VFD for 3hp 3ph air compressor motor

Hello Everyone

Im looking at purchasing an older air compressor that has a 3hp 3 phase motor.
I only have single phase in my shop. Im running my lathe with a vfd and am enjoying the variable speed it offers. I did buy a new inverter rated motor. I've been reading a lot of conflicting information regarding using vfd's for air compressors due to the large starting current required. The other alternative would be to simply buy a new single phase motor to replace the 3 phase motor. Im sure that would require more re-wiring of the starters and pressure switches though? Plus I would be losing the benefits of the smooth/reliable 3 phase motor.
What is everyone else doing with similar situations?

Thanks in advance!

This has indeed been a problem application for single phase VFDs. Too much starting load as you say.

We went over sized on the VFD with my son's installation. Works well EXCEPT in mid winter in his unheated Minnesota building.

I assume your compressor was purchased used. I also bought a used Quincy a couple years back. It blew fuses ocasionally on startup (generated 3phase). turns out the head needed a rebuild as the unloaders were all carboned up and did not function well. NO WAY would this machine have run on a VFD. Now that it is rebuilt I am thinking of moving it to my heated basement and starting it with an over size VFD. If I have trouble, I'll put a smaller pulley on the motor and then run it faster than 60 Hz.


So with these caveats, yes a VFD can work. I would most certainly get a 5 Hp VFD if you go this route.

An RPC would be a lot more appropriate. They'll take a lot more abuse than a VFD, work on multiple machines and are a lot easier to work on. A 20$ 3phase motor, several hours of research here on PM, a pull rope and you're good to go.

For 3 hp ????

Heck, Even HF is selling 3 hp single phase compressor duty motors now.

Hello Digger Dug

Yea that option is climbing to the top of my list. I just found a 5hp compressor rated single phase motor from northern tool for about as much as the VFD would cost. My question here is how will the single phase motor hold up as compared to the 3 phase? I havent bought the compressor bur am thinking about it as it is a good one at a decent price. Im sure I would have to go through it and clean/replace some parts including the unloaders.

Thanks

A single phase motor will draw more current, so your magnetic starter will have to be addressed. Some use replaceable heater elements to set the overload protection amps and others are adjustable with just a dial. Also, one leg of the single phase should be wired through two poles of the starter so that all three "see" the load.

Here's a picture of the starter on my champion single phase compressor. The red line input is connected to the center pole. The black wire at the bottom of the center pole loops back to the top (input) of the right hand pole, then that output at the bottom goes to the motor.

The yellow dial adjusts the overload amps.



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MeanMachine1980 said:

Hello Digger Dug

Yea that option is climbing to the top of my list. I just found a 5hp compressor rated single phase motor from northern tool for about as much as the VFD would cost. My question here is how will the single phase motor hold up as compared to the 3 phase? I havent bought the compressor bur am thinking about it as it is a good one at a decent price. Im sure I would have to go through it and clean/replace some parts including the unloaders.

Thanks

Click to expand...

Clean the unloaders ?
This wouldn't happen to be a Quincy pump ?

With the head unloaders, the starting is much easier, as they stay unloaded
until up to speed, and then the oil pump sends bleed oil to operate them.

a good 3-5 seconds on mine.

Buy a decent single phase motor and you should be fine. They can last a long time, Champion sells compressors with either single phase or 3 phase motors for their 3 and 5 Hp models. The factory Champion's come with Leeson or Baldor motors, mine has a 5 Hp Leeson. Getting a replacement contactor and thermal overload relay should not be a deal breaker, although you may have enough adjustment on the factory overload relay. The cost of wring up a VFD in this application would be more expensive. If you did go with a VFD, I would go with a 3 Hp as they can handle higher current for short periods of time and you can decrease the start-up load by programming something like a 3 second ramp up time. The compressor would be unloaded until it comes up to speed. If you oversized the VFD, there are very few defacto single phase input 5 Hp VFDs, buying a 3 phase VFD and derating it you would probably would need a 7.5 Hp. You are looking at significantly more cost in either case.

If you are going to change to a single phase motor, get one with a name brand. Some of the single ph motors made for compressors will have HP as "5Hp spl", the spl indicates special (or someone is probably lying), they supposedly have starting torque of the hp listed, but if you calculate volts x amps it is a lower hp. If you go with an spl motor, get the 5hp not the 3.

The "smooth running 3 phase motor" issue is essentially irrelevant on an air compressor. Go that route. The smoothness of a 3 phase motor does make a difference in machine tools where the slight torque pulsations from a single phase motor MIGHT show up on fine finishes. But on an air compressor, no. KISS...

A concern I have had about using a VFD for a compressor is related to the unloader. The comprssor is not hard to start if it is unloaded, but most small compressors (and 3HP is small) just unload a small length of tube between the comprssor and the tank. Pressure begins to build after just a revolution of the compressor. The advantage of a VFD is that you can soft start the compressor (ramp up frequency) to keep the starting current low. But this may not be doable unless an external unloader with timer is used to allow the motor to start freely. I have not verified this problem but I suspect it could be an issue. Another issue is do you leave the power on continuously to the vfd regardless of the compressor usage?

The RPC solution also either requires constant running of the RPC or another start control circuit. Constantly running the RPC while using the air intermittently will significantly increase the power bill.

The single phase motor is the simplest and most robust solution for your situation in my opinion.

My compressor had a working 5hp single phase motor on it until a start capacitor blew up and a start winding heated up until kaput.

I'm going to use a soft start approach with a VFD with a three phase motor.