Motor soft starters and VFD for off grid

I have already kind of done something like the unloader. You know how most air compressors have a very short tube between the compressor head and the check valve at the tank? Well I made this tube about 3 feet long on this compressor, to allow the air to cool some be for entering the tank.
It may work at delaying pressure build up, I can to see if my cool down tube has enough internal volume by just starting with the line hooked up like normal compared to starting with the pipe disconnected.

I also have the compressors staged. The one I am messing with at the moment is the lag compressor. It kicks on at 75psi and shuts off at about 95 psi. That's because the compressor is so over driven if it built up to normal 120psi it would burn up the compressor or motor.

 

How often does it start ?

 

There is a component you can install easily on that compressor that is called a Gene unloader.
When it get's to pressure it checks the air in the tank, and vents the air coming from the pump to the atmosphere in stead, there by relieving the load on the motor and letting things cool down, free wheeling so to speak motor continues running but with no real load has a chance to cool down as well between cycles.
What horse power motor are you working with ?

 

On the lag compressor it's got a 2.5hp motor that runs at about 5hp. It's pretty much hitting it's full load amp rating at shut off.
Depending on what I'm doing it can run almost continously or cycle on and off a few times an hour.
The other 240v compressor has a 1.5hp motor that runs at about 3 to 3.5hp at shut down. Runs right up to FLA.
I am going to convert my 120v compressor to a 4 pole 240v motor some time. It's got a 1.5hp motor and runs at about 1.5hp. Turning the compressor about 900 to 1000 rpm It's a lead compressor and shuts off at about 135psi, it's made to run slow and steady all day long if need be.

Unloading the motor and compressor for a short time before shut down is not a bad idea considering the punishment the lag compressors are put through.

 

Alright I did some more tests this time on my 1.5hp 240v compressor that has a bad compressor head. So it's just a 2 pole 1.5hp motor on sitting on an air tank with no belt.

When I across the line start this motor with nothing on it, no belt, just free spinning it draws 20 amps.
If I reduce voltage down to 150vac and free spin start, it peaks at 10 amps.
For comparison when this compressor was fully operational it would draw about 30 amps on start up and full load amps at shut off was around 10 amps.

If I can get starting amps to roughly equal shutoff amps on a single phase motor, I might be on to something, if, IF and thats a big if, I can automate the starting and shut down sequence.

I can kind of see why VFDs are so widely favored. This is basically a pre- digital/solid-state version soft starter like they may have used in the 1950s or 60s.

 

I must first state that I only understand about a third of what is being discussed here.

I had problems running a compressor on an extension cord too, so I understand that much. I was running a home made compressor with a HF pump and a 3hp electric motor. It was fine on an initial start with no load (air built up) but when it kicked back on it would be really slow to start, and eventually burned up the motor. I switched to a 6.5 hp motor, with a slightly smaller pulley, and have been running it for 2.5 years now with no problems. Gearing it down did not really make it take that much longer to build up air, and seemed to be the cheapest solution. The oversized motor also helped a lot before I changed the pulley.

That is the extension cord I am using.
100 ft. 10/3 SJTW Extension Cord with Lighted Plug-757-103100RL6A - The Home Depot
This is the pump
3 HP 145 PSI Cast Iron Twin Cylinder Air Compressor Pump

 

Did you have the unloader hooked up?

 

If by unloader, you mean the little pipe that releases pressure from the head at the pressure switch, then yes. I do not have an unloader like a gas compressor has that lets the motor or engine disengage and stay turning.

 

I hope to heaven you boys have safety valves on your equipment,
I hate to loose friends that way.
Extension cord kill motors . it starves the power, and causes over load. Run a longer hose instead.
If necessary add another air tank air can bank in closer to where you are working . It also gives a chance for the water to separate before your tool absorbs it all. put you air filter on the out put of that secondary tank and your work hose from there, and you will be surprised at how much water it collects.
By the way Husky makes a quick coupler that provides more volume than the old Cambel Hosfield couplers . makes a big difference in tool performance .
Every place you choke the power, or the air,, sacrifices performance.

 

Everything compressor I have has 145psi safety valves that are less than 3 or 4 years old.
I wanted to do like the military does with most of their air compressors and do a safety valve and burst disc setup but I could never find an appropriate burst disc setup.
Since my compressors are getting old I'm going to strip them down and take the tank to get hydrostatic tested, starting with the oldest one which also has a bad compressor unit.

I power factor corrected my air compressors which saves between 2 and 5 amps, all my 240v compressors use nema 10-30 dryer cords which are 10 gauge. My 240v extension cords go up to 6 gauge.
I use mostly 3/8 air line and a lot of hoses have air quick disconnects form Japan, they are almost the size of 3/8 heavy duty couplers.

 

Since I know I can free spin start a single phase motor with as little as 10 amps on my varrac I just need to figure out what the resistance is and use resistors because I am not dedicating my varrac to be a motor starter I'm thinking I should use wire wound resistors, hot water heater elements or clothes dryer heating elements.
I figure when across the line free spin starting starting the motor where it draws about 20 amps it has about 12 ohms of resistance. Free spin soft starting where the motor only draws about 10 amps it's more like having 25 ohms of overall resistance. Just need the resistors to drop the volts from 250 down to about 150, adding about a 12 ohm resistor should do the trick.

I have some wire wound resistors and hot water heater elements. A trip to the scrap yard should net me a dryer heating element or 2.
Dryer heating elements should have a resistance of around 10 ohms and so should hot water heating elements. My wound resistors I have would only add about 3 or 4 ohms if I series them up.

 

While the power used by the motor may be diminished, the resistor is going to dissipate the remainder as heat. Looking at your OP, you won't be reducing the actual start up power demand, just moving some of it to the resistive load. Not so sure this was what your original intent was or not... ?

 

I got a heating element out of a Kenmore clone. Has a resistance of almost 10 ohms.
I like the dryer heating element idea the best since it's a heating element for air and I should not have a problem over heating it by another start in rapid succession.

I tried to post that last one hours ago but it didn't send.

I built a 2 switch rig to simulate the soft start full voltage then full voltage.
Free spin resistance start draws marginally less amps than across the line starting. But it ramps up slowly.
Across the line starting is just bang, shots up to 20 to 22 amps and back to 3 amps since it's free spinning.
The resistance starting it goes 4, 6, 10, 14 and 16 amps then back down. I think that's the whole point of resistance starting, slow run up in amps to a slightly lower peak.
Free spin resistance start could work since it's a slow run up to less than what the running amps are.
The varrac starting was as low as 10 amps. The thing with the varrac is its about 90% efficient. The resistor is 0% efficient.
I can try inductively limited starting. The cheapest, easiest to get source of inductors I can think of is microwave oven transformers. Good thing I keep like a dozen or so on hand.

 

I pretty much have it narrowed down to a closed transition starter, either resistance or reactance. I have tested reactance starters made from microwave oven transformers and they worked pretty good.

I have written off open transition starters such as auto transformer or wye delta starters (only used on 3 phase motors) because of high transient power surges produced when the power is disconnected from starting power and reconnected to full line power. Power companies do not like when large industrial users have these starters because of the power surges produced when the transition is switched. I have a feeling it's bad for power inverters too.
When a closed transition starter switches a contactor just bypasses the resistor or reactor. There is no momentarily disconnection and reconnection like with open transition starters.
Closed transition reactance starters may produce a tiny power surge when the switch is made as peak amps are going through the coil but that can easily be fixed. By simply adding a light bulb or transient voltage surge suppression metal oxide varresistor. I like the latter option because it sounds cooler.

 

I think I'm liking the dryer heating element resistance starter more.
The amps ramp up slowly and no funky transients to worry about.

I'm testing it off grid now on my 7kw generator.
Across the line starting the unloaded motor makes the generator surge and sputter a little. I have not tried to quantify the voltage fluctuation yet.
Normal starting, full power with the belt on hits the 7kw pretty hard it really doesn't like that 45+ amp surge. Any advanced electronic stuff being ran on the generator wouldn't like that.

 

Well the 7kw genrrator could not normally start the larger lag compressor. The generator slowed down to 45hz or 2,700rpm and could not bring the compressor up to speed, which tripped the thermal overload.
Motor starting for a generator sucks.
My 17.5kw didn't even notice the lag compressor when it started. But it's got a twin cylinder 1L 30hp engine, grossly over powered for most applications.
The exhaust on the 7kw is shaking it's self apart so I may remake the exhaust system with a 3/4 inch inside diameter, 30 inch long primary pipe and better intake.

 

Something occurred to me the other day. When motors run unloaded or under loaded they usually run a very low power factor.
So I checked the power factor on my unloaded motor when I ran it up. It was only running at a 0.42 power factor. If I add in a starting power factor correction capacitor I may be able to reduce starting amp peak by 2 to 3 amps.
My 7kw generator seemed like it had a pretty tough time starting the motor even unloaded I'm sure this is why.
Only problem is I have never seen nor heard of resistance and capacitor starting. There may be a very good reason for not doing this.

 

My single phase motors have functioning start capacitor circuits on a centrifugal switch and a run capacitor. Plus an additional power factor correction capacitor that runs in parallel to the motor that I added a few years ago.

I am going to test adding an additional power factor correction capacitor in parallel to the motor, only for starting the unloaded motor.
It looks like starting the single phase motor while unloaded and at reduced voltage is exactly where I'm going with this since that is the only way to reduce starting amps on a single phase motor.
Starting the compressor and single phase motor together I was only able to reduce start amps slightly with an auto transformer.
The only way to really soft start a motor with the load attached is with 3 phase and VFD.

I thought the same thing when my generator couldn't start the 2.5hp motor connected to the compressor so I hooked up my 40 amp miller spectrum 625 plasma cutter to it and turned it most of the way up.
The generator still runs great it powers that plasma cutter which draws 25 to 27 amps of 240v power turned up to about 90%. My inverter based miller plasma cutter runs a very high power factor and it ramps up the power.
I have always heard that motor starting is very hard on generators.

The intent of this post and the very definition of a "soft starter" is to reduce the starting inrush and reduce mechanical stress by way of adding some electrical and/or mechanical devices to the motor power supply or the device power train.

So far on my 1.5hp compressor that had an inrush staring surge of 30 to 35 amps, so far had been reduced to a peak of 16 amps by unloading the motor and starting it on a resistor. It's shut off amps were around 11 amps, which is equal to the rated full load amps. This is the motor I have tried all the unloaded start testing.

The 2.5hp lag compressor that has a 45 amp starting inrush I have only reduced starting current to 35 amps with the auto transformer. This is the compresor that failed to start on the 7kw generator.
I have not tried further soft starting this one.

 

During lunch breakyesterday I designed the electrical schematic for a regular soft start version plus a soft start with soft shutdown version. So far I have electrical soft start, auto belt tension and slacking, auto power disconnect after a power failure or undervolt and unloader solenoid.
I could just control it all with a Programmable Logic Controller but I'm going purely electromechanical, simple relays, time delays and pressure switches.
It's more EMP proof this way.

To tension and slack the belt for motor free spin starting and unloading I have a 2 wire windynation liner 12v actuator left over from my abandoned wind power experiments.