things get and whether the relay passes the "smoke test."

The trouble with a resistor in series is that the current flow is MUCH highter before the relay operates than after it does. A resistor large enough to prevent overheating might keep it from operating.

Sounds like a good idea. The relay is made by Omron in Japan so I'm assuming it should be half decent. How hot should it normally get? I understand you can't exactly tell me a number, but should it be warm, hot, too hot to touch, etc.?

Sounds something like a locked rotor current situation? If the relay gets

Thanks for your swift reply.

Regards,

Robin

If it is an icecube relay, then why can t you get the right one for your application. My applications provide for the relays not to be EVEN warm because of current. Please notice relays not motor starters. Relays are not designed as motor starters. Motors draw 6X running current when starting and they can be overloaded that is why the NEC requires a over load protective device to be installed for protection. Have fun and I hope this is not a critical application.

I bought it surplus. It's not exactly an ice cube relay. It's got the 1/4" flat connectors on it and it's certainly beefier. Granted, it's not a contactor, but the contacts are rated for 20A at 115V.

Yup, you're right.

This is a motor starter for a 1/4HP fan motor that I'm using to run a small lathe. It is in my garage and I will be there 100% of the time in which it is operating.

I think the worst that will happen is that the contacts will wear out and I'll have to splurge for a contactor.

Thanks for all the comments. As a tool and die apprentice, it kills me to see people not doing it the "right" way so I understand your concern. I'm just too cheap to buy the correct product for my application. Because I can supervise it and I'm not to concerned if it fails, I think I should be OK.

Regards,

Robin

If your relay has an unused NC contact you could bypass the resistor with it for starting. Just a thought.

btw What frequency is your coil rated for?

This relay only has NO contacts...

I don't think it said. It's just got a painted "100VAC" which looks larger and rougher than the other ratings and info on the relay. Odd, but I guess they can do it any way they want.

I ran the relay for about 5 min today. It got very slightly warm. Hardly noticeable but I think it was warm. I think I'm going to run with it.

Regards,

Robin

This is actually trivial. First, a 100 VAC relay coil is not going to burn out quickly if it "sees" 120 VAC. It will happily take an overvoltage for quite some time before it dies. A simple power resistor will work fine. There is NO need to worry about inrush current harming the relay. You DO want to reduce the voltage to reduce the heat in the coil, not because it will burn out quickly, but to make the thing last for a long time. Next, your 100 volt relay will probably pull in as low as 80 volts, maybe lower, so precise computation of the resistor's value is not needed.

Try and test, as you mentioned, will work fine. Put the resistor in series with the coil and measure the voltage at the coil. You want it under 100 volts.

You DO need to use power resistors, say 10 watts or higher. And once you have determined the resistance, you want to make sure the resistor won't burn out. In this case, use the formula 400/R (where R is the resistance in ohms) and double that figure to come up with the wattage. Go to the next higher standard wattage value if the figure computed is not standard. Example: Say a 100 ohm resistor is selected. 400/R would be 4, double that would be 8, and the next higher standard wattage would be 10 watts.

By the way, you could buy 4 40 ohm, 10 watt resistors for $2.00 from All electronics. In series, or series/parallel or parallel you could get the following useable combinations: 40 ohms at 40 watts, 60 ohms at

In spite of my concerns regarding your "w/o the overload protection" I'll suggest that you consider the following site (and many others using "buck boost" as a search string in google) as a possible solution for experimentation purposes. Using a small control transformer of the proper current rating with a ratio of 120/16 or 120/24 connected in a "buck" arrangement could address your "over voltage" situation.

It is very important that you at least consider the reasons/necessities for overload protection, and many other safety issues before pursuing your quest. The energy available in the circuit you are working with is well beyond lethal. If you lack experience in this area you should at least have a competent electrician/engineer review your intentions before the actual exercise.

Watch out for word wrap.

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I get a kick out of this kind of post. We buy 9A contactors with overloads rated for motor loads for under $25 each. Trying to save yourself such a tiny amount of money by using an icne cube relay with an odd voltage coil just seems silly, even if you plan to watch it continuously.

OTOH - if its a 120V single phase motor why not just use a regular light switch rather than a relay in the first place, or just plug it in to an outlet with a regular old cord? Seems like a lot of trouble for little benefit.

Really?

Where?

EMWTK

I can assure you I would as well in your position.

My relay was about $4.00 (Canadian dollars).

Indeed.

That's true. I was originally going to wire the motor such that it could be be reversed. Unfortunately, the starter coil is not wired independently of the running coil (there are only three wires instead of four). Not that I would know, but it looks like that wiring does not allow reversal. Anyway, I thought I'd wire up a box to put it all in. Call it a learning experience.

Regards,

Robin

its about the standard price paid by those who buy them from distributors at oem level discounts. does not matter much what brand. often the oem price is 1/4 the list price, sometimes even less.