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Motor Amperage draw?

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Don Slinn

11-09-2002 14:59:44




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Could someone explain why some 1 HP 120 V motors draw 10 amps and some compressors can be 5 HP amd only draw 15 amps thanks




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Bill

11-16-2002 09:27:56




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 Re: Motor Amperage draw? in reply to Don Slinn, 11-09-2002 14:59:44  
How about 1 more theory? I belive they get away with it ( over rating motors/compressors ) using the duty cycle/load theory. True you may have that little motor on your single stage 5 horse power rated compressor. Included in the equation will be a low CFM rating on the compressor in ratio to amp draw. The motor MAY have a 5 horse rating.... at about a 2% duty cycle. Driving the small cfm compressor pump likely only requires in reality about a 1 to 1&1/2 horse motor at 100% duty cycle. Amp draw is always related to load. My 5 horse 2 stage Ingersoll Rand takes a full 20 amps of 220V but she puts out 19CFM @175psi. Your 5 horse on 120V ain't putt'in out anywhere near that/it don't take that much energy to spin the compressor at that cfm rating. Regardless of motor horsepower it will only draw enough power for the load. Understanding what the others have said here about the wattage/actual horsepower to power consumption info I did not know the formulas ( thank you!! information stored...)but to put the information into a format the naysayer will understand/can test: When I bought the new swimming pool the Dealer made this BIG thing out of selling me a 2 SPEED ENERGY SAVING FILTER PUMP!!! You could slow the motor down ( move less water ) & save ENERGY/$$$!!!! GREAT! Now I don't want that 2 speed motor, it's a I forget how much the $ option was , gimmeee something else worth equal... Grumble grumble & he did it. Once installed I put a ball valve on the pump RETURN ( MUST be on the pump return line or you will not get the savings. If you do it on the suction line the motor will work harder pulling/draw more amps.)line. When water is good & I'm just looking for circulation for whatever reason ( such as say water/pool clean but I want to use the chlorinator ) I just close down the ball valve half way, my flow drops & in ratio my power consumtion drops. To test theory put an amp meter on the line, start closing down the discharge valve on a CENTRIFICAL pump & the motor speed will stay the same & amp draw will drop. With the discharge valve throttled down the horsepower required to spin the pump drops & power draw drops as your not moving that much volume. That I believe is how they get away with these shyster small Air Compressor HP claims. The pump on the unit never pumps that much air so it never draws that much juice....

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JMS/MN

11-12-2002 23:09:26




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 Re: Motor Amperage draw? in reply to Don Slinn, 11-09-2002 14:59:44  
Not all of the 'spin' comes out of Washington. It's called lying! Like the others said- the laws of physics and electromotive force dictate what can really be achieved. Like the ad for your wife's vacuum cleaner that develops 3 hp on a 115 volt, 15 amp circuit- it just isn't so. Just like the 5 hp air compressor you plug in to your 115v shop circuit. The others got it right- basic laws of electricity don't support the advertising claims.

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Bus Driver

11-10-2002 08:44:43




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 Re: Motor Amperage draw? in reply to Don Slinn, 11-09-2002 14:59:44  
A true 5 HP motor for 240 volts single phase will draw about 25 amperes at a 5 HP load. Most electric motors will deliver about twice the rated horsepower for brief periods with a tremendous increase in the amperes required. Rapid overheating will result. The 5 HP motor with 15 amperes will develop briefly 5 HP, but calling it a 5 HP motor is very misleading. A true 5 HP motor can momentarily deliver almost 10 HP.

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D.L.

11-10-2002 00:22:45




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 Re: Motor Amperage draw? in reply to Don Slinn, 11-09-2002 14:59:44  
I'll bet the 1 HP motor is 120v and the 5HP is 240v. I've never seen a 5 HP motor that runs on 120v. Usually, anyhting that starts around the 2 HP mark and up goes to 240v.

A 5 HP motor means it needs 3735 watts (747 watts per 1 HP X 5) of power. At 240v, that means it draws 15.5 amps...so, if the 5 HP motor is 240v, then it looks like it is a 5 HP motor (assuming 100% efficiency). But, with the efficiency problems as the other folks have mentioned, it's probably really only going to output about 3.5 - 4 HP.

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G Taylor

11-10-2002 13:41:15




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 Re: Re: Motor Amperage draw? in reply to D.L., 11-10-2002 00:22:45  
D.L. you are assuming 100% efficiency & 1.0 power factor with that volts x amps equation. Even a 3 phase 600 volt motor on 60 Hz the best power factor & efficiency I've seen to date is .80PF and 94% eff.



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D.L.

11-10-2002 22:08:50




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 Re: Re: Re: Motor Amperage draw? in reply to G Taylor, 11-10-2002 13:41:15  
Right, that's why I said it wouldn't put out 5 HP because of the lesser efficiency and PF.

The manufacturers though, will put the rating on the motor as if it were 100% efficient.



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teadave

11-09-2002 19:14:04




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 Re: Motor Amperage draw? in reply to Don Slinn, 11-09-2002 14:59:44  
the manufactures have changed the way they rate hp. they used to rate them by the highest sustained power they could produce. now they get the motor up to speed, and stop the shaft in a split second. the hp is now how much they got in that split second. kinda crappy ain't it?

teadave



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Sam#3

11-09-2002 17:23:30




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 Re: Motor Amperage draw? in reply to Don Slinn, 11-09-2002 14:59:44  
Most units are configured with a small motor pulley and a large pump pulley. Just guessing I'd say, Maybe(?), the pumps do require 5hp.
Just a guess based on other advertising gimicks I've seen over the years.



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MarkB

11-09-2002 16:36:49




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 Re: Motor Amperage draw? in reply to Don Slinn, 11-09-2002 14:59:44  
Easy. They're lying when they say the 15 amp motor develops 5 hp.

1 horsepower equals 746 watts. So at 120 volts, it takes about 6.2 amps to generate 1 hp. But that assumes unity power factor, which is seldom the case. If you figure a power factor of .9, it takes about 7 amps per horsepower.

So for that "5 horse" compressor to really develop 5 hp, it would need about 35 amps at 120 volts, or about 17 amps at 240.

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G Taylor

11-09-2002 20:15:41




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 Re: Re: Motor Amperage draw? in reply to MarkB, 11-09-2002 16:36:49  
Unless that motor has a great hulking oil filled capacitor a single phase motor does well to achieve a .6 to .75 power factor at 100% of rated load, worse at part load. Efficiency of a single phase motor is at best 65-75%. It's stretching to get a true continous 1-1/2 HP from a 3600rpm, corrected PF to .9 at 120V and 15.0 amp. An easy rule of thumb is at best 62% of input volts x amps/746 makes it out as shaft HP for a small single phase motor.

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Steven M

11-09-2002 15:33:13




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 Re: Motor Amperage draw? in reply to Don Slinn, 11-09-2002 14:59:44  

That's not a real 5
HP. A real 5 HP has to be on 50 amps. The motor that is on my compressor looks like a 5 HP motor. Some have a motor that look only like a 1 HP. I don't know how they can call it 5 HP



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Randy

11-10-2002 07:47:33




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 Re: Re: Motor Amperage draw? in reply to Steven M, 11-09-2002 15:33:13  
The 5 hp motors that look like 1 hp are 3400 rpm. They double the motor speed to get the rated output out of a smaller frame.



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Dusty

11-09-2002 16:30:23




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 Re: Re: Motor Amperage draw? in reply to Steven M, 11-09-2002 15:33:13  
Amps X Volts = Watts 747watts = 1 hp. @ rated rpm. Some how some manufactors are allowed to rate motors at the point of stalling.



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steve

11-10-2002 15:14:49




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 Re: Re: Re: Motor Amperage draw? in reply to Dusty, 11-09-2002 16:30:23  
I dont know how they do that anymore. How do your local electrical inspectors look at something like that. It seems the AHJ would have final say so.

I think you can get a 3phase 4wire system in 208 volts and not have the wild leg. But I have been away from this busines for several(10) years. The system I liked to work with the best was a 480 volt 4 wire system and transformers for 240-120 volt applications.

steve

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John Humphrey Morocco Indiana

11-10-2002 10:37:55




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 Re: Re: Re: Motor Amperage draw? in reply to Dusty, 11-09-2002 16:30:23  
Just checked the slide rule from Square D and a 5 HP motor has a NEC FLA rateing of 56 amps/120volts and 28 amps/240 volts. Have not seen any new 5HP at 120 volts but have changed out some old ones. Power CO. wants higher HP run at the higher voltage, And they are making some of the farmers put in 3 phase. When we did the last big farm, we went from 200 AMP 120/240 single phase to 600 AMP 120/240 3 phase. Used special transformers so that we could run the 230 volt motors already on site. Drawback/downside= one leg has hot voltage,,,,just under 200 volts to ground as I recall. If this system was done in single phase, it would have to be 1038 AMPS. Even with the 600 AMP system they nad to power the truck unloading auger with a tractor with at least 100 HP PTO. (3 to 4 min unload time)

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G Taylor

11-11-2002 06:41:03




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 Re: Re: Re: Re: Motor Amperage draw? in reply to John Humphrey Morocco Indiana, 11-10-2002 10:37:55  
Must have been one of those nasty center tapped and grounded Delta transformers. Personally prefer the slightly more expensive but reliable,safe and even voltage from a 600v delta to 120/208 Wye system. Getting the voltage up to 600 from 480 helps get the conductor & conduit size down. Figure on 1 amp per HP. Those four wire with a neutral and 90 degree phase separation are getting pretty rare. Where did you see one? Takes a little tinkering getting the phase shift with a "T" tap transformer to feed one of those systems from a three phase supply.

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