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Old 01-11-17, 02:07 PM   #1
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Default 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.


Last edited by oil pan 4; 02-13-17 at 08:01 PM..
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Old 01-14-17, 09:11 AM   #2
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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.

Last edited by oil pan 4; 01-14-17 at 02:52 PM..
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Old 01-14-17, 11:18 AM   #3
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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.
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Old 01-14-17, 12:40 PM   #4
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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.
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Old 01-21-17, 01:11 AM   #5
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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.
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Old 01-22-17, 11:38 PM   #6
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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.
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Old 02-09-17, 09:14 PM   #7
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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.
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Old 02-12-17, 04:03 PM   #8
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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.
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Old 02-12-17, 09:06 PM   #9
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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.
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Old 02-13-17, 06:15 AM   #10
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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.

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