# 3/4 electric motor question



## Barnbikes (Mar 22, 2015)

Does a 3/4 hp have more power as a 220 volt motor then as a 110 volt motor?

Have a brand new motor but does not have enough to turn over my compressor  on 110 volt.


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## Brian Rupnow (Mar 22, 2015)

Make sure that the pressure relief valve has drained pressure from the cylinder before the motor tries to start it. Compressors have three valves built into the system. One shuts off electrical power to the motor when the pressure has reached the pre-set "Shut off" point. The second valve, called the "pressure relief" then drains any pressure off the cylinder head so the motor can start under a "no load" condition. The third valve is the safety valve which will blow off pressure to keep the tank from exploding if the "pressure shut-off' valve fails to function.


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## Ken I (Mar 22, 2015)

If you are trying to run a 3/4 horse 220V (rating plate) on 110V then you are only going to get 3/8 HP out of it.

Worse if the 220V was for 50Hz (as it may well be) and you are trying to run it on 60Hz then you will get another 20% less (approximately due to impeadance) and since it is trying to spin the load 20% faster then you only have (in total) about a third of the torque / power required to run the compressor.

Check if your motor has internal jumper settings for alternative voltages. These will be behind the terminal cover where there is also usually a diagram stuck to or moulded in to the cover.

If you don't understand electrical wiring get someone who does or post a photo for advice.

Otherwise you will have to get the motor rewound for 110V @ whatever Hz. or get another motor.

Also Brian's comments above - see my article on an unloading bottle.

http://http://www.homemodelenginemachinist.com/showthread.php?t=23906



Regards,


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## ShopShoe (Mar 23, 2015)

Barnbikes,

More information would be helpful, as has been stated above. Are you trying to put a motor on a compressor you have, or do you have a compressor you bought with a 220V motor that you want to run on 110V?

Without more information, If your motor is designed to run the compressor you have and has the ability to be set to run on either 220 or 110, you can change to 110 and it "should" work, but you will use twice as many amps, perhaps restricting what else you can do in your shop at the same time.

If the motor came with the compressor by design and can be wired to run on 110 and the compressor won't start, then there are other issues, including the ones Brian mentioned above. Be very careful, you can be hurt messing around with any pressurized system. More info .....!!!

If you are trying to put a motor on your compressor and you just looked for "3/4 HP," then you may not have a motor suitable for compressor duty or perhaps 3/4 HP is not enough to start and run your compressor. Once again, more information is needed.

Thank you for asking and please don't be put off by our responses, we will help you if we can but this hobby often leads to areas where slippery slopes lead to more than quick fixes and the chances for accidents and injuries.

--ShopShoe


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## aonemarine (Mar 23, 2015)

Surprised no one asked if you have a start capacitor wired in.....


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## ignator (Mar 23, 2015)

I'm going to assume the motor can be wired to run on 110 or 220, and if this is so, you do change the wiring for this to work. And No the HP will be exactly the same, as the power will be the same input. For 110 volt operation the current will be twice the 220 volt operation. But at 220, the current is less, and the starting current is half. Starting current also called LRA (Locked rotor amps) is typically 7 times FLA (full load amps). So given your wiring, the IR loss (current time resistance of the wiring) is half. So the motor will start easier with the 220 volt input configuration.
As Brian Rupnow indicated there should be an unloader valve. This is part of the pressure on off power switch. There is a check valve typically where the compressor pipe enters the pressure tank. When the pressure switch shuts the power off, it opens a small air valve connected typically via a 1/4" line to the compressor head. This bleeds off the head pressure so the next on cycle is just developing starting torque to overcome friction, and not having to work into the pressure of the system.
If your compressor does not have this unloader, then most likely the original motor was a repulsion start induction run, which develops enough torque to overcome the pressure head. This would be a fairly old compressor if that is the case.


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## Barnbikes (Mar 23, 2015)

Compressor was originally a gas engine model but previous owner took the motor off. Original motor was a 5hp - was told online a 3/4hp electric motor would work.

Motor starts compressor fine but stalls out around 75psi.

Motor is a brand new jet pump motor 3450 rpm.

If I open the drains and let it run it works fine.

I am guessing I just need a bigger motor.


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## tornitore45 (Mar 23, 2015)

Or a bigger pulley.  I think you may have the power but to much RPM and not enough torque.


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## Fixnair (Mar 30, 2015)

I think I see your problem. First of all what model compressor are we working with? Also what type of drive does it have? A V belt or a micro v belt?
Horsepower figures for a gasoline engine are roughly twice that of an electric motor. So if it were driven by a 5 HP engine you would need a 3 HP (they don't make a 2 1/2 HP motor) electric motor to drive it. Now, I see you are using a 3600 RPM motor. They only produce 1/2 the torque Of an 1800 RPM motor so it will struggle to start. If you are using the same diameter pulley as the engine did then you are way over speeding the compressor. You will need a pulley roughly 1/2 the diameter and a motor of three horsepower to make this thing work.
We need to know at what RPM the manufacturer designed the pump to operate.


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## velocette (Mar 30, 2015)

Hi 
If you intend to use a 3/4 hp 2 pole motor at 3600 you will need to run pump much slower and make do with the very reduced performance.

"Does a 3/4 hp have more power as a 220 volt motor then as a 110 volt motor?"

A  couple of figures to compare

3/4 hp = 562.5 watts @ 110 volts = 5.12 Amps

3/4 hp = 562.5 watts @ 220 volts = 2.55 Amps

Power is the same for equivalent RPM  2 pole motor 3600 RPM poor starting torque, 4 pole 1725  rpm better starting torque, 6 pole 900 rpm good starting torque.

To be realistic  as pointed out in a previous post by Fixnair a 3 hp will be adequate with a small compromise to the drive ratio.

Any machine tools will perform better on 220 volts as the amperage is half of that at 110 volts.

Assuming  "Mother of all Mistakes Is Assuming" that this is single phase supply or.

Have you access to 3 phase supply then invest in a 3 hp three phase motor that will start easier and run much smoother than single phase.

If the estimate for 3/4 hp will replace a 5 hp petrol engine then the roads would be jammed with electric cars you need to inform the compressor supplier of his error

Sorry you were misled by the power required. Keep posting on progress. 

No problems here just a solution not yet found.

Eric


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## Barnbikes (Mar 30, 2015)

The compressor is a Campbell Hausfeld VT617203.

Will put it in the back of the garage till I run across a better motor.


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