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Old March 7th, 2018, 11:13 PM   #1
Ih8Siemens
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PowerFlex 753 OL Current Protection?

Hey fellas,

Been getting absolutely hammered this week with tons of cool and exciting new weird problems so here's another one that I believe I know the answer but would love some other ideas.

The customer has 2 Powerflex 753 VFD's individually controlling 2 motors that operate their own tri plex plunger pumps. (PID controls speed reference for 1, if 1 cant reach setpoint after x amount of time a seperate PID controls both) but we haven't got to the fun of tuning that yet.

We start the first ~325 HP motor and tested it full speed with no problems.
we start the second one (~250HP) by itself and ran it full speed with no problems.

When we start both motors and set them to 44hz the bigger motor's powerflex drops down to 33hz on an OL current fault/alarm.

The only thing I can think of that could cause this is a spike in pressure which forces the drive to work harder? Its getting ~900psi on the discharge line but I almost want to say that the motor is undersized for the application. all the name plate information is correct in the drive, power is all good, I really cant find another explanation for this. Just want to collect a few more ideas before I respond to the customer that they need a new shiny expensive motor.

Thanks for the help fellas,
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Old March 8th, 2018, 12:16 AM   #2
rupej
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Yeah, the only thing I can think is exactly what you're suggesting- the load becomes too high for the motors when both pumps are on at the same time.

Only other thing could be if the feeder isn't sized right, the voltage could be sagging when both are on at the same time, raising the amp draw.
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Old March 8th, 2018, 12:37 AM   #3
Ih8Siemens
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Quote:
Originally Posted by rupej View Post
Yeah, the only thing I can think is exactly what you're suggesting- the load becomes too high for the motors when both pumps are on at the same time.

Only other thing could be if the feeder isn't sized right, the voltage could be sagging when both are on at the same time, raising the amp draw.
I didn't even think of that....I will have the electricians on site check it out tomorrow.
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Old March 8th, 2018, 09:39 PM   #4
Ih8Siemens
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Quote:
Originally Posted by rupej View Post
Yeah, the only thing I can think is exactly what you're suggesting- the load becomes too high for the motors when both pumps are on at the same time.

Only other thing could be if the feeder isn't sized right, the voltage could be sagging when both are on at the same time, raising the amp draw.
***Update***
The motor was in fact under sized.... Someone is taking an *** chewing.
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Old March 9th, 2018, 08:25 AM   #5
James Mcquade
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Something else to consider.
when 2 motors are connected to a vfd, the vfd must be sized to to accommodate both motors plus extra .

james
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