PDA

View Full Version : Electrical contact horse power question

rockrat
05-13-2009, 01:25 PM
So, I have a timer relay with multiple contacts that all operate together mechanically. The sticker in the box tells me that the timer can handle 1 hp per contact. I have a 220v 1.5 hp motor that I wish to apply to the timer.

If the motor is 1.5 hp and needs 2 contacts (220 volt = 2 legs) to operate, does that mean each leg of the motor is .75 hp? Or does one use the total hp per leg?

I'm thinking 1.5 hp per leg but I have made errors before. (edit)

Thoughts?

Duffy
05-13-2009, 01:43 PM
The electrical wizards here will probably tell you that a 1.5 hp motor needs a timer with 1.5 hp capacity contacts. They do not share the load. That being said, it is unlikely that your 1.5 hp motor will be starting under full load or possibly much load at all. Most motor loads are conservatively sized, and only hit the nameplate capacity when something goes wrong. You own the timer, so why not use it? the worst that will happen is that the contacts will eventually fail and THEN you can buy a correctly sized one But you may be dead by then! Duffy

Barrington
05-13-2009, 01:44 PM
I'm thinking 0.75 hp per leg...

I'm thinking 1.5 hp per leg... (edit)

Cheers

Carld
05-13-2009, 02:22 PM
You need a higher hp timer.

Henro
05-13-2009, 05:32 PM
You indicate your motor is a single phase 1.5 HP motor, so it will therefore have two leads going to it for power transmission.

You have a relay with four contacts. Each contact is rated at 1.0 HP.

If you parallel the contacts in pairs, so you have two contact feeding one of the motor leads, and two feeding the other, it will probably work. Anytime you paralled contacts you will not have an equal division of current, but the currents through the paralleled contacts will likely be close. Since you have 1.0 HP rated contacts, even if the division of currents is 66% and 34%, you are still within the rating of the contacts. It is unlikely in my opinion that the difference will be that great. I would try it and probably be happy with the result.

Your motor is likely an induction motor. While the point I am about to make is not a concern in your application, an interesting thing about induction motors is that the current that they draw is not proportional to the power they are delivering to their load. When running at NO LOAD they still may draw 80% (or more) of nameplate current. This is because they are inductive loads and as such when unloaded the current is out of phase with the applied voltage, because they appear as inductor to the voltage supply, and even though the motor current is high, the power delivered by the motor is low, due to the phase difference between the applied voltage and the current flowing through the circuit.

If I could afford to worst case have the relay fail, then I would try using it, with contacts paralleled. IF for some reason I did not want to take that chance then I would get a larger relay. I doubt your relay will fail if you got the specs right in your post...but the way you find out is to try and see...

Hope this helps...

Edit, you did not say you have four contacts, just multiple contacts. If you don't have four contacts, and only three, all bets are off. Get a higher HP rated relay. If you have four or more contacts, parallel them up equally...the more the better and the greater you probability of success!

rockrat
05-13-2009, 06:46 PM

Cheers

Ha, yea I kinda did. The last sentence is what I was shooting for.

I had thought a little today about an idea and it looks like Henro verified my thought. I do have multiple contacts so I could take the load side and spread it across the contacts. I think that I did that same thing on my phase converter. That may be why I was thinking about it.

Thanks for the verification. Any other comments welcome.

Cheers
rock~

J Tiers
05-14-2009, 12:14 AM
Everything that has been said is true.... You "should" use a correctly HP rated contactor.

Multiple contacts don't close together, or open together, so inevitably, all the load comes on one regardless of parallelling.

And it may not matter one tiny little bit as far as real-life usage.

The rest of this is based on a NEMA HP rating, NOT on an IEC type rating, which vastly overstates the capability.

I'd bet on the use of ONE contact per leg working for a LONG time, although I quite agree that the proper and reasonable course of action is to size the contactor for the motor HP..

Here is the "non-electrical code" explanation...

The "HP" rating is based on the ability of the contactor to CLOSE or OPEN the LOCKED ROTOR CURRENT of the motor, without welding contacts, or other damage. So it is based on a current that is not generally seen by the contacts.

The locked rotor current is several times the motor running current. There is a code on the motor that gives you the locked rotor KVA, from which you can derive the current. All you really need to know is that it may be 6 times the running current, or more.

Also, the HP rating is at a certain voltage. At a higher voltage, the HP rating is normally higher. So if your rating was 1 HP at 115V, it would be at least 1.5 HP at 220v, if it carries a 220V rating.

obviously if the 1 HP was at 220V already, that is the "real" rating.

The contactor does see the locked rotor current when starting, for a short time. But if the motor is not stalled, it is a very short time, and likely won't bother anything.

So the bottom line is that

1) the relay may be rated for over 1 HP at 220, you can look and see.

2) the rating is generally a worst case, and under normal conditions is overkill.

3) So, the relay probably will operate OK for many years.

4) if it fails sooner or later, you can have the satisfaction of realizing that it was, after all, probably under-rated for the application. Don't come to us and say "you said it will work fine"..... :)

rockrat
05-14-2009, 08:26 AM
So the bottom line is that

1) the relay may be rated for over 1 HP at 220, you can look and see.

2) the rating is generally a worst case, and under normal conditions is overkill.

3) So, the relay probably will operate OK for many years.

4) if it fails sooner or later, you can have the satisfaction of realizing that it was, after all, probably under-rated for the application. Don't come to us and say "you said it will work fine"..... :)

Good info, thanks for the edumacation. The box has a sticker in it that says specifically 1 hp at 110/220 which I thought was odd. I figured that if it has a rating at 110 then at 220 the rating would be different.

Most relays I have found tell me xx-hp at 110 and x-hp at 220. I have a few of these sitting around so it should not be an issue if one goes bad. I'll just have to remember that I am saving one for future use. And I'll also have to keep it somewhere that I can get to it.

cheers
rock~

J Tiers
05-14-2009, 09:00 AM
Yes, that rating is a little odd.

An example of the usual.... I am looking at an Omron G4B series relay.

120Vac rating = 1HP/15A

250 Vac rating = 2HP/25A

Generally one ought to believe the ratings...... the manufacturer normally has the largest rating they can get.