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OT (way OT) Compresser motors

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Old 10-01-2002, 02:07 PM
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Default OT (way OT) Compresser motors

The electric motor has given up the ghost on my compressor. I need a 5hp 3450 CCW motor. This use is not commercial and is used say 3-4 times a month. I've done a bunch of searching for different motors and I'm frankly rather baffled by all the options. I understand there are differences between the frame types and the shaft sizes, which I'm willing to deal with. My question really has to do with motor quality and if I'll be satisfied with a cheaper motor.

Here's a link to the cheapest motor that I can find. The page does not specify if it's for a compressor, but as long as it's a capacitor start, we're OK, right?

http://www.northerntool.com/webapp/w...=461&langId=-1

Here's a link to the Grainger page. The motor is a little more expensive (those couple of dollars was enough to get the Mrs. to balk) but I know that it's a quality motor and was designed for a compressor.

http://www.grainger.com/Grainger/pro...mId=1611621890

Here's a auction on E-bay. The ad sure looks a little hokey, but I'd use the credit card for protection.

http://cgi.ebay.com/ws/eBayISAPI.dll...058123662&rd=1

So can you guys offer some assitance to help guide my purchase?

Last edited by Cord; 10-01-2002 at 02:12 PM.
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Old 10-01-2002, 02:17 PM
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Ive bought several electric motors from Grainger and always had good luck. Dayton is their house brand and is plenty good enough for everything but heavy industrial use. They do sell Baldor and Emmerson for heavier duty applications.

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Old 10-01-2002, 02:40 PM
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here's the scoop..

A compressor runs best with a three phase motor. They have the most starting torque. If you do not have three phase juice in your box, you need a single-phase motor with capacitor start.

The different types of motors available differ in their frame style and are for different applications. An open-frame motor cools better than other styles but is not intended for use outdoors or in areas where flammable gases may be present. Open-frame motors are also cheapest. TEFC motors are for outdoor duty and typically last longer than other styles due to the sealed construction. They also cost more. All frame styles are available in different duty cycle levels (more duty cycle = more cost).

I would expect that for your non-industrial usage that an open frame motor will work fine. Just blow the dust out of it occasionally.

As far as your motor failure, heat is usually the culprit. If the motor was designed to be used with the compressor (if it came with it) then your compressor may be harder to start than it used to be. It is not uncommon for the check ball to be dirty or worn, which means the compressor is fully loaded when you try to start it. The check ball is usually located in the line from the compressor head to the tank (or even in the fitting where it connects to the tank). You can even add a universal one if you are unsure of where yours is (they are around $10). They hold pressure in the tank and let the head pressure bleed down so it can start unloaded next cycle.

The best way to make sure you have a setup that will last is to get a clamp-on amprobe and watch the current as the compressor pumps up the tank. The running current will be at its highest right before the pressure valve cuts it off. If the amp draw is above the rated amp draw, then you need to "pulley it down" or drop the pressure cutoff to a lower pressure. Lots of folks buy a homeowner compressor and then crank the cutoff pressure up higher. It will run this way only so long before heat eats the motor. You can pulley it down and keep the pressure up or vice versa.

Hope this helps.

Mike
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Old 10-01-2002, 02:58 PM
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I..uh...hopped up the comprsser. The new motor is going to be rated to the pumps specifications. It's rated either 5 or 7.5hp @ 1725rpm so I figured a 3450 5hp would be more than enough to run at the 7.5hp level.

Nope, I don't have 3ph. The cheepie motors are a "SPL" type. The good Grainger motor is a "OPDP" type. What do those designations mean?

Ultimately, which way should I go for the motor?
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Old 10-01-2002, 05:44 PM
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http://www.northerntool.com/webapp/w...ategoryId=6230

non commercial will do the trick but a commercial duty is better
but dont buy a motor with a low duty cycle like 20%
they should be 80 or 100% compressor rated only.
those walmart style ones dont last long "overrated hp"
and remember you dont get something for nothing
HP requires current and a 15amp motor at 110v cannot
produce a true 5hp or even 4hp"overrated
I use a cast Iron twin with a emerson 5hp continous duty
motor and yes its running Mobile 1
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Old 10-01-2002, 08:13 PM
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OPD -- Open Drip Proof style motor
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Old 10-01-2002, 11:09 PM
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The motor from Northern looks like a good choice- it has a 1.15 service factor (overbuilt for 15% momentary overloads) and appears to be a true 5 HP motor.

208*22 = 4576 watts

746 watts = 1 HP

4576/746 watts = 6.13 HP

Allowing for about 90% efficiency, output power is

6.13 * .9 = 5.52 HP


Those '5HP' motors that pull 12 amps at 120 volts are peak rated, and are only delivering about 2 HP.

You do need to check to see if the compressor is leaking pressure back past the discharge valves in the compressor head. This will put a strain on the motor when it is started, reducing life. Check the starting amps when there is no pressure in the tank, and check again when the motor starts with pressure in the tank.

If the valves are leaking, Grainger carries an unloader valve- "Load Genie????"- that you install in the discharge line between compressor and tank. Dayton also makes a pressure switch that has an unloader valve built into it to dump any head pressure when the switch closes.


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Old 10-01-2002, 11:45 PM
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Bull-here was my concept...I have a compressor that's rated for up to a 7.5hp motor. I had previously run it using the 3hp motor by slowing it down. This put the compressor at it's minimum speed. I was thinking that if I took a 3500rpm motor and halved the recommended pully size for a 1725 motor that it could turn the compressor at the 7.5hp level. Somebody was telling me that it wasn't going to work, but I keep on thinking that I'm doubling my power with the smaller pully.

BTW-I just had the compressor rebuilt with a new unloader and valves so we're good to go.
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Old 10-01-2002, 11:55 PM
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I buy motors from GRAINGERS all the time ( today in fact ) They are good motors but I would buy one with a higher service factor. The G motor only has a service factor of 1.00 -- I would go with at least 1.15 and you must have a capacitor start motor. Service factor is like headroom in an amplifier.
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Old 10-02-2002, 11:30 AM
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A 3500 rpm motor has HALF the actual torque of a 1725 rpm motor. If you run a 5 hp 1725 motor at the recommended pulley circumference, then you will need to run a 5hp 3500 rpm motor at HALF the recommended pulley circumference for the 1725 setup. This will net you no additional performance whatsoever.

You will get better pump output if you spin it faster (which the 5hp should definitely be able to do over the 3hp).

As has been mentioned, though, the motor needs to start the pump "unloaded", the max loaded amperage should not exceed the rating on the spec plate. If you try the recommended pulley and the amperage is less than the rating, you can cheat it up to the rating (with larger pulley on the motor) but no more.

Also check the voltage at the motor at full load. If it is less than 105 volts at full load, you need to upsize your wiring...
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