Need advice on compressor mag starter-heater size

[CGT80]

I need to add a magnetic starter to my 5hp compressor and want to make sure the amperage is correct for my motor. I'm not sure how close to the running amperage these should be.

Baldor L1508T
5hp 1725 rpm
208-230 28.5 amps
1.15 service factor

28.5x1.15=32.77 amps max

I run my pump up to 170 psi and I think it maxed out right at 30 amps as it reached full pressure. I need to recheck it to be sure and also because I just replaced the bearings, the start switch, and cleaned up the centrifugal switch, which was not working smoothly.

The other day, it tripped the breaker a couple times and it has not done this at all since I got it a year ago. The bearings were a bit rough.

The feed to this is small and I should probably re-run this line. It is a 30 amp breaker and #10 from the garage sub panel. The motor does not have a safety or reset and without a magnetic starter there is no way I'll run a larger feed until I do.

The startup amperage was 167 amps, last time I checked.

Here is what I was considering: https://www.amazon.com/dp/B004PHB5U8/?tag=atomicindus08-20

It has a b-40 heater element and 30 amp contactor.

I have read and google searched GJ for more detailed info on the heater element and I used to a lot of electrical work and trouble shooting when I was in the construction trades, so residential and small commercial electrical is nothing new to me.


Is the heating element a one shot deal, like a fuse, or do they "reset" to be used again? How close to the max amperage of the motor should they be?

I have seen some that were electronic and adjustable but the $60 units don't have the best reviews and Square D is a good name brand, but there are not a lot of options on Amazon. Other suppliers are ok and I could hit up an electrical supplier if needed.

Luckily, the old square D pressure switch that is on my compressor is well built and has big contacts (I dressed them when I got it) but I want to get this setup the right way and protect an expensive motor and really nice pump.

Thanks

 

[Norcal]

I would avoid any DP contactor, one poster on another board said that DP stood for don't purchase, instead of definite purpose, & frankly I agree. Another thing to avoid is off breed ChiCom starters, a SQ D starter w/ a type S contactor is a good choice, as Allen-Bradley, Furnas, (now Siemens) also, try eBay for used models, NEMA type starters are much more rugged then IEC type.

 

[wyliesdiesels]

What type of wire are you using- NM-b or THWN?

A 5HP motor needs 35a rated wired.

If you have NM-b, then its too small because #10 NM-b is limited to 30a.

 

[matt_i]

Is the heating element a one shot deal, like a fuse, or do they "reset" to be used again? How close to the max amperage of the motor should they be?

A discrete heating element doesn't get used up, ever, unless its a major arc-flash snafu-type event. The idea is that the bi-metallic-linked NC contact in the overload section gets tripped by a certain amount of heat. As long as the heat produced doesn't trip the overload contact you are good to go. The tables are developed experimentally and aren't 100% perfect, obviously it depends on the ambient, and the reliability of the FLA of the motor.

My personal tactic is to set the heater rating about 2A over the motor nameplated FLA (full load amps). In my opinion its completely proper to let the motor run at full load amps for years on end. Where you get into trouble is when a motor single phases (on 3ph) or fails to start or is pulled off the synchronous speed (basically down to a stall) due to an unexpected mechanical overload*. Then you want to either build extra heat on the 2 of 3 lines which are feeding the motor or build extra heat on all of them, in order to trip the overload contact.

If you were standing right beside the motor 100% of the time, you could probably shut it off in time. But consider the millions of motors that operate unattended on rooftops, in the high building steel running overhead conveyance, pump pits, etc, etc. Those need a watchdog.

The one advantage of an adjustable overload section is you need a screwdriver to tweak it into correction if you discover an error. If you have the discrete elements, its off to ebay or the supply house to buy a different set. But imo the "old school" are more reliable...its just simpler and more "mechanical". I've seen a couple adjustable overloads fail in industry and never ever saw a discrete element version fail.

* one more fail pathway is if you buy a piece of used 3ph equipment which was previously run on 480v and plug it into 240vac. Even if you do the motor-winding conversion and the control voltage conversion correctly, it draws double the amps when the motors starts. And the overload section *will* trip.

 

[metlmunchr]

I'd go with one of these Eaton starters from Amazon. https://www.amazon.com/dp/B01MTS46KC/?tag=atomicindus08-20 It has an adjustable overload relay section rather than heaters, and is adjustable to a max of 34 amps.

FWIW, this is the Eaton that manufactures all sorts of industrial equipment and not the one that markets imported air compressors. Eaton bought Cutler Hammer, an old line manufacturer of high quality electrical equipment.

The B40 heaters in the Square D you linked have a max current value of 27 amps. If your motor is running onto the service factor regularly, you could have nuisance trips. The B40 is the highest current heater available for this starter. Heaters are resettable, but multiple trips can cause them to need replacement over time.

I'd disagree with the need for a full blown NEMA starter. You'd need a single phase version of a size 2 starter which is going to be in the $600 to $700 range in a type 1 enclosure. Most everything I own is 3 phase so I don't know whether a single phase starter uses one heater or two, but the heaters will cost $30 to $40 each in addition to the cost of the starter.

I'm most familiar with Allen Bradley in NEMA starters because I've bought a couple hundred of them over the years. In looking thru used ones on Ebay, about half are model 709's which have been obsolete for at least 30 years and are obviously salvaged from junked equipment. The remainder are 509's which are current, but have been in production for at least 40 years. So most of the 509's are likely salvaged from junk as well. I'd assume the same is true for other brands as well. Given the cost of replacement parts for NEMA starters, regardless of manufacturer, buying used from an unknown source would be a poor bet IMO.

My Japanese CNC machine tools have at least 40 IEC type starters in them, and in 17 years in the machining business, I've never had a starter failure of any type. Most of my manual machines have A-B NEMA starters, and I've had no failures with any of those either. The difference is that an IEC starter from a major manufacturer costs around half of what a NEMA of the same capacity costs. IMO, a 150# bumper on a 68 Cadillac doesn't make the car more reliable than a new Toyota, and the massive chunk of plastic that makes up the bulk of a NEMA starter doesn't necessarily make it more reliable than an IEC starter from a known manufacturer.

 

[CGT80]

Yes, the wire is NM-b with a 30A breaker, and the motor was pulling 30 amps right before it hit full pressure, hence my comment about needing to go bigger in the OP.

That is great to know that the B40 maxes out at 27 amps. That is no good, as the nameplate on this motor is 28.5 and then add the service factor and it goes almost to 33 amps.

I was under the impression that it is ok to push it past the nameplate, as long as it is within the service factor amperage. It takes 60 second for the pump to run from 140 to 170 psi, so it is above the 28.5 for only a matter of seconds. Is this proper or safe for the motor?

Thanks for the clarity on Eaton. I saw those starters and figured it was the electrical company and not the compressor company, as I read about both and considered an Eaton compressor before.

While I like skookum parts that will out live me, this is a small business/hobby setup. On a busy day, the compressor might cycle 10-15 times and some days it may be none or a couple. That Eaton starter says it is made in the USA....a plus.

I will get this motor hooked up and recheck the draw while filling.

I really appreciate all the responses.