air compressor magnetic starter/relay single phase

[Jim Timber]

Just did a quick search and you're looking at about $1100.00 for a 20 hp VFD.

I got my 75HP (that's not a typo!) VFD for $750 shipped. The smaller ones cost more than the bigger ones. You can set load limits in the controls on the higher end drives.

As most of the guys have said, you want to de-rate the drive by half, but going bigger is perfectly acceptable.

I did the motor swap to single phase on my T30 IR compressor before I realized the VFD's soft starting would actually pay for the conversion in energy not used, and then later yet I've discovered that the reactive current from the VFD's (I run 3 motors on them now) actually confuses my digital electric utility meter into thinking we're putting back more than I'm using. It runs our bill backwards when I'm in the shop a lot; which just means we don't pay as much to keep the wife at 72F in the summer time.

Some VFD's will have a leg-loss fault that you'll need to program to ignore. Every VFD on the market made for 3Ph will run on single off two legs. You can't get a VFD made for higher voltage to run off 220-240 though, they'll error out on low input voltage and it's not user fixable (I've tried). So as long as it's the correct input voltage and 2x the power of your load, you're set. You can get universal transformers to bump 240v up to 480 etc - and then run machines that are wired with controls that would be too much trouble to convert. A VFD will run on the input side of a big transformer.

 

[hr3]

You basically made a static convertor. 8hp is a bit optimistic. How did you "figure" you are getting 8hp.

If you want.to get 10 HP, you need a 15 HP pony motor, use the caps to start it, and run the 10hp motor off that. That's a rotary and good for the full HP if you need it. Just cleaner and easier in the long run to go vfd's if you need the full power, and more adjustable too. Only downside is you can only run a single motor at a time

No pony motor needed it's a static convertor and they can be very efficient. 97% in fact.. I posted a link on page 2 I think...

 

[Jim Timber]

Static converters are only good for around 60% of the motor's capacity because you're not energizing 1/3 of the windings. You also end up with less torque than a single phase motor of the same size, and that's way less than the 3ph motor you're using when it's given good stiff power on all three legs.

I'd never run one on a high load application like an air compressor. I can't bring myself to run one on a 2hp mill.

 

[hr3]

Static converters are only good for around 60% of the motor's capacity because you're not energizing 1/3 of the windings. You also end up with less torque than a single phase motor of the same size, and that's way less than the 3ph motor you're using when it's given good stiff power on all three legs.

I'd never run one on a high load application like an air compressor. I can't bring myself to run one on a 2hp mill.

Check the specs. on Ronk's website they tell a different story.. I've used several of them and seem to be very reliable and inexpensive.. I have also used a lot of VFD's and love them but price here is the issue..

 

[Jim Timber]

Check the specs. on Ronk's website they tell a different story.. I've used several of them and seem to be very reliable and inexpensive.. I have also used a lot of VFD's and love them but price here is the issue..

ECONO-PHASE Shifter - Static Phase Converters - Ronk Electrical This?

Says right on it for light to medium loading: i.e., NOT full power.

 

[Mysfyt]

I got my 75HP (that's not a typo!) VFD for $750 shipped. The smaller ones cost more than the bigger ones. You can set load limits in the controls on the higher end drives.

As most of the guys have said, you want to de-rate the drive by half, but going bigger is perfectly acceptable.

I did the motor swap to single phase on my T30 IR compressor before I realized the VFD's soft starting would actually pay for the conversion in energy not used, and then later yet I've discovered that the reactive current from the VFD's (I run 3 motors on them now) actually confuses my digital electric utility meter into thinking we're putting back more than I'm using. It runs our bill backwards when I'm in the shop a lot; which just means we don't pay as much to keep the wife at 72F in the summer time.

Some VFD's will have a leg-loss fault that you'll need to program to ignore. Every VFD on the market made for 3Ph will run on single off two legs. You can't get a VFD made for higher voltage to run off 220-240 though, they'll error out on low input voltage and it's not user fixable (I've tried). So as long as it's the correct input voltage and 2x the power of your load, you're set. You can get universal transformers to bump 240v up to 480 etc - and then run machines that are wired with controls that would be too much trouble to convert. A VFD will run on the input side of a big transformer.

How about a picture of that VFD?

 

[bigdeano]

Static converters are only good for around 60% of the motor's capacity because you're not energizing 1/3 of the windings. You also end up with less torque than a single phase motor of the same size, and that's way less than the 3ph motor you're using when it's given good stiff power on all three legs.

I'd never run one on a high load application like an air compressor. I can't bring myself to run one on a 2hp mill.

All three legs of my motor are getting power. The third leg is getting power through the run capacitors, which provide the required phase shift. I don't know if commercial static converters have run capacitors or just start capacitors. The motor reaches full speed in less than a second and runs with no audible hum.

 

[bigdeano]

ECONO-PHASE Shifter - Static Phase Converters - Ronk Electrical This?

Says right on it for light to medium loading: i.e., NOT full power.

This is from the website above. Looks like they're rated for 3/4 load continuous with run capacitors.

Because of the run capacitor design in the ECONO-PHASE converter, they will generally operate most motors at full load as needed and provide ¾ load on a continuous basis.

 

[Jim Timber]

All three legs of my motor are getting power. The third leg is getting power through the run capacitors, which provide the required phase shift. I don't know if commercial static converters have run capacitors or just start capacitors. The motor reaches full speed in less than a second and runs with no audible hum.

It's not getting anything, it's providing a slur so the 2 hot legs have somewhere to go to flow power through them: it's a cobble Band-Aid to make a 3ph motor spin with a missing leg and 180 degree phase shift input. You need a start capacitor just to get the phases out of phase enough to buck each other. Without that, you get jack-spit at the armature.

Think of it like this: single phase = you have 2 guys pushing/pulling on a car that's stuck in the mud, but they're each throttling their effort in perfect harmony (180 degrees out of phase from each other). One guy pushes the other guy pulls at exactly opposite potentials. The car doesn't move!

Well, we can't get anything done with that kind of effort, so we're going to trick one of them into pushing and pulling slightly out of sync with the other one (this is what a starting capacitor does) and now all of a sudden we don't have systematic paralysis, and that little difference in forces allows us to move our car enough for the tires to hit traction and now the car gets itself moving. That's how a 1Ph motor works.

With 3ph we have 3 guys, but they're on different corners of the car pushing at 120 degrees to one another. As one pushes his hardest, the guy next to him is starting to throttle up and the other guy is starting to throttle down, but two of them are pushing at the same time all the time. This is why there's no starting or run capacitors on 3ph motors - they don't need them to create a bucking current to start the motor.


That you can delay the wave rise and fall enough to make a 3ph motor spin on 1ph is not an accomplishment, it's a detriment. You will overload the two winding's you're using and the third one has ZERO input. We had 3 legs with equal power inputs, now down to 2, and without that capacitor they're working 100% against each other to keep your motor turning. As you load that motor down, you lose more and more of your available power until it stalls. With a 3ph motor, as you load it down, it draws more and more current fighting to kick the car out of the mud.

 

[Jim Timber]

How about a picture of that VFD?

I bought it to run a 25hp (total load) CNC turning center, then discovered the control didn't like it, then bought a 18KW 3ph generator head and 40hp 3ph motor to make a motor-generator to run off my VFD (total cost of that entire system was about 2K) and ended up not ever hooking any of it up. 24hp on 240v 1ph is 89 amps btw.