air compressor magnetic starter/relay single phase

[hr3]

The Ronk site says 97% efficient .. All I know is the last set I put in "20 years ago" at the airport for there fuel tank field are still working and no motors have burned up yet.. I will agree a VFD is the way to go but the price for this application doesn't apply..

 

[bigdeano]

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.

Well I was bored, so I went out and measured the current on the capacitor leg. It's 22 amps no load, 20 amps full load. So obviously it's doing something. At no load the 240v draw is 11 amps. I used to be an electronics technician so you don't need to talk about guys pushing on a car. If I cared about it I'd put the scope on it and measure the phase shift, but it works, so that's good enough.

 

[Jim Timber]

Well I was bored, so I went out and measured the current on the capacitor leg. It's 22 amps no load, 20 amps full load. So obviously it's doing something. At no load the 240v draw is 11 amps. I used to be an electronics technician so you don't need to talk about guys pushing on a car. If I cared about it I'd put the scope on it and measure the phase shift, but it works, so that's good enough.

View attachment 453056

Put it on a dyno and tell me how much force it's capable of putting to the armature. That's information I care about. That it's carrying current is not important, it's how much force it makes at the output of the motor that is. I don't care what Ronk says their caps do energy wise. That they turn 3% into heat is pretty funny for a device that performs no work.

Sure, they make a 3ph motor spin, that's not in question. The only thing I dispute is how much effort that motor is capable of exerting on the load. Load it down and watch your current spike = not efficient at all.

 

[hr3]

Gee I guess I should call the airport and tell them to stop using them on there pumps. And maybe go on social media and tell people that Ronk and others that make like products are junk and don't work.. " I think not"

Disney your best bet is to change out the motor to 1 phase.. I knew this was going to go this way.. lol

 

[bigdeano]

The motor accelerates itself and the 4800 rpm alternator to full speed in about 1 second. Plenty of torque there. Sorry, I don't have a dyno.

 

[Jim Timber]

They work, at the cost of pulling a 20A draw with no load! (what you just posted)

As I said in my first post in this thread - the VFD will pay for itself in energy saved. My electric bill went down when I sold my 10hp RPC.

 

[Mysfyt]

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.

Yup. Your VFD is bigger than my VFD! But the message we're both trying to get across is don't be intimidated by 3ph motors. The first two pictures are my 40hp VFD running my 20hp motor for my lathe. The third and fourth is the 15hp VFD for my compressor which I installed a 7.5hp motor and larger pulley to get some more cfm. Don't worry, I consulted the manual before the upgrade to make sure the rpm was OK.

 

[Jim Timber]

I only needed a 50hp for my original intention, but 40-50hp vfd's are in greater demand than the larger ones, and no plant manager with downtime as a possible outcome of a used purchase would ever buy one of these things if they needed to keep a 75hp motor on-line. So they're cheaper the bigger you go. The drive is only going to use the current it needs to keep the load at the designated RPM, so there's no down-side other than physical size to worry about.

I got my 40hp electric motor as NOS for $350. Someone sized it too small so it was no longer returnable. I only needed about 26hp, but 40 was cheaper than 30. Running a 24hp load on a 40hp motor doesn't even begin to work it, so you get better efficiency there too.

It's not voodoo. VFD's are the way to go on small machinery. I have 3 of them directly running my Mill, lathe, and surface grinder. A surplus vfd will cost less than a new 1ph motor, and will result in lower electric bills in the long term. One is cheap now, the other is free down the road.

 

[LD1]

. One is cheap now, the other is free down the road.

Good saying

 

[bigdeano]

They work, at the cost of pulling a 20A draw with no load! (what you just posted)

As I said in my first post in this thread - the VFD will pay for itself in energy saved. My electric bill went down when I sold my 10hp RPC.

You should read more carefully. The current on the 240v line is 11 amps. Unloaded induction motors have a low power factor. Most of the amperage draw is not turned into heat nor does it turn the electric meter, so you don't pay for it.

A RPC is an extra motor running, so of course the bill goes down when it's eliminated.

I'd rather have a few capacitors to run my air compressor rather than complex electronics. You do as you want.