Soft-Starting a Compressor

      A properly located buffer tank in the air line can keep a compressor from stalling or tripping the breaker on start-up. June 13, 2011

I have a 25 hp phase converter in my shop. Just hooked up a 10 hp compressor. With an empty tank, the motor starts and builds to pressure, no problem. When the pressure drops to the low side, the motor tries to start but can't, and throws the reset. We moved this from a building with true 3 phase - no problems at all there. All other equipment runs with no problems on converter.

Forum Responses
(Solid Wood Machining Forum)
From contributor D:
I have a similar problem. My electrician said that either the heat sinks in the square D box needed to be upgraded or the power wire running to the machinery or converter was not large enough. He said at least 10 wire is sufficient.

From contributor L:
My electrician and the folks my converter came from said the same thing. I ran a 6 ft. cord of # 8 wire directly from the breaker. No difference, though.

From contributor G:
Does your compressor have a pressure unloader? It sounds as if your compressor is trying to start with the second stage fully loaded. The motor cannot get enough hp out to do it and the line trips out (with empty tank, this is not a problem). Check the unloader valve. (I am assuming that it has stopped functioning between the time it was connected to the old 3 phase and the new converter installation, or the wiring was changed, if electric.)

The only other thing that comes to mind is that the heaters in the starter need replacing, assuming here that the motor is not getting the full three phase power from the converter and is drawing heavier start current on the new installation. Check, with amp meter, the draw, and restart vs. empty start, and check starter chart to see if you have the right heater installed.

From contributor L:
The motor, under normal conditions, is starting under the load of the compressed air in the pipe before the tank. There is a check valve at the tank and a short pipe from the compressor head. It doesn't take many revolutions of the compressor before the pipe is loaded and the motor is still starting against a load. This is fine unless it takes the motor too long to come up to speed, the correct RPM, as found with motors starting on phase converters. A solution is to install a buffer tank between the compressor head, where the pipe is, and the check valve at the tank. This will allow the motor, starting on a phase converter, to come up to the correct RPM before the compressed air load is felt. The tank needs to be large enough to collect air for the delay.

From contributor R:
This is a common problem. One solution is to have the compressor unloaded until it reaches speed and then let it compress air. A time delay relay and an electric air valve should do the trick.

From the original questioner:
Just wanted to say thanks to contributor L. I put in a stack of three 3 inch x 12 inch pipes between pump head and tank this morning. Problem solved.

From contributor L:
I better run this one by you. I burned out a motor on a compressor that was starting under load on a phase converter. The problem was the glue pot heater coils on an edgebander that I started were draining a lot of the power from the converter. The compressor came on and stalled. More like fried - flames and all. That is when I discovered the buffer tank.

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