Sometimes I have to study for the test…

And this test was the final exam.

We were tasked with figuring out how to start two 480 volt 500 horse power motors, each pulling well over 2000 amps, on a 1600 amp breaker.

On any other day we would have ample power and switchgear to handle starting the large mulch grinder motors. But not today. This system was installed in a more rural area with limited power capacity and was previously used to grind tree stumps.

So to accomplish the feat we used a soft starter drive controller to ramp the first motor up to operating speed. I set the drive to limit the motor amperage to just under 2000 amps.

The next step was to tune the 1600 amp breaker to hold 2000 amps for about one minute. I needed approximately 45 seconds to get the grinder motor wound up to full speed. After the motor achieved full rpm’s the motor amperage dropped to a measly 300 amps.

After several attempts I found the balance between over loading the main breaker and getting the motor up to speed in under a minute.

After the first motor was at full speed we could then energize the second 500 hp motor. The two motors where coupled together and ran smooth as silk at idle.

The second it was loaded with wood chips the grinder roared to life, the ground shook and chips turned into mulch. I stepped back a few steps and turned away the first time the hopper was filled to capacity but proud when the motors didn’t seem to slow down at all.

At over 30 foot tall the grinder hopper could handle thousands of pounds of material at a time. It’s pretty spectacular to see it in operation. It was previously set up to grind entire trees so it handles the wood chips pretty good.

This project turned out to be a success but required some reading and studying on how to tune the breakers, soft starter controller and to calculate the load demands.

We had several visits from the power company while commissioning the grinder and they were very helpful in setting up the power systems.

A big thanks to everybody involved in pulling this one off.

Another interesting job in the books.

-Steve

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