Replacement Relay

[nat_wheatley]

The relay I use to turn my vacuum motors on and off is sticking, and needs to be replaced. I've attached a photo of it. Anyone know where I can get another? I probably don't want this same one, as it started sticking fairly early on, but something similar.

Thanks

[dana_swift]

Nat- relays for inductive loads need to have a rating for that purpose. The problem is motor start currents which are much higher than running currents. It can cause the contacts to weld, which is what you are probably experiencing.

If you have a 2hp dust collector, be sure to get a CONTACTOR with a rating for 3hp inductive, as well as the running amps your motor draws. Then it should last a long time.

D

[dana_swift]

I just looked at your relay image where it says "RESISTIVE RATED LOAD".

This is the wrong relay for your application! There is no HP rating at all.

D

[genek]

Nat: if this is inside of a control box go to your local hvac (heating and cooling) Distributor. You can get a good heavy duty one there for about 30.00. Also you can find that at any good electrical supply center or a Granger supply

you can get enclosed one there also.

[nat_wheatley]

Thanks Dana.

I'll check the vacuum motors in the morning. I'll post their specs, and then the question becomes what is (and where to find) the correct relay.

Nat

[Ken Sully]

Give this place a try, they have lots of surplus relays




http://www.surplussales.com/Relays/Relays.html

[ssflyer]

Actually, the image does show:
Motor Rated Load
15A 250VAC

The first time I looked, the resistive rating is what popped out at me, too.
Probably a bit small for your motors.

[nat_wheatley]

Will need some help once I post the vacuum specs, as I'm not really sure what I'd be looking for.

Thanks

[Burkhardt]

I use a 25A/240V SSR (Solid State Relay). No mechanical parts and no arcing, will last forever.
You can get them between $10 (China brand) and $35 (e.g. Opto22) from Amazon or Digikey. Unlike big contactors, they need very little drive current and have flexible drive voltage.
For full load rating they need to be mounted on a cooling surface, e.g. piece of aluminum sheet.
Not sure about the cheap models but the the one I bought from Opto22 has a snubber circuit built in for inductive loads.

[Bob Eustace]

This one is 40 amp and has a decent heat sink and is only $13.27.

http://www.amazon.com/Amico-Single-Phase-SSR-40DA-90-480V/dp/B00843IUG6/ref=sr_1_sc_2?ie=UTF8&qid=1371522718&sr=8-2-spell&keywords=Solidid+state+relay

[steve_g]

Read the reviews!

SG

[Burkhardt]

Read the reviews!

SG

Oh well, probably another example of China hit and miss. In any case I bought that 25 amp SSR from Opto22 and installed it (with heat conducting paste) on a larger piece of sheet metal. Works fine for over a year to control my dust extractor. Not sure where Opto22 has them built but they are a reputable company.

[nat_wheatley]

FWIW I have 4 of the LH7123-13 (240V) motors running through the relay...

[jerry_stanek]

Maybe the better way is use a contactor

[Burkhardt]

Maybe the better way is use a contactor
But that is like powering your CNC machine with a steam engine :D
Seriously, modern industrial control system don't use much contactors anymore. Semiconductor devices are so much more reliable. Just be careful with the no-name brands.

[jerry_stanek]

He said he is running 4 vacs off one relay. That is a lot of power through a relay

[Bob Eustace]

Even my wifes CNC quillting machine uses solid state on its tiny motors. I think the problem with s oldies is we cant see whats happening inside so we dont trust the technology I reckon?

[dana_swift]

Bob- solid state relays (SSR) are just like the mechanical ones. However when the current through the SSR gets over an amp or two (contactors instead of relays) things stop being quite so simple. Above that region the SSR dissipates power, where the old fashioned mechanical contactor effectively doesn't.

With a mechanical contactor it needs to be rated for inductive loads and able to handle the horsepower of the motor.

With a SSR, it must be mounted to a heat sink. How big does the heatsink need to be? That can be calculated, but the math is beyond what most "hook it up" applications are willing to bother with. The SSR's have an "on resistance" measured in milliohms, as well as a current and voltage rating. An SSR has to be sized larger than the PEAK current and PEAK voltage. How much does a given motor draw when starting? More messy math.

So .. at the sustained currents under load, there is some steady state current, with the lighthouse motors, lets say 8 Amps each. With 4 motors, that is 32 Amps (A little high I suspect), and lets assume a SSR with an on-resistance of 10 milliohms. V = I * R, so the voltage drop across the SSR will be 32 * .01 = .32 Volts. Then the power dissipated will be W = E * I, so the power will be .32 * 32 = 10.25 Watts.

A SSR dissipating 10.25 Watts MUST be on a heat sink, as a SSR case cannot dissipate that much power. Now start into the thermal resistance calculations of various materials and air temperatures..

Why engineers get paid.

Which is why Nat's relay was sticking. It was way under-rated for 4 motors whose running current is 8A each.

And why its probably much easier to use a mechanical contactor that is rated for 40 amps AC inductive. They are not cheap either, but the CNC quilting machines were made in large quantities justifying the engineering costs of SSRs, in this case use a contactor and just hook it up..

:)

D

[nat_wheatley]

I need to admit, I'm absolutely clueless in this area. Can someone post a link to one of these that would work for my application?

Nat

[dana_swift]

Nat- how about a simple solution- buy four of the relays you are using now, and hook them up one to a motor. The coils can be connected together or controlled separately. Then you can use a source you already know. That should handle the start currents and run a long time-

D

[coryatjohn]

Nat- how about a simple solution- buy four of the relays you are using now, and hook them up one to a motor. The coils can be connected together or controlled separately. Then you can use a source you already know. That should handle the start currents and run a long time-

D

This is what I did. I have two 2hp (240v) vacuum motors and use a relay for each one. The relays are rated for 15 amps inductive load. I got them at Grainger for about $20 each. It just seems to make sense to control each motor with its own relay.

[nat_wheatley]

Great idea, thanks very much for the help.

[richards]

SSR are nice. They interface easily with computers. Many have a zero-crossover detector so that they turn on/off at the zero point on the sine way (reducing arcing) and they seem to last forever; HOWEVER, most of them never completely turn off. They have "leakage", which can and will give you a major shock if you're not aware.

Whenever servicing equipment that use SSRs, I turn off the power before trusting an SSR.