Motor soft starters and VFD for off grid

Discussion in 'Off Grid Living' started by oil pan 4, Jan 17, 2017.


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  1. oil pan 4

    oil pan 4 Monkey+++

    Or at the very least I'm reducing power consumption while atempting to inch my way off grid.

    My first small step towards off grid was power factor correcting my air compressors and welding machines. Originally I did this to allow then to work better on long extension cords, be more efficient and make then draw less amps.

    Next step, will be to greatly reduce starting inrush current on air comoressors.

    To do this I am going to try the simplest solution first. And that is a using a closed circuit soft starter made for single phase capacitor start motors. But I have a feeling this may only be marginally effective. The soft starter I found is made by hyper engineering and claims to reduce inrush by at least 60%. I have a feeling if I really want to do away with inrush its going to involve a 3 phase variable frequency drive. Pretty much exactly like what inverter heat pumps and inverter refrigerators use.

    A single phase soft start for capacitor start motors is fairly simple. It just reduces voltage to the motor while it starts.
    But there are a few concerns.
    Single phase capacitor start capacitor run motors use a centrifugal switch to control the start windings. If the voltage is too low or the starting load too heavy and the motor doesn't build up to speed fast enough to disengage the start circuit with in a few seconds it will smoke the start circuit.
    Also the armatures of induction motors are not designed to work in excess of 6Hz, so the more time spent below about 3,300rpms will build extra heat in the rotor.
    This soft starter appears have terminals for the start circuit so I'm guessing it may be on a timer to prevent burning out the start cap.
    I am going to test each compressor on my 5kw, 0 to 300v varrac. Simple test, connect the air compressor to the varrac turn it on and quickly ramp up the power from 0 to 240v within 2 or 3 seconds. Just to figure out which one would works best with automatic reduced voltage starting.

    I built all my 240v air compressors out of junk I had laying around, or crap I bought from the scrap yard the only thing I really had to buy new for all of them was pulleys and the shut off pressure switch.
    The main problem with them is the single phase capacitor start, capacitor run motors have really bad inrush current.
    It also just so happens all my compressors are in the size range for the most common VFD.

    I have found that 240v single phase input, 3 phase 240v out put variable frequency drives have been getting a lot cheaper. Dont worry about getting 3 phase motors. Small single digit horsepower, delta wound, 3 phase motors that can be wired for 240 or 480v are everywhere because most average consumers cant use them and they are super common in industrial environments. I would prefer a 4 wire wye motor but I don't think its going to happen unless I want to cough up a lot of $.
    Reason why is I would like to take steps to move off grid, connecting big power hungry air compressors to an expensive split phase power inverter, just seems like its not a good idea.

    Right now the only way I have to eliminate compressor inrush current is use my 5hp gasoline powered air compressor which can more than match the output of any one of my 240v powered air compressors, but makes a lot of noise and uses some gas. Or use a 12v compressors my Gast air compressor uses a ton of amps and doesn't produce a lot of air and my ARB compressor is just tiny. Other way to protect an expensive inverter from compressor inrush is I could connect the air compressor to a huge generator like my 17.5kw generac or my 7kw troybuilt those do not care too much about high inrush across the line starting but those suck down a lot of gas and make a lot of noise and don't want to run them unless I really need a lot of air.

    I would like to at some point greatly reduce the inrush current on one of my compressors with a VFD. I figure I can have the motor kick on at 1 or 2Hz ramp up to the desired speed, throttle down to 20Hz then use the pressure switch to release pressure in the line and hopefully signal a turn off signal to power down second or 2 later.
    Depending on the features and inputs I may be able to have it run at 70Hz part of the time when its building up from 0 to 70psi, 60hz from say 70 to 90psi then 50hz tapering down to 20Hz from 90psi to shutoff pressure. Or something like that.

    I would need at least a 3 or 4hp, 4 pole motor that could be wired for 240. These are real common. Perfectly good 7.5hp motors are some times scraped where I work because they hire idiots who can't seem to comprehend how trouble shoot motors. One of these TC213 frame motors would work. I think at some point I will get a 3 phase motor and VFD to play with. It's only a matter of time.

    Another option may be just to get a 24 or 48 volt motor to use with a compressor and just run it straight off the batteries. But I have not made it that far yet.
     
    Tevin likes this.
  2. Tevin

    Tevin Monkey+++

    Wow, a lot going on here. But I think I follow.

    First, how much do you run the air compressors? And why do you have so many? By my count, you are running three air compressors? Do they cycle constantly, or just once in a while for home use?

    I'm trying to figure out exactly what problem you are solving. If this is only about energy savings, you seem to be jumping through a lot of hoops just to save a minuscule amount of juice.

    The soft start should work as long as you stay within the operating specifications of the device. I do not think you will burn anything out. I suggest contacting manufacturer tech support and tell them what you are looking to do. Electric motors do tend to behave differently when running air compressors, so that may make a difference. There might be good info on YouTube as well.

    I think a VFD is overkill unless there is some obvious need. Still, if that's how you want to go and are willing to spend the money (which could be substantial), I say just hook the sucker up and see what happens. Again, there should be nothing to fear if you have done your math correctly.

    You're dealing with very mature technology so this should not be that hard.

    Good luck, and thanks for the interesting post.
     
  3. Tempstar

    Tempstar Old and crochety Site Supporter+

    Why not run it like the gasoline model, with the motor running constantly and the unloader controlling the pressure?
     
  4. duane

    duane Monkey+++

    Soft start adds a lot of stuff to the motor and unless the motor was designed to use a soft start, can be a problem. The actual wire resistance of most small capacitor start electrical motors is such that if they do not develop some sort of back EMF to limit the current flow, they will smoke in a few seconds. Reducing the voltage to limit the starting current, will also limit the electrical field and decrease the output power of the motor and slow its starting way down. Tesla figured out that you could not produce major amounts of electrical power with a single phase supply. You can use brushes to make it act like a weird form of DC and develop power and starting power, or you can use tricks in the windings and capacitors and fool the motor into thinking there is more than one phase. A properly designed motor, built in an advanced society, and running under its designed load, power supply, and environmental conditions can not be easily improved on..I have had best luck working on the power source and reducing the starting load and with compressors just letting them run when started and unload the compressor instead.
     
    Last edited: Jan 17, 2017
    techsar likes this.
  5. oil pan 4

    oil pan 4 Monkey+++

    Sanding then sand blasting is the big one. The speed in which I can complete a sand blasting project is limited to the amount of air I can compress. Painting also uses a lot of air too.

    The problem with pressure unloading only is the motor stays running which isn't very efficient. The motor will still use up to 5 amps even unloaded. Then I run into the problem of the air compressor may use as much power unloaded as it does compressing air during times of lower demand. The unloader valves are also about $50.

    I have the gasoline powered air compressor but I try not to run it unless I have to. I used to run it mostly with my plasma cutter remotely.

    Right now for the big jobs I run 2 that are 240v and one 120v compressor. Several years ago when I didn't have any money I put together a few compressors instead of buying a $2,000 compressor with $ I didn't have. Plus those big compressors are difficult to move and I was planning to move at the time.
     
  6. Dunerunner

    Dunerunner Brewery Monkey Moderator

  7. Tempstar

    Tempstar Old and crochety Site Supporter+

    The original post was about reducing start load and inrush. Industrial compressors use unloading to do that...keeping the rotating mass moving to stop the start up inrush current, because most industrial users have to pay electric based on demand. If you are doing sandblasting then the very thing you need is an unloader. Running a VFD is not going to do what you think as the HP curve is also diminished with the freq and voltage. These are made for things like fans and conveyor systems that only need the HP at full speed, however a compressor requires much torque and HP even at low speed, especially if you are trying to keep pressure up. HP goes up as a cube of the pressure, so at 80 psi trying to restart with a VFD will never work.
     
    techsar likes this.
  8. oil pan 4

    oil pan 4 Monkey+++

    I built my 5.5hp gas air compressor out of junk for under $100 the only new parts were air filters and unloader valve. A 30+ year old Briggs and straton engine and a compressor I traded for some work I did. The tank was from the scrap yard had an engine and compressor that were full of water and seized.

    3 phase motor: should be free
    VFD $350 to $450
    Split phase power inverter I'm trying to protect $2,000 to $4,000
     
    Last edited: Jan 18, 2017
    Dunerunner likes this.
  9. oil pan 4

    oil pan 4 Monkey+++

    Where I work we use four 100hp screw type compressors on a VFD. They start up on a VFD but they don't run at less than 50% speed. Also the 700hp ammonia compressors are also on VFDs. Where I work they use around 300,000kwh per day, every day. That's enough power to supply a family of 4 with a 2,500 square foot home power for 8 to 12 years. So they are very conserned with with reducing power consumption.
    Running big compressors on unloaders seems like obsolete thinking.
    I bet they were started across the line or had some sort of simple closed circuit starter.

    You say the horse power isn't enough to run on a VFD thats why I'm replacing motors that run at 3 or 4 horse power with a 7.5hp rated motor.
    Worse case scenario I change the start profile and add an unloader on a timer.
     
  10. arleigh

    arleigh Goophy monkey

    Soft start does not reduce voltage ,reducing voltage will burn out the motor.
    Soft start is in inverter that 60 cycles are reduced to a much lower frequency at "full voltage" then the frequency is stepped up to consecutive cycles till it reaches 60 cycles again .
    I you are using a lot of air and running a business, constant speed control is far more practical ,thus providing a no load free wheel spin allowing the compressor and motor to cool off between loads. in place of stopping and cooking between loads. A hot compressor is less efficient .
     
  11. oil pan 4

    oil pan 4 Monkey+++

    The way the hyper engineering unit is wired in its impossible for it to be a variable frequently drive.
    Voltage reducing soft starting has been around almost as long as motors them selves have been around.
     
  12. arleigh

    arleigh Goophy monkey

    Only DC motors are controlled by voltage.
    AC motors can only be controlled by frequency, not voltage .
     
  13. ghrit

    ghrit Bad company Administrator Founding Member

    Not a positive statement. Newer vfd's vary voltage as well as frequency, very efficient. It is preferable that full wave inverters are used rather than pulse width or other wave form modulated devices, as less energy is wasted in heat.
     
    BTPost likes this.
  14. Tempstar

    Tempstar Old and crochety Site Supporter+

    Right on Ghrit. Most VFDs start with Hz at 10-15 and voltage at 45-55%, and dang near pure sine wave versus the old square wave of not too long ago. The whine is gone too.
     
  15. oil pan 4

    oil pan 4 Monkey+++

    I'm not trying to control the speed of an induction motor by reducing voltage. I went into depth as to why exactly that won't work on any induction motor in the original post. I am only trying to reduce starting amps by reducing volts.

    Voltage can automatically be reduced by modern VFD units to increase the power factor. I'm sure what I will end up getting won't be that advanced.

    The relatively new ABB VFDs they use where I work claim modified sign wave. I have not attached my fluke 123 to one while in operation. But I definitely know I'm not using one of those on my project because the small ones are around $1,700.
     
    Last edited: Jan 20, 2017
  16. oil pan 4

    oil pan 4 Monkey+++

    I was able test soft starting one air compressor with my 0 to 300v 5kw varrac.
    To get my auto transformer to do this test I needed to add a 30amp 2 pole switch.
    So I tested starting from 0 in the tank so the tests would be even. And gave the motor 10 minutes in between each start test.
    First test it was just normal starting, peaking at 45 amps for 3 starts. That will be the number to beat.
    The good news is pretty much every soft start recipe I tried reduced starting amps some.
    First one was wipping the varrac from 0 to 245v in well under a second. Seems the motor and transformer didn't like this, but it reduced peak amps to 40 on both tests.
    Then I tried flipping the switch on at 200v and quickly turning it up to 245v in about 0.1 to 0.2 seconds, still about 38 to 40 amps. Last I tried starting at 220v and turning the auto transformer up to 245v, that reduced the starting amps to 35 amps. Tried that 3 times to make sure. Then i got tired.
    But from 45 to 35 amps is only a reduction of 22%. No where near the 60% to 70% claimed by the motor starter tech info, but hand turning a auto transformer by hand isn't really a motor starter.
    So I will only expect the starter to reduce starting amps by 1/4. If it can do that automatically, every single time it could be good.
    Running amps at more or less no load is 16 amps. So I'm thinking start current being 3x no load current sounds a little low, but this compressor is on a 30 amp breaker but it's never tripped the breaker even hot starting. If it drew more than 60 amps for a second or 2 the breaker would trip. Also I have power factor corrected and swapped out the run capacitor for different ones until I got the lowest running amps. I was told not to do this because I would ruin the motor, but it draws less amps with a slightly bigger run cap and that was 3 years ago. So?
     
  17. oil pan 4

    oil pan 4 Monkey+++

    I was looking at exactly how I was going to wire the starter to the motor before I buy it and figured out that it won't work, different applications all together.
    Good thing I figured it out before I made the purchase.

    But still going try to reduce starting amps on single phase.
    I think there are 2 ways to do it.
    1 the easy way that might not work so well, connect say a 0.3 to 1 ohm resistor to each line using a normally open contact relay on a time delay. The motor will start motor plus resistor for a half second to a second, then the relay will bypass the resistor for normal operation. Aka the closed circuit starter. This might reduce inrush by up to 22%.
    1 the hard way, start the motor completely unloaded, as in completely uncoupled from the belt. Then engage the belt once the motor is up to speed.
    How much will this reduce inrush? I don't know yet, need to try starting an unbelted motor. My assumption is starting the motor unencumbered will greatly reduce inrush during soft starting. Then I think engaging the belt on a running motor won't draw nearly as many amps starting both the motor and compressor from a dead stop.
     
  18. Tempstar

    Tempstar Old and crochety Site Supporter+

    How about a 3 way solenoid valve on the compressor discharge with a 2 second time delay?
    I can fully appreciate what you're trying to do, but physics are fighting you. Adding resistance will reduce the inrush a bit but also lengthen the time to get up to speed. The old "energy in equals energy out minus losses" calculation. This is why industrial piston compressors use unloaders and keep the rotating mass moving. Even then, there are amperage spikes when the machine loads, but of very short duration as the mass is already in motion. Engaging a belt will still add the spike of energy required to start the mass of the compressor moving and against pressure. Inrush will be reduced by starting a motor with no load, because the inrush is the amperage consumed mainly by the start windings to get the motor to speed. I would investigate both unloaders (for restarts while in use) and a KVAR to reduce the reactive power and slide the amps down the power curve a bit.
    The math says that reducing the volts will increase the amps. AxVxPFxEFF/746 = BHP (single phase) and AxVxPFxEFFx1.73/746 = BHP (3 phase).
    Not beating you down at all, but trying to help you cover some ground I've already been over. I've been fortunate enough to have an oscilliscope and watch this all happen. The energy to start a given mass moving remains the same, you can only alter the time it takes to do it. Hope this helps a bit.
     
  19. arleigh

    arleigh Goophy monkey

    One thing you might do is alter the unloader system slightly on the compressor,
    wire in a pressure release solenoid on the unloader line to the capacitor , there by unloading continues till the starter is disengaged.
    I was going to design this while in the family business but I'm retired got other things to do.
    Some air compressors have a built in governor that either uses oil pressure to unload the system,or governor weights that either operate unloaders that force open the intake valves or simply a shredder valve that bleeds the air off when the RPM of the pump is dropped significantly.
    The other option is to run the compressor on a constant speed control with a delay for how ever long you determine before complete shut down.
     
  20. arleigh

    arleigh Goophy monkey

    What I have done for my self at times is to have 2 compressor units . the larger was for major work ad the lighter was for lite work as soon as the pressure dropped at say 120, the small one came on ,but if the pressure continued to drop the larger started and made up the difference .
    Many smart companies run several compressors for more than this reason .
    Oft times if something happens to the primary unit, the company is dependent on the secondary unit.
    If they don't have one, they are loosing money, with men waiting for the rental unit air to come back up so production can continue.
     
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