3 Phase compressor, single ph service: What options?

[Fl_Key_Guy]

The higher fan speed of a faster motor is a good consideration.

Harbor Freight is selling a 7.5 hp Marathon Electric (Wasauwa, Wisconsin) single phase electric motor for about $450. They also have a 6hp (probably Chinese) for $200. The HF 7.5 one is 1800 rpm with a 1-3/8" shaft- a close match in speed and same pulley can be used, so I'm leaning that way but need to check if it will fit.

There are advantages and disadvantages to every solution, so I'm still considering the options.

I moved the rig from my van to a trailer today. i can;t believe it is only about 300 lbs- seemed like 800! It sure loaded easily with the forklift.... Took ramps and a hydraulic hoist and two people to get it off the van and two people to get the hand truck to tilt back to go up the trailer ramp. not exactly portable...

 

[mike_s]

Harbor Freight is selling a 7.5 hp ...

I've never bought an electric motor from Harbor Freight before, but if its got as much wiggle in the shaft that they've got in their sliding compound miter saws, then I'd stay away from it.... I mean they aren't exactly known for their precision engineering in the products at that store...

 

[Fl_Key_Guy]

I've gotten good quality and horrific from Harbor Freight, but Marathon Electric is a US motor manufacturer so I am not too concerned if the vendor also sells junk. (Off the thread: I bought 8 spark checkers from HF and only 2 worked. I'm currently the Owner's Rep on an $11million sewer plant and the prime contractor uses a lot of HF power tools. If he can get 2 days from a concrete vibrator, it is cheaper than renting one, and so far the HF one has been more reliable than the big name brands. But not quite as powerful and it is much newer. Plus, HF tools are not as likely to "walk off"! This is a very competent Superintendent.)

 

[mike_s]

you don't cry as much when you drop a Harbor Freight tool off the side of you boat by accident either....

Harbor Freight tools have their place, but I'd just be leary of thier motors and other stuff like that....

 

[CraigAClark]

Marathon is not a bad motor. I have used them in the past and Bauer used them on some of their machines before they went "cheap". Stay far away from Dayton as well.
Be aware that the pulley you will need for that 1800 rpm motor will have to be 12.32" to turn the Mariner at full rpm. Some motors have a max limit on pulley size. Plus you may not have the space on that frame to accommodate a pulley of that size.

Equation for determining pulley size-

Driven Speed divided by the driver speed, multiplied by the Driven pulley size = Needed pulley size.

1250 divided by 1800 = .694 X 17.75 (Mariner Flywheel diameter) = 12.326. You can go under size but don't exceed the motor service factor in pulley size.

Craig

 

[Fl_Key_Guy]

Thanks for the motor advice, Craig.

The Yachtpro currently has a Baldour 1750 rpm motor with roughly a 12" pulley, so it is really close.

I'm a little inexperienced on motors and had not noticed any reference to pulley size in any specs I had read. Also do not know how one would determine that the service factor would be exceeded by a certain pulley. What spec do you look for that gives you that info? (I checked the Leeson site and found the motor you used, but could not find an obvious reference to pulley size. Service factor was just 1.0, so easily exceeded)

Another maybe dumb question, but looking at the shaft end, which way does the Mariner rotate?

Also, what is the stock Mariner operation speed? (The Yachtpro delivers with 1200 rpm operation)

 

[Isaac-1]

Just wait until you try moving a 15 CFM 2100 pound off road trailer mounted, diesel powered Davey compressor, just wheeling it out of the shed to run is exercise for 2 people.

Ike

 

[pescador775]

The Leeson 5 hp rotates CCW. That is the correct rotation. Service factor has nothing to do with pulley size. It is a measure of efficiency. A service factor of 1.0 means that the Leeson operates at full rate hp.

Craig was talking about the clearance for such a large pulley. A pulley provides torque multiplication which is like leverage. A slow rotating motor provides high torque at low rpms which is needed for a large pulley. A high rpm motor needs a smaller pulley as indicated by Craig.

The 5 hp is the way to go when discussing amperage available at commonly available dryer hook ups.

IMO,
pesky

 

[CraigAClark]

Single phase motors generally can operate either CCW or CW. In most cases, you swap lead 5 and 8.

Craig

 

[Scuba Duck]

Be careful hooking up a 5 HP motor to a load designed for 7.5 horse power. You should see if you can get the torque/HP curves for the compressor block.

Bus frequency is 60 HZ * 60 seconds = 3600 revolutions per minute. Based on how the motor is wound, they usually turn at around 1800 or 1200 rpm which are 1/2 and 1/3 of bus frequency. The motor will turn near this synchronous RPM at no load. As the load increases, the motor slows down. The difference between motor speed and bus frequency is called slip. Slip is what drives current through the motor. As the motor slows down, the slip increases, the current through the motor increases, and the delivered power (HP) increases. If you hook up a 5 HP motor to something designed to run with a 7.5 HP motor, and use the same pully ratio, the motor will slow down, current and HP will increase until it finds the ballance between the applied load and motor output, but the designed rating of the windings in the motor will likely be exceeded, resulting in insulation breakdown and shortened life.

In short, if you want to under power the compressor, use a smaller pully ratio and turn the compressor slower. Don't try to turn the compressor at design speed with an undersized motor... the motor won't last.

Changing the direction of a single phase motor involves changing wires in the motor starting circuitry. Depending on the motor, this may not be an option.

3 phase motors do not require any starting circuitry to establish direction of rotation (one less thing to fail), the 3 phase power is rotational by nature. Changing direction the direction of a 3 phase motor simply involves swapping two of the power wires.