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Limited Power To Test Pump


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#1 BRIAN MAY

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Posted 14 December 2007 - 04:52 PM

I NEED TO TEST A 300 HP PUMP AND MOTOR COMBINATION AT 85% OF ITS NORMAL 1770 SPEED. I HAVE STARTERS AND VFD CAPABLE OF RUNNING A 200 HP MOTOR. I NEED TO KNOW IF THE STARTERS AND VFD WILL BE ABLE TO DO THIS IF I DON'T EXCEED 200 HP REQUIRED

#2 jraef

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Posted 14 December 2007 - 06:13 PM

1st off, please take off your All Caps switch. I know it makes it easier for you to type, but it makes it more difficult for us to read. Human reading is not based solely on letter combinations, but also on shapes of words. By using All Caps, you force our brains to slow down and read every individual letter.

If you use a VFD, the motor HP output is proportional to the speed. So at 85% speed, your output will be 85% of the maximum of 300HP, or 255HP, but only IF your motor is fully loaded. So without knowing what the motor loading is, you can't really determine if the 200HP VFD will work. So your first step is to determine, if you can, whether the motor at full speed would be using all 300HP or not. If it is, the VFD will not work. If not, how much less? With a 200HP drive, the equivalent full speed HP would be about 235HP. It's not likely to work though, because if you really only needed 235HP, the pump design engineer probably would have selected a 250HP motor, not a 300HP.

If on the other hand by "test" you mean something under 60 seconds, then the 200HP VFD might do it for that long by using the VFD's overload capacity. But again, that will depend on the VFD you have. A "Constant Torque" rated VFD will be rated for 60 seconds of 150% overload, but a "Variable Torque" rated VFD might be rated for only 115% OL for 30 seconds.

I don't know what the starters issue is. Why would you have starters if you have a VFD?
"He's not dead, he's just pinin' for the fjords!"




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