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Compressor Amps Too High

stevon

Plastic
Joined
Apr 8, 2016
Question about my recently purchased used compressor ($250). It is an old store bought 60 Gal, Sanborn 2 stage with a T29 pump head, currently running at 1400RPM! the motor running it is a 3470 rpm HF (3) HP S.F. 1.15 (a bogus 3HP, probably a 2HP overrated). I turned compressor on and the amps were at 18 amps @245 volts @ 50 lbs pressure in the tank. the SFA on the motor says 14.2 amps. Shut it down when it reached 70 lbs or so. I need help slowing/cooling this baby down! Pump flywheel Dia is 14.5 and motor sheave is currently 6 inches. I don't expect the motor to last long in it's current dragster setup.
Question is will a 4.75 size pulley be too big? That will slow the pump to 1100 rpm. Since this is a compressor overloading the motor slightly with compressor run at 50% or less duty cycle I figure I'll get away with it. I can always lower the shutoff pressure if amps become too high right?
Any advice? anyone ever do this?
 
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Upon looking at a similar air pump head T29S specifications I see pump (at my current setup)is running at the 5HP speed settings. The old motor must have been a true 5HP (220volt @ 22amp) motor. This explains the crazy current draw on the HF 3 HP motor. Pump is demanding 5HP and 3 HP motor is trying to give it! the 6 inch motor pulley is only OK for a True 5HP motor. They must have been too lazy or cheap to resize the drive pulley and smaller belt required to use HF 3HP motor.

Model pump T29S
HORSEPOWER 3.00 5.00
BORE 3.74 x 1.97
STROKE 1.97
R P M 1000 1400
PISTON DISP. 14.73 18.59
MAX PSIG 175 175
ACFM @ 100 12.05 15.33
ACFM MAX 10.08 13.75 (@175 psi)
VALVING FINGER REED
LUBRICATION SPLASH SPLASH
FLYWHEEL O.D. 14.50
BELT(SECT.-GROOVE)A-1
MAX RPM 1400
MIN RPM 700
OIL CAPACITY (QT) 1.49

Quote Originally Posted by stevon View Post
bosleyjr,

this is how compressor came used and "repaired" with a Harbor Freight so called "3HP" motor. GOD only knows what scrapyard the original motor is in, wasn't laying around when I bought this compressor. I already know the motor sheave size is too big! question is will a 4.75 motor sheave work? Because this compressor will be used at a 50% duty cycle or less, I may be OK with a 4.75 sheave. The T29 pump is listed to run anywhere from 700 rpm to 1400 max so I will be within the pump rpm limits.
Guess I'm the professor on this one and will have to see full load amps used at highest pressure (175 psi)and motor temperature after a few on-off cycles

Stephen
 
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Use a smaller sheave and check amps, if your needs are minimal you might get by with that. Personally, I'd advise replacing motor with something larger and of better quality. Stated HP on HF motors is optimistic, and from experience diagnosing a similar problem they always pull too many amps and run hot under a real load. Tore one apart a few years ago just to see what internals looked like, windings were sloppy and appeared to have been hand wound by less than skilled labor.
 
Jim,

Removed the 5.75" inch pulley and installed the 4.75. Motor amps now is 10A @ 10psi and 15 @ 175psi. The motor was just barely warm after 1 run, maybe 100 degrees. The current draw is within reason now and so far no heat problem has been detected. I know the HF motor is a POS but for part time hobby/home use it will work safely for now, thanks for the help! I will put the belt guard back on (came loose with purchase)and the next question would be in the mounted disconnect switch, to change out the 30 amp fuses with something less 25 or 20 amp maybe. Oh and I ordered a Solex filter/silencer to install

Stephen

If the 6" sheave was optimum for a 5 HP motor and you have a 3 HP motor now, then I'd say 'no'. Scaling it linearly from 5HP down to 3 HP would indicate using a 3.6" sheave.
 

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Jim,

I found a handy dandy pulley rpm calculator:

Pulley Calculator. RPM, Belt Length, Speed, Animated Diagrams

Jim,

Removed the 5.75" inch pulley and installed the 4.75. Motor amps now is 10A @ 10psi and 15A @ 175psi. The motor was just barely warm after 1 run, maybe 100 degrees. The current draw is within reason now and so far no heat problem has been detected. I know the HF motor is a POS but for part time hobby/home use it will work safely for now, thanks for the help! I will put the belt guard back on (came loose with purchase)and the next question would be in the mounted disconnect switch, to change out the 30 amp fuses with something less 25 or 20 amp maybe. Oh and I ordered a Solex filter/silencer to install

Stephen
 
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What fuse should I run for this setup with the Harbor freight motor? I have 30amp fuses now.

Stephen
 
Doug,

I looked up the recommended litlefuse values for a 3 HP single phase motor. the smallest recommended for a 3HP at 17 amps is a 25 amp fuse. Guess I'll just leave the 30 amp fuses there for now and future use, they aren't blowing now, even previously with the 5.75 inch pulley setup before. The house circuit breaker is 30 amp with # 10 gauge wire to the dryer plug and cord I'm using.

Stephen

What is easier to repair ?
1. Changing blown fuses ?

OR


2. Changing out the HF motor ?
 
Fun fact: fuses don't protect you. They don't necessarily protect your equipment. They MUST protect the wiring from fused location to the next terminal. 30A is appropriate for 10 gauge wire, so would anything smaller. If you put something smaller in, you're just wasting capacity of the conductors. Something larger and you have a potential fire hazard. If you want the motor protected, you can undersize the whole circuit if you know you won't need it for anything else, or you can add a fuse between the motor and the terminal where it's plugged in.
 
BoxcarPete,

The dedicated dryer house circuit is not in question, the fuses I'm asking about are inside the quick disconnect switch mounted on the compressor itself between the house outlet and electric motor. The motor itself has thermal overload protection built in but not fusing. On a 30 amp circuit and the motor drawing say 25 amps, burning, on fire, setting the house on fire too I might add, the 30 amp fuses will happily do nothing in that scenario.

Stephen


Fun fact: fuses don't protect you. They don't necessarily protect your equipment. They MUST protect the wiring from fused location to the next terminal. 30A is appropriate for 10 gauge wire, so would anything smaller. If you put something smaller in, you're just wasting capacity of the conductors. Something larger and you have a potential fire hazard. If you want the motor protected, you can undersize the whole circuit if you know you won't need it for anything else, or you can add a fuse between the motor and the terminal where it's plugged in.
 
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BoxcarPete,

The dedicated dryer house circuit is not in question, the fuses I'm asking about are inside the quick disconnect switch mounted on the compressor itself between the house outlet and electric motor. The motor itself has thermal overload protection built in but not fusing. On a 30 amp circuit and the motor drawing say 25 amps, burning, on fire, setting the house on fire too I might add, the 30 amp fuses will happily do nothing in that scenario.

Stephen

Not necessarily. It depends on the type of fuse, but most slow-blow types can sustain 110% of their rated current indefinitely. However, that value drops as temperatures go up. Since it's directly attached to the motor, it will warm up if the motor gets hot. Around 200°F, the fuse can only handle 85% of its normal load, this brings us right back to around 100% of its rating. So if the unit is sitting there, humming along at 200, 300, 400 degrees (bad news but fire unlikely), the fuse will eventually decide that it's had enough. It is for this reason that fuses, and therefore wiring, are generally recommended to be sized for an expected load equal to 70% of the fuse rating. That's where the 25A Recommendation comes for 17A on a full-load "3 HP" motor.

Regardless, you said yourself that the motor already has thermal overload. Therefore, you only need to protect the wires for fusing. What are you concerned with? If the circuit has a 30A interrupter, either fuse or breaker, any 10 gauge wires downstream of there will be considered protected. The fuses on the motor itself are most likely there in case of a full stall condition if you hook it up to a large industrial circuit. In your case, it's anyone's guess if the 30A slow-blow fuses sitting next to the hot motor or the relatively faster-acting Circuit Breaker on your home circuit are going to go first. Depends on the overload condition, but 135% (40 amps) would be where I put my money on the breakpoint. 110%-135% and I'd expect the fuses getting cooked by the motor to pop, more than that and the breaker will probably respond first. You could put copper slugs in there and still be protected, as long as you have it on that same 30A circuit.
 
BoxcarPete,T

Hmmm, you make a good point, fire (if the thermal overload works) would not be a problem. The fuses in question (mounted on compressor) are about 18 inches from the motor inside of the quick disconnect box. I'll use it as is for now.

Stephen

Not necessarily. It depends on the type of fuse, but most slow-blow types can sustain 110% of their rated current indefinitely. However, that value drops as temperatures go up. Since it's directly attached to the motor, it will warm up if the motor gets hot. Around 200°F, the fuse can only handle 85% of its normal load, this brings us right back to around 100% of its rating. So if the unit is sitting there, humming along at 200, 300, 400 degrees (bad news but fire unlikely), the fuse will eventually decide that it's had enough. It is for this reason that fuses, and therefore wiring, are generally recommended to be sized for an expected load equal to 70% of the fuse rating. That's where the 25A Recommendation comes for 17A on a full-load "3 HP" motor.

Regardless, you said yourself that the motor already has thermal overload. Therefore, you only need to protect the wires for fusing. What are you concerned with? If the circuit has a 30A interrupter, either fuse or breaker, any 10 gauge wires downstream of there will be considered protected. The fuses on the motor itself are most likely there in case of a full stall condition if you hook it up to a large industrial circuit. In your case, it's anyone's guess if the 30A slow-blow fuses sitting next to the hot motor or the relatively faster-acting Circuit Breaker on your home circuit are going to go first. Depends on the overload condition, but 135% (40 amps) would be where I put my money on the breakpoint. 110%-135% and I'd expect the fuses getting cooked by the motor to pop, more than that and the breaker will probably respond first. You could put copper slugs in there and still be protected, as long as you have it on that same 30A circuit.
 
I took off the pump and motor to deal with the surface rust on the tank. The T29 pump is amazingly light. Pump is mostly aluminum and the only steel parts seem to be the cylinder bores, bearings and crankshaft. Flywheel is more of a light weight Alloy pulley (no counterweight built in) and everything else is aluminum The counter balance must be on the crankshaft itself. It's amazing this light duty pump has lasted so many years! I see why it is considered a throw away pump though and parts aren't available to repair it. Good enough for a home owner and inexpensive enough too. Has been doing the job for almost 40 years so I'm impressed with it's original design. I see why there is only a one groove pulley needed when other iron compressor pumps (Qwincy, IR, etc.) of 5 HP and up require 2 grooves.
Removed stickers, taped off the rest, ground off the surface rust and coated tank and platform with Rust-Destroyer. This is the only primer I know of that you can actually paint on top of rust! You can use it on steel, iron, galavanized, aluminum, brass ect, sand off top of rust. It is an active coating sealing rust down and can be top coated with almost any paint in 24 hours. Rust-Destroyer actually works (not a gimmick), have used it for years. Next I top coated compressor with Rustoleum epoxy appliance black and the disconnect box and air filter/silencer with black stainless steel color. Reassembled compressor and replaced check valve. The belt guard was sent out for sandblasting and powder coating.

Jim,

Removed the 5.75" inch pulley and installed the 4.75. Motor amps now is 10A @ 10psi and 15 @ 175psi. The motor was just barely warm after 1 run, maybe 100 degrees. The current draw is within reason now and so far no heat problem has been detected. I know the HF motor is a POS but for part time hobby/home use it will work safely for now, thanks for the help! I will put the belt guard back on (came loose with purchase)and the next question would be in the mounted disconnect switch, to change out the 30 amp fuses with something less 25 or 20 amp maybe. Oh and I ordered a Solex filter/silencer to install

Stephen
IMG_1272.jpgIMG_1269.jpgIMG_1273.jpgIMG_1274.jpgIMG_1389.jpg
 
Stevon, I know you've already got a new sheave/belt combo, but it sounds like you don't worry much about pump CFM. If you mainly get your job done with stored air, going slower still on the pump can greatly reduce both noise and wear (might be relevant if its already got 40 yrs on it). The min speed spec, as you probably know, is for adequate lube splash and if your compressor is in the same room you are any reduction in noise should be welcome.
 
Gordon,

I did worry about CFM output, my drive pulley size choice was calculated using the suggested speed for a particular HP motor by the T29 pump manufacture's guidelines. And I am exceeding recommendation of 1000 pump rpm for a 3 HP motor (running it at 1137.7 RPM), the previous 6" sheave was running pump at 1400RPM. The compressor is now outputting enough CFM without overheating. Have been using compressor to run an 8" National Detroit DA (MAX 23 CFM, AVG 14CFM) an animal sanding tool non-stop for a couple of hours, and compressor keeps up with demand (13 CFM @ 85 PSI) without stopping and no over heat problems so far. It is cool here though in San Diego at 75 to 85 degrees tops. The HF replacement motor shows an install date of 6-16-2015 so I'm guessing it has been using the 6" 5HP pulley since then apparently without problems. When I recently purchased it , I just measured the amps used under load and reacted accordingly, 14.2 is motor nameplate Max amps, not the 18 amps I measured at 50 PSI. I can get reasonable CFM output without excessively overloading the HF Smith + Jones electric motor. The most amps used now is 14.85 @ 165 PSI so I'm Just over the top edge, but only for less than a minute at the end of the tank filling cycle.
My original goal was to get a 10CFM @ 90PSI compressor used for
$200 to $300. 2-stage was not a requirement but I wouldn't turn it down. The other concerns were physical size and the amount of amps drawn when using it. In southern California power rate is determined by a base limit but after you go over that your electric cost literally double! So power consumption is always a concern here! At peak service rates 12:00pm to 9:00pm We pay up to $0.49 a kilowatt! some of the highest rates in America! With a few new parts and paint I kept the bill right around $300 so I succeeded in meeting my goals.


Stevon, I know you've already got a new sheave/belt combo, but it sounds like you don't worry much about pump CFM. If you mainly get your job done with stored air, going slower still on the pump can greatly reduce both noise and wear (might be relevant if its already got 40 yrs on it). The min speed spec, as you probably know, is for adequate lube splash and if your compressor is in the same room you are any reduction in noise should be welcome.
IMG_1267.jpgIMG_1278.jpgIMG_1398.jpg
 
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Gordon,

Thank you for the compliment, I can't take credit for all of the paint, the belt guard was pretty rusty too living by the Ocean for so many years. Sent it out to be sand blasted and powder coated, they gave me a break and only charged me $50 !
Looks like you're all set. The compressor looks great too!
 
TomB,

In a fan load maybe but with an air compressor the load or HP required peaks near the shutoff switch setting, 165 psi in my case. At the start pressure the motor only draws about 11 amps but at 165 PSI it draws 15 amps. the load and therefore required HP or amps increase at a linear curve. The pump manufacturer knows this and lists different speeds for different drive HP motors. The pump manufacturer lists the peak required HP to run the pump at it's highest rated pressure. HP = Torque x RPM ÷ 5252. By lowering the drive RPM more torque is available to the load. It is a very linear equation. And in AC electric motors actual HP output is calculated directly with amps being drawn x power factor. AC motor Horsepower (hp) = voltage x amperage x % EFF x power factor x 1.73/746

on a fan when you 2x speed it takes 2x2x2 or 8x more hp.
.
just saying its probably not a 1 to 1 ratio, rpm change to hp change and thus amps change
 








 
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