Understanding Relay Spec's.

[amdx]

I'm looking at the relay below to drive a 120 volt,7 amp, 1 HP pump motor.
The contacts are rated at 10 amps 120 volt 1/3 hp.
I don't understand the discrepancy in the HP rating.
http://www.electronicsurplus.com/ItemView.aspx?ItemID=7060

Or cheaper with slightly higher current,
http://www.allelectronics.com/make-a-store/item/RLY-453/120-VAC-RELAY-DPDT-12-AMPS//1.html

Is this a 10 amp resistive load and much lower for inductive load situation?

Also any comments about using a SSR to drive a 1 HP motor?

Thanks, Mikek

 

[Winston]

I'm looking at the relay below to drive a 120 volt,7 amp, 1 HP pump motor.
The contacts are rated at 10 amps 120 volt 1/3 hp.
I don't understand the discrepancy in the HP rating.

http://www.reliance.com/prodserv/motgen/b9652new/b9652.htm
"INRUSH CURRENT
When started with full line voltage, AC motors draw line currents
substantially greater than their full load running current rating."

Looks like your motor can draw ~7460 W while it is
spinning up to RPM.

http://www.electronicsurplus.com/ItemView.aspx?ItemID=7060

Or cheaper with slightly higher current,
http://www.allelectronics.com/make-a-store/item/RLY-453/120-VAC-RELAY-DPDT-12-AMPS//1.html

Is this a 10 amp resistive load and much lower for inductive load situation?

Also any comments about using a SSR to drive a 1 HP motor?

Sure! Just specify it to tolerate the maximum current you'll need.


--Winston

 

[Uwe Hercksen]

I'm looking at the relay below to drive a 120 volt,7 amp, 1 HP pump motor.
The contacts are rated at 10 amps 120 volt 1/3 hp.

Hello,

you may try to switch on the motor over a fast blow fuse with 8 to 10
amps. If those fuses will blow often, you may think again about the
relay contacts.

Bye

 

[amdx]

I'm gradually replacing the SSR's in my swimming pool controller with
contactors as they each fail ;-)

...Jim Thompson

I'm going to try this for my 1 HP pump motor. It only comes on
20 to 50 times a year.
http://www.skycraftsurplus.com/120vacdpst30amp.aspx $4.50
Mikek

 

[Grant]

I'm going to try this for my 1 HP pump motor. It only comes on
20 to 50 times a year.
http://www.skycraftsurplus.com/120vacdpst30amp.aspx $4.50
Mikek

Add contact snubbers to reduce arcing too?

At least a plug in relay is easy to replace

Grant.

 

[Jamie]

I'm looking at the relay below to drive a 120 volt,7 amp, 1 HP pump motor.
The contacts are rated at 10 amps 120 volt 1/3 hp.
I don't understand the discrepancy in the HP rating.
http://www.electronicsurplus.com/ItemView.aspx?ItemID=7060

Or cheaper with slightly higher current,
http://www.allelectronics.com/make-a-store/item/RLY-453/120-VAC-RELAY-DPDT-12-AMPS//1.html

Is this a 10 amp resistive load and much lower for inductive load situation?

Also any comments about using a SSR to drive a 1 HP motor?

Thanks, Mikek

Starting motors is a tricky thing, at times you can get inrush currents
along with EMF kick back that generates a high potential of HV&Plasma
around the contacts. The plasma becomes a conductive path and thus
serves to fuel the flame which makes the arc last longer.

Contacts for inductive load usage's, usually have a moon type surface
which helps extinguish the flame, along with harden material.

You should get a more starter relay like this one.

http://www.powerqualityanddrives.co...ize-00-Starter-Open-5-to-8-Amps_16451260.html

Although that one is a 3-phase unit, you simply series the contacts.


Jamie

 

[amdx]

Add contact snubbers to reduce arcing too?

At least a plug in relay is easy to replace

Grant.

Is that an offer to send a socket?
Mikek

 

[John KD5YI]

I'm looking at the relay below to drive a 120 volt,7 amp, 1 HP pump motor.
The contacts are rated at 10 amps 120 volt 1/3 hp.
I don't understand the discrepancy in the HP rating.
http://www.electronicsurplus.com/ItemView.aspx?ItemID=7060

Or cheaper with slightly higher current,
http://www.allelectronics.com/make-a-store/item/RLY-453/120-VAC-RELAY-DPDT-12-AMPS//1.html

Is this a 10 amp resistive load and much lower for inductive load situation?

Also any comments about using a SSR to drive a 1 HP motor?

Thanks, Mikek

It is harder to break the connection to an inductive load than to a
resistive load. Trust the manufacturer's statement of 1/3 HP.

John

 

[ehsjr]

Wrong statements should be corrected.

The relay contacts can't tell whether a 10 amp sine wave was created
by a resistive load or an inductive load. An ammeter in series with
the contacts couldn't tell, either.

Motors (and incandescent lamps) stress relay contacts more than
resistive loads because of startup surge.

John

And motors can stress the relay contacts with a high voltage arc
when the relay opens.

Ed

 

[Phil Allison]

"John Fields"

---
Yes, but it's not just about the start-up surge, it's because the
motor is a reactive load and sends the energy it stores back to the
power company through the same contacts used to power the motor, the
end result being that the relay contacts run hotter than they would if
it was a purely resistive load.
---

** Huh ???

Wanna explain how the contacts know ( or care) which way energy is flowing
??


..... Phil

 

[amdx]

Wrong statements should be corrected.

The relay contacts can't tell whether a 10 amp sine wave was created
by a resistive load or an inductive load. An ammeter in series with
the contacts couldn't tell, either.

Motors (and incandescent lamps) stress relay contacts more than
resistive loads because of startup surge.

John

I was thinking of the arc across the contacts when they open
with an inductive load. Phase angle was not on my mind,
I had thought your statement was aimed at someone else.
Mikek

 

[Chieftain of the Carpet Crawlers]

I like the knife blade switches because the grasp the entire blade of
conductor with more conductor.

I just posted this because that is what came to mind when I envisioned
contact bounce events.

It was the first thing I thought of. Frankenstein.

 

[Chieftain of the Carpet Crawlers]

In that case, 10-amp average resistive loads and motors and
incandescents would apply equal stress to relay contacts. Except, of
course, they don't.

They also do not exhibit the same bounce characteristic, idiot.

So, you just shot yourself in the foot, dumbass.

Try for the head now, boy.

 

[ehsjr]

The worst-case scenario could be an induction motor that acts like a
generator if the AC power line fails in a low-Z mode. Then the motor
can briefly push essentially its stall current back through an opening
relay contact... if the contact opens while the motor still has enough
rotor field to generate power.

John

You don't need worst case to stress the contacts that way. Any time
the contacts open L*di/dt can - and often does - create a high
voltage which can arc and stress the contacts.

Ed

 

[Chieftain of the Carpet Crawlers]

OK, explain to us why the contact bounce pattern depends on the load
impedance.

John

You explain how an inductive load can cause a different current
signature to be present at the time of initiation, which is when the
contacts bounce.

Don't forget to describe what the high current influx does as the
conductors that make up the contacts bounce open and closed several times
in the first milliseconds of operation.

With a resistive load, the contacts almost meld together (molecularly
speaking) at the moment they contact. It is a "controlled short". Ever
seen the points on a lawn mower engine? They only 'bounce' once, each
time they are operated. They bounce 'open',and there is an arc and a
material transference.

For AC, regardless of load, the best way to fire a relay is to time it
such that it closes while the voltage is crossing through zero volts.

 

[[email protected]]

OK, I figured out where my relationship with Thompson went wrong. I
sent him a 6-pack of Widmer. It was the wrong beer.

ftp://jjlarkin.lmi.net/Jim_Beer.JPG

Hmm, is Jim Beer anything like Billy Beer?

 

[Phil Allison]

"John Larkin"

AC doesn't sustain an arc like DC does.

** In the time span of a few mS - there is no big difference.

How much a pair of contacts arc depends on how much current has to be broken
and the voltage available after opening - all AC power does is provide a
good opportunity for the arc to self extinguish within 8 to 10 mS. The
amount of damage done to the contacts in that time may not be great - but
if repeated 10,000 times it is often fatal.

Relays often have three contact current ratings, for resistive loads,
incandescent, and motors. The resistive is highest, and incandescent
loads don't have much inductance.

** With incandescent loads, the inrush surge is higher ( around 10 to 12
times the normal Ipk ) - but comparatively brief at only a few cycles
instead of a few seconds. Contacts generally have longer thermal time
constants than a few cycles.

DC contact voltage ratings are usually much less than AC.

** Yep - typical relays rated for 240VAC switching at 10 amps are also
rated for 24 or 32 volts DC at 10 amps. When tested, there is a dirty great
flash from the contacts when doing the latter and a total failure to break
the circuit if the voltage is raised above about 40 volts.

I tried this few times with a 50 volt DC supply (cap filtered) and a 4 ohm
resistive load - a self sustaining arc developed across the relay contacts
* first shot * and destroyed them in a few seconds.

Using a relay with a larger opening gap helps somewhat as does using a
strong magnet near the contacts - but is still not usable over many
operations.


..... Phil

 

[Grant]

Warning! Apostrophe police!
Q: WHAT belongs to the spec???

Period!

 

[Grant]

Is that an offer to send a socket?
Mikek

Sorry, no. It's part of the decision when replacing a relay, how often will
I need to do this again? Last time I used relays, I socketed them, and none
of the relays failed. Good magic

 

[amdx]

Warning! Apostrophe police!
Q: WHAT belongs to the spec???

I've been looking into this!
I find the following on several sites, which is why I inserted the
apostrophe.
"We use an apostrophe when letters are missing.
I will look at the obvious cases first. These are where we deliberately
shorten a word

or phrase and then use an apostrophe to show that letters are missing."

HOWEVER, all the examples I see are when letters are missing AND two words
are combined.

So, I made a horrible grammatical error and won't do it again!

Until I do.

Mikek