Understanding Relay Spec's.

"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.

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

--Winston

amdx schrieb:

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

--
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.

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

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
         Obama is not the solution, Obama is the problem!

"Jim Thompson" wrote in message news: snipped-for-privacy@4ax.com...

I'm going to try this for my 1 HP pump motor. It only comes on

20 to 50 times a year. $4.50 Mikek

Add contact snubbers to reduce arcing too?

At least a plug in relay is easy to replace :)

Grant.

How do the relay contacts know the phase angle of the 10 amp current?

John

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.

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

Jamie

Is that an offer to send a socket? 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

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

--
Troll questions are best left unanswered.

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

situation?

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're right, of course, but the stress isn't due to the startup
current surge, which could be easily accommodated by the I²R loss in
the contacts, it's caused by contact bounce.

"John Fields"

** Huh ???

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

.... Phil

motor.

situation?

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.

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