Motor soft starters and VFD for off grid

A system I had been toying with was to set up a solenoid on the compressor discharge manifold, and tie it to the capacitor start circuit, so that while the motor is starting the compressor would be spinning unloaded till it reached the full run speed of the motor till the start circuit disengaged.

 

Yes that is exactly one of the things I was going to do.
Use line to neutral 120v signal off the centrifugal switch to energize a time delay relay and a normal relay, this time delay will give the motor 1 more second to come up to full speed before the motor receives full power and also starts the liner actuator to tighten the belt.
The liner actuator takes about 4 seconds to fully extend so it doesn't need to be on a time delay.
A 120v normally closed solenoid on the centrifugal switch circuit could definitely reduce some of that starting torque on an otherwise normal compressor.
I was looking through my solenoids and they are all 12v. So I will have to order a 120v one.

 

I had my 20+ year old speedy air compressor tank hydrostatic tested. It developed a pin hole leak around the time the compressor went bad. I drilled into the pin hole to make sure the metal wasn't rotted away and paper thin. It was not so I welded it up and had it tested to 250psi for 4 minutes and wached with a plastic mallet. It's only goes up to 135psi at shutoff.
This is the one with the bad compressor that I was just soft start testing the motor. Since I found the best way to soft start this motor this is where I will start.

 

More things I was told not to do:
I wanted to test the hypothesis that the start capacitor that comes with the motor, you know the one that is designed to start at full power, give full 2 to 3 times max rated running torque may not be the best start capacitor for starting the motor while unloaded and at reduced voltage.
I compared the 594uf start capacitor to larger and smaller values by parallel different capacitors together. First off, don't go bigger, the motor was very unhappy when I slightly increased the start capacitor value. Since I have had good results replacing the run capacitor on other motors to get lower running amps I was expecting to go smaller to see good results on the starting capacitor also.
I replaced the factory 594uf start capacitor with a 340-408uf start capacitor. The motor drew the same amps and came up to speed just as fast as the 594uf capacitor. So I figured I would try to split the difference between the 340-408uf cap and the 594uf capacitor by adding a 80uf motor run capacitor in parallel with the 340-408 capacitor.
With the 340-408 +80uf start capacitor the motor came up to speed noticeably faster than it did with the lone 594 or 340-408 capacitor. Coming up to speed faster, while drawing the same amps means it's more efficient so that's what I'm going with.
So I ordered the closest size I could find to what I tested, I picked up an $11 temco brand 400-480uf motor start capacitor.
Now the motor is firing up drawing a consistent 16.5 amps every time with ceramic wound resistors and slightly smaller start capacitor.
It was usually drawing around 16 amps but could as little as 15 amps some times as much as 18 amps other times.

 

I tested engagement of the belt between the motor running at full speed and the still unloaded compressor on my lag compressor.
This is the one that has a 2.5hp motor with the 45 amp inrush, runs with compressor unloaded at 11 amps and cuts off at between 15 and 16 amps.
I was only able to get up to 20 amps by jerking the belt to tension. Engaging the belt at the rate I expect the slow linear actuator to move at it peaked between 15 and 18 amps.
It does smoke the belt a little but that's to be expected.
So belt engagement surge should be about equal to or greater than shut off amps plus up to an additional 20% to 33%.

So on my smaller 1.5hp compressor that had a 30 to 35 amp start surge, runs unloaded at 7 amps and has an 11 amp shutoff current. It will have a 16.5 amp start up peak, see 11 to 14 amps of belt engagement surge.
That's a pretty good improvement from a 30 to 35 amp instantaneous and violent starting surge that is probably peaking a lot higher than 35 amps but I can't measure it.

 

For all this your going through ,it might be advisable to go to an engine driven air compressor in stead.
It is never advisable to run an air compressor on a generator , however engine driven air compressors are common and far less trouble than what your going through .

 

I can see why it's not advisable to run a compressor on a generator.
For the usual application these capacitor start motors need almost unlimited 60hz inertia from say the power grid or a way over powered generator to start correctly.
I'm looking to make my smaller 1.5hp compressors more generator and off grid friendly.
I already have a gasoline powered one with 30 or 40 year old 5.5hp Briggs and Stratton.
It's getting updated too. I have a Honda 6.5hp OHV motor and a go cart clutch to make cold starting easier to put on it.

 

Finally got around to testing something I have been thinking about for a while.
4 pole motors. Why? Because they have double the torque and come up to speed in half the time and your angular velocity is half that of a 2 pole.
Pretty much have to accept that full power no load starting a 2 pole motor has a momentary power draw of around 200% full load amps. Resistive starting lowers that peak to about 150% of FLA.
The 4 pole motors use substantially less when no load full power starting, they have a momentary peak of 130% to 150% of FLA. Resistive starting lowers a 4 pole motors starting amps to just about equal to its full load amps.
I expected 4 pole to have slightly lower starting amps, not this much.

It would seem the the motor choice for off grid is 4 pole, unless you have to have 3,600rpm speed for say a direct drive pump, squirrel cage fan or rotory converter.

 

I found 2 motors at work I'm trying to get.
A Baldor 3 phase 240/480v motor, 4 pole, 7.5hp, 213TC frame sever duty, inverter rated industrial motor. Pretty bad ass.
And a no thrills 1.5hp 3 phase 240/480v 4 pole lesson, think it's a 56 frame
They appear to have good bearings and check out electrically with a multimeter.
I figure I'll try a haas-kamp conversion to make sure the motor works.

 

I got the 1.5hp 4 pole motor from work, it's a 56C frame which means it's a gear box motor, it has a C face but no mounting base. Not really a problem.
It's getting an ABB single phase input VFD.
It's going to replace the 1.5hp 2 pole speedy air compressor motor. That's the one with the bad compressor.
The replacement compressor finely came in after the place waited 6 days to ship, now it's got a "3hp" 11cfm at 90psi unit.
I'll just adjust pulleys and the VFD to get it running up to FLA.
Starting inrush on this one: none.

The final solution for my lag compressor, you know the one that has a 2.5hp 2 pole single phase motor that draws a momentary inrush of 45amps and can't be started by my 7kw 13hp 420cc generator.
It's getting a 4 pole 2 horse motor with resistive starting and magnetic clutch. I would like to reduce its peak inrush down to about 20 amps or less when the clutch engages the compressor. Then this lag compressor is going to become a lead compressor since it will draw the most power starting and running.
Thats just basic rock quarry generator logic: start the biggest machine first and the smallest one last.

 

I ordered a used 1.5hp ABB drive off ebay, it should be here next week.
You could get a new cheap knock off drive a lot cheaper than a used ABB but that's what we use where I work and I wanted to use this opportunity to learn a lot more about ABB drives. A new ABB drive like this is up around $500.
A cheap knock off 1.5hp drive can be had for around $150 or less.
I for sure decided to go with ABB after the substation E2 EMP we had took out something like $10,000 worth of PLCs where I work, but it only fried a single 50hp ABB drive out of the roughly 700 of them we use (we have all shapes and sizes of ABB drives up to 700hp)

 

This bit got a chuckle...although they are decent engines, how many running 30 - 40 year old Honda engines have you seen?

I enjoy seeing the different approaches you are considering. Interesting, to say the least.

 

Techstar you might like to re read your post .

 

Nope...it is correct. I have seen many B&S engines run for many decades with minimal repairs, quite different from Honda. Go cart clutches are common for B&S, so....?

 

I'll figure out what the most effective ways are to do this then it will be a lot easier for someone to adapt it to their application.
And not just for air compressors, could be adapted to any application, such as pumps, fans, sawmills, grain elevator, grain milling, hydraulic power units, rotory converters where someone is stuck with trying to run a single phase motor off grid and the obvious solution of a VFD and 3 phase motor isn't really an option.

The old Briggs and Stratton engines are pretty solid. The only thing wrong with mine is the gas tank filler neck has a dent and leaks a little bit while running. Obviously operator error. Just need to replace the gas tank some day.

 

Alright my ABB drive came in.
First problem I had is it wouldn't turn on. Turns out it was missing the LAN cable stub that connects the local control panel to the drive. So I replaced it with a regular LAN cable.
The drive was setup to use devicenet so I had to set it up for local control.
I have the the motor and drive working in manual mode.

It looks like the drive has digital inputs that could be used to control speed settings by pressure switches.
It has on/off, constant speed 1, constant speed 2, acceleration and deceleration selection. I can work with that.

 

I think I may have found the cheapest, easy-to-use, most effective, monkey proof 1 step method to reduce inrush current. And it was sitting right in front of me the whole time.
I put the gocart clutch from my gasoline compressor on my 2.5hp 2pole electric compressor. I knew it was going to need a smaller belt. The cart clutch is a 3.2 inch pulley and the solid pulley I had on there was a 4 inch. I had a slightly smaller belt, but it was too small.
The original belt was way too big with the smaller pulley and I didn't have enough adjustment built in to my design to take out the slack, the next size smaller belt was just too small with the motor adjusted all the way in. That was last week and I finely got around to taking both belts to the store and selecting a belt size between the 2.

The results are that the motor which had a start up inrush of 45 amps is now down to 23 amps with the smaller clutch pulley.
Now if I welded on a bigger pulley added mass and lowered the mechanical advantage the motor has on the compressor even further with a bigger pulley it might peak at 25 amps.
Still down 20 amps from the original inrush is amazing.
I may even be able to use resistor start to further lower inrush if needed.
But I'm replacing this 2.5hp 2pole motor with a 2hp 4pole and electromagnetic clutch. They're already on the way.
I should say the magnetic clutch is already here, just waiting on the motor.
The end?
No not even close I still have a 3 phase 4pole motor on an ABB drive to get going, put together the 4pole motor on an electromagnetic clutch unit and try to put a go cart clutch on a 4 pole motor which means I will have to monkey with the springs on the centrifugal weights and weld on a 6 inch pulley.
Going from 45 amps down to 22, and maybe back up to a hypothetical 25 amps with the desired pulley size is good, but not good enough.

 

Where is all this energy coming from ?

 

Usually the power grid or generator when remote.

 

For the energy compressing air it is too much to ask of a generator . the end cost is high.
In the long run a Gas engine powered air compressor is the best use of equipment especially for your needs.
The standard rig has a pilot control that either; vents the excess to the atmosphere,, or there are unloaders on the intake valves of the compressor that prevent it from compressing air during that cycle.

If you use a clutch and throttle down, the work it will take to get back up to speed is too much for the clutch ,(something else to were out) you will still have to unload the compressor head to get started any way.

Besides the free wheeling on the air compressor unloaded, has a cooling effect that contributes to it's over all performance.
Remember that there are no oil filters in these smaller compressors and engines , and oil level is critical.
They do not use the same oils either.
The engine can tollerate detergent oils, however air compressors need non detergent 20wt oils ,as detergent oils ( in a splash lubricated atmosphere) tend to foam, and in an air compressor the breathers tend to expel the foamy oil ,or those with a tube to the intake draw the foamy oil right out of them .
It is recommended oils are replaced every 100 hours ,your millage may vary .
Air filters and location are also important . Higher altitudes need a larger filter .
Air being drawn in ends in the tank , and humid atmospheres collect a lot of water and will rust out tanks.
As a rule I have a hose attached to the bottom of my tanks I use for blowing the dust off things , but first I direct the air where it don't matter to drain all the water out first. kills three birds with one stone.