# Compressor Amp draw Too high Question



## stevon

Question about my recently purchased used compressor 
paid ($250). It is an old store bought 60 Gal, Sanborn model 50-17-60V, 2 stage with a T29 pump head, currently running at 1400RPM! the newish electric motor running it is a 3470 rpm Harbor Freight (3) HP S.F. 1.15 (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 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?

Stephen


----------



## Osviur

Hi Stephen,

I found these specs. :

https://mastertoolrepair.com/air-compressor-pump-parts-t29-31-15-33-15-31b-15-33b-17-51-17-p-51.html

They indicate:

Speed : Max 1400RPM - Min 700RPM
Motor : From 3 HP to 5 HP
Press. : Max 175 PSI
Pulley : 13.78"

So, with a 3 HP motor , the pump must go at 700 RPM to get the max possible pressure.
3470 RPM / 700 RPM = 5 Ratio 
14.5" / 5 = 2.9" Diam. or with a 3" sheave : 717 RPM
Set the cutoff pressure when the motor current reaches 14 A

Regards


----------



## stevon

Osviur,

at 700 pump rpm, not all of the available HP would be used. The T29 Pump RPM listed for a 3 HP motor is 1000rpm. Plan to get the most CFM available while using this electric motor. If I wanted this pump and motor to last 50 or more years and would be happy with the lower CFM output, a 3 inch pulley would work. If I do the calculations of running this pump at 1000 rpm and the motor at 3470 (listed rpm on motor) pulley drive sheave size comes to 4.18. 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 well 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. the maximum temperature to safely run the electric motor is listed as long as I don't exceed that I'm sure I will be OK. Haven't run compressor long in it's current configuration but now it does not stall with current 6" pulley but I'm afraid at 18 plus amps current draw at 50 PSI (4HP or so) motor would overheat engaging the thermal overload protection. I get the impression that I will be OK with the 4.75" motor sheave. After the 4.75" sheave install, will do some test runs and measure the amps and winding temperatures. the listed motor insulation is "F" class or 311 degrees maximum running temperature according to NEMA codes.

Hp. of
Motor	Current - Full-Load (Amperes)
Single-phase Motors

volts 115 230 (single phase)
¼ 4.8	2.4
½ 7	3.5
¾ 9.4 4.7
1	11	5.5
1 ½	15.2	7.6
2	20	10
3	28	14
5	46	23
7 ½	68	34
10	86	43


----------



## stevon

*4.75 Pulley installed!*

Osviur,

Removed the 6" inch pulley and installed the 4.75. Motor amps now are 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.

Stephen



Osviur said:


> Hi Stephen,
> 
> I found these specs. :
> 
> https://mastertoolrepair.com/air-compressor-pump-parts-t29-31-15-33-15-31b-15-33b-17-51-17-p-51.html
> 
> They indicate:
> 
> Speed : Max 1400RPM - Min 700RPM
> Motor : From 3 HP to 5 HP
> Press. : Max 175 PSI
> Pulley : 13.78"
> 
> So, with a 3 HP motor , the pump must go at 700 RPM to get the max possible pressure.
> 3470 RPM / 700 RPM = 5 Ratio
> 14.5" / 5 = 2.9" Diam. or with a 3" sheave : 717 RPM
> Set the cutoff pressure when the motor current reaches 14 A
> 
> Regards


----------



## stevon

*T29 Specs*

Osviur,

I found some useful Specs as well:

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



Osviur said:


> Hi Stephen,
> 
> I found these specs. :
> 
> https://mastertoolrepair.com/air-compressor-pump-parts-t29-31-15-33-15-31b-15-33b-17-51-17-p-51.html
> 
> They indicate:
> 
> Speed : Max 1400RPM - Min 700RPM
> Motor : From 3 HP to 5 HP
> Press. : Max 175 PSI
> Pulley : 13.78"
> 
> So, with a 3 HP motor , the pump must go at 700 RPM to get the max possible pressure.
> 3470 RPM / 700 RPM = 5 Ratio
> 14.5" / 5 = 2.9" Diam. or with a 3" sheave : 717 RPM
> Set the cutoff pressure when the motor current reaches 14 A
> 
> Regards


----------



## Osviur

OK Stephen, it seems that you found the solution. In your case you need to obtain the maximum CFM at the max PSI possible. This implies to work in the motor outer limits, because the pump is well into the specs.

The SFA (service factor amps ) is the max current allowable for the motor SF (1.15). In this case SFA = 14.2 A so the full load amp is 14.2A / 1.15 = 12.35A. Nema code recommends to use the SFA in a short term basis. Other way the expected motor life will be reduced for insulation thermal stress, especially in the internal hot spots.

Regards.


----------



## stevon

*Setup OK*

Osviur,

Yes, thank you for the informed advice. Sorry I wasn't specific in my question as to what I actually wanted to do. Since I will not be using this compressor at near 100% service duty, and who knows how long the 5.75 pulley has been on there already, Think I will be OK with the 4.75 pulley installed. In the summer and IF air requirements need to provide more than 10 CFM for long periods of time, this setup might be pushing the limits too far. I have a 3" pulley to install if I ever need more of a duty cycle with hot ambient temperatures. San Diego rarely gets over 80 degrees though. I can also lower the top pressure cutoff on the pressure switch. No air tools I own or use need more than 90 psi. The other option is to purchase a true 5 HP motor

Stephen



Osviur said:


> OK Stephen, it seems that you found the solution. In your case you need to obtain the maximum CFM at the max PSI possible. This implies to work in the motor outer limits, because the pump is well into the specs.
> 
> The SFA (service factor amps ) is the max current allowable for the motor SF (1.15). In this case SFA = 14.2 A so the full load amp is 14.2A / 1.15 = 12.35A. Nema code recommends to use the SFA in a short term basis. Other way the expected motor life will be reduced for insulation thermal stress, especially in the internal hot spots.
> 
> Regards.


----------



## Osviur

You are welcome Stephen, nice to share opinions in this case.

Regards.


----------



## stevon

Osviur,

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.



Osviur said:


> You are welcome Stephen, nice to share opinions in this case.
> 
> Regards.


----------



## stevon

So I can see that at 175psi motor is running at 15amps @245 volts (measured under load). Using this nifty amp to HP online tool : https://www.inchcalculator.com/amps-to-horsepower-calculator/ and giving the Harbor Freight motor a generous 75% efficiency rating HP produced at peak load (175psi) is 3.69 HP pulled from the HF 3HP motor. Don't know how long I can pull this with the motor but for now that is where I'm at.


----------



## Osviur

Stephen,

I think that there is a misunderstanding with respect to the formula application to find HP. It is correct if applied to a universal motor, an special one, with a power factor= 1.

In DC power is enough to multiply Volts applied by Amps drawn to obtain Watts. 745 Watts makes 1 HP (absorbed) and, as all the electrical power is not converted in mechanical power, the efficiency reduces this power in a certain percentage.

In AC power, all the Amps are not "consumed" ( except in resistive loads), a percentage of them are returned to the grid. This in known as Power Factor or cos θ. The real power consumed is a product given by Volts x Amperes x Power Factor= Watts (or VA x PF = Watts).

All the equipment having wire coils ( motors, Transfrmrs, solenoids), store energy during part of the AC cycle and then returns it later (msecs) to the power source. The Power Factor varies (as the cos function) from 0 to 1. Induction motors developing their nominal power output (full load) have a high PF, to say 0.8, 0.85. The same motors at idle (no load) have a lower PF, to say 0.4, 0.3).

In the mentioned calculator PF is not taken in consideration:

245V x 15A x .75eff = 2756 VA (volt amps not Watts)
2756VA / 745 = 3.69 HP 

BUT, 745 is a factor with units: 745 Watts per HP, not VA per HP.

If we consider the PF:

2756VA x .8 = 2205 Watts
2205W / 745 = 2.96 HP

Which is very near to the 3 HP.

.75 Eff is a little lower, a more generous .8, .82, I think would be nearer to the normal Eff. found in a 3 HP induction 
motor. 

Regards.


----------



## stevon

Osviur,

I forgot about inductive loads with A/C, thanks! 

Stephen


Osviur said:


> Stephen,
> 
> I think that there is a misunderstanding with respect to the formula application to find HP. It is correct if applied to a universal motor, an special one, with a power factor= 1.
> 
> In DC power is enough to multiply Volts applied by Amps drawn to obtain Watts. 745 Watts makes 1 HP (absorbed) and, as all the electrical power is not converted in mechanical power, the efficiency reduces this power in a certain percentage.
> 
> In AC power, all the Amps are not "consumed" ( except in resistive loads), a percentage of them are returned to the grid. This in known as Power Factor or cos θ. The real power consumed is a product given by Volts x Amperes x Power Factor= Watts (or VA x PF = Watts).
> 
> All the equipment having wire coils ( motors, Transfrmrs, solenoids), store energy during part of the AC cycle and then returns it later (msecs) to the power source. The Power Factor varies (as the cos function) from 0 to 1. Induction motors developing their nominal power output (full load) have a high PF, to say 0.8, 0.85. The same motors at idle (no load) have a lower PF, to say 0.4, 0.3).
> 
> In the mentioned calculator PF is not taken in consideration:
> 
> 245V x 15A x .75eff = 2756 VA (volt amps not Watts)
> 2756VA / 745 = 3.69 HP
> 
> BUT, 745 is a factor with units: 745 Watts per HP, not VA per HP.
> 
> If we consider the PF:
> 
> 2756VA x .8 = 2205 Watts
> 2205W / 745 = 2.96 HP
> 
> Which is very near to the 3 HP.
> 
> .75 Eff is a little lower, a more generous .8, .82, I think would be nearer to the normal Eff. found in a 3 HP induction
> motor.
> 
> Regards.


----------



## stevon

Finally got the belt guard from the powder coat shop R.W Little in San Diego. It was like opening a Christmas package! painted a couple of brackets so now I'm done. Hopefully this compressor will last a few more years.


----------



## ritetools

*How to Determine the Power Requirements of Air Tools*

Hi I thought I'd add something I found useful https://www.ranch-rite.com/blog/What_Size_Air_Compressor/


----------



## stevon

ritetools,

Thanks for the additional information. 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 parts and paint I kept the bill under $300 so I succeeded meeting my goals. Just barely, 3HP Smith + Jones motor overheats if you use this for more than an hour at my current setup.

PS I lowered shutoff limit to 160 PSI


----------



## stevon

I guess I'm pushing the limits too hard on the 3HP SMITH + JONES Harbor Freight electric motor. It seems that if you use the listed 3 HP advertised on the motor for a short length of time it will overheat! I ran the compressor for about 2 hours at about 80% run [email protected] 75 degrees ambient temperature. When I shut it off the thermal overload tripped by itself just sitting there. The next time I used the compressor it wouldn't turn on until I reset it. Guess this is not a real 3HP motor that in an industrial application, should be able to run all day. The rumor that this motor really is an overated 2HP must be true. If the running amps that is listed on the motor is 12.2 amps @ 230 volts and the Power Factor is about .75, it really is a 2 HP motor actual output. Technically you can pull 3 HP from this from this motor or any motor, but not for long! unless it is designed to produce 3HP or more from the start. Guess you get what you pay for lol!


----------



## stevon

*New WEG Motor*

Well I solved the motor overheating problem, sort of. I purchased a new WEG light duty, 5 HP compressor motor for $160 (open box) and installed it with a 5" pulley. https://www.ebay.com/itm/WEG-135212...e=STRK:MEBIDX:IT&_trksid=p2057872.m2749.l2649
The new WEG motor runs quieter somehow and definately has more torque than the HF motor but it does seem to run hot. I filled the compressor tank from empty and I could barely hold my hand on the motor near the pulley end. I'm glad I chose a conservative size pulley at 5.25 outside diameter (5" sheave). Motor draws 17.25 amps @ 160 PSI. The motor is rated at 5HP 20.3amps. It does have class F insulation rating and my guess is that you need it! Listed as "light duty" probably means compressor use only (on-off) in cycles not continuous use like in a machinine tool machining center. With the ducted and internal fans, guess I'll be OK for my application @ 4HP load, T29 pump. Maybe the belt is too tight, the holes on the mounting barely reach and the belt has no slack at all, will have to remedy that and see if thats the problem now. If I had a Quincy or Ingersol Rand compressor with a real 5 HP load and used this motor, it probably would have a short life.



stevon said:


> I guess I'm pushing the limits too hard on the 3HP SMITH + JONES Harbor Freight electric motor. It seems that if you use the listed 3 HP advertised on the motor for a short length of time it will overheat! I ran the compressor for about 2 hours at about 80% run [email protected] 75 degrees ambient temperature. When I shut it off the thermal overload tripped by itself just sitting there. The next time I used the compressor it wouldn't turn on until I reset it. Guess this is not a real 3HP motor that in an industrial application, should be able to run all day. The story that this motor really is an overated 2HP must be true. If the running amps that is listed on the motor is 12.2 amps @ 230 volts and the Power Factor is about .75, it really is a 2 HP motor actual output. Technically you can pull 3 HP from this from this motor or any motor, but not for long! unless it is designed to produce 3HP or more from the start. Guess you get what you pay for lol!


----------



## stevon

*hot motor?*

this weekend I adjusted the motor mounting bolts and belt tension. Used the compressor and it cycled 30% on and 70 % off, a typical use pattern for me. The WEG motor did get warm but well within the 40 degree heat rize listed for the motor. So guess things will work out for a few years at least now. The new motor is great! plenty of power and torque, a good fit for my application, and it didn't cost $500!


stevon said:


> Well I solved the motor overheating problem, sort of. I purchased a new WEG light duty, 5 HP compressor motor for $160 (open box) and installed it with a 5" pulley. https://www.ebay.com/itm/WEG-135212...e=STRK:MEBIDX:IT&_trksid=p2057872.m2749.l2649
> The new WEG motor runs quieter somehow and definately has more torque than the HF motor but it does seem to run hot. I filled the compressor tank from empty and I could barely hold my hand on the motor near the pulley end. I'm glad I chose a conservative size pulley at 5.25 outside diameter (5" sheave). Motor draws 17.25 amps @ 160 PSI. The motor is rated at 5HP 20.3amps. It does have class F insulation rating and my guess is that you need it! Listed as "light duty" probably means compressor use only (on-off) in cycles not continuous use like in a machinine tool machining center. With the ducted and internal fans, guess I'll be OK for my application @ 4HP load, T29 pump. Maybe the belt is too tight, the holes on the mounting barely reach and the belt has no slack at all, will have to remedy that and see if thats the problem now. If I had a Quincy or Ingersol Rand compressor with a real 5 HP load and used this motor, it probably would have a short life.


----------



## okmarts

i had the same problem


----------



## stevon

okmarts,

On the used compressor bought on Craigslist, apparently the original motor was replaced with a cheap HF 3HP motor (really an overated 2 HP). What was not replaced was the original pulley size of (5.75") which required around a 4HP electric motor to run the compressor load! this caused the motor to go into "stall" or "startup" loading, drawing an additional 8 amps over it's FLA rating! The motor quickly overheated being forced to pull that load. A higher HP motor replacement and the correct size pulley, solved my problems.

Stephen



okmarts said:


> i had the same problem


----------

