Torque1st is right. Most compressors have a minimum RPM of 400 to splash the lubricant around.

Most circuit breakers will handle a few seconds of over current. Some are instantaneous trip though and I don't know how to tell the difference . Maybe an electrician can advise you on that

 

Not to insult anyone but I recommend you hire an electrician to take care of this for you. Your lack of knowledge/experience on this conversion/installation will put you in a situation where you may get hurt or killed if it's not done correctly.

You may be able to save some money by purchasing a used or reconditioned motor from a local rebuild shop. Just make sure they give you a warranty on it and test run it for you at their shop. The open drip style just means there are openings in each endframe for air to be drawn through the motor for cooling. As long as the motor/comp is inside or protected from the weather, it will work fine.

The ratings on the electrical breakers, fuses, and/or magnetic starter are designed to stand the momentary high amperage to get the motor up to speed.

In the 20 years I worked in an electric motor repair facility, on average, I encountered this situation roughly 3 or 4 times a year. Someone finds a "bargain" and just wants it converted from 3 phase to single phase to run in the garage. Once they get a final cost, they were usually ready to dump it in our junk pile. Assuming your compressor is good, you still have a chance of coming out ahead of the game on your "bargain".

Again, I'm not trying to insult or belittle anyone. For your on safety, get it done right.

 

Breakers and starters have some of that magical property called smoke and mirrors - actually called a response curve - they will take many amps for a short period of time, few amps for a long period of time type thing. For example a 30 amp breaker could take 80 amps for a half second or so, but will hold at 30 amps all day. At 40 amps it might take a full minute to trip, at 50 perhaps 45 seconds, and so on.

Manufacturers size breakers and starters for the predicted use. Many electrical devices are rated for "resistive" loads, and loads that are "inductive" such as your motor will be rated less for the same device. The "heaters" or coils in your starter are interchangable, and determine what startup and max as well as running current are allowable. They work by heating a bimetal strip according to current draw, and trip the coiloff at a given temperature.

More than you ever wanted to know at:
http://www.e-t-a.com/fileadmin/user_..._vs_Shorts.pdf

 

A magnetic starter contains a relay (contactor) and thermal overload detection circuits. The relay is closed when you push the start button (or, in your case, when the pressure switch calls for air). This supplies power to the motor and to the overload heaters.

If the motor draws excessive current for long enough, the overload heaters will trip and open the relay. The higher the current, the quicker the overload will trip.

It is still necessary to provide short circuit protection for the motor. This is typically the function of the circuit breaker or fuse in the branch circuit supplying the motor (and motor starter). Since the magnetic starter is supplying overload protection, it is feasible to select a circuit breaker big enough to avoid nuisance trips due to starting current. Similarly, the branch circuit need only be large enough to supply the normal loaded running current of the motor, not the starting surge current.

Most circuit breakers are "inverse time breakers", much like motor overload heaters. They will supply currents much higher than their ratings, provided the current is of short duration. This enables most 30 amp breakers to successfully start a 5 HP motor, even though the motor will draw a brief surge of 80 amps or more. One type of breaker, labled "HACR", is especially rated for starting hermetic motor compressors, such as those used in air conditioners. (Such motor-compressors have very high starting loads). A HACR type breaker can be sized fairly close to the running load of an air compressor, given its tolerance for high start currents.

Wire sizing is governed primarily by thermal effects. The starting load of a motor is drawn only for fractions of a second, and the branch circuit conductors will not overheat during that time. So it is not necessary to use 4 AWG wire to supply the starting current -- 10 AWG is fine for the normal running current. If the branch circuit is fairly long, it may be prudent to go with the next size larger wire, due to voltage drop reasons. This will improve the starting performance of the motor.

It is not absolutely necessary to use a motor starter for a typical single phase air compressor. One can use a motor with integral thermal overload protection (it has that little reset button sticking out the side of the motor) and a pressure switch rated to operate to motor directly, thus dispensing with the contactor function. This is the way a typical "Home Depot" air compressor is designed.

Article 430 of the NEC goes to great lengths to detail all the issues in selecting the circuit ampacity for motor branch circuits, and the overcurrent protection for same.

 

I am getting an electrician to do the hook up, but he charges $40 bucks and hour so I am trying to get everything together a head of time so I am not paying him 40 bucks to shop. Also I am trying to learn as much as I can. As for the price of converting I think it will cost me about 400 bucks with new motor and starter so since this type of compressor cost over 1000 bucks I should still come out ahead.

 

I like to say thanks to all of you that have answered. Like I said I am getting an electrician to do the work but I like to know whats going and and trying to save some money. Last question. The place where I am putting the compressor is about 40-50 feet from the breaker box. If the motor pulls 21 amps and I use a 30 amp breaker what sixe wire should I use. I saw that 10awg is rated at 30 amps but with that much distance will I need to go the the 40 amp 8 awg? Again thanks for all the help, I am just trying to learn as much as I can. I am eager to start using this thing after all the work I had to do just to move the heavy thing( it was a nightmare).

 

Rule of thumb:
14Ga - 15 Amp
12 Ga - 20
10 Ga - 30
8 Ga - 40
6 Ga - 60

for runs under 50 feet. Over 50 feet, move up a guage......

Now if you need 0000 Ga, you're drawing some serious amps, and when the copper bus gets to be 2 inch by 6 in - look out!

 

check out w.w.grainger for your motor they have great prices and are in most areas. i think thier web site is www.grainger.com

im doing the same project with a two stage compressor dual head right now also. but i am not using motor starter im using pressure switches to start the motors. the pressure switch life may not be that great over the long run but it is less money than new motor starters. Make sure you have the correct frame size for you motor also. that is what determins mounting. you can do this for less thatn the cost of a new compressor. the one i am building would cost about 3k

i think ill have enough air to float a blimp....if they use air that is..

jeff

 

Man! Thats what I like about this forum So much expertise available.

 

40 to 50 feet from the box might be about 120 electrical feet. (Allow for vertical runs plus consider the total length of L1 -> motor -> L2.)

The resistance of 10 AWG solid wire is listed as 1.21 ohms per 1000 feet.
At 21 amps (RLA) there will be a voltage drop of 21 * 120 * 1.21 / 1000 or about 3 volts.

This is well within the 3 % voltage drop "goal" of the NEC for branch circuits (3% of 240 volts is 7.2 volts). The 4 to 5x starting current will result in a momentary drop of 15 volts, but this should not affect starting performance of the compressor.

10 AWG should be fine for this application.

Grainger has a lot of stuff, but their complex discount schedule has always been a problem for me. If you can buy material through a company with an account, you will save a lot of money vs. being Joe Schmoe off the street. Also, you need to sound like you know what you are doing, or they won't sell to you, and will send you on to Home Depot. See that sign in the door that says "Wholesale Only"?

 

If you can't get a electric motor to work you may have to get a 4 cycle gas engine like a 11hp honda it will have sufficient power to run the compressor.

At our shop we have 440 3 phase its very expensive to have but we need it to run the welders we have.

 

it's to bad that most houses don't hace 3 phase .. 3 phase is so much more efficent compared to a single phase

 

Yes three phase is efficient if you use enough power B.C. Hydro charges us a surcharge because we don't use enough power at our shop. So we leave all the lights on plus we have 500 watt spot lights we use we leave them on our electric bill is around 1500 dollars a month.

In the day when work was good we had 10 400 amp welders going all day long.

You can buy old 3 phase compressors cheap because not many people have 3 phase power and changing the electric motor with a single phase unit is very expensive.

 

I paid $250 for a 5 HP Quincy compressor that's 3 phase and run it off a 10 hp rotary converter as well the 3 phase Bridgeport. An old welder friend had 3 phase to his garage added as the pole was 30' away besides his house with single phase in a residential area.
.....=o&o>.....

 

I forget the exact figure on what it costs for the hook up for 3 phase but its around 10,000 dollars. You also need a good size electrical room for the main breaker panel etc that was another 5000 just for the panels and breaker and transformer.