[micromind]

I'm doing the MSHA required ground continuity test at a local rock crushing plant, likely more than 100 motors from 2 - 400HP. My connection to the ground bus at the MCC was right below a size 2 NEMA starter which was labelled '40HP Tunnel Belt'.

The bucket didn't look like a size 3 so I opened it up and sure enough, it was a size 2 feeding an actual 40HP motor. I've done work at this plant for 6 years and I know for a fact that this starter and motor have been this way for at least that long.

So, being the curious person that I am, during my ground check, I wrote down the HP of every motor and looked at the starter size when I cleaned the buckets.

4 - size 1 (10HP max) controlling 15HP motors. 2 - size 2 (25HP max) controlling 30HP motors, and 1 controlling a 40HP. Everything else was ok.

So here's my question, a size 1 starter is rated at 27 amps and can control a 7 1/2HP motor on 208 or 230 but only a 10HP on 460. Why? It seems to me that it could (and some certainly do....) control a 15HP on 460.

Same with a size 2. 45 amps and 15HP @ 230 volts but only 25HP @ 460. It seems like it could handle a 30HP.

Size 4 and larger, the 460 HP is 2X the 230 HP. This makes sense, the smaller sizes do not.

I can't help but wonder just exactly how these ratings came to be, they don't make much sense.

Anyone else see starters controlling motors larger than their ratings?

 

[Jlarson]

Occasionally. Like a lot of other "wrong" stuff it's been like that for ever and in a lot of cases will probably keep working for a long time.....

The ratings also consider LRA. I think the rating for a size 2 @ 460v is 87 or 88 amps. Assuming it's not plugging/jog duty.

 

[frenchelectrican]

I have see that few time in both NEMA or IEC verison and useally with NEMA they can handle oversize in some degrees without overheating the concats.

However you have to keep in your mind is the AIC rating on them is not the best when you load it to max. espcally on 400 volts and higher.

but some of them been running like that for years without hiccups.

And the cycle duty if they are on constant most of the time it dont really affect too much at all but jogging or plugging that is different story. ditto with frequent reversing.

 

[Cow]

No, I don't see any undersized starters in my neck of the woods.

I know I'd feel a lot better if the starter were the right size or a little over, then I'd know those contacts stand a better chance of lasting a little longer.

I'm sure the only reason they've lasted that long is they are NEMA starters, I don't think an IEC would have lasted that long.

 

[JRaef]

The NEMA size rating is based on the absolute worst case scenario of what a user might want to do with a given motor, namely inching and plugging duty (plugging, for those who don't know, it reversing a motor to stop a load). So when you size a starter as NEMA size 1, that is capable of starting a 10HP motor 5 times in a 10 minute period without a specific rest time between starts. If you are never starting it that often, the contacts are going to basically last forever. Some people decide that also allows for connecting them to larger motors. While that might be technically possible, it's still a code violation; 110.3(B)

(B) Installation and Use.
Listed or labeled equipment shall be installed and used in accordance with any instructions included in the listing or labeling.

 

[micromind]

Which labeling do we use? It can be either amps or HP.

Of course, we have always used HP and I think we should continue to do so. But I also think that NEMA should either justify their ratings or have the backbone it takes to say they're wrong, if they actually are.

 

[Big John]

The rule of thumb is IEC is used when it's precisely engineered: If you are certain you won't exceed the maximum power and duty cycle, it works. You start pushing design ratings, it's gonna fail.

NEMA is bomb proof: If you know it's gonna have severe duty and needs to perform reliability, then you pay for that. NEMA contactors will take a lot of abuse.

 

[triden]

NEMA starters are massive and rely on thermal mass in the contacts to save them from damage and give them better heat tolerance. IEC starters of a similar rating are much smaller because they use more modern science (such as arc quenching contacts) in their design. NEMA will take more abuse because they are massive, but if either is used as designed I believe they will both last just as long.

 

[Jhellwig]

Which labeling do we use? It can be either amps or HP.

Of course, we have always used HP and I think we should continue to do so. But I also think that NEMA should either justify their ratings or have the backbone it takes to say they're wrong, if they actually are.

Always horsepower. None of the ratings are linear most of the time. It all has to do with the designs of the contractor. If the voltage is up and it isn't starting or stopping under a heavy load frequently it likely isn't going to have a problem.

Most likely you will run into problems with being under fused than anything.

 

[frenchelectrican]

Which labeling do we use? It can be either amps or HP.

Of course, we have always used HP and I think we should continue to do so. But I also think that NEMA should either justify their ratings or have the backbone it takes to say they're wrong, if they actually are.

Most case useally Horsepower rating useally take care majorty of the time but if have mixed load you will need to look at both HP and amp load.

For non motor load the amps rating do come in play.

 

[micromind]

In the past, I never paid much attention to the discrepancy in the HP ratings of starter, I always installed them according to their ratings and will continue to do so.

But I thought it was interesting that the size 1,2 and 3 the 230 volt ratings are more than 1/2 of the 460 ones but on size 4 and larger, they are exactly half.

Also, it confuses me that on size 1 and larger at 230 volts, the next higher HP than the rating would likely result in more current than the rating but on size 1,2 and 3 at 460 volts, the next HP is well within the amp rating.

 

[JRaef]

Motor HP rating of a contactor has to do with interrupting current flow in an inductive circuit, the amp rating of a contactor has only to do with the ohmic heating capacity of the contacts under load. So you can get away with using a contactor at a higher current rating than a motor's FLA, as long as you don't ask the contactor to open when the motor is under load. Of course, when an OL trips, that's EXACTLY when you are going to want that contactor to open under load.

Opening under an inductive load is harder on the contacts because the inductance of the load circuit will resist the change in current and voltage, sustaining the arc that is created when the contacts begin to open. Then once they do start, the separation begins to look like a capacitor and raises the voltage across the gap, which further assists in sustaining the arc until the dielectric of the air gap is greater than the distance the arc can jump. All the while, the contacts are being subjected to temperatures that approach the surface of the sun, albeit very briefly.