Soft Start For Dummies

In my shop I run a 5HP Ingersol Rand 2 stage compressor. The first motor lasted just a little longer than the warranty, but until it let the magic smoke out it started the compressor every single time.

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I put this PZEM-061 on my mongrel 80 gallon compressor to observe and compare the current draw to the motor's rating, to match the motor pulley to a replacement compressor pump.

Might your unloader have failed and be forcing the motor to start against pressure?

My 1/2HP compressor wouldn't start from a 3KW generator until I modified a blowoff valve into a manual lever controlled unloader. The genny won't start my ancient Maytag unless I push the spring-loaded motor inward to loosen the belt.

Would that be able to show if there was a significant voltage sag while it was trying to start? Transient events like that (lasting just seconds) can be hard to spot...

It sure sounds like the motor is a tad too small for the task. And/or needs the pulley system to run the pump slower for a given motor speed.

Joe Gwinn

Would that be able to show if there was a significant voltage sag while it was trying to start? Transient events like that (lasting just seconds) can be hard to spot... Leon Fisk

------------------------- They aren't as fast or good as analog meters for detecting glitches, but they will indicate high circuit resistance that could cause a voltage sag by the current and voltage drop when running. One reason is that the changing digital display isn't as noticeable in your peripheral vision as a sudden needle jump. I leave an analog Amprobe on troublesome old motors to show when the start switch has stuck open. The PZEM-061 like is a 240V, 100A Kill-A-Watt. Installing one in an electrical box cover is a good test of layout and filing skills. I'll just say that I've gotten better at it.

A voltage sag is more a symptom of a problem in your supply circuit. Although it takes more test equipment I prefer to monitor the current draw to tell if something is operating normally or has a problem.

My UT61E DVM supposedly can capture peak transients but its readings of them don't relate well to oscilloscope captures. My datalogger uses it and other RS232-output DVMs as sensors and won't capture events shorter than a second.

The best I have to capture current spikes on a scope is a Hantek Hall effect probe for DC and a Fluke current clamp for AC. They were fairly inexpensive and good enough for hobby use such as measuring refrigerator starting surges, to select a large enough inverter.

I assure you its very unlikely I will ever buy another IR compressor.

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Thanks, good info👍

Analog meters were still widely used when I started working. Switched to a digital (bought my own) a few years in. The analog meter spike/twitch was sorely missing. Maybe 10 years later I bought a Fluke

87 which had the bottom bar in the display that updated 4x? faster than the digital display. Was quirky but helpful once you got used to it...

My compressor motor was originally wired for 115 and it kicked out on start-up about hnalf the time when cold and about 10% of the time when warm. I switched it over to 230 and it hasn't had a single issue since. I even put a bigger pulley on it after converting to 230. My original motor was a repulsion start monster(with brishes) but the start switch went bad and parts were unobtanium so I switched to the cap-start

In my shop I run a 5HP Ingersol Rand 2 stage compressor. The first motor lasted just a little longer than the warranty, but until it let the magic smoke out it started the compressor every single time.

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Here is a better controlled alternative to a mechanical unloader. It might reduce the starting surge enough to make the motor last much longer.

Their indirect valve operates like the water valves on a lawn sprinkler system. It does the same thing as my manual cammed-open pressure relief valve, lets the motor start without a load. A mechanical unloader releases the air between the compressor head and the tank check valve when the pressure sensing diaphragm in the control unit rises to shut off the motor, but the head outlet pipe's volume is low and the unloader vent valve closes when the diaphragm drops and pressure builds again on the first piston upstroke. A timed unloader can vent the air until the motor reach full speed.

I think you could add one in parallel with the mechanical unloader, the compressor won't build pressure until both unloaders have stopped venting.

Today at the ham radio flea market I picked up and examined a 1-10 second time delay relay of the type I had used in industrial relay logic controllers in the 70's. The seller explained using it to time a solenoid valve unloader on a compressor, which suggests he had seen it done. I was just trying to remember if I already had one like it. I built a 240V spotwelder timer with another one that I set to 0.2 seconds for 22 gauge galvanized steel.

My solution was to increase the volume of the line between the outlet of the compressor and the check valve at the inlet to the tank, providedof course, that this line is de-pressurized when the motor is awitched off.

My solution was to increase the volume of the line between the outlet of the compressor and the check valve at the inlet to the tank, providedof course, that this line is de-pressurized when the motor is awitched off.

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I did that too, by adding a loop of Parker Push-Lok air hose that won't degrade from vibration as unsupported copper tubing might. My HF compressor pump has a built-in air cooler attached to the head, that adds some volume.

I think the best approach is to capture and compare the motor starting surge with a digital storage scope and current clamp, both as-is and with the compressor outlet disconnected to simulate a timed unloader.

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I've been investigating soft start options and mainly found $200-$300 commercial units and much cheaper DIY solutions that require knowledge, construction and test equipment. It seems the common requirement is reducing the air pump starting load so I posted what I found about that. Reducing the motor start current is a separate issue that might be solved or helped with an unloader.

like everybody else says reduce the pulley. Your line voltage may also be too low. A three phase motor at 5hp should cost less than 1 single phase one as no other junk like starters or capacitors are needed. The do make soft starters for 3 phase motors at under $100. They mess with the power going to one phase, but it works. The starter coils on single phase motors draw a ton of power and could be the overheating cause if you're constantly starting and stopping that motor. 3 phase motors don't have this issue at all.

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This reminds me that motor startup is critically dependent on the AC wiring gage; that '230' conversion using similar gage wiring has less resistive power loss at any given load than the 120V connection, because the higher voltage uses less current. A long, not very heavy, wiring connection from the breaker box to the socket that motor plugs into, would have the overheats-at-start effect, even on a properly designed motor and compressor with working unloader.

If you get overheating during starts, suspect the wall wiring.