Air compressor motor

[Whiskey Bent]

I am building 2 air compressors not sure what motor to use. The pumps are the same except 1 has 1 longer stroke so I need to use the better motor on this 1. The 1 with the shorter stroke calls for a 2hp motor and the larger pump calls for a 3-5 hp motor. I have 2 motors both 3 hp but the amps are different 14.5 and 22. Why are they so much different when both 3hp? I know NOTHING about electricity so maybe a dumb question.

 

[funky_g00]

I can not tell you why they are so different in amps but I do know that the higher amp one should be the better of the two. This is purely based on 15 amp power tools are considered better/more powerful than the 12, 10 and 7.5 amp models. HTH

 

[Janicholson]

Some motors are more efficient than others. I would just put the lower amp motor on the shorter stroke compressor. Then run it runs well, put the other motor on the
remaining compressor. They should have nearly identical mounting hole and shaft size. If it also runs well and has solid usability fine and dandy. If the longer stroke
compressor is used more than the other, I might try swapping out the higher amp motor with the other to gain efficiency. Jim

 

[Stephen Newell]

You might check the motor requirements on the pump you plan to use. A lot of them use a motor that is 3450 RPM.

 

[Whiskey Bent]

Got the rpm figured out, found an online calculator I do need a 1750 rpm motor. Also noticed 1 motor is 1740 and other is 1750 rpm. Actually what you need to get right is the compressor rpm, so have to get the right size pulleys.

 

[Steve@Advance]

That is really strange there is that much difference.

I've seen those type discrepancies on cheap compressors with false HP ratings, but those appear to be standard real HP motors.

It must have to do with the efficiency, which is not listed on the label. You will want to go by the FLA listed on the motor.

When you get the compressor set up and running at the recommended RPM, the final test will be a FLA reading as the compressor reaches the pressure cut off setting.

A well designed compressor/motor relationship will push the motor to the edge of the FLA limit, but not go over. Under is OK, but it can't go over. Be sure to use the proper (or larger) wire gauge for the test run.

 

[bc]

I have to wonder if the 22 amp motor is wound with a heavier guage wire and also looking at the fins, it looks like they are big enough to dispel heat better. I'm guessing that the 22 amp is bigger and heavier due to the winding, armature, magnets, body material which isn't as efficient as newer ones.

 

[buickanddeere]

The 14.5 amp motor probably has a low resistance rotor and run capacitors to correct the power factor .
The 22 amp motor probably has no run capacitors to correct the power factor . It also probably has a high resistance rotor to increase starting torque , however this reduces run efficiency .

What do you have planned for short circuit protection and thermal protection ?
Are you going to measure the running currents once the compressor systems are built ?

 

[MarkB_MI]

If the horsepower ratings are indeed equivalent, then the two motors must have different power factors. The Dayton motor has a PF of .67, while for the GE it's only .44. That would make the Dayton the "better" motor. But that assumes both motors do indeed have comparable ratings; it's possible the Dayton was rated more optimistically than the GE.

You'll note that most new compressors come with capacitor-start/capacitor-run motors; as B&D mentioned a run capacitor results in lower running amps.

You don't say where you're going to install your compressors, but if one will have a longer wiring run from your service panel you should probably put the motor with the lower power factor on that circuit to reduce voltage drop.

It should be noted that an induction motor will, to a point, draw whatever current is required to handle its load. If you slightly overload a motor, it will handle the load but will eventually overheat. So it matters if the compressors will run intermittently or continuously.

 

[John T]

Its NOT a dumb question. The difference has basically to do with (among other things) 1) Motor design and quality 2) The use of capacitors
which can improve efficiency and starting torque. A motor can be designed for better performance but perhaps at an increased cost. My very
first engineering job was with The Century Electric Motor Co where I learned of all the engineering trade offs and battles between the
engineers and bean counters. The "marketing" decision was build a better motor or build one more price competitive.

John T Nooooooo warranty that was years ago lol

 

[MarkB_MI]

> You don't say where you're going to install your compressors, but if one will have a longer wiring run from your service panel you should probably put the motor with the lower power factor on that circuit to reduce voltage drop.

Oops, I should have said HIGHER power factor.

 

[Determined]

(quoted from post at 20:53:06 11/08/20) Why are they so much different when both 3hp? I know NOTHING about electricity so maybe a dumb question.

In simple terms it's the same reason a fuel injected 4 cylinder can make the same HP as a carbureted V-8.

Electricity MPG so to speak.

Improvements in technology and efficiency.

As far as longevity goes many will stick with the old heavy motors because they seem to last forever and the money one would save on electricity by upgrading is not great enough to justify the cost of the new motor.

 

[JMOR]

(quoted from post at 10:22:24 11/09/20)

(quoted from post at 20:53:06 11/08/20) Why are they so much different when both 3hp? I know NOTHING about electricity so maybe a dumb question.

In simple terms it's the same reason a fuel injected 4 cylinder can make the same HP as a carbureted V-8.

Electricity MPG so to speak.

Improvements in technology and efficiency.

As far as longevity goes many will stick with the old heavy motors because they seem to last forever and the money one would save on electricity by upgrading is not great enough to justify the cost of the new motor.

he original motor (repulsion start/induction run) on my 1927 Curtis compressor is still running today and it's first use was service station duty. New brushes once about 50 years ago.

 

[Whiskey Bent]

(quoted from post at 03:49:14 11/09/20)
The 14.5 amp motor probably has a low resistance rotor and run capacitors to correct the power factor .
The 22 amp motor probably has no run capacitors to correct the power factor . It also probably has a high resistance rotor to increase starting torque , however this reduces run efficiency .

What do you have planned for short circuit protection and thermal protection ?
Are you going to measure the running currents once the compressor systems are built ?

oth motors have thermal protection I think, that is the small red reset button? How do you measure running current?

 

[Whiskey Bent]

My panel is in the compressor room so will only need 4-5 feet of wire. Using 10 gage wire. Sound ok? Not sure what amp breakers to use?

 

[MarkB_MI]

> My panel is in the compressor room so will only need 4-5 feet of wire. Using 10 gage wire. Sound ok? Not sure what amp breakers to use?

AWG 10 wire should be adequate. The breaker protects the wiring, not the motor. For 10 gauge wire, 30 amp breakers are appropriate.

 

[Whiskey Bent]

I also have this 5hp motor it seems big but maybe bigger is better? The larger compressor calls for 3-5hp.

 

[Whiskey Bent]

Forgot picture.

 

[Whiskey Bent]

Took another look at the 3ph motors, the Ge has 2 round capacitors and the Dayton has 3 capacitors 2 round like the Ge and also a oval capacitor prolly twice the size as the round 1.

 

[John T]

Whisky bent, your question "so will only need 4-5 feet of wire. Using 10 gage wire. Sound ok? Not sure what amp breakers to use?"

SHORT ANSWER if the motor is listed at 28 Amps Id use 40 amp rated wire and a 40 amp breaker and proper sized thermal overload motor
protection.......

CAUTION Ifffffffff you're talking about the Baldor Motor that states 28 Amps ??????? (Cant say exact it depends on load)

NOOOOOOOOOOOOOOOOOOOOOOOOO 10 Gauge (if 30 Amp rated) Wire is NOTTTTTTTTTTTT proper per the NEC as I learned it, even if sure it
will "work"

NOOOOOOOOOOOOOOOOOOOOOOOOO it does NOT "sound okay" at least per the NEC as it was taught to me

REASON 1) Its been years and Im rusty on the latest codes so no warranty, but when I was a practicing electrical power distribution design
engineer and attended several NEC Seminars we were taught to size the conductors to have a MINIMUM AMPACITY of 125% of the Maximum
Continuous Current.

2) If your conductors (depends on several factors including if in free air or enclosed and if so how many and temperature and
insulation etc etc) were typical 10 Gauge 30 Amp rated, THE MAXIMUM CONTINUOUS CURRENT WOULD BE 80% OF 30 OR 24 AMPS (28 amps is more DUH)

3) If the Max Continuous Current is 28 amps (more then 24) then 30 amp rated wire is insufficient (at least per NEC was then)

NOTE A) The "Max Continuous Current" per the NEC definition may or may not be 28 listed amps, it depends on the load amps. B) Sure 30 amp
rated wire will "work" C) HOWEVER The NEC in my day clearly called for the wire to be rated to have an ampacity of at least 125% of the
MAXIMUM CONTINUOUS CURRENT (IE 80% OF 30 AMPS= 24 not 28)

SOOOOOOOOOOOOOOOO if I were the design engineer and knew (or best approximation) the load could be 28 amps I WOULD SPECIFY 8 GAUGE (40
Amp) WIRE as 80% of 40 = 32 Amps

BRANCH CIRCUIT DESIGN per the NEC when I last practiced was as follows:

1) Compute the Maximum Continuous Current......
2) Size the conductors to have a MINIMUM ampacity of 125% of the MCC
3) Size the Overcurrent Protection Device (fuse or breaker) TO PROTECT THE WIRE NOTTTTTTTTTT THE MOTOR
4) In a Motor use Thermal Overload Protection to protect the MOTOR NOT THE WIRE.
5) The breaker protects the wire the motors thermal overload protects the motor

DISCLAIMER Codes may have changed so NO WARRANTY this is ONLY how the NEC was taught years ago,,,,,,,,Sure 30 amp rated wire would "work"
even if the max cont current was 28 amps,,,,,,,,,Im ONLY saying based on the NEC as it was then for a motor listed at 28 amps I would NOT
specify only 30 amp rated wire.

NOTE to each their own choices, use what wire you like, even if 10 gauge would work fine, Im ONLY saying how I was taught the NEC years
ago when I was a practicing power engineer BUT THINGS MAY HAVE CHANGED....DO NOTTAKE MY WORD FOR IT where fire and life safety is at stake
at least consider consulting with the NEC and professional electricians and engineers, the life you save may be your own !!! even if sure
30 amp rated wire would work okay.........

John T Longgggggggg retired n rusty electrical power distribution design engineer

 

[John T]

"How do you measure running current?"

By using an ammeter

John T

 

[John T PS]

Whiskey, one more PS to your question "Not sure what amp breakers to use?"

In addition to my post below please be advised:

1) Id you use a 30 or even perhaps a 40 Amp breaker subject to start up load and due to the initial high inrush motor current A STANDARD
BREAKER MIGHT TRIP

2) There are three ways to prevent that:

A) On motor circuits the NEC used to allow the use of a bigger (above 30 or 40 amps) standard breaker to allow the motor to start
IFFFFFFFFFF THE MOTOR HAS ADEQUATE THERMAL OVERLOAD PROTECTION

B) Use a Duel Element Time Delay fuse rated for motor starting.

C) Use a special circuit breaker that acts similar to a time delay fuse (seems like HACR rated) which allows the motor to start
but then acts normally

Hope this and my post below helps, while many methods may suffice and "work" I suggest you consult local professional authority and the
NEC even if no codes are applicable, its YOUR choice, do it right or do with what can work...

John T

 

[buickanddeere]

(quoted from post at 12:52:09 11/09/20) I also have this 5hp motor it seems big but maybe bigger is better? The larger compressor calls for 3-5hp.

Bigger is better

 

[Whiskey Bent]

Ok, thank you John T and others. For the 10 gauge wire and 30 amp breaker I was referring to the 14.5 and 22 amp 3hp motors, so I think Im ok with these motors. If I use the 5hp Baldor 40 amp 8 gauge wire and 40 amp breaker. Its been mentioned several times about thermal motor protection, isnt this the little red button on the side of the motor? All 3 motors have this so Im ok there? Again thank all of you, I just dont understand electricity!

 

[buickanddeere]

You are wiring these motors with physical protection and vibration protection for the wiring ? With proper connectors at the junction boxes/pressure switch/ breaker panel . Motors and boxes grounded .
Dont have the code book here at the moment. 250% is the usual with breakers .

 

[John T]

Ok, thank you John T and others. For the 10 gauge wire and 30 amp breaker I was referring to the 14.5 and 22 amp 3hp motors, so I think Im
ok with these motors. If I use the 5hp Baldor 40 amp 8 gauge wire and 40 amp breaker. Its been mentioned several times about thermal motor
protection, isnt this the little red button on the side of the motor?

Youre welcome,

YES youre on top of this. For a 22 amp motor since 80% of 30 is 24, 30 amp 10 Gauge wire is okay. For a 28 amp motor 40 amp wire (80% =
32) is okay

YES often a little red re set button is the thermal protection device

My description below explains circuit breaker sizing

John T

 

[buickanddeere]

14.5 amp is 18.75 Amp wire and a 30 amp breaker
22 amp is 27.5 amp wire and a 50 amp breaker .
28amp is 35.0 amp wire and a 70 amp breaker .