Eliminating start up inrush current
 

I may try closed circuit soft starts but the way capacitor start motors work this may only be marginally effective. I have a feeling if I really want to do this its going to involve a 3 phase VFD.

I have found that 240v single phase input, 3 phase 240v out put variable frequency drives have been getting a lot cheaper.
I built all my 240v air compressors out of junk I had laying around, the only thing I really had to buy 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 VFDs.
Don't 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 people 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, seems like 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 power 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 greatly reduce the inrush current on one of my compressors with a VFD. I figure I can have the motor kick on at 20 or 25Hz 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.

I would need a 3 or 4hp, 4 pole motor that could be wired for 240. These are real common.

Some terms to clear up.
Unloaded motor = no belt, motor not driving anything.
Unloaded compressor = starting with 0psi in the tank or using a dump valve to release pressure while the compressor is turning.
Soft start = any mechanical or electrical means to reduce starting inrush current and/or mechanical stress at startup.

Alright I think I found a way to do single phase soft start.
Hyper Engineering | Single Phase

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 and it will smoke.
Also the armatures of induction motors are not designed to work in excess of 6Hz 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's on a timer to prevent burning out the start cap.

I am still going to look for a cheap free used 2 or 4 pole 230v 3 phase T or TC 213 frame motor. Some times they scrap perfectly good motors because people don't know how to trouble shoot motors where I work.

With single phase I think I will only be able to greatly reduce start up inrush, cut it just about in half. Hyper engineering claims up to a 70% reduction in inrush and guarantees a 60% reduction.
I think it will take a VFD and 3 phase to mostly eliminate it.

Or just build an air compressor designed to run on DC to begin with? Forklift motors were once a popular option for 48V motors, but now there are also motors for hybrid bicycles if you prefer something brushless.

I can use any motor that I can get a pulley onto. Which means it needs to have a keyed straight shaft or a pulley already on there.
I will keep that in mind for later on for running straight off DC power. That isn't a bad idea at all. I just haven't made it that far yet.

I tested starting my air compressor with my 5kw varrac. I was only able to reduce starting amps from 45 amps across the line starting down to 35 amps by switching the varrac on at 220v and very quicky turning it up to normal 245 volts.
Effectively soft starting a capacitor start, capacitor run induction motor is proving difficult.
Some reduction is possible but a dramatic reduction looks to be elusive.

I was looking at the hyper engineering single phase soft start figuring out exactly which wire was getting connected where. Then realized there isn't a capacitor start circuit on the soft start, since it's for A\C compressors.
Oops. At least I didn't buy it.

I'm rebuilding my 20 some year old speedy air compressor as an off grid soft start unit. The compressor went bad and it developed a pin hole leak on one of the welds. So I welded over the pin hole and took it to the local fire fighting supply store that does hydrostatic testing. They filled the tank with water, pressurized the tank to 250psi (the working pressure is 135max), held it at 250psi for 4 minutes and smacked it with a plastic mallet. So now that I know the tank is safe I'm building it back up.
I have found the only way to soft start a capacitor start motor is to uncouple the motor from the load, start the motor on a resistor, bring it up to full speed and full voltage, engage the belt with the compressor unloaded, close the unloader solenoid and build up air preasure.

This shouldn't be too hard to do.
Using the existing pressure switch as the on/off control signal, linear actuator to slack and tension the belt, using the motors centrifugal switch, a magnetic motor control to control power, 2 or 3 time delays depending on how I want to do the soft shutdown.

You might try an inductor. I've used them on welders to keep the inrush of current down.

You could make one from an old current transformer. The ferrite core torrids seems to work best.
I use the same awg wire as the extension cord I'm using to wind the toroid.

My 1.5 hp electric mortar mixer needs one now. It will start for a few seconds, then stop. It's powered from a 4 kw inverter. I'm thinking about 3 milli Henrys should do it. In line capacitors can interfere with starting and also make the motor run hot. I'll let you know how well it works.

I have tried inductors. The inductors that I could get ahold of didn't preform as well as the resistors I could get.
I used micro wave over transformers that had their secondary windings removed.
The resistor allowed the current to slowly build up amps, the inductors I tried still allowed inrush and a large spike in starting amps.
If the inductor had too much reactance it would start the motor slower but the amps would surge as high if not higher than just putting full power to the motor. If I used less reactance the motor would start faster, not draw as many amps but would still surge pretty hard.

I may parallel a capacitor in with the motor during startup because I have observed lower power factor during starting and running unloaded. It may save 2 to 3 amps during starting.

My 5ohm 200w resistors came in. So I put them on on my manual soft start experiment board.
I suspected that running the starter with power factor correction capacitors removed would make more heat in the resistors, which it did. Massively more heat. So much heat the resistors had to have at least 2 or 3 minutes of cool down time minimum between starts.
With power factor correction I could start the motor 2 times back to back and it still wouldn't get them as hot as one uncorrected start.

I was also hypothesizing that a slightly different size motor start capacitor may work better starting an unloaded motor on reduced voltage, since capacitor start motors are designed to give maximum starting torque at full voltage.
It has a 600uf capacitor from the factory. I found that with a 420-488uf capacitor (actually a 340-408uf start cap plus an 80uf motor run cap) it did better. So I ordered the closest thing to that size I could find, a single standard size 400-480uf cap.
On reduced voltage starting the motor draws 1 to 2 less amps and comes up to speed faster with the smaller capacitor.

This motor would draw 35 to 40 amps originally, starting with full power and the belt driving the compressor.
Just unloading the motor, removing the belt, starting the motor at full power, it would violently jump to 22 amps.
Unloading the motor, full power starting with the smaller start cap and power factor correction it draws 18 to 19 amps and still dims the lights in the house.
So I believe full power starting draws a lot more amps than what's showing up on the amp meter.
With resistive starting, a smaller start cap and powerfactor correction it peaks at16 amps. It quickly climbs to 16 amps, with no violent inrush. I can see it build to a peak of 16 amps from lower numbers on my amp meter. And most importantly no lights dim upon starting so there is no violent current inrush.

So I have at least cut starting amps in half.

The best way would be to use a three phase motor with a soft start vfd which can be had for around $100 these days depending on size. Then you should also use a centrifugal clutch on the motor or an electromagnetic clutch. That way you can not only soft start the motor but get it up to speed before the load is introduced.
Now if you don't use the clutch make sure your unloader works properly and find a way for it to stay open longer so the motor can get up to speed before it gets a load on it. Or use another valve as an unloader.

Soft starting a single phase induction motor doesn't work well. You end up burning up the start windings which aren't meant to run long and are frail.

Oh and old washers used to have centrifugal clutches on them because their motors were way undersized for the job. Just enough to spin the tub but not enough to get it going without burning up. And for electric there are the ones on riding mowers however neither is going to like the beating they get on a compressor. Lots of cycles and lots of heat.

I will do 3 phase eventually because I can score a free good motor from work. Anything smaller than 10hp goes in the recycling bin and doesn't get sent off to be rebuilt.
The 3 phase VFD units are between $350 and $450 for what I would need.
I have added an unloader valve on a time delay to keep the compressor unloaded until it's up to speed and to possibly unload it before shut down if I chose to do so.

The key to soft starting a single phase motor is to start the motor unloaded, as in not coupled to anything and on a resistive starter.

Then engage the load swiftly but not quickly enough to drag motor RPMs down.
I have already tested engaging the stopped compressor to a motor running at full speed. If I jerk the slack out of the belt the motor will surge will far exceed soft start and running amps at shut off.
If something like a magnetic clutch is used that instant on may slow the motor enough to reengauge the start circuit, potentially making amps even higher than what my simple belt pull test showed.
If I tension the belt more gradually I can keep the motor amps less than the start surge and equal to or slightly above shutdown amps.

Also I got the temco 400-480uf start capacitor in, to replace the original 594uf start capacitor. It is replicating the results of my 2 capacitor test, where the reduced start capacitance slightly reduced, stabilized and brought the motor up to speed faster than the 594uf capacitor did.

What about a CVT from a 49cc scooter?

The compressor already has an unloader as part of the cycling switch so you won't need to replicate that.

Had a thought... what about an old comet drive for a go kart? They were the first widely used successful CVT. There were others but they were the only to be popular and have sold the test of time. That would allow you to change the gear ratio at start up, then as the motor comes back up to speed you can start speeding the pump up.

A flywheel is also an idea.


I know how it is with compressors. Got a big one in the shop that I haven't even hooked up yet and won't since we are going to move. Been using my smaller compressors for nail guns etc but miss having air for sanders and drills etc. I love air tools but they are horribly horribly inefficient.
I have a version of the California air tools compressor that I use for hauling around now. It's extremely quiet and airflow is quite good for the style of compressor.

The only 3phase motors under 10hp worth rebuilding are the special purpose ones like the ones used for the pump on a power wash in restaurants. They run upwards of 2k for a new one because they are watertight. Usually they are three phase 1 to 3 hp motors.

Now I do have a coke machine pump and co2 mixing tank I don't know what to use for yet.. I saved it from a Wendy's remodel as they are all going to freestyle machines. Also have an order counter and sign....

I need an additional unloader valve because the mechanical one closes as soon as the mechanical pressure switch closes and sends power to the motor. I need the unloader to stay open until the compressor is up to full speed.

At some point I was going to buy a go-cart clutch for my gasoline air compressor to make cold starting a lot easier, but before I put it on the gasser I was going to put it on my electric air compressor, just to see what happens.

Any clutch or CVT would need to have a 5/8 keyed shaft.

I have also found a nother way to reduce start up in rush.
Some testing has shown unloaded motor starts on 4 pole versus 2 pole motors is reducing starting inrush between 30 and 40% on the 1/2 speed motor.
So I may just switch to 4 pole motors and sell off my 2 pole units.

The magnetic clutch idea may work on a 4 pole motor.
Because I know I can get smaller horsepower 4 pole motors with 7/8 inch shafts. That's important because 7/8 inch keyed shafts have a wide variety of magnetic belt clutches.
Where 5/8 keyed shafts have almost nothing available as far as magnetic belt clutches.

Here is a quick link I found of an import version. Just a CVT which is a mechanical governor.

https://www.aliexpress.com/item/For-Go-kart-Parts-Comet-30-Series-Torque-Converter-Clutch-Replacement-Driver-QPL32/32621649614.html?source={ifdyn:dyn}{ifpla:pla}{ifd bm:DBM&albch=DID}&src=google&albch=shopping&acnt=7 08-803-3821&isdl=y&aff_short_key=UneMJZVf&albcp=653482935 &albag=32646801963&slnk=&trgt=68684682745&plac=&cr ea=en32621649614&netw=g&device=c&mtctp=&gclid=CjwK EAiA_p_FBRCRi_mW5Myl4S0SJAAkezZrTFSyHHzbOib4oEyi60 lSwM8aXuUCMKr_Px1IvTm8zhoCpyTw_wcB

I converted one of my compressors to 240v.
Instead of using a normal 2pole 3,600rpm air compressor motor I found a 4 pole 1,800rpm motor.
I have found that high speed 2 pole motors typically have a momentary start up in rush of 4 times shut off current.
This motor is hitting double shut off amps on startup.
This is normal starting with the belt on the compressor.
4 pole motors have a momentary inrush of 2 or 3 times shut off current.

Alright I have made 3 more advancements towards startup inrush reduction.
First I picked up a 1hp 3 phase wye wound 4 pole motor. free. Only problem is the ABB VFD I want to use isn't free.
I went ahead and bought the centrifugal go cart clutch, only problem with it I see is its only got a 3.2 inch pulley. For a 4 pole motor driving a 10 to 11 inch compressor pulley you want at least a 4 inch pulley, a 5 or 6 inche motor pulley would be better. But I bet I can weld a bigger pulley on to go-cart clutch.
Then of course 5 minutes later I found a magnetic clutch for a 56TC frame pump face motor. Only problem is I don't have a pump face motor with mounting base.
But I can get one.
Free spin starting the motor then slam engaging the motor with say a magnetic clutch should only equal a current surge equal to full power free spin starting which for a 4 pole motor is only about 100 to 120% of full load amps. So that could work.

I did buy the single phase powered ABB drive it should be here next week.
Along with that 4 pole motor.
Looks like the go-cart clutch will be here first thing next week. I'm going test putting it on my 2.5hp 2 pole compressor first. If it helps soften start up current I will adapt it to 4 pole motor use.
So I'll have a 4 pole 1.5hp compressor on a VFD, a 1hp 4 pole motor with a go-cart clutch and a 2hp 4 pole compressor with a electromagnetic clutch.
My prediction is the go-cart clutch will give you the most bang for you buck as far as taming violent startup inrush current. The best thing overall thing you can do is is stick with 4 pole motors.
I will test the magnetic clutch against the go-cart clutch, if the go-cart clutch is better I will abandon the magnetic clutch idea and resell it on ebay.

The clutch was just an idea.... But will be interested to see how it works out and what works best.

I have never liked the magnetic clutches on mowers not just because they readily fail and are expensive but because of the harsh engagement. I can see that need on something that needs a big jolt to get it going but not on a couple blade as they aren't that heavy.

I pretty much have to start the single phase motors unloaded so it needs a clutch of some sort.
The 3 phase motor on a VFD is going to just be belted to the compressor with solid pulleys and no clutch. Just like a normal compressor with solid pulleys.

Alright I have my ABB drive and motor working in hand control only.
The ABB drive has digital input that could be controlled with pressure switches and used to ramp up the speed.

Turns out the cheapest most effective way to reduce start up inrush was sitting in front of me the whole time.
I put the gocart clutch on my 2.5hp 2pole 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 wasn't small enough.
The original belt was way too big 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. I finely got around to taking both belts to the store and selecting a 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 and the 4pole electromagnetic clutch to make happen.

The 2hp 4 pole motor I ordered came in.
Full power starting it free spinning with no power factor correction it hit 14 amps. Resistor starting reduced it to 11 amps. Power factor correction should reduce that by an amp or 2.
That's half of what the 2.5hp 2 pole motor was drawing with the go cart clutch.

I tried putting the go cart clutch on a 4 pole motor and as I expected running the clutch at half speed would not engage the clutch at all.
I took the very strong spring out of the clutch and replaced is with a much weaker spring and it works very well. It looks like it will reduce inrush current from 15 amp down to 9 or 10 amps.

I was told that there wasn't anything I could do to soft start or reduce starting current on an air compressor with a single phase motor (not on here, you guys are npt so negative).
Well on my original 45 amp inrush compressor, just adding a simple go cart clutch cut its inrush in half. For $50 to $60 this will likely be the simplest one step, most bang for the $ way to reduce starting amps.
I am going to replace the 45 amp inrush 2.5hp 2 pole motor with a 2hp 4 pole motor and magnetic clutch and likely further reduce inrush to between 10 and 15 amps. Yes I will have to give up about a half horse power but I will be reducing inrush by up to something like 77%. It wasn't cheap, it wont be super simple but it will happen.
So its not looking good for the nay sayers.
I'm glad I don't listen very well.

The armatures of induction motors are not designed to work in excess of 6Hz 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's on a timer to prevent burning out the start cap.

I needed a much larger than normal 56 inch belt to connect the 2hp 4 pole motor with 6 inch pulley to the compressor. It came in and I finally tested it.
The motor inrush full power starting the compressor and motor together is about 17 amps.
That's a huge reduction from the 2 pole 2.5hp motor that was hitting 45 amps on start up.
Free spin starting the motor hits 14 amps.
Free spin starting the motor with resistors is hitting 10 to 12 amps. But I have not tried all my methodsto reduce starting amps like trying a slightly smaller start capacitor or different resistance values.
When I engage the clutch on the motor running at full speed to the unloaded compreasor the motor only surges up to about 6 amps.

Compared to go cart clutch starting the go cart clutch start hits about 110 to 120% of unloaded starting amps. Which works pretty good for only changing one $50 or $60 part.
Only thing is the go cart clutch comes in one size, 3.2 inches which doesn't work real well with most 4 pole motor setups.

I know I said that on post #2.

I took the 2hp motor put in a smaller start capacitor, with some testing I found the smaller capacitor starting amps trending slightly lower and we're being more consistent.
I also took my big 8 amp 25 ohm rheostat and hooked it up to my resistor start test rig in place of one of the 5 ohm resistors. Found that this motor likes closer to 15 ohms of start resistance.
This lowered staring amps to a consistent 8 amps on the unloaded motor.
Energising the magnetic clutch to the motor running at full speed only peaks at 7 amps.
Tested the pneumatic unloader it drops running amps to 4.5
Full load shut off is at 7.5 amps. With a .96 powerfactor.
Going from 45 inrush amps down to 8 is pretty darn impressive.

That's a nice big drop in amps. Been looking at a new planer and cyclone and the 3 hp and 5hp both pull the same amps on the label... The import motors are horribly inefficient and pull more and most than they should for a given hp.

I run my motors well above rated horsepower.
For example on my 2hp 4 pole compressor it runs a .95 power factor, so 6.5 amps is equal to 2hp.
I run it up to 7.5 amps which is close to 2.5hp. The full load amps rating on the motor is 10 amps, which would make just over 3hp.