New Single-Phase Compressor Motor fails in 2 weeks on 2 legs of 208V 3 phase Wye...?

So we just installed a new single phase motor on our 3HP air compressor (to replace a failed motor). The new motor failed in 13 days. The explanation that we got from the "air compressor repair guy" is that the compressor motor is rated for 230V and it's getting 208V (our power is 208V 3Ø Y), compounded by the fact that it was powered by 12 gage wire, and of course the compressor is working too hard (not big enough). The fact that we're in the San Fernando Valley and it has been over 100º F for several days recently may be a factor too (compressor is outside in a "shack").

The air compressor guy recommended upgrading the compressor with a 3 phase motor, but the cost would be around $450.-

I don't do facilities at the company I work for, but I do advise as I'm the only person that really understand this stuff (electrical power, air compressor,s etc). The operations manager is going to want my advice, so I'm posting here because you guys know your stuff. I was really impressed by the knowledge of the members of this forum 5 years ago when I was building a RPC so this is my first and only stop for advice.

Thanks in advance for your help.

If you have 3-phase avail why go with single phase ?

Much simpler construction (3-phase motor) no mechanical
starting switch (seen a couple get launched thru the windings)
no capacitor to fail either.

Your electrical supplier would like to keep the loads balanced on the
line as well.

The explanation you got for the motor failure sounds pretty sensible. If I were in your shoes, I'd go for the 3-phase motor replacement, but you might find a compressor-rated single phase motor with a nameplate rating at 208V. Don't know it would be that much cheaper than the 3-phase motor.

One consideration: Presumably your compressor has a pressure switch. That switch either directly, or through a contactor, controls the existing single phase motor. You (or the air compressor guy) will have to do a bit of control work so that the switch can control a replacement 3-phase motor. Maybe that's part of the $450? If so, I'd say the 3-phase replacement was a really good deal.

I'd get a 3 ph motor and motor starter, etc from Automationdirect, and it WON"T cost anywhere near 450! Maybe get a 5hp motor if you start and stop a lot.

It's a logical conclusion, and might even be right. It's not really a slam-dunk.

at 1 kW per HP, you should be drawing around 14.5A single phase. Well within the 80% for 20A wiring, and unlikely to be a horrible source of under-voltage BY ITSELF.

208 is right at 90% of 230, which should be acceptable for any UL device, but maybe not for your motor and compressor. Especially if it really wants a bit bigger motor in reality. It leaves you no reserve for voltage drop from wire losses.

Now, what is the service factor of the motor? If above 1.0, it should not really be unhappy, even a 1.05 would help. A 1.1 should just plain work.

A different motor of the same HP might well have the same problems, although a 3 phase motor has better torque etc, and should be a little happier, besides drawing a bit less current per wire.

It sounds like the 3 HP may be an optimistic rating, for the motor (barely makes it), or for the compressor (really wants more), or both.

Edited much later after looking at my table of motor currents: The "statutory" value for 3 HP single phase is 17A. Probably a newish motor does not pull that much, but that's what the code value is. So the 12 gauge is just under the correct value per the table. One would have a hard time arguing that a compressor is not a useage falling under the 80% rule.

nameplate current is 15A, so the 125% does indeed go over.

Service factor is definitely an issue. That's a large part of what "compressor-duty" is about. And I'd bet the OEM cost-engineered the original compressor motor to just barely be sufficient.

I suppose it would be good also to ask HOW the motor "failed".

Obviously it stopped working.... But in what way? Burnt windings? Unknown cause of open coil?

Burnt smelly windings is one thing. After checking that the unloader is working, it probably suggests the motor stalled for some reason, or was just plain severely overloaded. May need a different motor type, maybe a bigger one.

12ga is barely too small for a 3hp, it should be 18~amps. So by adding the 125% in you are at 22~amps which is over for 90 deg thhn. Are the windings burning up? Caps failing? Like said above what is the service factor of the motors. The current per phase will be lower on the 3 phase #12 wire would be good. With the temps the service factor needs to be addressed. $450 sounds cheap for a motor and starter.

It sounds like the installation may have been a low bid job. The best solution is to oversize - go with 10 gauge wire (particularly if it's more than ~50 ft or so to the main breaker panel, and go with a higher grade motor (watch the service factor as mentioned above). A 3-phase motor would also be a good idea.

Overkill can be a lot cheaper than trying to save the last cent possible on an installation.

BobRenz said:

It sounds like the installation may have been a low bid job. The best solution is to oversize - go with 10 gauge wire (particularly if it's more than ~50 ft or so to the main breaker panel, and go with a higher grade motor (watch the service factor as mentioned above). A 3-phase motor would also be a good idea.

Overkill can be a lot cheaper than trying to save the last cent possible on an installation.

Click to expand...

To quote Tom Lipton, "Nothing too strong ever broke." What's a couple hundred bucks extra over the life of the motor as long as it does what you need it to do?

Thanks for the replies. Here are the specs on the compressor and motor:

Compressor:
2 cylinder
5HP "special"
125 PSI, 14 CFM, 60 gal tank

Motor:
DOERR, St. Louis, MS USA
Definite Purpose Compressor Motor
Frame 56 T
Model: T638XBSS1083
240V, 60Hz, 3450 rpm, single phase
15 A, 1.0 SF
Output 2.98 kW

I took a quick look and noticed that 3 phase 56 T motors of 5 HP don't seem to be available. So this 5 HP special motor is really an overworked 2-3 HP motor?

My operations manager needs to make a decision soon because we are negatively affected by not having compressed air.

I think going to three-phase for the motor makes sense. I'm just wondering if getting a new compressor might be necessary. The company has expanded a great deal and the compressor is over-worked.

To answer one question: The compressor shack is right next to the panel box, and that is very close to the transformer (the compressor actually sits in-between). When I measured our power at no load, it was around 215V.

"I suppose it would be good also to ask HOW the motor "failed"."

Unless this question is answered, there is no way to say what should be done
to remedy the situation. If the original motor was faulty, the best possible
response is to return it for a good, new single phase motor.

The other question of course is, why was the original motor replaced in the
first place? How did that one fail?

cinematechnic said:

..............
I think going to three-phase for the motor makes sense. I'm just wondering if getting a new compressor might be necessary. The company has expanded a great deal and the compressor is over-worked.................

Click to expand...

Well, there are two statements that almost but not quite contradict one another....

a) "we are negatively affected by not having compressed air"
b) "The company has expanded a great deal and the compressor is over-worked."

So it is over-worked, but not so essential that it's a slam-dunk plant manager decision "get another compressor in here by 2PM".

I'll go with the over-worked, and assume that what is the issue is at least in part that the motor is getting WAY too many starts per hour.

Could also be forced to start into pressure due to a bad unloader.

It can be hard to fix the too many starts issue. Sometimes a SMALLER pump is needed, one that has to keep working and doesn't get so many motor starts. Or, the hysteresis on the switch can be set up to allow more pressure drop before starting the motor, which probably works best if there are a lot of smaller nuisance loads that keep draining it little by little. If it's a big load that comes on regularly, that may not work. Same if the pressure can't get so low due to the load.

All guessing, but that's pretty typical for on-line diagnosis.... It's like cheating if you can walk into the place and SEE the problem, right?

I'll +1 on the starts-per-hour... but will consider that high ambien temps and being inside a shed, along with heat radiating from the compression process (P*V/T) is making it unlikely that the indicated motor size is up to the task of providing the power required. Having an improperly functioning unloader mechanism, and too high a drive ratio is murderous, even with the single-phase motor's higher starting torque, but don't expect the start cap or start cap centrifugal switch to take that punishment.

I would take it one step further... that it sounds like the compressor system being used is not sufficient for the condition under which it was placed (both environmental and workload), and under the premise that 'negatively affected' implies critical process need for compressed air, that the more prudent path would be to step up to an industrial-duty machine of higher capacity, such that neither motor, nor pump, nor tank capacity is ever yields more than 25% duty cycle or exceeds the motor's starts-per-hour rating by 50%.

I used to have issues with air compressors just running my small blast cabinet, and one of my solutions was to run multiple machines, but even with that added capacity, I was STILL overheating motors due to excess duty cycle and starts-per-hour. JST's suggestion of a SMALLER pump could very well be a suitable solution, however, the compressor would be running 100% duty cycle to keep up.

Industrial supply systems dealt with this problem well over ten decades ago, and one of the simplest solutions, was to leave the motor run all the time, and just activate the unloader so the pump would free-wheel. I have two double-barrel'd jobsite compressors that run on that very same premise... the engines constantly spin the pumps, but the pumps' unloaders kick in to vent the output. IN doing so, several things happen... First, you don't overpressure the reservoirs, and don't stall the engine... second, you don't overheat or overstress the compressor pump... next, you don't shut the prime mover down... and finally, since the pump is still spinning, AND the pump-sheave includes a cooling fan, it continues cooling the pump even when it's not pumping.

5HP "Special" is suspect, especially for 15A on a 5HP motor. My 5HP Dayton compressor motor lists 20A@230VAC.

Yes, all of what Dave said.

As for the "special".... Table motor current for 5HP 3 phase is 15.2A.... For 5HP single phase, 28A, both 230V not 208. For 3 phase 208, more like 17A.

Good advice so far, so I will just add a comment. One place I work for kept adding little compressors until they finally got a big one and solved the problem permanently. My next employer was going down the same path, so I told them about the previous experience. They got serious and paid the price for an industrial class 10 hp unit and that was the end of the tale. Managers and maintenance people have better things to think about than fretting about substandard compressors.

Bill

a 56 frame motor is the same side that used to be used for furnace blower motors. I have seen them rated up to 5 hp, but this gets into the bs of "ratings". I have a 5 hp leeson motor on my compressor, and it is a 5 hp motor with a 184 or 213 frame. No issues in 8 years, single phase. I run it at 3hp, so it runs very cool. A friend runs a 56 frame 5hp and blows one every year or so, but he gets them free from a test lab.
Joe

+1 on the duty cycle issues as well, that's the #1 killer of compressors. The compressor has to run too long (for it's design) to keep up with the demand. Might have been right on the margin of acceptable if the voltage was correct, but it is marginal there too. So you have a size that was likely marginal to begin with, if all other things are perfect, then a supply voltage that is marginal. You can't consume the same fudge factor twice. Either give it the right voltage, or use a bigger motor. Either way, it means changing the service to it. If you are going to do that, just go with 3 phase. There is no more torque (common myth), no more power etc., because 3HP is 3HP is 3HP, and HP is just a notation meaning "xxx lb.-ft. of torque at yyy RPM", so "phase" does not play a part in that. However, on a single phase motor you have a centrifugal starting switch and capacitors to make it work. Those are parts that wear out and/or get damaged with use, so going with a 3 phase motor makes the entire system more reliable.

And, to the issue of it being 12ga, that's actually making everything WORSE. Yes, the motor is 3HP so the NEC chart says 17A at 230V, but you do not have 230V! Common mistake. You must size the conductors for the HP from the chart at the SERVICE voltage, not the nameplate voltage. So the service voltage would be 208V, the FLA for determining conductor size would use 19.6A, x 1.25 = 24.5A for conductor sizing. You cannot load a 12ga wire to more than 16A continuous, it was supposed to be 10ga. Your wire is too small, which is going to cause voltage drop, which makes the marginal motor size even WORSE. If you change to a 3 phase 5HP 3450RPM motor, the NEC wire size would be based on 17.5A, x 1.25 = 21.85A, you will still need 10ga wire, but that would be a better way to go. Get your fudge back!

Thanks for the replies. I appreciate everyone's input.

jim rozen said:

"I suppose it would be good also to ask HOW the motor "failed".

Click to expand...

I'm told the capacitor blew, and apparently there was fluid leaking. Don't have more exact info.

JST said:

Well, there are two statements that almost but not quite contradict one another....
a) "we are negatively affected by not having compressed air"
b) "The company has expanded a great deal and the compressor is over-worked."

Click to expand...

Good observation. I'm a technician, not a manager. I give advice to management which gets ignored 95% of the time. When we expanded to double the original footprint, I told them that the one 5HP compressor was not enough. I even offered to sell them my 3HP Ingersoll-Rand that I had in storage so they could run two compressors (I advised two separate air distribution systems). That fell in the the 95% category of my advice that they ignore.

We use comprised air for dust blow-off. We have about 20 blow-gun ports in the building. For most of the techs it's not that critical. They are blowing sand off of tripods and the like. For me it is critical. I service the high end optics and my clients will reject a job for a few specs of dust. The lenses I work on are assembled by the manufacturer in a clean room, and the clients expect to get them that clean from us. But we don't have a true clean room. I've been able to improve things substantially but I still can't get it to clean room standards.

Right now, the rest of the guys are using that "canned air" which is garbage. I refuse to use that crap. It leaves residue. I had to push hard for a highly filtered system with anti-static ionizers for my very specialized blow-off needs. I tried compressed nitrogen (from the welding supply house) but that wasn't good enough. So if the problem doesn't get solved I will be the one most negatively affected.

To be continued...