I’m sorry, 7.5 Schulz compressor pump***

[howland]

I posted yesterday for help with questions about my air compressor setup. I was a bit judgmental. I am sry for that and for the people that I upset. I have been reading a few posts on here before I became a member and a guy that had the same pump I have and there was a bit of negativity from members on there. That was not everyone and I know that. That was the reason for my post yesterday. I am sry.

This is the help that I need. Again I am sry. Hope u all are well.

The issue I am having is that I have a Schulz 7.5hp, 2 stage, 30 cfm, 5.5 kw, 960rpm
Pump, I’m trying to figure out what motor I need and specs preferred for it. I only have a 220 outlet so I am guessing that I need a single phase motor? I have seen people talk about using a phase box?

 

[larry_g]

I used to dive into the compressor threads a lot but now do not have the time. If I may suggest, go to the search function here and search 'compressor' and my user name. It will bring up a lot of threads on your area of need.

welcome to the group

lg
no neat sig line

 

[howland]

Thanks bud.

 

[GeoBruin]

I posted yesterday for help with questions about my air compressor setup. I was a bit judgmental. I am sry for that and for the people that I upset. I have been reading a few posts on here before I became a member and a guy that had the same pump I have and there was a bit of negativity from members on there. That was not everyone and I know that. That was the reason for my post yesterday. I am sry.

An apology goes a long way in my book.

It's been said already but generally folks here want to help. There are a minority that can be unapologetic in their posts, but the same is true in the "real world" and you must learn to filter those people out. I will end my meta discussion here as the mods have already locked the other post so as far as this forum is concerned, it's time to move on.

That said, because the other post is locked, you won't get any more responses there, so maybe edit your post here with your original question, along with as much detail as you can.

 

[howland]

Thanks bud. I’ll do that. Hope ur day is going well.

 

[PoorUB]

The issue I am having is that I have a Schulz 7.5hp, 2 stage, 30 cfm, 5.5 kw, 960rpm
Pump, I’m trying to figure out what motor I need and specs preferred for it. I only have a 220 outlet so I am guessing that I need a single phase motor? I have seen people talk about using a phase box?

I would look for a 1725 RPM, 7.5 HP motor, do not buy a 3450 RPM motor as the motor pulley will be terribly small. The motor pulley diameter will depend on the compressor pulley diameter. What diameter pulley is on a compressor?

Also, do you have electrical service to run a 7.5 HP motor? You are probably needing around 6 gauge wire and a maybe a 50 amp breaker, but I am guessing, but I will bet I am not far off. You can not just plug it in! It will need to be hard wired. I will bet your 220 volt outlet is way too small to run 7.5 HP.

If you find a 7.5 HP 3 phase motor you can buy a variable frequency drive (VFD) to convert the single phase to three phase power, but it adds to the complexity, and they can be difficult to set up if you are not very technically oriented.. I would just find a 1 phase motor.

 

[howland]

Thank u for the info

 

[GeoBruin]

I would look for a 1725 RPM, 7.5 HP motor, do not buy a 3450 RPM motor as the motor pulley will be terribly small. The motor pulley diameter will depend on the compressor pulley diameter. What diameter pulley is on a compressor?

Also, do you have electrical service to run a 7.5 HP motor? You are probably needing around 6 gauge wire and a maybe a 50 amp breaker, but I am guessing, but I will bet I am not far off. You can not just plug it in! It will need to be hard wired. I will bet your 220 volt outlet is way too small to run 7.5 HP.

If you find a 7.5 HP 3 phase motor you can buy a variable frequency drive (VFD) to convert the single phase to three phase power, but it adds to the complexity, and they can be difficult to set up if you are not very technically oriented.. I would just find a 1 phase motor.

I'll add that you will likely need a magnetic starter for a motor load that size, regardless of the size of the circuit you have available.

 

[Packard V8]

If you find a 7.5 HP 3 phase motor you can buy a variable frequency drive (VFD) to convert the single phase to three phase power, but it adds to the complexity, and they can be difficult to set up if you are not very technically oriented.. I would just find a 1 phase motor.

Better do some serious research before attempting to operate a 3-phase 7.5hp compressor motor on a VFD. There are many threads warning against it.

jack vines, who wants to but has been hesitant to try it, because VFDs that large are very expensive.

 

[PoorUB]

Better do some serious research before attempting to operate a 3-phase 7.5hp compressor motor on a VFD. There are many threads warning against it.

jack vines, who wants to but has been hesitant to try it, because VFDs that large are very expensive.

It can be done, but I would not recommend it. I would hook up some sort of unloader valve so the compressor does not start pumping against pressure until the motor is nearly up to speed. Many VFD's have additional control contacts that you can program to close by RPM or hertz. When the motor reaches maybe 50 hertz (on a 60 hertz power) close the contacts that closes the unloader valve. The next thing it trying to find and unloader valve that can deal with the heat of the compressor supply air, that air gets nasty hot!

I went through this trying to get a 5 HP 3 phase motor running on my shop compressor. I had a 10 HP VFD that would run the motor just fine, but I had no way to unload the head and the VFD didn't have a way to control it. Plus I had inrush power problems. The incoming voltage would drop to about 90 volts per leg on startup and the VFD would trip out on low voltage. If I ramped the motor up slowly, maybe 5 seconds it would start it unloaded.

I had a 5 HP 1 phase motor and it works just fine.

 

[PoorUB]

I'll add that you will likely need a magnetic starter for a motor load that size, regardless of the size of the circuit you have available.

Right around 5 HP requires a magnetic starter. There are some pressure switches that will handle 5 HP, but the motor needs to be thermally protected and many are not.

 

[howland]

Thanks a lot everyone. Very much appreciated

 

[Cruzan80]

So the 7.5HP and 5.5Kw part are referring to the motor, not the pump. Pulley sizes can vary, but sounds like you need 960 at the pump, so as mentioned, a 1725rpm motor is a better bet than a 3450. Lots of pulley calculators online.

 

[howland]

Thank u

 

[LopezBart]

Remember, if you don't need the full output, you can just turn the compressor more slowly with a five hp motor. Since the compressor takes about the same torque to run whatever the speed, the calculation becomes simple:
5 hp/7.5 hp * 920 rpm = 613 rpm.
To figure out the pulley size on the motor, you can use the ratios of the speeds:
613 rpm/1725 rpm * compressor pulley = motor pulley diameter needed.
If you end up using a VFD, set it up for a soft start with an unloader and it will work w/o dimming the lights.

 

[dutchgray]

Remember, if you don't need the full output, you can just turn the compressor more slowly with a five hp motor. Since the compressor takes about the same torque to run whatever the speed, the calculation becomes simple:
5 hp/7.5 hp * 920 rpm = 613 rpm.
To figure out the pulley size on the motor, you can use the ratios of the speeds:
613 rpm/1725 rpm * compressor pulley = motor pulley diameter needed.
If you end up using a VFD, set it up for a soft start with an unloader and it will work w/o dimming the lights.

You can also run a little more slowly with a 7.5hp motor and give the whole system an easier time. Motor will last longer, pump will last longer, it will run quite and cooler.
Need to look up minimum speeds spec on the pump though.