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

[Robin S.]

I bought a relay to make a simple motor controller (w/o the overload
protection). The relay's coil is rated for 100VAC and I want to run it on
120VAC. Is this a big issue? I've been told that because a coil is a highly
inductive load, one cannot simply find the correct value for the resistor
(guess and test would be a good idea).

This is for a hobby machine tool, not an industrial installation.

Thoughts?

Regards,

Robin

 

[John Gilmer]

I bought a relay to make a simple motor controller (w/o the overload
protection). The relay's coil is rated for 100VAC and I want to run it on
120VAC. Is this a big issue?

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.

 

[Robin S.]

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."

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.?

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 something like a locked rotor current situation? If the relay gets
*too hot*, do I have any options (other than to go and buy a real
contactor)?

Thanks for your swift reply.

Regards,

Robin

 

[SQLit]

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
*too hot*, do I have any options (other than to go and buy a real
contactor)?

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.

 

[Robin S.]

If it is an icecube relay, then why can t you get the right one for your
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.

Please notice relays not motor starters.

Yup, you're right.

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.

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

 

[Robin S.]

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

This relay only has NO contacts...

btw What frequency is your coil rated for?

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

 

[[email protected]]

I bought a relay to make a simple motor controller (w/o the overload
protection). The relay's coil is rated for 100VAC and I want to run it on
120VAC. Is this a big issue? I've been told that because a coil is a highly
inductive load, one cannot simply find the correct value for the resistor
(guess and test would be a good idea).

This is for a hobby machine tool, not an industrial installation.

Thoughts?

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
30 watts, 80 ohms at 20 watts, 120 ohms at 30 watts and 160 ohms
at 40 watts.

 

[Louis Bybee]

I bought a relay to make a simple motor controller (w/o the overload
protection). The relay's coil is rated for 100VAC and I want to run it on
120VAC. Is this a big issue? I've been told that because a coil is a highly
inductive load, one cannot simply find the correct value for the resistor
(guess and test would be a good idea).

This is for a hobby machine tool, not an industrial installation.

Thoughts?

Regards,

Robin

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.
http://www.electrical-contractor.net/ubb/Forum15/HTML/000058.html

Louis

 

[John Gilmer]

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.

Really?

Where?

EMWTK

 

[Robin S.]

I get a kick out of this kind of post.

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

We buy 9A contactors with
overloads rated for motor loads for under $25 each.

My relay was about $4.00 (Canadian dollars).

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.
Indeed.


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.

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

 

[Bob Peterson]

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.