Compressor HP Question

[pelletman]

Hello everyone. I have a Champion VRV7-8 compressor that has a 7.5 HP 3 phase motor on it. I need to change the motor to single phase. I am wondering what I would lose if I went to 5HP with the same RPM? I don't exactly understand what the benefit of the larger horsepower if you are running the same RPM. Can somebody explain? If you have any input on brands of motors I would like to hear it also. Obviously I would like to spend as little as possible so the asian motors are attractive, but this is probably going to be a rest of my life tool and I am guessing I probably should spend the extra for a USA motor. Thoughts?

 

[rlitman]

...I am wondering what I would lose if I went to 5HP with the same RPM?...

Assuming the RPM at 7.5HP was matched to the motor, what you'd lose would be the 5HP motor when it burns up.

 

[PoorUB]

The pulley sizes on the 5HP motor will be smaller that the 7HP so the pump runs slower, also less CFM.

The 5HP is 2/3rds the size of the 7.5HP so the motor pulley will need to be roughly 2/3rds the diameter. Minimum pump RPM for that model is 400 RPM. The CFM will drop roughly by 1/3rd, so from around 22CFM to 15 CFM. If the lower CFM is no issue, buy a different pulley and belt and go with the 5HP.

 

[joe_padavano]

Get a 7.5 HP single phase motor. That's what I have on my compressor. Leeson and Baldor both make them.

This is the one I have on my compressor, though I found it for less money. Obviously you need at least a 40A circuit to run it (I used 50A). These come in both 3450 and 1725 RPM versions. Be sure you get the one that matches your current motor.

By the way, I grew up in the Worcester area.

 

[The Tool Tyrant]

Think of compressor motor HP / RPM along these lines...(hypothetical scenario) you have a truck with a manual transmission and a 500 HP engine, pulling a 10 ton load, you can use 4th gear. Now switch out the engine to a 300 HP unit and you would need to use a lower gear to haul the same load.
All pumps have a minimum and maximum RPM range. Being as the RV-30A pump RPM shown on the Champion website is 570 with the 7.5 HP motor, if you were to keep the same HP to RPM ratio (5 divided by 7.5 = .66666 x 570 = 380 RPM, which is less than the 400 rpm minimum. Listen to joe_padavano and stay with a 7.5 HP motor.

 

[PoorUB]

Huh! You are correct on the RPM. The 5HP will be under the low RPM limit. I never did the math on it. Best to stay with 7.5 HP.

 

[pelletman]

Thanks for the replies guys! I will be sticking with the 7.5 HP, I don't want the CFM loss.

The compressor I am going to replace is a very old Champion 5HP. I am assuming I am going to have to get a new motor starter for the 7.5? I would love it if I could use the one from the old compressor...

 

[Firebrick43]

Thanks for the replies guys! I will be sticking with the 7.5 HP, I don't want the CFM loss.

The compressor I am going to replace is a very old Champion 5HP. I am assuming I am going to have to get a new motor starter for the 7.5? I would love it if I could use the one from the old compressor...

depends on the amp rating of the starter. We don't know it, its in your shop.

 

[pelletman]

depends on the amp rating of the starter. We don't know it, its in your shop.

Well, I am going from 3 phase to single phase on the motor so I am just guessing the starter is different

 

[Firebrick43]

Well, I am going from 3 phase to single phase on the motor so I am just guessing the starter is different

ok, not the one on the 5hp champion?

 

[CraigStu]

This is one of those deals where it might be worth it to go to a local electric motor repair shop. You may pay a little more vs an internet source but they can answer all your questions.

 

[The Tool Tyrant]

Well, I am going from 3 phase to single phase on the motor so I am just guessing the starter is different

You can still use the 3 phase starter... https://springercontrols.com/news/single-phase-motor-3-phase-contactor/
I've done it once or twice in the past. You may also need to resize the heaters.

 

[pelletman]

ok, not the one on the 5hp champion?

Well, I am going to 7.5 HP on the new(er) compressor so I was assuming the starter on the 5HP old compressor would be too small

 

[pelletman]

You can still use the 3 phase starter... https://springercontrols.com/news/single-phase-motor-3-phase-contactor/
I've done it once or twice in the past. You may also need to resize the heaters.

Thanks!

 

[Firebrick43]

Well, I am going to 7.5 HP on the new(er) compressor so I was assuming the starter on the 5HP old compressor would be too small

Check, don't assume

 

[tdkkart]

This is one of those deals where it might be worth it to go to a local electric motor repair shop. You may pay a little more vs an internet source but they can answer all your questions.

And, they might make you a sweet deal to trade your old motor in.

 

[The Tool Tyrant]

Well, I am going to 7.5 HP on the new(er) compressor so I was assuming the starter on the 5HP old compressor would be too small

Again, all you would need to do is resize the heaters for the increased amperage.

 

[larry_g]

You have one other knob to turn. If you want to run the pump at the same RPM with less HP then you will have to lower the cutout pressure to prevent over-currenting the motor. So you have ~ 2/3 the hp you will have to drop the cutout pressure by 2/3. For any given system if you have a given HP then you can have pressure or volume (CFM) but not both. You want high pressure then slow the pump down and have less CFM. You want high CFM then you have to give up high pressure.

No free lunch here.

On edit, Study the specs for RPM. HP, and pressure on the following pump, https://www.aircompressorcfm.com/product/fs-curtis-e71-two-stage-bare-pump/

If you know the replacement pump for your compressor then you may be able to look your pump specs up at the above site.

lg
no neat sig line

 

[larry_g]

I don't exactly understand what the benefit of the larger horsepower if you are running the same RPM. Can somebody explain?

An aircompressor presents an ever increasing load to the motor as it builds pressure. As pressure builds the load on the motor increases. Your system is designed with the proper HP to carry the max load and reach cutoff without over-currenting the motor. Drop to a lower HP motor and your going to over-current the motor and bad things happen if you make no other changes.

lg
no neat sig line

 

[Bert_]

Again, all you would need to do is resize the heaters for the increased amperage.

Most of the time the starter is too small. A size 1 starter is good for up to 7.5hp on 240 3 phase but only good for 3hp on single phase

 

[The Tool Tyrant]

Thank you Bert...I stand corrected.

 

[pelletman]

OK, I am going to go with a new (or at least different) motor, I can get a "rebuilt" Baldor, and I don't yet know what the rebuild entails, with a one year warranty for about the same price. Baldor is about 1/2 new. The other option is a WEG brand new for about the same price as the rebuilt Baldor. What would you do? Thanks for all the input everyone

This is the WEG motor https://www.ebay.com/itm/292612561921?epid=8031090035&hash=item4421115c01:g:oEMAAOSwF25bK7f0

 

[PoorUB]

WEG is a good motor.

 

[like2wheel]

Consider a VFD. Some are compressor rated. I put one on my 5hp 3ph Quincy almost 2 years ago & couldn't be happier. Pretty cool having a variable speed air compressor. Also programed a 5 sec ramp up speed so no big jolt when it starts up.

https://www.amazon.com/gp/product/B077KSN4C5/?tag=atomicindus08-20

 

[nadogail]

Before investing in a motor, I feel yo should get your tank checked; it nay be rusted thin from undrained condensation.

 

[pelletman]

Consider a VFD. Some are compressor rated. I put one on my 5hp 3ph Quincy almost 2 years ago & couldn't be happier. Pretty cool having a variable speed air compressor. Also programed a 5 sec ramp up speed so no big jolt when it starts up.

https://www.amazon.com/gp/product/B077KSN4C5/?tag=atomicindus08-20

Huh, I did a bit of searching on that and I thought the general consensus was that it was better to just replace the motor. Does it replace the magnetic starter?

 

[pelletman]

Before investing in a motor, I feel yo should get your tank checked; it nay be rusted thin from undrained condensation.

It is a fairly new compressor, I am not worried about that

 

[PoorUB]

Huh, I did a bit of searching on that and I thought the general consensus was that it was better to just replace the motor. Does it replace the magnetic starter?

Yes, you can remove the magnetic starter and use the VFD in its place. Personally, I would just use a motor starter. The next five guys will say VFD!

Now, I tried this a couple years ago when I bought my used 5HP Curtis. It had a 208 volt 3 phase motor and I have a good VFD. I had an issue where the VFD would not start the motor under load because the inrush voltage would drop and the VFD would shut down on low input voltage. Pop the belts off and it would start the motor just fine. I assume as I have a detached shop with only a 60 amp panel the wiring is a bit suspect to handle the inrush on the motor/VFD. I had a used 5 HP single phase motor and used it and it works fine. Perhaps a newer or different VFD would work, but I had the motor so I bolted it up and i worked, so I went that way.

 

[csp]

Now, I tried this a couple years ago when I bought my used 5HP Curtis. It had a 208 volt 3 phase motor and I have a good VFD. I had an issue where the VFD would not start the motor under load because the inrush voltage would drop and the VFD would shut down on low input voltage.

How many HP was your VFD rated for?

 

[PoorUB]

How many HP was your VFD rated for?

It was rated for 10 HP, trying to run a 5 HP motor. It should have worked. I monitored the incoming voltage to the VFD and it would drop down to something like 190 volts and that was below the lowest setting on the VFD, and I couldn't just shut off the limit.

It is an older VFD, one I inherited from a job site. It was originally installed around 2003 by the company I worked for. We were remodeling the building it was in and the VFD was headed for the dumpster so it disappeared one night after we shut down and ended up in my shop.

The funny thing I am using it. I have a drill press with a 208 Volt -3 PH motor, 1HP! Yep, 10 HP VFD running a 1 HP motor. A bit over kill and I can't set the over amp limits low enough to protect the motor, but for a drill press it doesn't run unless I am standing there. If it had worked on the compressor I was going to buy one that better matched the drill press motor. As it didn't, it is back on the drill press waiting for the next opportunity.

 

[like2wheel]

I'm no expert, but it seems like vfd's rated for motors with a high start up load is a fairly recent phenomenon.
I know in the brand I used, it was the GT series. Also, I think the old rule of thumb of 1/2 the rating for high start up load no longer applies.

I would be interested if your vfd would have been happier with better supply.

 

[PoorUB]

I think it would have started it if it had been a 100 amp or larger panel over the 60 amp panel I have. The inrush amps were over 125 amps if I remember. Larger wire size to the panel would have held the voltage better and probably less peak amps so less load on the wire on startup, kind of a catch 22. I have roughly 60 feet of wire from the meter to the shop, probably 4 gauge, I don't know for certain, but the peak amps was probably twice of what the wire was rated, although we generally don't worry about the inrush amps. but the feeder wire was once big resister at that point.

 

[redmondjp]

As stated above, look at the price on a NEMA size 2 starter, which you will need for a 7.5HP single-phase motor running on 240VAC. Pardon the pun, but you may be shocked! I have both a 7.5HP motor and starter which I bought used for a reasonable amount.

 

[PoorUB]

Does it have to be a NEMA starter? You can buy UL listed 7.5 HP starters on Amazon for $150, but yes a quality NEMA starter will cost more than the motor.
I worked for an HVAC company and we put in a lot of VFD's simply because they were less money that the starter we would have to use in a commercial setting.

 

[redmondjp]

Does it have to be a NEMA starter? You can buy UL listed 7.5 HP starters on Amazon for $150, but yes a quality NEMA starter will cost more than the motor.
I worked for an HVAC company and we put in a lot of VFD's simply because they were less money that the starter we would have to use in a commercial setting.

I am old-school I guess - I can't find it online now, but I remember watching a NEMA vs. IEC failure test video where they run short-circuit current through each device mounted on a CMU wall. The NEMA device stayed intact, and the IEC device, with the same fault current flowing through it, was nothing more than a black spot on the wall afterwards.

So I tend to prefer the NEMA equipment for this reason. There are lots of inexpensive IEC devices out there which probably work just fine. Maybe mount it inside of a metal junction box or enclosure just to be safe.

 

[PoorUB]

So I tend to prefer the NEMA equipment for this reason. There are lots of inexpensive IEC devices out there which probably work just fine. Maybe mount it inside of a metal junction box or enclosure just to be safe.

No doubt the NEMA rated it better, but for a home shop and low usage?

Well, I would expect the starter to be mounted inside a box.

I have a cheap starter on my 5 HP compressor. It is mounted right to the motor mounting frame. I think I paid $60-$70. It has been working fine for three years.

 

[The Tool Tyrant]

Electrical guru's...will this work for the OP? https://www.ebay.com/itm/114897132289?hash=item1ac0671b01:g:Z1IAAOSwazhg9HbO

Price is right if so...even with high shipping.