Eliminating start up inrush current

[oil pan 4]

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.

[Elcam84]

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.

[oil pan 4]

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.

[Clev]

What about a CVT from a 49cc scooter?

[Elcam84]

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....

[oil pan 4]

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.

[Elcam84]

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}{ifplala}{ifd bmBM&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

[oil pan 4]

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.

[oil pan 4]

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.

[oil pan 4]

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.